Progress of High Temperature Niobium Alloy for Aerospace Applications

Niobium metal has a low density (8.57g/cm3), high melting point (2741 K), high plasticity, good corrosion resistance and low vapor pressure, etc.,. Niobium alloy has a higher temperature (600 ~ 1600 ℃) than the strength and good hot and cold machining performance, you can make the shape of the complex parts, is one of the important candidates for aerospace structural components. It can be used to manufacture critical components for rocket engines, space-earth round-trip airships, supersonic aircraft, satellites, missiles, and nuclear reactors, including shields for combustion chambers of high-thrust space engines, combustion chambers, small-vector or attitude-control nozzles, and extended shields for orbit-control engines, and so on.

In the 1960s, traditional niobium-based alloys began to be used in the aerospace and nuclear industries, with the most widely used alloy being the C-103 (Nb-10Hf-1Ti) niobium alloy, which can be used for high-temperature valves, rocket booster tops, and damper blades for turbine booster units. The extension of the radiation cooling nozzle of the descent engine of the lunar module of the Apollo 11 spacecraft of the United States is also processed with C-103 niobium alloy and coated with anti-oxidizing alumina. the combustion chamber of the R-4D reaction-control engine is processed and made of SCb-291 (Nb-10W-10Ta) and C-103 niobium alloys. In the 1990s, along with the development of aerospace technology, various countries successively developed C-103 niobium alloys for rocket engines, satellite attaches, and rocket engines. In the 1990s, with the development of aerospace technology, countries carried out a new round of competitive development in the fields of rocket engines, satellite attitude control engines and ultra-high speed airplanes, etc., which put forward higher requirements for high-temperature structural materials, and niobium alloys have received further attention again.
In response to the aerospace industry’s needs, the United States and the former Soviet Union have developed more than 20 types of niobium alloys, forming their own system. The niobium alloys of the United States are mainly reinforced with W, Mo and Hf, while those of the former Soviet Union are mainly reinforced with W, Mo and Zr. The second-phase dispersion reinforcement is mainly carbide, which are primarily used in the two-component liquid rocket motors with high specific impulse and adjustable thrust that can be started many times. Among them, the United States to C-103 alloy-based, temperature up to 1200 ~ 1400 ℃; the former Soviet Union to niobium alloy 5 ΒΜЦ (Nb-5W-2Mo-1Zr) alloy based on the density of the alloy and C-103 similar, but the use of the temperature can be reached 1200 ~ 1650 ℃, for a short period of time can be reached 2000 ℃. Based on American and Russian niobium alloys, China has copied and researched and developed niobium alloy structural materials for aerospace engines such as C-103, Cb-752, C-129Y, D43, SCb-291, and Nb521, among which C-103 and Nb521 alloys are the most widely used.
Based on the classification of traditional niobium-based high-temperature alloys, this paper highlights the research progress of C-103, PWC-11 and Nb521 alloys, as well as low-density niobium alloys, discusses the current problems of niobium alloys for aerospace applications, and looks forward to the direction of future development.

1. Classification of Niobium Alloys

Niobium alloys are usually classified into six categories according to their strength levels and functional properties: the first category of high-strength low-plasticity niobium alloys, the second category of medium-strength medium-plasticity niobium alloys, the third category of low-strength high-plasticity niobium alloys, the fourth category of high-strength antioxidant niobium alloys, the fifth category of plastic antioxidant niobium alloys, and the sixth category of corrosion-resistant niobium alloys. According to density, they are divided into high-density and low-density niobium alloys. According to different functions, they are divided into structural alloys, functional precision alloys and corrosion-resistant alloys. The main structural alloys used in the aerospace field are roughly divided into three categories of high strength, medium strength and low strength according to their strength, low-density niobium alloys have been developed in consideration of weight reduction, and high-strength niobium alloys reinforced with interstitial compounds (carbides, oxides, and nitrides) have been developed in order to improve the comprehensive performance of niobium alloys continuously. Niobium alloys of various strengths are currently produced or have been studied as listed in Table 1.

1.1 High-strength, low-plasticity niobium alloys

Generally, high-strength, low-plasticity niobium alloys are strengthened by adding the alloying elements W, Mo and Ta, small amounts of Hf and Zr, and traces of C. These niobium alloys are also known as Cb-niobium alloys. These niobium alloys include Cb-1, As-30, Cb-132M, F48, Su-31, etc., mainly used for gas turbine blades. The W, Mo and Ta elements in these alloys are solid solution strengthened, and Hf and Zr can form a diffuse second-phase strengthening phase with C, giving the alloys higher creep strength. Their solid-phase temperature is higher than that of pure niobium, and their recrystallization temperatures are also higher. Their high-temperature strengths are significantly higher than those of medium- and low-strength niobium alloys, and their operating temperatures are generally in the range of 1,300 to 1,600°C, with even higher temperatures for short periods of time. The antioxidant property is improved somewhat because of W, Hf and other elements. However, with the increase in the content of W, Mo and other high melting point reinforcing alloying elements, the plastic-brittle transition temperature will also rise, and its plastic processing performance deteriorates, deformation processing is difficult. To ensure a good match between high-temperature strength and low-temperature plasticity, the thermomechanical processing of these alloys must be strictly controlled. In addition, high-strength niobium alloys are primarily in the development stage, and the niobium alloys currently in industrial production are primarily medium- and low-strength niobium alloys.

1.2 Medium strength, medium plasticity niobium alloys

Medium-strength and medium-plasticity niobium alloys are mainly composed of niobium as a base with the addition of not more than 10% of metal elements such as W, Mo, Ta, V, Ti, Zr, Hf and a small amount of C. These alloys include C-129Y, SCb-291, D31, D43, FS85, Cb-752, PWC-11, 5ВНЦ and Nb521. The strength of these alloys at room temperature is 400-600 MPa and the elongation is 20-30%; at high temperatures of 1000-1400°C they are still quite strong and can work efficiently, and if the time is short, the working temperature can be higher. Since this kind of alloy contains moderate amount of W, Ta, Ti, Zr, Hf, the recrystallization temperature is increased to 1150~1250℃. At the same time, because the plastic-brittle transition temperature of this type of alloy is higher than pure niobium, the transition temperature in the welded state is generally above room temperature. It is more sensitive to the interstitial elements such as O, N, H, etc., so it is necessary to control the contamination of O, N, H strictly, and the oxygen content in the alloy must be controlled at less than 80 ppm. This kind of alloy has a certain degree of plasticity and good process performance can be used to manufacture various parts and components, such as skin, bolts and, nuts, and other components.

1.3 Low-strength, high-plasticity niobium alloys

Low-strength, high-plasticity niobium alloy is niobium as a substrate, the addition of Ⅳ of the periodic table of elements such as Ti, Zr, Hf and other metal elements to form a solid solution strengthened alloy. The alloys belonging to this category are Nb-1Zr for liquid metal containers and pipelines, C-103 alloy for rocket engine thrust chambers, radiation sleeves, and heat shielding, and Cb-753 alloy for ion engines with honeycomb structures. The recrystallization temperature of these niobium alloys is about the same as that of pure niobium, which is generally 1000-1100°C. The room-temperature strength of the alloys is generally 3,000-3,000°C. The room-temperature strength of the alloys is 320-420 MPa, and the elongation at break is 20-40%. Such alloys have good welding performance. The plastic-brittle transition temperature is low, in the (0.37 ~ 0.47) T melting temperature range and vacuum; after aging treatment of plastic-brittle transition temperature is still lower than room temperature. These alloys, compared with medium and high strength alloys, have good plasticity at room temperature, with excellent process performance, and can be made for liquid alkali metal piping, space and power generation equipment turbopumps, satellites, spacecraft, and missiles, attitude control/orbit control engine thrust chamber body extensions and other components.

Table 1 Niobium alloys of various strength grades

Classification Alloy Designation Nominal Composition Semi-finished Product State Temperature (°C) Rm​ (MPa) Rp0.2​ (MPa) A (%)
High Strength Cb-1 Nb-30W-1Zr-0.06C Annealed rod 1315
B88 Nb-28W-2Hf-0.067C 1315 372
VAM-79 Nb-22W-2Hf-0.067C
Cb-132M Nb-15W-5Mo-2Ta-2.5Zr-0.13C Annealed rod 1315 407
As-30 Nb-20W-1Zr-0.1C Extruded rod (ε80-90%) 1315 633 580 9
F-48 Nb-15W-5Mo-1Zr-0.1C Cold working plate (ε80-90%) 1095 450 295 18
Su-31 Nb-17W-3.5Hf-0.1C Annealed rod 1315 281
C-3009 Nb-22.4Hf-5.9W Ingot 1200 588
Medium Strength and Plasticity Su-16 Nb-11W-3Mo-2Hf-0.08C 1100 333 231
Fs-85 Nb-10W-28Ta-1Zr Annealed 1315 161
F-50 Nb-15W-5Mo-1Zr-5Ti-0.1C Cold working plate (ε80-90%) 1205 146 90 35
D-31 Nb-10Mo-10Ti Annealed rod 1095 260 184 8
D-43 Nb-10W-1Zr-0.1C Extruded rod 1260 253 225 5
Cb-752 Nb-10W-2.5Zr Annealed plate 1649 70.3 63.3
SCb-291 Nb-10W-10Ta Annealed sheet 1649 70.3 59.3
C-1297 Nb-10W-10Hf-0.1Y Annealed plate 1649 74.4 60.7
B-66 Nb-5Mo-5V-1Zr Annealed plate 1315 372
B-77 Nb-10W-5V-1Zr Annealed plate 1315 391
As-55 Nb-5W-1Zr-0.1Y-0.06C
PWC-11 Nb-1Zr-0.1C 927 160
5BHt1 Nb-5W-2Mo-1Zr Annealed rod 1250 120-140 80-110 8-10
Nb521 Nb-5W-2Mo-1Zr-0.07C Annealed rod 1600 ≥70 ≥60 ≥4
Low Strength and High Plasticity Cb-753 Nb-5V-1.25Zr
D-369 Nb-10Hf-1Ti-0.7Zr Annealed plate 1095 199
1427 703
D-36 Nb-5Zr-10Ti
Cb-1Zr Nb-1Zr Rotary swaging rod 927 130

2. Progress and applications of niobium alloys for aerospace applications

At the end of the 1950s, there was a strong interest in nuclear power, aviation and space flight, and the development of related niobium alloys began, and the development of commercial conventional niobium-based alloys began in the 1960s. In the United States, the development of niobium alloys in the early units are mainly CANEL, Wright Patterson AFB (WADD), Boeing Aircraft Company, General Electric Company, Union Carbide Corporation and Westinghouse. Among them, CANEL is primarily engaged in Cb-1Zr and PWC-11 alloy research, Boeing and TWCA developed C-103 and C-129Y alloy, General Electric Company developed As-55, As-30, F48 and F50 niobium alloys, Union Carbide developed Cb-752 alloy. Around 1970, the development of medium and low-strength niobium alloys in foreign countries has been relatively mature, and the niobium alloys that have been used in rocket nozzles or thrust chambers are C-103, SCb-291, C-129Y, FS85, etc. Among these alloys, C-103 alloy has been used in rocket nozzles and thrust chambers, and C-129Y alloy has been used in rocket nozzles and thrust chambers. Among them, C-103 alloy processing, welding performance is excellent, although the room temperature strength is low, but the overall performance is good, especially at high temperatures, can meet the working conditions of the nozzle. SCb-291 alloy plasticity is good and high temperature strength is relatively high, C-129Y alloy also has a good plasticity and welding performance, but the creep strength is low, FS85 alloy creep strength is high, plasticity, welding performance is very good. Performance is excellent. In terms of overall performance, Cb-752 and D43 alloys have high-temperature strength, moderate plasticity, and good machining and weldability and can be selected for rocket nozzles. However, niobium alloys also have a fatal weakness: is easy to oxidize in the atmospheric atmosphere at high temperatures, that is, at about 600 ℃ will be very easy to “pest” oxidation phenomenon, the need to prepare a layer of antioxidant protective coatings on the surface, to meet the high-temperature requirements of the use of the thrust chamber of the space engine.

In the 1970s, people began to study high-strength niobium alloys, the primary strengthening method is still solid solution strengthening or dispersion strengthening two ways mainly; during this period, the former Soviet Union and the United States have carried out a lot of research on the development of high-strength niobium alloys, China’s research in the field of this type of material is still in the blank. With the continuous development of new aerospace products and the need for upgrading, the orbit control/attitude control liquid engine’s specific impulse and weight reduction have put forward higher requirements. Consideration is given to starting from the weight reduction of the base material, which in turn develops the low-density niobium alloy with a density of less than 8g/cm3.
In aerospace, the United States is the most widely used C-103 alloy, using temperatures between 1200 ~ 1400 ℃, followed by PWC-11, Nb-1Zr, SCb-291, FS85, and other alloys. The coatings are mainly of the silicide system, such as R512A (Si-20Cr-5Ti) and R512E (Si-20Cr-20Fe). The most used alloy in Russia is 5ΒΜЦ alloy with a service temperature of 1200-1650°C, usually coated with molybdenum silicide (MoSi2). China’s most used alloys are C-103 and Nb521. The current orbit control/attitude control engine refractory metal materials thrust chamber has formed a “two-generation” series of products. The “first generation” is a niobium-hafnium alloy (i.e., C-103) and “815” coating system, and the “second generation” is a niobium-tungsten alloy (i.e., Nb521) and “056” coating system. The “second generation” is a niobium-tungsten alloy (i.e., Nb521) and a “056” coating system. Physical photos of the thrust and combustion chambers of the two generations of niobium alloy engines in China are shown in Figure 1. The physical properties of the two niobium alloys are compared in Table 2, and their high-temperature tensile properties are shown in Figure 2. From Fig. 2, it can be seen that the high-temperature mechanical properties of Nb521 are much higher than those of C-103 alloy, and its strength is 3-4 times higher than that of C-103 alloy at 1600℃, and it has been successfully applied to many kinds of orbit/attitude control type engines. Some examples of niobium alloys used in the aerospace industry at home and abroad are listed in Table 3.

Fig. 1 Photos of thrust chamber and combustion chamber of China's first and second generation engines

Fig. 1 Photos of the thrust chamber and combustion chamber of China’s first and second-generation engines

(a) The first generation of thrust chambers with different specifications(C-103 + 815 coating) ;

(b) The second generation engine thrust chamber(Nb521+ 056 coating);

(c) The second generation engine combustion chamber(Nb521+ 056 coating);

(d) The First (C-103,the specific impulse in 305 seconds)and second (Nb521,the specific impulse in 315 seconds)generation 490N engines

Table 2 Physical properties of typical niobium alloys

Alloy Melting Point (°C) Density (g/cm³) Thermal Conductivity (W/(m·K)) Coefficient of Thermal Expansion (10⁻⁶ K⁻¹) (20–1205°C) Working Temperature (°C) Ref.
C-103 2350 8.86 41.9 8.10 1100–1450 [16,27]
Nb521 2452 8.85 48.7 7.40 1370–1650 [27,40]
Fig. 2 Tensile properties of two niobium alloys: (a) C-103 and (b) Nb521

Fig. 2 Tensile properties of two niobium alloys: (a) C-103 and (b) Nb521

Table 3 Applications of niobium alloys in the aerospace industry

Nationality Alloy Application Examples
USA C-103 R-4D (490N), R-1E (110N), and R-6C (22N) thrust chambers of rocket engines, attitude control of Apollo lunar module and service module, Agena auxiliary propulsion system, combustion chamber, and nozzle, wings of the Apollo engine, compression cone of a launch vehicle, and structural material for the orbital control engine of the space shuttle.
PWC-11 The reactor for satellites, space nuclear power systems, and high-temperature reactors.
Nb-1Zr Minuteman III MK12 attitude control engine, injector. Poseidon’s final boost control system. Exhaust gas regenerator for Poseidon engine system.
SCb-291 2670, 1330, 90N shuttle attitude control engine. MinutemanⅢ MK12 attitude control engine and thrust chamber.
Cb-752 Booster glide manned vehicle. Space shuttle and manned aircraft, screw joints.
FS-85 Space nuclear energy system, liquid metal container.
C-129Y Space shuttle and manned aircraft, screw joints.
Russia 5BMЦ 12, 20, 100, 135, 200, 400N engine thrust chamber is used in satellites such as “Cosmos”, “Crystal”, “Quantum”, “nature” and “Spectrum”, in spacecraft such as “Progress” and “Soyuz-T”, and in space stations such as “Salute” and “Peace”.
China C-103 490N engine, used for all kinds of satellites, Shenzhou spacecraft, Tiangong and carrier rockets, etc.
Nb521 The nozzle of the gas-oxygen kerosene engine of a new generation of the high-thrust launch vehicle. 25, 120, 150, 250, 490N engine, Attitude and orbit control engine for Sinosat 6, Nigeria Satellite, etc. China Lunar Rover project engine.

The following is a summary of the progress of C-103, PWC-11, Nb521 alloys, and low-density niobium alloys under research and development.

2.1 C-103 niobium alloy

C-103 (Nb-10Hf-1Ti) alloy is a kind of low-strength niobium alloy with good high-temperature strength, excellent molding performance and welding performance jointly developed by ATI Wah Chang and Boeing, which is widely used in rocket propellers and other fields, and is a kind of niobium alloy with a wide range of applications. Our country imitated this alloy used in two-component liquid rocket engines, and using niobium silicide high-temperature anti-oxidation protective coating, the working temperature can reach 1200 ~ 1300 ℃.

The alloy is a solid solution strengthened by adding Hf, Ti, and Zr alloy elements. Zr strengthening is more significant; Hf and trace W are mainly used to improve high-temperature performance. Panwar et al. investigated the tensile properties and fracture behavior of C-103 alloy in the temperature range between room temperature (RT) and 1200℃ (see Fig. 3), the stress-strain curve of the alloy shows a sawtooth shape and occurs in a high-temperature range from RT to 1200℃, the stress-strain curve of the alloy shows a sawtooth shape and occurs in a low-temperature range. The stress-strain curves of the alloys between room temperature and 1200°C show a sawtooth shape, and the dynamic strain aging (DSA) phenomenon occurs. Among them, dynamic strain aging plays a dominant role in the tensile properties at 900°C, and dynamic recovery and oxidation significantly influence the tensile properties beyond 900°C. The fracture mechanisms were ductile fracture, disintegration fracture and along-crystal fracture from room temperature, 600~900°C and 1000~1200°C, respectively. In addition, the addition of Hf element has another function to improve the resistance of the alloy to internal oxidation. Sankar et al. investigated the effect of internal oxidation on the organization and mechanical properties of C-103 niobium alloy. The microstructure and mechanical properties of alloy samples with different oxygen contents (100~2500 ppm) were characterized, and it was found that the strength and plasticity of the alloy decreased with the increase of the average oxygen content, and the internal oxidation led to surface embrittlement of the alloy. Dhole et al. found that industrial C-103 alloys prepared using the vacuum arc remelting (VAR) method were characterized by a pronounced internal oxidation, that is, at all grain boundaries, there was no internal oxidation in the alloys. Monoclinic HfO2 was present at all grain boundaries and coarsened HfO2 second-phase particles were present at some of the large-angle grain boundaries. Cold rolling cracking occurs even when a small deformation is applied during cold rolling (see Fig. 4). Further microscopic observation reveals selective cold cracking at the Nb-HfO2 interface. Density Functional Theory (DFT) calculations on the metal-oxide interface model indicated that the chemical nature of the termination layer significantly affects the work of separation and depolymerization. In the termination layer of the Nb-HfO2 interface, their separation energies differ significantly due to the difference in atomic bonding, especially the separation work of the single-atom layer interface between Nb and the oxygen bound in HfO2 is very low, about 8% lower than that of the Nb-Nb interface of the substrate. That is, DFT simulations reveal the decisive role of the termination layer at the metal-oxide interface for interfacial depolymerization.

Adding a small amount of Ti to the alloy not only improves the machinability of the alloy, but also improves the antioxidant properties. The interstitial impurities C, N, H, and O elements have a great influence on the mechanical properties of the alloy, and when the oxygen content exceeds 0.1%, it will lead to difficulties in subsequent cold working. In addition, the oxidation resistance of the alloy deteriorates above 450°C. Therefore, care should be taken to avoid thermal oxidation during thermal processing, and inert gas protection or vacuum equipment is usually used. The metal surface is usually coated with silicide antioxidant protective coatings to prevent oxidation when used at high temperatures. Sankar et al. used slurry coating and vacuum diffusion technology to prepare Fe-Cr alloy silicide coating on the surface of C-103 alloy, and observed that the coating microstructure of the three-layer structure, the outer layer of the NbSi2 phase, the inner layer of Nb5Si3 and Nb3Si and other low silicide, the intermediate layer of Fe-Cr alloy is composed of low silica, and the middle layer is composed of Nb5Si3 and Nb3Si, and the middle layer is composed of Nb5Si3 and Nb3Si. The middle layer comprises the Fe-Cr alloy niobium silicide phase and NbSi2 (see Figure 5). The results show that the coating provides good short-term protection of the substrate against high-temperature oxidation in air at 1100 and 1300 °C, and the presence of the coating also increases the tensile strength of the alloy.

Fig. 3 Tensile properties of C-103 alloys[47]: (a) Comparison of plastic region of engineering stress-strain plots at various temperatures;Schematic drawing of stress-strain plots from (b) 600~900℃ and (c) 1000~1200℃

Fig. 3 Tensile properties of C-103 alloys[47]: (a) Comparison of plastic region of engineering stress-strain plots at various temperatures; Schematic drawing of stress-strain plots from (b) 600~900℃ and (c) 1000~1200℃

Fig. 4 EBSD maps of C-103 alloy before and after cold rolling and the schematic of Nb-HfO2 interface(a) EBSD inverse pole figure (IPF) maps showing grain structures and boundaries before and after cold rolling; (b) The schematic of the Nb-HfO2 interface

Fig. 4 EBSD maps of C-103 alloy before and after cold rolling and the schematic of Nb-HfO2 interface (a) EBSD inverse pole figure (IPF) maps showing grain structures and boundaries before and after cold rolling; (b) The schematic of the Nb-HfO2 interface

Fig.5 Microstructure and composition of silicide coated Nb-alloy C-103(a) Cross-sectional microstructure of overall coating and (b) magnified view of L1, L2 and IDZ; (c) EMPA concentration profiles of various elements across the coating; (d) X-ray diffractograms obtained from surface of uncoated and coated alloy

Fig.5 Microstructure and composition of silicide coated Nb-alloy C-103 (a) Cross-sectional microstructure of overall coating and (b) magnified view of L1, L2 and IDZ; (c) EMPA concentration profiles of various elements across the coating; (d) X-ray diffractograms obtained from surface of uncoated and coated alloy

ATI Wah Chang prepares C-103 niobium alloy ingots by electron-beam secondary remelting. These are then machined, clad and sheathed, hot-extruded into slabs, and then cold-rolled and processed into thin plates. For smaller components or propellers, the bars are generally processed directly. For the manufacture of larger combustion chambers requiring larger diameter bars, or the use of bar taper die reverse extrusion molding process can effectively improve the components’ yield and the formation rate. The good forming performance and stable reliability make C-103 niobium alloy have excellent cost performance and further promote its application in the aerospace field.

2.2 PWC-11 niobium alloy

PWC-11 (Nb-1Zr-0.1C) alloy is a niobium alloy developed by Pratt & Witney Aircraft Company for high-temperature applications with moderately strong plasticity, good resistance to high-temperature creep, and resistance to corrosion by liquid alkali metals, and it is widely used in high-temperature reactors and space nuclear power systems.

The alloy is simultaneously strengthened by solid solution and precipitation by adding Zr and C elements to Nb to increase the high temperature strength of the alloy. Therefore, the carbide precipitation strengthening of PWC-11 alloy has attracted much attention.The transformation of Nb and C to a mixture of ZrC and NbC during heat treatment has been confirmed by thermodynamic analysis by Farkas et al. Free energy calculations showed that the maximum NbC concentration exists in the mixed carbide particles at a certain aging temperature. The higher the aging temperature, the higher the equilibrium NbC content in the particles.The microstructures of Nb-1Zr-0.1C alloy in as-cast, extruded, and annealed states were studied by Vishwanath et al. The results showed that: the as-cast state organization is a two-phase organization in which the Nb2C precipitation phase is distributed in the matrix in the form of needles; there are two types of precipitation phases in the extruded state specimens (see Fig. 6(a)~(c)), one is needle-like ((Nb, Zr)2C with an orthorhombic crystalline structure, and the other one is rectangular ((Nb,Zr)3C2 with hexagonal structure); and the organization in the annealed state shows an equilibrium structure, i.e., the Nb matrix and (Nb, Zr)C precipitates (see Figs. 6(d)~(f)). Vishwanath et al. investigated the formation of γ-Nb2C and the interrelationships of the other Nb2C carbide phases (α, β) in the Nb-1Zr-0.1C alloy. The results show that the transformation of Nb to γ-Nb2C is realized by carbon atoms occupying octahedral positions in the Nb bcc lattice, whereas the sequence of phase transitions γ-Nb2C → β-Nb2C → α-Nb2C involves the arrangement of vacancies in a lattice that is essentially the same for all three structures (see Fig. 7). Lattice strain calculations show that the strain due to the carbon atoms occupying octahedral positions arises mainly along the [0001] direction of the Nb2C lattice, and this strain also contributes to the dissolution of the substable disordered structural phase, γ-Nb2C phase, at lower temperatures. In order to further clarify the formation of various carbides and the Nb2C→NbC carbide transition in Nb-1Zr-0.1C alloy, Vishwanadh et al. prepared specimens using two methods (i.e., solidified specimen extrusion + recrystallization treatment and solidified specimen + heat treatment) and found that the alloy contains only α-Nb2C carbides in the solidified state, and the Zr phase is uniformly distributed between Nb matrix and Nb2C phase. When Zr is uniformly distributed in the Nb matrix and Nb2C phase, α-Nb2C remains stable; when the alloy is extruded or heat-treated, Zr preferentially diffuses into the α-Nb2C carbide phase, destabilizing the carbide phase, and precipitates out in the subsequent annealing treatment as a more stable (Nb,Zr)C phase with a spherical or acicular morphology, which is specifically related to the thermo-mechanical processing of the alloy; if the precipitated phase nucleates before recrystallization and the If the precipitated phase nucleates before recrystallization, and the growth of the precipitated phase occurs at the same time as recrystallization, it is spherical and has no specific orientation relationship with the matrix phase. If the precipitated phase nucleates and grows after recrystallization or in the uncrystallized sample, it is needle-like and follows a specific orientation with the matrix phase (see Figure 8).

The PWC-11 alloy has very good machinability and fabrication properties compared to other niobium, tantalum, and molybdenum alloys, and Sarkar et al. used vacuum uniaxial compression experiments to study the high-temperature thermal deformation behavior of the Nb-1Zr-0.1C alloy in the range of 700-1700°C and 10-3~ 10s-1 strain rate, and found that the alloy was stable at temperatures greater than 1400°C and at a wide range of strain rates ( 10-3~ 10-1s-1) range will occur dynamic recrystallization, with better processing performance; at the appropriate temperature and strain rate, the dynamic recrystallization grain size of the material can be changed, while below 1000 ℃ and in the range of 10-3~ 10s-1 strain rate are occurring in the region of strain localization, which should be avoided as much as possible in the industrial thermal processing process.Behera et al. also used the Behera et al. also used uniaxial compression experiments to investigate the evolution of dynamic recrystallization with strain in Nb-1Zr-0.1C alloys at 1500 and 1600°C and a strain rate of 0.1s-1. The results show that at strains of 0.6 and 0.9 and temperatures of 1500 and 1600 °C, dynamic recrystallization grains are distributed in a necklace-like manner along the jagged large-grain grain boundaries; at 1500 and 1600 °C, fine grain distribution can be observed at a strain of 0.9, and equiaxial organization is seen at a strain of 1.2 (see Fig. 9); at 1500 and 1600 °C, and at all the measured strains, the dynamic recrystallization evolution of the alloys with strains of 0.1 s-1 and 0.1 s-1 is observed, and the dynamic recrystallization evolution of the alloys with strains of 0.1 s-1 and 0.1 s-1 is observed. at 1500 and 1600 °C and at all measured strains, the recrystallized grains have a strong <001> structure.
Welding is essential for the application of Nb-1Zr-0.1C alloys. Welding of this alloy is a difficult task due to its high melting point and reactive nature. Badgujar et al investigated the electron beam welding process parameters and microstructure of Nb-1Zr-0.1C alloys and found that fine carbides precipitated along the grain boundaries in both the heat-affected zone (HAZ) and the base metal, while carbides dissolved in the weld zone; the microhardness distributions over the width of weld showed that the slight increase in hardness in the weld zone and heat affected zone; and the thermal and residual stress fields of square butt electron beam welded joints were estimated using the finite element method, and a good qualitative match between the calculated and experimental values was obtained. Gupta et al. studied butt laser welding of Nb-1Zr-0.1C alloys by using inert gas protection at the top and bottom. They found that the average fusion zone hardness is much higher than the hardness of the base material. This larger increase in hardness may be due to grain refinement, dissolution of precipitated phases, and formation of brittle phases such as carbides, and established a laser welding technique for preparing niobium alloys under ambient atmosphere.

Similarly, applying Nb-1Zr-0.1C alloys requires corresponding antioxidant coating protection, and sprayed NbSi2 remains the focus of coating design for this alloy. Vishwanadh et al. have shown that a two-layer coating structure is formed on the Nb-1Zr-0.1C alloy substrate above 1300°C, with a thin inner layer of Nb5Si3 and a thicker outer layer of NbSi2 . outer layer. Majumda et al. investigated the static isothermal oxidation of single (NbSi2) and bilayer (Nb5Si3 and NbSi2) coatings between 800 and 1300 °C and derived a kinetic model to predict the growth kinetics of the NbSi2 phase during the filler silicification process. In addition, Majumda et al. prepared MoSi2 coatings on the surface of Nb-1Zr-0.1C alloys by deposition of Mo using magnetron sputtering followed by deposition of Si using chemical vapor deposition (i.e., silica-in-packing), and it was found that a bilayered coating with an outer layer of MoSi2 and an inner layer of NbSi2 was formed. The MoSi2 coatings were formed at 1100 °C with a thickness of about 25 μm consisting of a fine crystalline organization.

Fig. 6 OM and TEM micrograph of the deformed and the annealed(1300℃/3h) Nb alloy(a) OM of the deformed sample; TEM micrograph of the (b) needle and (c) rectangular shape precipitate present in the deformed sample; OM (d) and EBSD images (e) of the annealed samples; (f) TEM images of annealed samples

Fig. 6 OM and TEM micrograph of the deformed and the annealed(1300℃/3h) Nb alloy (a) OM of the deformed sample; TEM micrograph of the (b) needle and (c) rectangular shape precipitate present in the deformed sample; OM (d) and EBSD images (e) of the annealed samples; (f) TEM images of annealed samples

Fig.7 Schematic diagram of carbides Nb2C phase transition in Nb-1Zr-0.1C alloy(a) Crystal structures of γ, β and α-Nb2C carbide phases; (b)The unit cells of all the Nb2C phases are drawn with Nb atom positions as the origin; (c) The dimensions and atom positions of Nb atoms in these structures are similar in certain directions

Fig.7 Schematic diagram of carbides Nb2C phase transition in Nb-1Zr-0.1C alloy
(a) Crystal structures of γ, β and α-Nb2C carbide phases; (b)The unit cells of all the Nb2C phases are drawn with Nb atom positions as the origin; (c) The dimensions and atom positions of Nb atoms in these structures are similar in certain directions

Fig.8 Microstructure and carbide morphology of Nb-1Zr-0.1c alloy during solidification, extrusion and recrystallization (a) Bright field STEM micrograph of the as-solidified sample; (b) Bright field TEM image of the extruded alloy; (c) BSE micrographs of the recrystallized sample and spherical morphology of carbides; (d) ) HAADF micrograph of the carbide particle showing the presence high concentration of Zr at the interface; HRTEM micrograph of the interface between Nb matrix and (e)spherical and (f) needle morphology of (Nb,Zr)C carbide, respectively

Fig.8 Microstructure and carbide morphology of Nb-1Zr-0.1c alloy during solidification, extrusion and recrystallization (a) Bright field STEM micrograph of the as-solidified sample; (b) Bright field TEM image of the extruded alloy; (c) BSE micrographs of the recrystallized sample and spherical morphology of carbides; (d) ) HAADF micrograph of the carbide particle showing the presence high concentration of Zr at the interface; HRTEM micrograph of the interface between Nb matrix and (e)spherical and (f) needle morphology of (Nb,Zr)C carbide, respectively

Fig.9 EBSD map showing recrystallized grains for samples deformed at strain rate of 0.1 s−1 for different temperatureand true strains of (a)1500℃, 0.3, (b)1500℃,0.6, (c)1500℃,0.9, (d)1500℃,1.2,(e)1600℃, 0.3, (f)1600℃,0.6, (g)1600℃,0.9 and (h)1600℃,1.2

Fig.9 EBSD map showing recrystallized grains for samples deformed at strain rate of 0.1 s−1 for different temperature and true strains of (a)1500℃, 0.3, (b)1500℃,0.6, (c)1500℃,0.9, (d)1500℃,1.2,(e)1600℃, 0.3, (f)1600℃,0.6, (g)1600℃,0.9 and (h)1600℃,1.2

2.3 Nb521 Niobium Alloy

Due to operating temperature limitations and high-temperature mechanical properties, C-103 alloy and niobium silicide antioxidant coatings have been challenging to meet the ever-developing needs of spacecraft. For this reason, China has developed Nb521 (Nb-5W-2Mo-1Zr) niobium alloy based on 5ΒΜЦ niobium alloy of the former Soviet Union. The alloy adopts molybdenum silicide high-temperature anti-oxidation protective coating, making the working temperature increased to about 1550 ℃, significantly reducing the flow rate used to cool the combustion chamber of the propellant, thus effectively enhancing the engine-specific impulse, and becoming the second generation of niobium alloy substrate for the thrust chambers of the refractory metal materials of China’s current orbit control/attitude control engines.

Nb521 niobium alloy is a medium-strength plastic niobium alloy with the addition of W, Mo, Zr alloying elements and a small amount of C elements in the niobium matrix, which further improves the room-temperature and high-temperature mechanical properties of niobium alloys through a combination of solid-solution strengthening and precipitation strengthening. It is a medium-strength plastic niobium alloy. The alloy can be prepared as ingots by vacuum electron beam melting or vacuum electron beam + vacuum self-consumption melting, and bars, forgings and plates of various specifications can be prepared by hot extrusion, forging, cold spinning, stretching and rolling. The addition of W and Mo elements to the alloy is mainly due to their high melting point and similar atomic radius, which can form a replacement solid solution to improve the alloy matrix’s creep resistance and high-temperature strength. Zr and C elements are added to the alloy because C can easily generate diffuse precipitated reinforcing phases with Zr and Nb, which can further improve the high-temperature strength of the alloy by playing the role of precipitation strengthening.

Usually, niobium alloys are diffusely strengthened by stable and fine carbides, oxides and nitrides, and this strengthening method is very effective in improving the high-temperature strength. Chunji Zhang et al. investigated that the carbide strengthened phases of Nb521 alloy are diffusely distributed (Nb,Zr)C, ZrC and Nb2C phases, ZrC is a diffusely distributed stabilized carbide phase while Nb2C is a sub-stabilized carbide phase. Xia Mingxing et al. studied the effect of different contents and sizes of Nb2C particles on the organization and properties of Nb-W-Mo-Zr-C niobium alloy, and concluded that the addition of Nb2C particles with a content of 0.4 wt% and a size of about 5 μm can make the alloy reach the highest high-temperature strength.

Nb521 niobium alloy has good room-temperature forming properties, and the uniform transition nozzle extensions can usually be prepared by plate spinning, but most of the nozzles are obtained by machining the bars, which is difficult and the material utilization is relatively low. Therefore, the study of near-net-shaping and additive manufacturing of Nb521 niobium alloy is an effective way to improve the material utilization of this alloy. Zhang et al. successfully prepared nano Nb521 alloy powder by using a high-energy ball mill at room temperature and carried out the corresponding analytical characterization and the results show that the ball milling speed is decisive when the ball milling speed reaches 450 rpm, after 60 hours, the grain size of 14 nm nanopowder can be obtained. The results show that the ball milling speed is the decisive factor. Liu Baoqi et al. studied the Nb521 alloy powder prepared by the plasma rotary atomization method, and found that the alloy powder prepared by this method is mostly spherical and has a high degree of sphericity, the surface of the large particle powder is relatively rough, and the surface of the small-sized particles of the powder is relatively smooth (see Fig. 10), and the distribution of the particle size is in accordance with the standard normal distribution; analyzed by the XRD and nano-indentation experiments, the results show that only Nb diffraction peaks exist, and the powder size decreases with the decrease of the particle size of the powder. As the powder particle size decreases, the nano-hardness and the maximum load increase.Yang et al. prepared Nb521 alloys by the electron beam selective melting (EBSM) method and compared the microstructure characteristics of the prepared specimens with those of the ingot specimens, as shown in Fig. 11. The results show that in the Nb521 samples prepared by EBSM, the content of precipitated phases increases gradually from top to bottom. The point-like or rod-like precipitated phases with different morphologies are distributed inside the grains or along the grain boundaries. In contrast, needle-like precipitated phases have large aspect ratios in the ingots of Nb521 alloys. The precipitated phases in the specimens prepared by the EBSM method are mainly (Nb, Zr)C and Nb2C. With the extension of the thermal equilibrium holding time, the finer and the more compact precipitated phases are found. In the specimens prepared by the EBSM method, the precipitated phases are mainly (Nb, Zr)C and Nb2C, and with the prolongation of the thermal equilibrium holding time, the elongated precipitated phases are partially fragmented and the grains become fine and uniform.

Fig. 10 Surface and XRD patterns of Nb521 alloy powder with different sizes(a) >105μm; (b) 45~105μm; (c) ≤45μm; (d) XRD patterns of alloy powder with different sizes

Fig. 10 Surface and XRD patterns of Nb521 alloy powder with different sizes
(a) >105μm; (b) 45~105μm; (c) ≤45μm; (d) XRD patterns of alloy powder with different sizes

Fig. 11 Comparison of precipitation phase between Nb521 prepared by EBSM method and ingot sample(a) Schematic diagram of powders before and after remelting; (b) Schematic diagram of sampling positions S1, S2, S3 of the EBSM alloy; (c) XRD patterns of S1, S2, S3 and ingot samples; SEM images of EBSM sample (d)S1, (e) S2, (f) S3 and (g) ingot samples; (h) Schematic diagram of precipitates morphology of EBSM alloy; TEM bright field image of (i) C-rich rod-like precipitates in the grain of S2 sample, and HRTEM and SAED (inset) of (j) region I and (k) region II in (i)

Fig. 11 Comparison of precipitation phase between Nb521 prepared by EBSM method and ingot sample
(a) Schematic diagram of powders before and after remelting; (b) Schematic diagram of sampling positions S1, S2, S3 of the EBSM
alloy; (c) XRD patterns of S1, S2, S3 and ingot samples; SEM images of EBSM sample (d)S1, (e) S2, (f) S3 and (g) ingot samples;
(h) Schematic diagram of precipitates morphology of EBSM alloy; TEM bright field image of (i) C-rich rod-like precipitates in the
grain of S2 sample, and HRTEM and SAED (inset) of (j) region I and (k) region II in (i)

In addition, the molybdenum silicide high-temperature antioxidant coating prepared on Nb521 alloy material has a certain self-healing ability, and the coefficient of linear expansion of the alloy material is relatively close to that of Nb521 alloy material, so it has good antioxidant protection and bonding properties. In order to further improve the comprehensive performance of the molybdenum silicide coatings, Sun et al. prepared multiphase MoSi2 coatings on the surface of niobium-based alloys by using two plasma spraying techniques (SAPS and SPS) and defined the melt index (i.e., M.I. value) of the plasma-sprayed MoSi2 coatings in order to characterize the multiphase effect of the coatings, and it was found that the higher the M.I. value was, the stronger were the mechanical properties of the coatings, and the better were the antioxidant properties. Sun et al [80] investigated the preparation of MoSi2-based/NbSi2 double coatings on the surface of Nb521 alloy, which could form a continuous SiO2 barrier on the coating surface after pre-oxidation at 1500°C for 10h, and could effectively retard the thermal corrosion between MoSi2-based coatings and Na2SO4 salt. Xiao et al. used a new two-step method to prepare a MoSi2-NbSi2 dual coating on the surface of Nb521 alloy. MoSi2-NbSi2 coating and Ce-modified MoSi2-NbSi2 coating, and the effective protection times of the two coating specimens at 1600 °C were 24.7 h and 28.5 h, respectively, indicating that the Ce-modified coating exhibited better antioxidant properties. Zhang et al. prepared mullite-modified MoSi2(MM) coating, WSi2 and mullite on the surface of Nb521 alloy Co-modified MoSi2 (WMM) coatings and their antioxidant properties were investigated, and it was found that the antioxidant properties of MM coatings with the addition of 10 wt% mullite were very good, whereas the antioxidant properties of WMM coatings were even better, with the effective protection time at 1500 °C (500 h) being at least 2.8 and 1.5 times longer than that of the MoSi2 single coatings (175 h) and MM (346 h) coatings, respectively. as shown in Fig. 12. In order to further develop ultrahigh-temperature niobium alloy coatings for higher temperatures, Zhang et al [84] used a novel three-step method to prepare 10% ZrB2 + 5% YSZ modified Si-Mo-18%W coatings with boride diffusion barrier, and found that the coatings with the NbB2-Nb3B2 diffusion barrier could effectively protect the Nb521 alloy up to 1850 °C for 8 h above (as shown in Fig. 13), while the lifetime of the coating without diffusion barrier is only 3.5 h. The contribution of the superior coating performance is mainly due to the diffusion barrier layer and the formation of the self-healing SiO2-B2O3-ZrSiO4-ZrO2 oxide scale, which effectively prevents the mutual diffusion of the coating and the substrate and can reduce inward depletion of the silicon element, thus improving the lifetime of the coating.

Fig. 12 Schematic diagrams of oxidation mechanism of three different coatings: (a) M coating; (b) MM coating; (c) WMMcoating

Fig. 12 Schematic diagrams of oxidation mechanism of three different coatings: (a) M coating; (b) MM coating; (c) WMM coating

Fig. 13 The schematic diagram of section microstructure before and after oxidation and failure process of the coating[84](a) Cross-sectional schematic diagram of the coating; (b) Cross-sectional schematic diagram of the coating after oxidation; (c) schematic diagram of the evolution process of the coating cross-section with oxidation time

Fig. 13 The schematic diagram of section microstructure before and after oxidation and failure process of the coating[84]
(a) Cross-sectional schematic diagram of the coating; (b) Cross-sectional schematic diagram of the coating after oxidation; (c)
schematic diagram of the evolution process of the coating cross-section with oxidation time

Through the practice and application in recent years, Nb521 niobium alloy and its supporting high-temperature antioxidant coating have been successfully used in the new generation of high-thrust carrier rocket gas-oxygen kerosene engine nozzles, Nigerian satellites, Xinnuo 6, etc. Attitude control/orbit control engine and Chinese lunar vehicle engine and other projects, and it is the first choice of the domestic attitude control/orbit control engine refractory metal materials for the thrust chamber.

2.4 Low-density niobium alloy

Low-density niobium alloy is also known as lightweight niobium alloy. Due to the addition of a large number of Ti, Al and other lightweight elements and W, Mo and other reinforcing elements, has a low density (<7g/cm3), high room temperature and high-temperature strength, and room temperature plasticity is good (grain refinement can be cold machining molding) and other excellent characteristics, is a can not be coated in the atmosphere at 550 ~ 800 ℃ direct use, coating can be applied to the atmosphere at 800 ~ 1200 ℃ It is a new type of high-temperature structural material that can be used directly without coating in 550~800℃ atmospheric environment and with coating in 800~1200℃ atmospheric environment. The United States and the former Soviet Union have developed dozens of low-density niobium alloys in accordance with the requirements of the use of temperature and strength, with Nb-Ti-Al, Nb-Ti-Al-Cr, Nb-Ti-Al-Hf, Nb-Ti-Al-Cr-Hf and other systems. In foreign countries, low-density niobium alloys have been used in rocket and aviation engine heated parts, such as the U.S. Pratt & Whitney company to manufacture military aircraft engines with plates for pressurized nozzles, Russia for aircraft engine exhaust piping, and similar new stamping engine thermal structure components and other aspects of the application has also been used. Table 4 shows some of the foreign low-density niobium alloy grades and their properties of the domestic since 2005, the development of low-density niobium alloys, so far has been in the research and development stage, there is no engineering application of the alloy grades and products. Up to now, the research units of such alloys are Zhongnan University, Nanjing University of Aeronautics and Astronautics, Northwestern Polytechnical University, Northwestern Nonferrous Metals Research Institute and Ningxia Oriental Tantalum Industry Co. Among them, Zhongnan University, Northwest Research Institute of Nonferrous Metals and Ningxia Oriental Tantalum Industry Co., Ltd. are all using the Nb-Ti-Al system, mainly for the development of a material that can be used in the side skirt part of the thrust chamber of the liquid rocket engine, the use of the temperature of 1100 ~ 1200 ℃, in order to meet the urgent requirements of the liquid rocket engine on the lightweight. Table 5 shows the composition and properties of some domestic low-density niobium alloys.

Table 4 Tensile properties and density of some low-density niobium alloys abroad

Alloy Density (ρ/g/cm³) Test Temp. (°C) Tensile Strength (Rm/MPa) Yield Strength (Rp₀.₂/MPa) Elongation (A/%)
Nb-40Ti-12Al 6.20 25 811 799 25
1200 56 56 166
Nb-40Ti-10Al-10Cr 6.35 25 1010 1010 14
1200 56 56 153
Nb-40Ti-10Al-10Cr-C-Y (at%) 25 1050 1000 22
1000 150 130 75
Nb-38Ti-12Al-12Hf 6.76 25 901 901 15
1200 77 77 91
Nb-34Ti-8Al-7Cr-2Hf 6.04 25 915 915 23
1200 97 62 51
Nb-27.5Ti-5.5Al-6Cr-4.5Hf-2.5V 5.49 25 930 930 24
1200 141 141 83
Nb-35Ti-6Al-5Cr-8V-1W-0.5Mo-0.3Hf (at%) 23 1117 1107 16
1000 280 280 244
Nb-18Al-20V (at%) 25 1200 8
1000 550
Nb-11Al-41Ti-1.5Mo-1.5Cr (at%) 25 788 11
1000 875
Nb-35Ti-6Al-5Cr-8V-1C (at%) 1000 137

Northwest Institute of Nonferrous Metals (NIN) developed low-density niobium alloys are mainly Nb-31Ti-7Al-(3~10)V-1.5Zr, Nb-35Ti-6Al-(2~10)Cr-(3~8)V-1W-0.5Zr and Nb-37.5Ti-5Al-4.5V-0.5Zr alloys and alloy rods, plates and machining processes. were studied. Among them, Cai Xiaomei et al. studied the hot rolling process of Nb-37.5Ti-5Al-4.5V-0.5Zr alloy plate, and concluded that the alloy has good room temperature and high-temperature properties when hot rolled at temperatures of 1200 and 1100 ℃ and that the tensile strength decreases and the plasticity increases as the rolling temperature increases, and that when hot rolled at 1000 ℃, the mechanical properties of the alloy have lower room temperature and high temperature properties, and the room temperature tensile strength is lower than that of the alloy, and the mechanical properties of the alloy have lower than that of the alloy when hot rolled. Lower, and the room temperature tensile fracture showed brittle fracture. Wang Feng et al. Nb-35Ti-5Al-5V-1Zr and NbW521 alloy vacuum electron beam welding process research, the results show that the alloy and NbW5-2 alloy have good welding performance in the state of weld without heat treatment after welding the room temperature tensile strength of weld samples reached 468 MPa, close to the tensile strength of the Nb521 alloy matrix. Cai Xiaomei et al. on the Nb-30Ti-5Al-4.5W-0.5Mo alloy bar forging process research by changing the forging process to prepare a room temperature mechanical property of good low-density high-strength niobium alloy bar.

Ningxia Oriental Tantalum Industry Co., Ltd (OTIC) has successively developed a variety of alloys such as Nb-40Ti-15Al, Nb-40Ti-10Cr, Nb-40Ti-20Cr, and Nb-40Ti-10Cr-10A1, etc. The micro-alloying process of the alloys and the effect of the B element on the refinement of the alloy grains were investigated, and the alloys Nb-Ti-Al-Al-Cr and Nb-Ti-Al-Al-Cr and Nb-Ti-Al-Al-Al-Cr and Nb-Ti-Al-Al-Al-Cr and Nb-Ti-Al-Al-Al-Cr alloys were formed. The Cr and Nb-Ti-Al-Zr series of new low-density niobium alloys have a production capacity of φ30~150mm alloy bars and 1~3mm×500mm×500mm plates. Fig. 14 shows the actual parts of Nb-Ti-Al-Cr-M3 and Nb-Ti-Al-Zr-M8 plates, which were pressed and spun at room temperature. In recent years, Nb-Ti-Al-Mo-W-Zr niobium alloys have been developed based on the original. Zhao Hongyun et al. studied the heat deformation behavior and microstructure evolution of low-density Nb-Ti-Al-Mo-W-Zr niobium alloy. They obtained the flow stress curves and constitutive equations of the new low-density niobium alloy for high-temperature deformation. Liu Yanchang et al. studied the annealing process of Nb-Ti-Al-Mo-W-Zr niobium alloy plates. They concluded that the alloy can obtain good comprehensive performance by annealing at 900-950℃.

Table 5 Tensile properties and density of some low-density niobium alloys in China

Alloy Density (ρ/g/cm³) Test Temp. (°C) Tensile Strength (Rm/MPa) Yield Strength (Rp₀.₂/MPa) Elongation (A/%) Institution
Nb-31Ti-7Al-xV-1.5Zr 6.0 25 937 16.2
1100 81 56.8
Nb-35Ti-6Al-xCr-yV-1W-0.5Zr 6.0~6.9 25 1088 21.0 NIN
1200 152 62.6
Nb-37.5Ti-5Al-4.5V-0.5Zr 6.01 25 895 12.0
1100 90 39.4
Nb-Ti-Al-Cr-M3 6.15 25 900 877 32.8
1100 87 86 62.0
1200 53 52 62.0 OTIC
Nb-Ti-Al-Zr-M8 6.15 25 804 804 28.0
1100 77.8 73.2 59.5
1200 47.8 45.2 74.0
Fig. 14 Stamping and spinning parts of low density niobium alloy: (a) Nb-Ti-Al-Cr-M3 ; (b) Nb-Ti-Al-Zr-M8

Fig. 14 Stamping and spinning parts of low-density niobium alloy: (a) Nb-Ti-Al-Cr-M3 ; (b) Nb-Ti-Al-Zr-M8

In addition, Zhao et al. investigated that the addition of Si could enhance the strength of Nb-35Ti-15Al alloy. Shi et al. prepared Nb-23Ti-15Al (at%) alloy by mechanical alloying (MA) and hot pressing (HP) method and studied the organization evolution of powder particles during the MA process and its effect on the organization and mechanical properties of the hot-pressed alloy. Meanwhile, Shi [108] et al. also investigated the laser forming of this alloy and its microstructure and mechanical behavior, and the results showed that almost defect-free Nb-Ti-Al alloys with fine dendrites could be obtained by laser forming. The presence of β, δ, and Ti(O,C) phases in the alloys (see Fig. 15), and the relatively high microhardness and fracture toughness when the β/δ phases are refined and Ti(O,C) dispersed indicate that laser forming is a potential preparation method that can be used to prepare high-performance Nb-Ti-Al alloys. Chaia [109] et al. deposited aluminum compounds on the surface of Nb-Ti-Al alloys using chloride, fluoride, aluminum-silicon, and aluminum-silicon coatings on the surface of Nb-Ti-Al alloys. Silicide coatings and the high-temperature oxidation test of the Al-Si coatings in the air at 1000°C proved that the coatings have specific protective properties. Wei et al. investigated the microstructure evolution and mechanical behaviors of Nb-35Ti-4C, Nb-35Ti-4C-15Al, Nb-25Ti-8C, and Nb-25Ti-8C-15Al. They found that the use of C and Al elements C and Al elements were found to significantly improve the mechanical properties of the as-cast and heat-treated specimens at room and evaluation temperatures (see Fig. 16). The plasticity of the alloys was reduced by the appearance of large brittle carbides and the formation of Nb3Al with the introduction of the strengthening phases (Nb, Ti)C and Nb3A. In addition, Xiao Lairong’s team at Central South University has also developed low-density niobium alloys such as Nb-40Ti-7Al and Nb-38Ti-12Al and investigated the hot deformation behavior, recrystallization kinetics, high-temperature oxidation behavior, compatibility with Si-Cr-Ti coatings, and the role of the coating oxidation resistance, and the effect of the addition of the C element on the organization and properties of the Nb-20Ti-16Al alloys. The impact of the addition of C elements on the organization and properties of Nb-20Ti-16Al alloy was studied. It was determined that the recrystallization temperature of Nb-Ti-Al alloy was 880~1000℃. The C atoms were solidly dissolved in the form of replacement atoms in Nbss and Nb3 Al, and with the increase of C elements, the volume fraction of the heat-treated alloys of Nbss decreases, the high-temperature compression strength increases, and the room-temperature fracture toughness gradually decreases.

Fig. 15 TEM images showing different kinds of microstructures in the laser formed Nb-23Ti-15Al alloy: (a) a region ofβ and δ strips; (b) a region of equiaxed βand δ; (c) a region of Ti(O,C) twin

Fig. 15 TEM images showing different kinds of microstructures in the laser formed Nb-23Ti-15Al alloy: (a) a region of β and δ strips; (b) a region of equiaxed βand δ; (c) a region of Ti(O,C) twin

Fig. 16 TEM images and corresponding SAED patterns of Nb-35Ti-4C-15Al alloy, and compressive stress-strain curves ofNb-Ti-C and Nb-Ti-Al-C alloys at room temperature and 1000℃ (a) The bright field image of Nbss, (Nb, Ti)C and Nb3Al; (b) SAED pattern of Nb3Al; (c) SAED pattern of Nbss; (d) SAED pattern of the interface between Nb3Al and Nbss; (e) Stress-strain curves of as-cast samples at RT and (f) samples at 1000℃

Fig. 16 TEM images and corresponding SAED patterns of Nb-35Ti-4C-15Al alloy, and compressive stress-strain curves of Nb-Ti-C and Nb-Ti-Al-C alloys at room temperature and 1000℃
(a) The bright field image of Nbss, (Nb, Ti)C and Nb3Al; (b) SAED pattern of Nb3Al; (c) SAED pattern of Nbss; (d) SAED pattern of the interface between Nb3Al and Nbss; (e) Stress-strain curves of as-cast samples at RT and (f) samples at 1000℃

3. Problems and prospects

With the progress and development of aerospace technology, the performance of the engine (such as combustion efficiency, payload and in-orbit operation life) requirements continue to improve, which requires further improvement of the engine-specific impulse performance so that the temperature of the combustion chamber body is getting higher and higher, resulting in the use of the temperature of the engine body materials and mechanical properties are also more and more demanding. Compared with other kinds of high-temperature alloys, niobium alloy has the advantages of low density, high specific strength, good cold forming, and welding performance, and it can be processed to form thin-walled and complex-shaped parts. Therefore, niobium alloy has been widely used in the aerospace field. But at the same time, there are some unavoidable problems:

1) Currently used niobium alloys are mostly low—and medium-strength niobium alloys. To upgrade engine body materials, in-depth research on the mechanism of high-temperature strengthening of high-strength niobium alloys and toughening mechanism is needed to develop a new generation of niobium alloys of higher strength, higher toughness, or higher strength plasticity to provide theoretical support.

2) With the increase in operating temperature and life time requirements, the current high-temperature antioxidant protective coating technology is also difficult to meet such requirements. In order to meet the requirements of the new generation of niobium alloys with higher strength or toughness in higher temperature environments, it is necessary to develop long-life protective coatings with higher operating temperatures and better antioxidant properties.

3) Although niobium alloys are among the lightest of the high melting point metals, they are still slightly heavier in density compared to the development of aerospace engineering. The lightness of niobium alloys is of great importance for increasing the specific impulse of space engines, further extending the range and increasing the spacecraft payload. Therefore, developing low-density niobium alloys and the thinning and lightweight of high-strength niobium alloys is also an essential direction for the future development of aerospace engineering.

Therefore, in order to meet the growing demand for high performance of space engines, niobium alloys, which are commonly used in engine thrust chambers, need to continuously improve and solve their own problems and further develop new niobium alloys with higher strength, higher toughness, and lighter weight, as well as higher temperature and long-life high-temperature oxidation-resistant protective coatings, so as to promote further the application of niobium alloys in the aerospace field.

Ti-6Al-4V, AMS 4928 AMS 4965

Ti-6Al-4V: AMS 4928 vs AMS 4965

Ti-6Al-4V is a titanium alloy commonly used in aerospace, medical, and high performance applications for its excellent strength-to-weight ratio, corrosion resistance, and high temperature stability. When comparing AMS 4928 and AMS 4965, the two specifications cover different forms of Ti-6Al-4V and differ in processing and intended applications.

Product form: AMS 4928 vs AMS 4965

AMS 4928 Ti-6Al-4V, UNS R56400 Form

This specification covers a titanium alloy in the form of bars, wire, forgings, flash welded rings, drawn shapes up through 6.000 inches (152.40 mm) inclusive in diameter or least distance between parallel sides and stock of any size for forging or flash welded rings.

AMS 4965 Ti-6Al-4V, UNS R56400 Form

This specification covers a titanium alloy in the form of bars, wire, forgings, and flash welded rings 4.000 inches (101.60 mm) and under in nominal diameter or least distance between parallel sides and of stock for forging and flash welded rings.

Chemical composition: AMS 4928 vs AMS 4965

AMS 4928 Ti-6Al-4V, UNS R56400 Chemical Composition

Element Min. Max.
Aluminum 5.50 6.75
Vanadium 3.50 4.50
Iron 0.30
Oxygen 0.20
Carbon 0.08
Nitrogen 0.05 (500 ppm)
Hydrogen (3.1.1) 0.0125 (125 ppm)
Yttrium (3.1.2) 0.005 (50 ppm)
Other Elements, each (3.1.2) 0.10
Other Elements, total (3.1.2) 0.40
Titanium Remainder

AMS 4965 Ti-6Al-4V, UNS R56400 Chemical Composition

Element Min. Max.
Aluminum 5.50 6.75
Vanadium 3.50 4.50
Iron 0.30
Oxygen 0.20
Carbon 0.08
Nitrogen 0.05 (500 ppm)
Hydrogen (3.1.1) 0.0125 (125 ppm)
Yttrium (3.1.2) 0.005 (50 ppm)
Other elements, each (3.1.2) 0.10
Other elements, total (3.1.2) 0.40
Titanium Remainder

Mechanical properties: AMS 4928 vs AMS 4965

AMS 4928 Ti-6Al-4V, UNS R56400 Mechanical Properties

Nominal Diameter or Least Distance Between Parallel Sides (mm) Tensile Strength (MPa) Yield Strength at 0.2% Offset (MPa) Elongation in 50.8 mm or 4D % Long. Elongation in 50.8 mm or 4D % L.T. Elongation in 50.8 mm or 4D % S.T. Reduction of Area % Long. Reduction of Area % L.T. Reduction of Area % S.T. (2)
Up to 50.80, incl (1) 931 862 10 10 XXV 20 20
Over 50.80  to 101.60, incl 896 827 10 10 10 XXV 20 15
Over 101.60 to 152.40, incl. (3) 896 827 10 10 8 20 20 15
Note: Long. = longitudinal LT = long horizontal ST = short horizontal
1. Tensile strength of 130 ksi (896 MPa) minimum and yield strength of 120 ksi (827 MPa) minimum are permitted for wire or rod for fastener applications and for flash welded rings made from extrusions up to 2.000 inches (50.80 mm), inclusive, in distance between parallel sides.
2. Short-transverse reduction of area is waived for flash welded rings made from extrusions.
3. See 8.3.

AMS 4965 Ti-6Al-4V, UNS R56400 Mechanical Properties

Nominal Diameter or Distance Between Parallel Sides Tensile Strength Yield Strength
at 0.2% Offset
Elongation (1)
in 50.8 mm
or 4D, L
Elongation (1)
in 50.8 mm
or 4D, T
Reduction
of Area, L
mm MPa MPa (%) (%) (%)
Up to 12.70, incl. 1138 1069 10 20
Over 12.70 to 25.40, incl. 1103 1034 10 20
Over 25.40 to 38.10, incl. 1069 1000 10 20
Over 38.10 to 50.80, incl. 1034 965 10 20
Over 50.80 to 76.20, incl. 965 896 10 8 20
Over 76.20 to 101.60, incl. 896 827 8 6 20
NOTE: For forgings, the elongation in all dimensions shall not be less than 8% (L) and 6% (T).
Nominal Thickness Nominal Width Tensile Strength Yield Strength
at 0.2% Offset
Elongation in 50.8 mm or 4D, L Elongation in 50.8 mm or 4D, T Reduction
of Area, L
mm mm MPa MPa (%) (%) (%)
Up to 12.70, incl. Over 12.70  to 203.20, incl. 1103 1034 10 10 25
Over 12.70 to 25.40, incl. Over 25.40 to 101.60, incl. 1069 1000 10 10 20
Over 12.70 to 25.40, incl. Over 101.60 to 203.20, incl. 1034 965 10 10 20
Over 25.40 to 38.10, incl. Over 25.40 to 101.60, incl. 1034 965 10 10 20
Over 25.40 to 38.10, incl. Over 101.60 to 203.20, incl. 1000 931 10 10 20
Over 38.10 to 50.80, incl. Over 50.80 to 101.60, incl. 1000 931 10 10 20
Over 38.10 to 50.80, incl. Over 101.60 to 203.20, incl. 965 896 10 10 20
Over 50.80 to 76.20, incl. Over 76.20 to 203.20, incl. 965 862 10 8 20
Over 76.20 to 101.60, incl. Over 101.60 to 203.20, incl. 896 827 8 6 20

Melting Practices: AMS 4928 vs AMS 4965

AMS 4928 Ti-6Al-4V, UNS R56400 Melting Practice

Alloy shall be multiple melted. The first melt shall be made by vacuum consumable electrode, nonconsumable electrode, electron beam cold hearth, or plasma arc cold hearth melting practice. The subsequent melt or melts shall be made under vacuum using vacuum arc remelting (VAR) practice. Alloy additions are not permitted in the final melt cycle.
The atmosphere for nonconsumable electrode melting shall be vacuum or shall be argon and/or helium at an absolute pressure not higher than 1000 mm of mercury.
The electrode tip for nonconsumable electrode melting shall be water-cooled copper.

AMS 4965 Ti-6Al-4V, UNS R56400 Melting Practice

Alloy shall be multiple melted; melting cycle(s) prior to final melting cycle shall be made using consumable electrode, nonconsumable electrode, electron beam, or plasma arc melting practices. The final melting cycle shall be made under vacuum using consumable electrode practice with no alloy additions permitted.
The atmosphere for nonconsumable electrode melting shall be vacuum or shall be argon and/or helium at an absolute pressure not higher than 1000 mm of mercury.
The electrode tip for nonconsumable electrode melting shall be water-cooled copper.

Heat treatment: AMS 4928 vs AMS 4965

AMS 4928 Ti-6Al-4V, UNS R56400 Heat Treatment

Bars, wire, forgings, drawn shapes and flash welded rings shall be heat treated as follows; pyrometry shall be in accordance with AMS2750.
1. Solution Heat Treatment
Except as specified in 3.4.3 when solution heat treatment is used, heat to a temperature within the range 50 to 150 °F (28 to 83 °C) degrees below the beta transus, hold at the selected temperature within ±25 °F (±14 °C) for a time commensurate with section thickness and the heating equipment and procedure used, and cool at a rate equivalent to an air cool or faster.
2. Annealing
Except as specified in 3.4.3, heat to a temperature within the range 1300 to 1450 °F (704 to 788 °C), hold at the selected temperature within ±25 °F (±14 °C) for not less than 1 hour, and cool as required.
3. Continuous Heat Treating
Wire 0.125 inch (3.18 mm) and under in diameter may be continuously heat treated provided that process parameters (e.g., furnace temperature set points, heat input, travel rate, etc.) for continuous heat treating lines shall be established by the material producer and validated by testing of product to requirements of 3.5.

AMS 4965 Ti-6Al-4V, UNS R56400 Heat Treatment

Bars, wire, forgings, and flash welded rings shall be solution heat treated by heating in a suitable atmosphere to 1750 °F ± 25 (954 °C ± 14), holding at heat for 1 to 2 hours, and quenching in agitated water, and aged by heating to a temperature within the range 900 to 1150 °F (482 to 621 °C), holding at the selected temperature within ±15 °F (±8 °C) for 4 to 8 hours, and cooling in air. Pyrometry shall be in accordance with AMS 2750.

Application: AMS 4928 vs AMS 4965

AMS 4928 Ti-6Al-4V, UNS R56400 Application

These products have been used typically for parts requiring moderate strength with a maximum service temperature in the 750 to 900 °F (399 to 510 °C) range depending on time at temperature where the product is to be used in the annealed condition, but usage is not limited to such applications.

AMS 4965 Ti-6Al-4V, UNS R56400 Application

These products have been used typically for parts which are machined after solution heat treatment and aging and are suitable for parts requiring high strength-to-weight ratios up to moderately elevated temperatures, but usage is not limited to such applications.

MONEL, INCOLOY, INCONEL ALLOYS

Everything You Need to Know: Monel/Incoloy/Inconel Alloys

Introduction

In various industries, including oil and gas, aerospace, chemical processing, and marine engineering, extreme conditions—such as high temperatures, corrosive environments, and mechanical stresses—necessitate using materials that exhibit exceptional durability and resistance. Consequently, nickel-based alloys such as Monel/Incoloy/Inconel Alloys are specifically engineered to withstand these demanding conditions. As a result, they ultimately provide effective solutions for challenges related to corrosion, heat, and mechanical wear.

This blog will explore the most widely used Monel/Incoloy/Inconel Alloys and their specific applications across various industries, offering guidance for material selection to address common concerns. This guide focuses on the user experience and delivers practical information and expert insights into how these Monel/Incoloy/Inconel Alloys solve critical challenges.

1. Monel Alloys

Monel is a family of nickel-copper alloys that provides excellent resistance to corrosion and high mechanical strength. Therefore, these alloys prove particularly effective in marine environments and industries consistently exposed to harsh chemicals and extreme temperatures.

UNS N04400/Monel 400

  • Challenges: Corrosion in seawater, salt solutions, and acids.
  • Solution: Monel 400 offers excellent resistance to seawater, brine, and many acids, including hydrofluoric acid. Its combination of corrosion resistance and high strength makes it a popular choice for marine and chemical processing applications.
  • Critical Applications include marine engineering (pump shafts, propeller shafts), chemical plant equipment, and heat exchangers.
  • Limitations: Prone to stress corrosion cracking in environments containing ammonia or mercury.

Monel R-405

  • Challenges: Manufacturing challenges require machinability with corrosion resistance.
  • Solution: Monel R-405, a variant of Monel 400, offers enhanced machinability primarily due to its sulfur content. As a result, it is particularly well-suited for fast-turnover production and precise machining operations while maintaining excellent corrosion resistance.
  • Critical Applications: Valve stems, screw machine parts, and other precision-machined components.
  • Limitations: Slightly lower mechanical strength compared to Monel 400.

Monel K500

  • Challenges: Strength limitations in high-stress applications.
  • Solution: Monel K500 combines the corrosion resistance of Monel 400 with significantly higher strength and hardness due to its age-hardenable properties. It maintains excellent performance in seawater and other harsh environments.
  • Critical Applications: Marine fasteners, oil and gas tools, pump shafts, and springs.
  • Limitations: Susceptible to stress corrosion cracking in some harsh chemical environments.

2. Incoloy Alloys

Incoloy is a family of nickel-iron-chromium alloys designed for high-temperature service in corrosive environments. These alloys are especially suitable for applications requiring oxidation and carburization resistance.

Incoloy 25-6Mo

  • Challenges: Corrosion in highly acidic environments.
  • Solution: Thanks to its high molybdenum content, Incoloy 25-6Mo provides excellent resistance to sulfuric and phosphoric acids and is highly effective against pitting and crevice corrosion. Consequently, it is an ideal choice for environments with aggressive chemicals.
  • Critical Applications: Chemical processing equipment, flue gas scrubbers, and pollution control systems.
  • Limitations: High alloy content increases material cost.

Incoloy 800, 800H, 800HT

  • Challenges: High-temperature oxidation and stress rupture.
  • Solution: Incoloy 800 series alloys provide excellent strength and resistance to oxidation, carburization, and nitridation at elevated temperatures. The H and HT versions offer enhanced creep and stress rupture properties, making them suitable for long-term exposure to high temperatures.
  • Critical Applications: Heat exchangers, furnace parts, and petrochemical processing.
  • Limitations: Not ideal for environments with aggressive acids or chlorides.

UNS N08825/Incoloy 825

  • Challenges: Corrosion from sulfuric and phosphoric acids.
  • Solution: Incoloy 825 provides excellent resistance to reducing and oxidizing acids, including sulfuric and phosphoric acids, and chloride-induced stress corrosion cracking.
  • Critical Applications: Acid production plants, chemical processing, oil and gas piping systems, and pollution control.
  • Limitations: Lower strength than some other nickel alloys at elevated temperatures.

3. Inconel Alloys

Inconel is a family of nickel-chromium alloys known for maintaining mechanical strength and corrosion resistance at extremely high temperatures. These alloys are crucial for industries that operate in high-heat and highly corrosive environments.

UNS N06600/Inconel 600

  • Challenges: High-temperature oxidation and chloride stress corrosion cracking.
  • Solution: Inconel 600 is highly resistant to oxidation and corrosion at elevated temperatures and effectively resists chloride-induced stress corrosion cracking. These properties are particularly useful in nuclear reactors and high-heat environments.
  • Critical Applications: Nuclear reactors, furnace components, and chemical processing equipment.
  • Limitations: Prone to pitting in specific environments, particularly in seawater.

Inconel 601/UNS N06601

  • Challenges: High-temperature oxidation and sulfuric acid exposure.
  • Solution: Inconel 601 is optimized for high-temperature oxidation resistance and is used in environments where heat and chemical exposure are concerned.
  • Critical Applications: Heat treatment equipment, furnace parts, and chemical processing.
  • Limitations: Less resistant to reducing environments compared to Inconel 625.

UNS N06617/Inconel 617

  • Challenges: Creep resistance and oxidation at extreme temperatures.
  • Solution: Inconel 617 offers exceptional creep strength and oxidation resistance at temperatures as high as 2000°F (1093°C), making it ideal for gas turbines and other high-temperature applications.
  • Critical Applications: Gas turbines, petrochemical processing, and nuclear reactors.
  • Limitations: Higher cost due to alloy composition.

Inconel 625/UNS N06625

  • Challenges: Severe environments involving acid, alkali, and seawater corrosion.
  • Solution: Inconel 625 is undoubtedly one of the most versatile corrosion-resistant alloys. It exhibits excellent strength and remarkable resistance to a wide range of aggressive chemicals. Furthermore, it is particularly effective in seawater environments and excels at combating pitting and crevice corrosion.
  • Critical Applications: Marine engineering, oil and gas, chemical processing, and aerospace components.
  • Limitations: Not as substantial at very high temperatures as other Inconel grades.

UNS N06690/Inconel 690

  • Challenges: Corrosion in nuclear and high-temperature steam environments.
  • Solution: Inconel 690 offers excellent corrosion resistance in nuclear steam generators and other environments with high-temperature water and oxidizing chemicals.
  • Critical Applications: Nuclear reactors, steam generators, and chemical processing.
  • Limitations: High alloy content leads to increased material costs.

Inconel 718/UNS N07718

  • Challenges: High strength at cryogenic and elevated temperatures.
  • Solution: Inconel 718 is a precipitation-hardenable alloy with excellent strength across a wide temperature range—from cryogenic levels to over 1300°F (704°C)—widely used in aerospace and gas turbines. Furthermore, its outstanding fatigue and creep resistance make it a preferred choice in demanding applications.
  • Critical Applications: Aerospace engines, gas turbines, cryogenic tanks, and high-strength fasteners.
  • Limitations: It is more expensive due to its alloying elements.

Inconel X750

  • Challenges: Long-term high-temperature exposure in high-stress environments.
  • Solution: Inconel X750 is a precipitation-hardenable alloy that maintains strength and oxidation resistance at high temperatures. Therefore, it is ideal for use in high-stress, high-temperature environments, such as gas turbines and nuclear reactors.
  • Critical Applications: Gas turbines, rocket engines, and nuclear power plants.
  • Limitations: Slightly lower corrosion resistance than Inconel 625 in some aggressive chemical environments.
Monel/Incoloy/Inconel Alloys

Monel/Incoloy/Inconel Alloys

Industry Challenges with Monel/Incoloy/Inconel Alloys

1. Corrosion in Marine and Chemical Processing

Nickel alloys like Monel 400, Incoloy 825, and Inconel 625 provide exceptional resistance to seawater, brine, and aggressive chemicals. For offshore platforms, marine engineering, and chemical processing plants, these materials help prevent pitting, crevice corrosion, and general material degradation, ensuring long-term durability.

2. High-Temperature Strength in Aerospace and Power Generation

Inconel alloys, particularly Inconel 718, 601, and X750, are commonly used in high-temperature applications. They are crucial in gas turbines, aerospace engines, and power generation equipment. Moreover, their remarkable ability to retain strength at elevated temperatures makes them indispensable in environments where mechanical stresses are extreme.

3. Stress Corrosion Cracking in Oil and Gas

Stress corrosion cracking is a common challenge in oil and gas extraction. Incoloy and Inconel alloys strongly resist chloride-induced stress corrosion, particularly Incoloy 825 and Inconel 600. Consequently, this property makes them ideal for downhole tubing and piping systems.

Conclusion

Nickel-based alloys, including Monel/Incoloy/Inconel Alloys, are critical solutions in various industries and play a vital role in sectors such as oil and gas, aerospace, chemical processing, and marine engineering. Their exceptional resistance to corrosion, high-temperature oxidation, and mechanical stresses provide a comprehensive range of solutions for the most challenging environments. By selecting the appropriate Monel/Incoloy/Inconel Alloys for the specific operating conditions—whether high-temperature strength, chemical corrosion resistance, or a combination of both—industries can ensure reliable, long-lasting performance. For decision-makers and engineers, understanding the strengths and limitations of each alloy is essential for optimizing performance, safety, and operational efficiency in critical applications.

What is BA Stainless Steel Tube?

Introduction of BA Stainless Steel Tube

BA Stainless Steel Tube, known for its excellent surface finish, cleanliness, and high dimensional accuracy, is essential for industries requiring ultra-clean, high-purity conditions. These include sectors like biopharmaceuticals, semiconductors, liquid crystal displays (LCD), solar photovoltaic systems, and other high-tech fields. BA tubing undergoes a vacuum or controlled atmosphere heat treatment, producing a bright, reflective surface without oxidation, making it ideal for applications that require purity and corrosion resistance.

Materials and Standards of BA Stainless Steel Tube

Manufacturers typically use 304 and 316L stainless steels to produce BA stainless steel tubes, ensuring excellent corrosion resistance, strength, and versatility. These tubes adhere to various international standards, such as:

  • ASTM A249, ASTM A269, ASTM A270, 3A, and BPE standards for the US.
  • JIS G3447 and G3459 for Japan.

Manufacturing Process of BA Stainless Steel Tube

BA Stainless Steel Tube is primarily cold-rolled, offering high dimensional precision (up to 0.2%) and superior surface quality (Ra < 0.45 µm). Unlike polished tubes, the inner surface of BA tubing retains its smoothness, thanks to the advanced annealing process. Raw materials must comply with ASTM A269; in some cases, double-melted steel is employed for enhanced purity.

Characteristics of BA Stainless Steel Tube

BA Stainless Steel Tube is highly prized for its exceptional dimensional accuracy, smooth surface, and cleanliness. Moreover, their inner surface is softer than polished alternatives, making them ideal for ultra-clean applications. Additionally, the process ensures high precision, with dimensional accuracy reaching up to ±0.05 mm and surface roughness (Ra) approaching 0.4 µm.

Heat Treatment (Bright Annealing)

The heat treatment process, or bright annealing, occurs in a reducing atmosphere, where oxygen is minimized to prevent oxidation. This controlled environment maintains the tube’s smooth, reflective surface and optimizes its mechanical properties. As a result, the tubes are free of scale and, therefore, do not require further pickling or passivation.

Cleaning and Inspection of BA Stainless Steel Tube

Before leaving the production facility, BA tubes undergo an extensive cleaning process. Ultrasonic cleaning with degreasing agents like SC2286 removes oils and contaminants, ensuring ultra-clean surfaces are suitable for high-purity environments. Inspections involve methods like eddy current testing and hydraulic pressure tests to guarantee the absence of defects.

Finishing and Packaging of BA Stainless Steel Tube

After production, BA stainless steel tubes are straightened, cut to precise lengths, and polished to remove minor imperfections. Packaging typically includes several protective layers, such as PE bags, end caps, and anti-static materials, ensuring the tubes remain contamination-free during transport and storage.

Applications of BA Stainless Steel Tube

BA stainless steel tubes find extensive use in industries requiring ultra-high purity gas or fluid transport. Specific industries and applications include:

  • Biopharmaceuticals
  • Semiconductors
  • Clean rooms and laboratories
  • LCD and solar photovoltaic manufacturing
  • High-purity gas systems
  • WFI (Water for Injection) systems
  • Compressed air and ultra-pure chemical systems
AP, BA, MP, EP Stainless Steel Tubes

AP, BA, MP, EP Stainless Steel Tubes

Additional Surface Treatments

Electropolished (EP) Tubing

Electropolished (EP) tubing is an extension of BA stainless steel tubing, where the internal surface undergoes an additional step of electrochemical polishing. This process further enhances smoothness and reduces the risk of contamination. EP tubing is the preferred choice for ultra-high purity applications, such as those in the semiconductor and pharmaceutical sectors.

Critical properties of EP tubes include:

  • Diameter tolerance: ±0.02 mm (0.1%)
  • Length tolerance: ±0.05 mm
  • Straightness: ≤0.1 mm/m
  • Surface roughness: Ra ≤ 0.15 µm
  • Concentricity tolerance: ±0.01 mm

AP Tubing (Annealed and Pickled)

AP Tubing undergoes a less complex finishing process, including annealing, pickling, and passivation. This treatment removes surface oxides and enhances the tube’s mechanical properties for general applications.

MP Tubing (Mechanical Polish)

We mechanically polish stainless steel tubing to enhance internal and external surface finishes. While MP tubing does not achieve the smoothness of EP tubing, it remains suitable for less demanding clean applications requiring high surface quality but not ultra-high purity.

Key Properties

Corrosion Resistance

BA stainless steel tubes, particularly those made from 304 and 316L stainless steel, offer excellent corrosion resistance in most environments. The bright annealing process eliminates the need for pickling and passivation, further enhancing the material’s corrosion resistance by removing impurities and oxide layers.

Physical and Thermal Properties

Stainless steel BA tubes have impressive mechanical strength and moderate flexibility, making them ideal for bending and forming applications. Their thermal expansion coefficient and thermal conductivity are consistent with standard austenitic stainless steels, ensuring reliability in both high—and low-temperature environments.

Chemical Composition

  • 304 Stainless Steel: 18-20% chromium, 8-10.5% nickel, with small amounts of carbon and manganese.
  • 316L Stainless Steel: 16-18% chromium, 10-14% nickel, and 2-3% molybdenum for enhanced corrosion resistance.

Mechanical Properties

Stainless steel BA tubes exhibit high tensile strength and excellent elongation properties, allowing them to withstand high pressures and mechanical stresses daily in applications like gas lines and cleanroom systems.

Forming and Welding

Forming

Stainless steel BA tubes are relatively easy to form thanks to their high flexibility and controlled surface finish. They are suitable for bending, flaring, and other fabrication processes used in pipefitting and assembly.

Welding

Welding stainless steel BA tubes requires precision to avoid contamination and maintain surface integrity. TIG welding (Tungsten Inert Gas) is the preferred method, as it minimizes the introduction of impurities. After welding, proper passivation or electropolishing may be necessary to restore corrosion resistance and smoothness.

Corrosion of Welds

To maintain the tube’s overall corrosion resistance in critical applications, you must thoroughly clean and electropolish the welds. Welds can be vulnerable to corrosion, especially if the surface finish isn’t properly restored.

Descaling, Pickling, and Cleaning

Due to the bright annealing process, descaling and pickling processes are typically unnecessary in applications requiring stainless steel BA tubes. To ensure cleanliness, the appropriate chemical solutions clean tubes exposed to potential contaminants during fabrication, followed by rinsing with high-purity water.

Surface Finish Options

BA (Bright Anneal)

The standard finish for high-purity applications offers a smooth, reflective surface.

EP (Electropolished)

The manufacturing process achieves a superior internal surface finish, which is ideal for ultra-high purity systems where minimizing contamination is critical.

AP (Annealed and Pickled)

The bright annealing process is followed by chemical pickling to remove oxides, creating a bare finish.

MP (Mechanical Polish)

Mechanical polishing creates a polished surface, making it suitable for applications requiring cleanliness but not necessarily ultra-high purity levels.

Conclusion

BA Stainless Steel Tube is indispensable in high-tech and critical industries that require cleanliness, dimensional precision, and corrosion resistance. From biopharmaceuticals to semiconductors, these tubes offer unmatched reliability, ensuring the safety and integrity of ultra-pure systems. By understanding the unique properties of BA tubes and the various finishes available, industries can select the most appropriate material for their specific needs, ensuring efficiency and longevity in their operations.

UNS N08367 AL-6XN

UNS N08367 (AL-6XN®): A Comprehensive Guide for Critical Industrial Applications

Introduction

UNS N08367 (AL-6XN®) is super-austenitic stainless steel with superior corrosion resistance and strength in some of the most demanding industrial environments. Its exceptional ability to withstand chloride-induced pitting and crevice corrosion makes it widely used in industries such as chemical processing, pulp and paper, marine and offshore, air pollution control, power generation, food processing, and advanced fields like space biospheres and high-efficiency residential furnaces.

In this blog, we’ll explore UNS N08367’s key properties and advantages and provide users with detailed information on its specifications, applications, corrosion resistance, and more.

1. Overview of UNS N08367 (AL-6XN®)

UNS N08367 (AL-6XN®) is a nitrogen-bearing super-austenitic stainless steel designed for extreme environments. It offers superior resistance to pitting, crevice corrosion, and stress-corrosion cracking (SCC) in chloride-bearing environments, making it a cost-effective alternative to higher-cost nickel-based alloys. This material suits industries that handle aggressive chemicals and seawater or require long-term durability in harsh environments.

2. Available UNS N08367 (AL-6XN®) Products and Specifications

UNS Number: N08367
Common Name: AL-6XN®
W.Nr.: 1.4529
ASTM/ASME Standards: A240, A312, A249, A479, B688, B675, among others.

AL-6XN® is available in various product forms, including:
Pipe
Tube
Plate
Sheet
Strip
Bar
Rod
Wire
Forging Stock

These product forms make it versatile and suitable for various industrial applications.

3. Applications of UNS N08367 (AL-6XN®)

Due to its outstanding corrosion resistance and mechanical strength, UNS N08367 (AL-6XN®) is widely used in industries that demand reliable performance in harsh environments:

Chemical Processing: Ideal for handling corrosive chemicals like sulfuric and hydrochloric acids.
Pulp and Paper: Resistant to the harsh chemicals used in bleaching.
Marine and Offshore: Excellent for seawater piping systems, desalination plants, and offshore platforms.
Air Pollution Control: It is suitable for flue gas desulfurization (FGD) systems and scrubbers and offers resistance to acidic condensates.
Power Generation: Commonly used in heat exchangers, condenser tubing, and service water systems.
Space Biosphere: Utilized in advanced environmental systems like Biosphere 2, where long-term exposure to recycled air and water is critical.
Food Processing: Certified for food contact applications, including brewing and dairy equipment.
High-Efficiency Residential Furnaces: Used in condensing furnaces to resist acidic condensates.

4. Corrosion Resistance Properties

AL-6XN® is engineered to provide top-tier resistance to localized corrosion, including:
Pitting and Crevice Corrosion: Its high molybdenum content (6.2%) and nitrogen enhance resistance to these forms of corrosion in chloride environments.
Stress-Corrosion Cracking (SCC): The high nickel content (24%) helps prevent SCC, which is common in chloride-laden environments.
Resistance to Acidic Environments: AL-6XN® can withstand exposure to sulfuric, phosphoric, and hydrochloric acid.

5. Physical and Thermal Properties

AL-6XN® maintains its physical stability even in extreme conditions:
Density: 8.03 g/cm³
Melting Range: 1300°C to 1350°C (2370°F to 2460°F)
Thermal Conductivity: Lower than standard stainless steels, making it suitable for thermal management in specific applications.
Thermal Expansion: 8.1 × 10⁻⁶/°F (14.6 × 10⁻⁶/°C) from 70°F to 1000°F.

6. Chemical Composition

The chemical composition of AL-6XN® is carefully balanced to optimize its corrosion resistance and mechanical properties:
Nickel (Ni): 24.0%
Chromium (Cr): 20.5%
Molybdenum (Mo): 6.2%
Nitrogen (N): 0.22%
Iron (Fe): Balance
Others: Trace amounts of carbon, silicon, manganese, and copper.

7. Mechanical Properties

AL-6XN® offers excellent mechanical strength:
Tensile Strength: 690 MPa (100 ksi)
Yield Strength: 310 MPa (45 ksi)
Elongation: 30% (in 2 inches or 4D)
Its high tensile strength and elongation make it ideal for durability and flexibility applications.

8. Heat Treatment

AL-6XN® undergoes a solution annealing heat treatment to achieve optimal mechanical and corrosion resistance properties:
Solution Annealing Temperature: 2050°F (1120°C), followed by rapid cooling.
This heat treatment helps maintain its fully austenitic structure. It ensures that the material remains resistant to detrimental phases such as the sigma phase, which can weaken both corrosion resistance and strength.

UNS N08367 AL-6XN SMLS TUBE

UNS N08367 AL-6XN SMLS TUBE

9. Forming

AL-6XN® demonstrates excellent formability and can be formed using both hot and cold working techniques:
Cold Forming: Its high flexibility makes it easily cold-formed into complex shapes without risk of cracking.
Hot Forming: It should be done between 1700°F and 2150°F (927°C to 1177°C) and followed by rapid quenching to preserve corrosion resistance.
Its formability is critical for complex manufacturing processes in demanding industries.

10. Welding of UNS N08367 (AL-6XN®)

The weldability of AL-6XN® is one of its strengths, making it highly suitable for complex fabrication:
Welding Processes: TIG (GTAW), MIG (GMAW), SMAW, and SAW are commonly used.
Filler Metals: Over-matching filler materials is recommended to ensure the welded joint maintains corrosion resistance equivalent to the base metal.
The low carbon content reduces carbide precipitation, a common cause of intergranular corrosion in welds.

11. Corrosion of Welds

If not correctly treated, welds can be vulnerable to corrosion. AL-6XN® minimizes this risk with low carbon and high nitrogen content, reducing carbide precipitation and leading to intergranular corrosion. Proper heat input control and post-weld cleaning are essential to maintaining corrosion resistance.

12. Descaling, Pickling, and Cleaning

Post-weld cleaning is critical to preserve the corrosion resistance of AL-6XN®. Descaling and pickling are commonly performed using nitric-hydrofluoric acid solutions, followed by thorough rinsing with water. These processes remove oxides and surface contaminants that could otherwise initiate localized corrosion.

13. Surface Hardening

Surface hardening is generally not recommended for AL-6XN® because it could compromise the material’s corrosion resistance. The alloy’s natural strength and toughness are sufficient for most applications, eliminating the need for additional hardening treatments.

Conclusion

UNS N08367 (AL-6XN®) is a versatile, high-performance alloy with unparalleled corrosion resistance, strength, and formability for demanding industrial applications. Its ability to withstand harsh environments and cost-efficiency makes it a reliable choice across chemical processing, marine, pulp and paper industries, and power generation. Whether dealing with corrosive chemicals, seawater, or high temperatures, AL-6XN® delivers superior performance, long service life, and reduced maintenance costs.

If you have questions or need more information on how AL-6XN® can benefit your application, please get in touch with us at [email protected] for guidance and support.

ASME Material Standards

List of Commonly Used ASME Material Standards for Pressure Vessels and Equipment

Introduction

Adhering to stringent material standards is essential for ensuring safety, reliability, and efficiency in pressure vessels and equipment. The American Society of Mechanical Engineers (ASME) has established a comprehensive set of material standards that govern the use of various materials in the design and construction of pressure equipment. This blog post aims to provide valuable insights into these standards, focusing on commonly used plates, pipes, heat exchanger tubes, bars, flanges, and fittings.

Understanding ASME Material Standards for Pressure Vessels

ASME material standards are crucial in defining the specifications and performance requirements for materials used in pressure applications. These ASME Material Standards for Pressure Vessels help engineers, fabricators, and manufacturers select the appropriate materials based on the intended service conditions, ensuring that the components can withstand operational demands.

Importance of Compliance

Compliance with ASME standards is not just a regulatory requirement; it is a best practice that enhances safety, minimizes risks, and prolongs the lifespan of equipment. By carefully selecting the proper materials, industries can prevent failures, leaks, and catastrophic incidents and ensure long-term safety and reliability, which are crucial considerations in sectors such as oil and gas, chemical processing, and power generation.

ASME Material Standards for Pressure Vessels (Materials Compatibility Table)

Material Plate Pipe Tube Bar Flanges Fittings
Nickel 200 SB162 SB161 SB163 SB160 SB564 B366 WPN
Monel 400 SB127 SB165 SB163 SB164 SB564 B366 WPNC
Inconel 600 SB168 SB167 SB163 SB166 SB564 B366 WPNCI
Incoloy 825 SB424 SB423 SB407 SB425 SB564 SB366 WPNICMC
Hastelloy C-4 SB575 SB574 SB622 SB574 SB575 SB366 WPHC4
Carpenter 20 SB463 SB464 SB468/SB729 SB472 SB564 SB366 WPH20Cb
Stainless Steel SA240 SA312 SA213/SA249 SA479 SA182 SA403
Carbon Steel SA516 SA106 Gr.B SA179 SA36/SA675 SA105 SA234 WPB
Titanium SB265 SB861 SB338 SB348 SB381 SB363
Aluminum 6061 SB209 SB241 SB210 SB211 SB247
Cr-Mo Alloy SA387 SA335 SA213 SA739 SA182 SA234
FSS405 12Cr Use SST Designations Use SST Designations
MSS410 13Cr Use SST Designations Use SST Designations
FSS430 17Cr Use SST Designations Use SST Designations
3½% Ni SA203-D SA333-3 SA350 LF3 SA420 WPL3
Hastelloy G-30 SB582 SB619 SB622 SB581 SB462 SB366 WPHG30
Nitronic 50 (UNS N20910) SA240-XM19 SA312-XM19 SA213-XM19 SA479-XM19 SA182-XM19 SA403-XM19
Incoloy 800 SB409 SB407 SB163 SB408 SB564 B366 WPNIC
ASME Material Standards for Pressure Vessels

ASME Material Standards for Pressure Vessels

ASME Material Standards for Pressure Vessels: Selecting the Right Material

When selecting materials for pressure vessels and equipment, it is important to consider the following factors:

  1. Service Conditions: Understand the temperature, pressure, and chemical environment where the equipment will operate.
  2. Mechanical Properties: Ensure the chosen materials meet the required tensile strength, yield strength, and flexibility.
  3. Weldability: When evaluating the selected materials, assessing the ease of welding and the fabrication process is essential, particularly in the context of more complex designs.
  4. Corrosion Resistance: Select materials that provide adequate corrosion resistance based on the service environment.

Conclusion

ASME Material Standards for Pressure Vessels are crucial for professionals involved in designing, fabricating, and maintaining pressure vessels and equipment. By understanding these standards and their applications, you can ensure the safety and efficiency of your systems. Always consult with experienced engineers and adhere to the latest codes to make informed decisions regarding material selection.

Please seek our professional assistance for more guidance on ASME standards and their application to your projects. Your commitment to quality and safety is the foundation of effective pressure equipment design and operation.

INCOLOY vs. HASTELLOY

INCOLOY vs. HASTELLOY: A Comprehensive Comparison for Industrial Applications

Introduction

In industries like oil and gas, aerospace, chemical processing, marine engineering, and heat treatment, the choice of material can significantly impact equipment’s efficiency, safety, and longevity. INCOLOY vs. HASTELLOY is two high-performance alloys often compared in these fields. Both nickel-based alloys offer exceptional resistance to corrosion and heat, but their specific applications and characteristics vary based on environmental demands. This post will detail the essential differences between INCOLOY and HASTELLOY, focusing on their chemical composition, properties, advantages, and typical applications. By the end, you will clearly understand which alloy suits your particular needs.

Overview of INCOLOY

SMC developed INCOLOY, a family of nickel-iron-chromium alloys. These alloys offer high strength and resist oxidation, carburization, and sulfidation, making them ideal for high-temperature environments. Industries that require resistance to corrosive environments commonly use INCOLOY alloys, though extreme heat resistance, like that of HASTELLOY, is not always needed.

INCOLOY Grades

INCOLOY 800: Known for its strength and stability in high-temperature applications, commonly used in heat exchangers, chemical and petrochemical processing, and furnace components.
INCOLOY 825 offers excellent resistance to reducing acids like sulfuric and phosphoric acids, making it suitable for chemical processing, marine engineering, and oil and gas production.

Chemical Composition

INCOLOY alloys typically contain:
Nickel: 30-46%
Chromium: 19-23%
Iron: Balance
Molybdenum and Copper (in specific grades for enhanced corrosion resistance)

Properties

Corrosion Resistance: Excellent in acidic environments, especially against sulfuric and phosphoric acids.
Oxidation Resistance: Good at elevated temperatures.
Mechanical Strength: High strength and resistance to stress corrosion cracking.
Ease of Fabrication: INCOLOY alloys are more straightforward to machine and fabricate than other high-performance alloys.

Overview of HASTELLOY

HASTELLOY is a family of nickel-molybdenum or nickel-chromium-molybdenum alloys produced by Haynes International. These alloys resist various chemical environments, especially under extreme temperatures and pressures. HASTELLOY is particularly effective in environments involving strong acids and high temperatures, where other materials corrode rapidly.

HASTELLOY Grades

HASTELLOY C-276: Known as the most versatile corrosion-resistant alloy, it is widely used in chemical processing, pollution control, pulp and paper production, and waste treatment.
HASTELLOY C-22: Offers superior resistance to both oxidizing and reducing environments, making it ideal for industries such as pharmaceuticals, marine engineering, and flue gas desulfurization.

Chemical Composition

HASTELLOY alloys typically contain:
Nickel: 50-70%
Chromium: 14-16%
Molybdenum: 15-17%
Iron, Tungsten, and Cobalt (depending on the specific grade)

Properties

Corrosion Resistance: Exceptional resistance to a broad range of chemicals, particularly in harsh acidic environments.
Oxidation Resistance: Excellent at high temperatures, especially in reducing environments.
High-Temperature Strength: Retains strength and stability even at extreme temperatures.
Ease of Fabrication: Due to its high strength and resistance, machines and fabricates are more challenging

Heat Exchanger & Pressure Vessel

Heat Etheiranger & Pressure Vessel

INCOLOY vs. HASTELLOY: A Comparative Analysis

Feature INCOLOY HASTELLOY
Primary Elements Nickel, Iron, Chromium Nickel, Chromium, Molybdenum
Corrosion Resistance Suitable in oxidizing and mildly corrosive environments Excellent in harsh chemical environments (acids, chlorides)
Heat Resistance Moderate to high Exceptional at high temperatures
Mechanical Strength High Very high
Cost Lower compared to HASTELLOY Higher due to superior performance
Ease of Fabrication More accessible to machine and weld More difficult due to the toughness
Typical Applications Heat exchangers, chemical processing, marine Chemical processing, pollution control, marine engineering

Applications: INCOLOY vs HASTELLOY

INCOLOY Applications

Oil and Gas: Oil and gas extraction widely uses INCOLOY alloys, such as INCOLOY 825, especially for components exposed to sour gas and brine.
Marine Engineering: The alloy’s resistance to corrosion in seawater makes it ideal for marine environments.
Heat Treatment: INCOLOY alloys are commonly used in furnace parts and heat exchangers due to their high-temperature strength.

HASTELLOY Applications

Chemical Processing: HASTELLOY C-276 is the material of choice for equipment handling corrosive chemicals, especially in producing sulfuric, hydrochloric, and phosphoric acids.
Aerospace: HASTELLOY alloys are used in aircraft components that withstand extreme heat and corrosion.
Marine Engineering: HASTELLOY is often preferred in marine environments where chloride stress corrosion is a concern.
Heat Treatment: HASTELLOY’s ability to maintain mechanical properties at high temperatures makes it suitable for heat exchangers and reactors in extreme environments.

Selection: INCOLOY vs HASTELLOY

The decision to use INCOLOY or HASTELLOY largely depends on the application’s specific environmental and operational conditions. Here are some general guidelines:
HASTELLOY is the better choice for applications with high levels of acidic or corrosive chemicals, particularly under extreme temperatures, because it offers superior chemical resistance.
INCOLOY delivers excellent performance at a lower cost in high-temperature applications that don’t involve highly corrosive environments.
When easier fabrication is required without sacrificing corrosion resistance, INCOLOY is generally preferred for its machinability.
HASTELLOY is the best option for situations involving extreme heat and harsh chemicals, such as chemical reactors or pollution control equipment.

Conclusion

Both INCOLOY and HASTELLOY offer distinct advantages depending on the industry and environment. INCOLOY excels in less harsh environments, providing high strength and moderate corrosion resistance at elevated temperatures. In contrast, HASTELLOY is the go-to material for extreme conditions involving high temperatures, corrosive chemicals, and demanding mechanical stress.

When choosing between these two alloys, consider your application’s specific chemical and thermal conditions. Consulting with materials experts and suppliers can also help tailor the alloy selection to your operational needs, ensuring long-lasting, reliable performance.

By understanding the critical differences between INCOLOY and HASTELLOY, industries like oil and gas, aerospace, and chemical processing can make informed decisions that optimize performance, safety, and cost-efficiency.

MONEL vs. INCOLOY

MONEL vs INCOLOY: A Comprehensive Guide for Industrial Applications

Introduction

Selecting the suitable material for critical industrial applications can be complex, particularly when faced with various alloys offering unique properties. Two well-known nickel-based alloy families, MONEL and INCOLOY, often emerge as contenders in industries like oil and gas, aerospace, chemical processing, marine engineering, and heat treatment. Both alloys exhibit remarkable corrosion resistance and mechanical strength, but their distinct compositions make them suitable for different environments.

In this post, we will compare MONEL and INCOLOY, focusing on their chemical compositions, properties, and the environments in which they excel. By the end, you will better understand which alloy is best suited for your application.

1. Overview of MONEL

MONEL is a family of nickel-copper alloys known for its excellent corrosion resistance, particularly in marine environments and chemical industries. MONEL alloys are highly resistant to saline environments and a range of corrosive chemicals, making them popular in applications requiring protection from saltwater and acidic substances.

Key MONEL Grades

  • MONEL 400: The most widely used grade, MONEL 400, offers excellent resistance to corrosion in a variety of environments, particularly against seawater and hydrofluoric acid.
  • MONEL K-500: Similar to MONEL 400 but with added aluminum and titanium, offering increased strength and hardness through age-hardening.

Chemical Composition

MONEL alloys typically contain:

  • Nickel: 63-70%
  • Copper: 20-29%
  • Iron, Manganese, and other elements in trace amounts

Properties

  • Corrosion Resistance: Exceptional resistance to seawater, acids, and alkalis.
  • Mechanical Strength: High tensile strength, particularly in MONEL K-500, which heat treatment can further strengthen.
  • Oxidation Resistance: Good at moderate temperatures but less effective at extremely high temperatures than INCOLOY.
  • Ease of Fabrication: MONEL alloys are relatively easy to machine and weld, though work-hardening can be a concern.

2. Overview of INCOLOY

Engineers and manufacturers recognize INCOLOY as a family of nickel-iron-chromium alloys known for their high strength and resistance to oxidation and corrosion in high-temperature environments. Unlike MONEL, which primarily resists seawater and acids, INCOLOY alloys are designed to perform well in these environments.

Key INCOLOY Grades

  • INCOLOY 800: Known for its high-temperature stability, it is ideal for furnace components, heat exchangers, and petrochemical processing.
  • INCOLOY 825: Provides excellent resistance to reducing and oxidizing acids, making it ideal for chemical processing, pollution control, and oil and gas production.

Chemical Composition

INCOLOY alloys typically contain:

  • Nickel: 30-46%
  • Chromium: 19-23%
  • Iron: Balance
  • Other elements such as molybdenum, copper, and titanium (depending on the grade)

Properties

  • Corrosion Resistance: Excellent resistance to oxidizing and reducing acids and chloride stress-corrosion cracking.
  • Heat Resistance: Superior resistance to oxidation at elevated temperatures, making it a go-to material for high-temperature applications.
  • Mechanical Strength: High strength and durability under stress, particularly at elevated temperatures.
  • Ease of Fabrication: INCOLOY alloys are relatively easy to machine and weld compared to other high-performance alloys, though not as simple as MONEL.
UNS N08800 Incoloy 800, UNS N08810 Incoloy 800H, UNS N08811 Incoloy 800HT

UNS N08800 Incoloy 800, UNS N08810 Incoloy 800H, UNS N08811 Incoloy 800HT

3. MONEL vs INCOLOY: Main Differences

Feature MONEL INCOLOY
Primary Elements Nickel, Copper Nickel, Iron, Chromium
Corrosion Resistance Exceptional in marine and acidic environments Excellent in high-temperature, corrosive environments
Heat Resistance Moderate High
Mechanical Strength High, can be enhanced with age-hardening (K-500) High, particularly at elevated temperatures
Cost Typically lower than INCOLOY Higher due to chromium and advanced performance
Ease of Fabrication More accessible to machine and weld Slightly more challenging to fabricate
Typical Applications Marine, chemical processing, oil and gas High-temperature applications, chemical processing, oil and gas

4. Applications: MONEL vs INCOLOY

MONEL Applications

  • Marine Engineering: MONEL’s outstanding resistance to saltwater corrosion makes it a top choice for marine applications, including propeller shafts, seawater valves, and pump components.
  • Oil and Gas: In environments with high levels of hydrogen sulfide or hydrofluoric acid, engineers frequently use MONEL for downhole tools, valves, and other oilfield equipment.
  • Chemical Processing: MONEL alloys resist the corrosion of strong acids and alkalis, making them suitable for use in heat exchangers, process piping, and chemical storage tanks.

INCOLOY Applications

  • Heat Treatment: INCOLOY’s ability to retain strength at high temperatures makes it ideal for furnace parts, heat exchangers, and thermal reactors.
  • Oil and Gas: Oil and gas extraction commonly uses INCOLOY alloys, particularly INCOLOY 825, especially in environments that involve sour gas, brine, or hydrogen sulfide.
  • Aerospace: Engineers use INCOLOY alloys in jet engines, exhaust systems, and other high-temperature components where both strength and corrosion resistance are critical.
  • Chemical Processing: INCOLOY’s resistance to oxidizing and reducing acids allows it to perform well in demanding chemical environments.

5. Selecting the Right Alloy: MONEL vs INCOLOY?

When choosing between MONEL and INCOLOY, consider the following factors:

  • Marine Environments: If your application involves prolonged exposure to seawater or brine, MONEL is the superior choice due to its high resistance to saltwater corrosion.
  • High-Temperature Applications: INCOLOY offers better performance in environments where temperatures exceed 500°C (932°F) and oxidation resistance is critical.
  • Chemical Processing: For applications involving sulfuric, phosphoric, or nitric acid, INCOLOY alloys such as INCOLOY 825 excel, though MONEL, perform well in hydrofluoric acid environments.
  • Cost Considerations: MONEL may be a more cost-effective option if your budget is constrained and you don’t require high-temperature performance.
  • Mechanical Strength: MONEL K-500 can be a good fit for applications requiring high mechanical strength with the option of age hardening, while INCOLOY alloys offer higher strength at elevated temperatures.

6. Conclusion

MONEL and INCOLOY have distinct advantages depending on the specific needs of the application. MONEL excels in marine environments, offers resistance to a wide range of acids, and is cost-effective for many chemical processes. On the other hand, engineers choose INCOLOY for high-temperature applications requiring strength and corrosion resistance.

Choosing the suitable alloy depends on balancing your requirements for corrosion resistance, temperature tolerance, mechanical strength, and cost. Careful consideration of these factors will ensure long-lasting and reliable performance for applications in industries such as oil and gas, aerospace, marine engineering, and heat treatment.

By understanding the key differences between MONEL and INCOLOY, engineers and decision-makers in critical industries can make informed material choices, ensuring safety, efficiency, and durability in demanding environments.

AP, BA, MP, EP Stainless Steel Tubes

Surface Treatment of Stainless Steel Tubes: AP, BA, MP, and EP

Introduction

Various industries, including biopharmaceuticals, food processing, high-purity gas systems, chemical systems, and semiconductors, widely use stainless steel tubes. The performance and cleanliness of these tubes are crucial, and selecting the appropriate surface treatment can significantly impact their durability, corrosion resistance, and suitability for specific applications. This blog explores the Surface Treatment of Stainless Steel Tubes—Annealed and Pickled (AP), Bright Annealed (BA), Mechanically Polished (MP), and Electropolished (EP)—detailing the relevant standards, grades, manufacturing and finishing methods, heat treatments, key characteristics after surface treatment, inspections, packaging methods, and application recommendations.

Surface Treatment of Stainless Steel Tubes

AP (Annealed and Pickled) Stainless Steel Tubes

AP (Annealed and Pickled) Stainless Steel Tubes

1. AP (Annealed and Pickled) Stainless Steel Tubes

Standards and Grades:

  • Common Standards: ASTM A213, ASTM A269, ASTM A249
  • Grades: 304/304L, 316/316L, 317/317L

Manufacturing and Finishing Methods:

  • Manufacturing: Manufacturers produce AP stainless steel tubes through either seamless or welded processes. After forming, they subject the tubes to annealing and pickling.
  • Annealing: A heat treatment process heats the tubes to high temperatures and then slowly cools them, restoring ductility, reducing hardness, and improving toughness.
  • Pickling: After annealing, the tubes are chemically treated with an acid solution to remove any surface oxides or scale formed during heat treatment.

Characteristics:

  • Corrosion Resistance: The pickling process removes surface impurities, ensuring superior corrosion resistance.
  • Surface Finish: AP tubes have a matte, rough finish, which may not be suitable for applications requiring high cleanliness.
  • Surface Roughness: Ra typically ranges from 0.5 to 1.5 μm.

Inspection Items:

  • Tolerance of Outer Diameter & Wall Thickness: Verified per the standards, ensuring dimensional accuracy.
  • Surface Roughness/Cleanliness: Surface roughness is checked to confirm it meets requirements for basic industrial applications.
  • Non-Destructive Testing (NDT): Ultrasonic or eddy current testing is performed to detect any internal flaws or defects.

Packaging Method:

  • Producers generally bundle and secure AP tubes to prevent surface contamination or mechanical damage during transportation.

Applications:

  • Boilers and Heat Exchangers: AP tubes are ideal for high-temperature and high-pressure applications due to their superior corrosion resistance.
  • Chemical Processing: Resistant to a range of chemicals, AP tubes are widely used in chemical plants and refineries.
BA (Bright Annealed) Stainless Steel Tubes

BA (Bright Annealed) Stainless Steel Tubes

2. BA (Bright Annealed) Stainless Steel Tubes

Standards and Grades:

  • Common Standards: ASTM A213, ASTM A269, ASTM A249, ASME BPE
  • Grades: 304/304L, 316/316L, 317/317L

Manufacturing and Finishing Methods:

  • Manufacturing: BA tubes can be produced through either seamless or welded processes, followed by bright annealing.
  • Bright Annealing: Carried out in a controlled atmosphere (hydrogen or nitrogen) to prevent oxidation during heat treatment. To avoid any moisture that could cause oxidation, producers maintain the atmosphere at low dew points.

Characteristics:

  • Surface Finish: The result is a reflective, bright, and smooth finish, free from oxides and scales.
  • Corrosion Resistance: Enhanced due to the smooth, oxide-free surface.
  • Surface Roughness: Ra typically below 0.8 μm.

Inspection Items:

  • Tolerance of Outer Diameter & Wall Thickness: Dimensional accuracy checked according to specifications.
  • Surface Roughness: A profilometer is used to measure the surface smoothness, ensuring it meets the requirements of clean industries.
  • Visual and Cleanliness Inspection: Checked to ensure the bright surface is uniform and free of impurities.
  • NDT: Ultrasonic or eddy current testing for weld integrity and defect detection.

Packaging Method:

  • Producers individually wrap or cover BA tubes in plastic to maintain their cleanliness, preserve the bright surface, and prevent contamination or surface damage during handling.

Applications:

  • Pharmaceutical and Biopharmaceutical: Due to the smooth, clean surface, BA tubes are used in sterile environments and piping systems.
  • Food and Beverage: Hygienic applications where surface cleanliness is critical, especially in sanitary piping.
  • High-Purity Gas Systems: BA tubes are used in systems where contaminants could affect the gas purity.
MP (Mechanically Polished) Stainless Steel Tubes

MP (Mechanically Polished) Stainless Steel Tubes

3. MP (Mechanically Polished) Stainless Steel Tubes

Standards and Grades:

  • Common Standards: ASTM A270, ASME BPE, DIN 11850
  • Grades: 304/304L, 316/316L, 317/317L, 321

Manufacturing and Finishing Methods:

  • Manufacturing: Similar to other manufacturing methods, the tubes are either seamless or welded. The manufacturer applies mechanical polishing post-manufacture.
  • Mechanical Polishing: A mechanical process that grinds and polishes the surface, removing surface irregularities and achieving a smooth finish.

Characteristics:

  • Surface Finish: A smooth, visually appealing finish that offers lower Ra values than standard AP or BA finishes.
  • Corrosion Resistance: Excellent, particularly in applications where smooth surfaces help reduce crevice formation.
  • Surface Roughness: Ra values range between 0.2 to 0.6 μm, depending on the degree of polishing.

Inspection Items:

  • Tolerance of Outer Diameter & Wall Thickness: Manufacturers check tolerances in line with the relevant standards for mechanically polished tubes.
  • Surface Roughness: We measure roughness to confirm that it meets the polishing specifications, often focusing on cleanliness and smoothness for high-purity applications.
  • NDT: Various non-destructive testing methods, including visual and ultrasonic inspections, ensure tube integrity and performance.

Packaging Method:

  • Typically, manufacturers pack MP tubes in sealed plastic covers or wrap the polished areas in protective materials to prevent scratches and surface damage during transport.

Applications:

  • Pharmaceutical: Widely used in bioprocessing and sterile applications due to the smooth surface and ease of cleaning.
  • Semiconductors: Critical in systems requiring ultra-clean surfaces, where contamination could lead to product failures.
  • Food and Beverage: Used in sanitary applications, including dairy and beverage systems where a smooth, easy-to-clean surface is essential.
EP (Electropolished) Stainless Steel Tubes

EP (Electropolished) Stainless Steel Tubes

4. EP (Electropolished) Stainless Steel Tubes

Standards and Grades:

  • Common Standards: ASME BPE, ASTM A270, ASTM A269
  • Grades: 304/304L, 316/316L, 317/317L, 321

Manufacturing and Finishing Methods:

  • Manufacturing: EP tubes are typically produced through seamless or welded processes and then electropolished.
  • Electropolishing: This is an electrochemical process where a small layer of the material is removed from the surface. This results in a mirror-like finish, reducing the number of surface defects and contaminants.

Characteristics:

  • Surface Finish: Highly reflective, smooth, and ultra-clean, with Ra values as low as 0.1 μm.
  • Corrosion Resistance: Maximized due to the removal of micro-defects and impurities from the surface.
  • Surface Roughness: Typically between 0.1 and 0.4 μm, ideal for ultra-clean environments.

Inspection Items:

  • Tolerance of Outer Diameter & Wall Thickness: Checked with high precision to ensure consistency in dimensions.
  • Surface Roughness: Rigorous testing using profilometers ensures the surface finish meets ultra-high purity standards.
  • Cleanliness: Inspected microscopically to ensure no residual contaminants remain on the surface.
  • NDT: Advanced methods like eddy current or ultrasonic testing for internal defects, along with a visual inspection to ensure uniformity.

Packaging Method:

  • Manufacturers pack EP tubes in sealed plastic, often within sterile environments, to maintain cleanliness and give special attention to ensure the tubes arrive contamination-free.

Applications:

  • Ultra-High Purity Gas Systems: EP tubes are essential for maintaining gas purity and preventing particle contamination in industries like semiconductors.
  • Pharmaceutical: Critical for WFI (Water for Injection) systems, biopharmaceutical processing, and cleanroom environments.
  • Semiconductor: Ideal for applications requiring the lowest contamination levels, such as in liquid crystal displays, solar photovoltaics, and microelectronics.

Conclusion

AP, BA, MP, and EP surface treatments offer distinct advantages and suit different applications. AP tubes provide robustness and corrosion resistance for boilers and chemical processing systems, while BA tubes offer a clean, bright finish suitable for pharmaceutical and food industries. Manufacturers mechanically polish MP tubes to create smooth, easy-to-clean surfaces, while EP tubes offer the highest level of cleanliness and corrosion resistance, making them essential for ultra-high purity environments in semiconductors and biopharmaceuticals.

Users can make informed decisions that align with their specific operational needs by understanding the manufacturing processes, characteristics, and inspection protocols for each Surface Treatment of Stainless Steel Tubes.

NIMONIC 75 vs. NIMONIC 80A vs. NIMONIC 90

NIMONIC 75 vs. NIMONIC 80A vs. NIMONIC 90: A Comprehensive Comparison

Introduction

Material selection is critical in high-performance engineering, particularly in gas turbines, industrial thermal processing, furnace components, and heat-treatment equipment. NIMONIC 75 vs. NIMONIC 80A vs. NIMONIC 90, all among the most commonly used materials in these high-stress, high-temperature environments, are the nickel-chromium alloys of the NIMONIC family—specifically NIMONIC 75 vs. NIMONIC 80A vs. NIMONIC 90. These superalloys are known for their excellent strength at elevated temperatures, oxidation resistance, and long-term durability under harsh conditions.

In this article, we’ll explore the fundamental properties, differences, and applications of NIMONIC 75, 80A, and 90, guiding you to the right material choice for your engineering needs.

1. Overview of NIMONIC Alloys: NIMONIC 75 vs.NIMONIC 80A vs. NIMONIC 90

NIMONIC alloys are primarily nickel-chromium-based, with various alloying elements added to enhance specific properties. They are renowned for retaining strength and resisting oxidation at temperatures where traditional metals fail, often exceeding 700°C. These alloys are commonly employed in applications where high-temperature strength, creep resistance, and durability in corrosive environments are critical.

NIMONIC 75 vs.NIMONIC 80A vs. NIMONIC 90, all three are used in similar applications, they possess distinct properties that make them suitable for different operating conditions.

2. NIMONIC 75: General-purpose alloy for Elevated Temperatures

Composition:

  • Nickel: ~75%
  • Chromium: 20%
  • Titanium, Carbon, Manganese, and Trace Elements

Key Properties:

  • Temperature Resistance: Up to 980°C (1800°F)
  • Oxidation Resistance: High resistance to oxidation and scaling in high-temperature environments
  • Creep Resistance: Moderate creep resistance
  • Mechanical Strength: Moderate tensile strength compared to other NIMONIC grades

Applications:

  • Gas Turbines: Used in lower-temperature components such as ducting, transition liners, and turbine blades.
  • Thermal Processing: Ideal for heat exchangers, exhaust systems, and components in industrial furnaces.
  • Nuclear Power Generation: Utilized in structural components and fixtures.

Why Choose NIMONIC 75?

NIMONIC 75 is an ideal choice when a combination of oxidation resistance and moderate strength at elevated temperatures is needed. It performs well in environments up to 980°C, offering an economical option for less critical high-temperature applications. However, for more demanding environments, particularly where high stress and long service life are required, stronger alloys like NIMONIC 80A and NIMONIC 90 may be more suitable.

3. NIMONIC 80A: Improved Strength and Fatigue Resistance

Composition:

  • Nickel: ~75%
  • Chromium: 19-21%
  • Titanium: 2.2-2.7%
  • Aluminum: 1.0-1.8%
  • Carbon, Manganese, Trace Elements

Key Properties:

  • Temperature Resistance: Up to 815°C (1500°F) for long-term use, higher for short-term.
  • Creep Resistance: Excellent creep and fatigue resistance due to the addition of titanium and aluminum, which form gamma prime precipitates that strengthen the alloy.
  • Oxidation Resistance: Comparable to NIMONIC 75.
  • Mechanical Strength: Higher tensile and yield strength than NIMONIC 75, particularly at elevated temperatures.

Applications:

  • Aerospace (Gas Turbines): Extensively used in turbine blades, discs, and other hot-section components.
  • Heat-Treatment Equipment: Excellent for furnace parts that require long-term stability and resistance to thermal fatigue.
  • Industrial Gas Turbines are ideal for combustion chambers, flame holders, and afterburners.

Why Choose NIMONIC 80A?

NIMONIC 80A is a go-to material for components that must withstand high stress and temperatures over long periods. Its enhanced creep resistance and high-temperature strength make it suitable for critical components in gas turbines and heat-treatment equipment, where long-term stability is crucial. This alloy offers improved performance over NIMONIC 75 in more demanding environments.

4. NIMONIC 90: The High-Strength Workhorse

Composition:

  • Nickel: ~57%
  • Chromium: 18-21%
  • Cobalt: 15-21%
  • Titanium, Aluminum, Carbon, Trace Elements

Key Properties:

  • Temperature Resistance: Can withstand continuous service temperatures up to 920°C (1690°F).
  • Creep Resistance: Excellent creep resistance, even under high stress.
  • Oxidation Resistance: Similar to NIMONIC 80A, with enhanced performance due to the presence of cobalt.
  • Mechanical Strength: The highest strength of the three alloys, particularly at elevated temperatures, is due to the higher cobalt content.

Applications:

  • Gas Turbines: Used in the hottest, most stressed parts, such as turbine blades and discs.
  • Nuclear Power Plants: Ideal for high-stress, high-temperature components.
  • Aerospace and Industrial Thermal Processing: Extensively used in parts requiring high strength and long-term thermal stability.

Why Choose NIMONIC 90?

NIMONIC 90 provides superior strength and creep resistance for the most extreme high-temperature applications. Its ability to maintain performance under severe conditions makes it the preferred choice for the most demanding applications in gas turbines and high-temperature processing. If you’re dealing with temperatures above 900°C and need maximum durability, NIMONIC 90 offers unparalleled reliability.

NIMONIC 75 vs.NIMONIC 80A vs. NIMONIC 90

NIMONIC 75 vs.NIMONIC 80A vs. NIMONIC 90

5. Comparison Chart: NIMONIC 75 vs.NIMONIC 80A vs. NIMONIC 90

Property NIMONIC 75 NIMONIC 80A NIMONIC 90
Nickel Content ~75% ~75% ~57%
Max Continuous Temp 980°C (1800°F) 815°C (1500°F) 920°C (1690°F)
Creep Resistance Moderate High Excellent
Oxidation Resistance High High High
Strength at High Temp Moderate High Highest
Best Applications Ducts, Furnace Parts Turbine Blades, Discs Turbine Blades, Critical Components

6. Guidance for Material Selection: NIMONIC 75 vs.NIMONIC 80A vs. NIMONIC 90

When selecting the suitable NIMONIC alloy, several factors must be considered:

  1. Operating Temperature: If the application involves long-term use above 900°C, NIMONIC 90 is your best option due to its superior strength and creep resistance. For moderate temperatures, NIMONIC 75 or 80A might suffice.
  2. Mechanical Stress: For applications involving high mechanical stress at elevated temperatures, such as turbine blades or heat-treatment equipment, NIMONIC 80A or 90 is recommended, as both have excellent fatigue resistance.
  3. Cost Considerations: While offering good performance, NIMONIC 75 is more cost-effective for less critical applications. In contrast, NIMONIC 80A and 90, with their higher strength and durability, come at a higher cost but are necessary for more demanding environments.
  4. Service Life Requirements: If long-term durability and minimal maintenance are priorities, particularly in gas turbines or industrial thermal processing, investing in NIMONIC 90 offers the most return on investment due to its superior performance under stress.

7. Conclusion

NIMONIC 75 vs.NIMONIC 80A vs. NIMONIC 90, choosing the right material can significantly improve the performance and longevity of your equipment in high-temperature and high-stress environments. NIMONIC 75, 80A, and 90 each offer distinct advantages, depending on your application’s specific needs.

  • NIMONIC 75 provides a cost-effective solution for moderate-temperature applications with good oxidation resistance.
  • NIMONIC 80A offers improved strength and creep resistance for more demanding environments, particularly in turbine blades and furnace components.
  • NIMONIC 90 excels in extreme conditions, providing the highest strength and durability for critical gas turbine and high-temperature processing parts.

By understanding each alloy’s unique properties and applications, engineers and designers can make informed decisions, ensuring optimal performance and cost-efficiency in high-temperature operations.