Fabrication Guidelines: Duplex Stainless Steels

Introduction

Duplex stainless steels are a family of grades combining good corrosion resistance with high strength and ease of fabrication. Their physical properties are between the austenitic and ferritic stainless steels but tend to be closer to those of the ferritic and carbon steel. The duplex stainless steel’s chloride pitting and crevice corrosion resistance is a function of chromium, molybdenum, tungsten, and nitrogen content. They may be similar to Type 316 or range above the seawater stainless steels, such as the 6% Mo austenitic stainless steels. All duplex stainless steels have chloride stress corrosion cracking resistance significantly greater than that of the 300-series austenitic stainless steels. They all provide significantly greater strength than the austenitic grades while exhibiting good ductility and toughness.

There are many similarities in the fabrication of austenitic and duplex stainless steels, but there are also significant differences. The high alloy content and the high strength of the duplex grades require some changes in fabrication practice. This manual is for fabricators and end users responsible for fabrication. It presents, in a single source, practical information for the successful fabrication of duplex stainless steel. This publication assumes the reader already has experience with the fabrication of stainless steels; therefore, it provides data comparing the properties and fabrication practices of duplex stainless steels to those of the 300-series austenitic stainless steels and carbon steel.

History of Duplex Stainless Steels

Duplex stainless steels, meaning those with a mixed microstructure of about equal proportions of austenite and ferrite, have existed for nearly 80 years. The early grades were alloys of chromium, nickel, and molybdenum. The first wrought duplex stainless steels were produced in Sweden in 1930 and were used in the sulfite paper industry. These grades were developed to reduce the intergranular corrosion problems in the early, high-carbon austenitic stainless steels. Duplex castings were produced in Finland in 1930, and a patent was granted in France in 1936 for the forerunner of what would eventually be known as Uranus 50. AISI Type 329 became well established after World War II and was used extensively for heat exchanger tubing in nitric acid service. One of the first duplex grades explicitly developed for improved resistance to chloride stress corrosion cracking (SCC) was 3RE60. In subsequent years, wrought and cast duplex grades have been used for various process industry applications, including vessels, heat exchangers, and pumps.

These first-generation duplex stainless steels provided good performance characteristics but had limitations in the as-welded condition. The welds’ heat-affected zone (HAZ) had low toughness because of excessive ferrite and significantly lower corrosion resistance than the base metal. In 1968, the invention of the stainless steel refining process, argon oxygen decarburization (AOD), opened the possibility of a
broad spectrum of new stainless steels. Among the advances made possible with the AOD was the deliberate addition of nitrogen as an alloying element. Nitrogen alloying of duplex stainless steels makes possible HAZ toughness and corrosion resistance approaching that of the base metal in the as-welded condition. With increased austenite stability, nitrogen also reduces the rate at which detrimental intermetallic phases form.

The second-generation duplex stainless steels are defined by their nitrogen alloying. This new commercial development, which began in the late 1970s, coincided with offshore gas and oil fields in the North Sea and the demand for stainless steels with excellent chloride corrosion resistance, good fabricability, and high strength. 2205 became the workhorse of the second-generation duplex grades and was used extensively for gas-gathering line pipe and process applications on offshore platforms. The high strength of these steels allowed for reduced wall thickness and reduced weight on the platforms and provided considerable incentive for their use.

Like the austenitic stainless steels, the duplex stainless steels are a family of grades ranging in corrosion performance depending on the alloy content. The development of duplex stainless steels has continued, and modern duplex stainless steels have been divided into five groups in this brochure according to their corrosion resistance. Other ways to group these steels have been proposed, but no consensus has been reached on the definition of these groups.

• Lean duplex without deliberate Mo addition, such as 2304;
• Molybdenum-containing lean duplex, such as S32003;
• Standard duplex with around 22% Cr and 3% Mo, such as 2205, the workhorse grade accounting for nearly 60% of duplex use;
• Super duplex with approximately 25% Cr and 3% Mo, with PREN of 40 to 45, such as 2507;
• Hyper duplex with higher Cr and Mo contents than super duplex grades and PREN above 45, such as S32707.

The resistance of stainless steel to localized corrosion is strongly related to its alloy content. The primary elements contributing to the pitting corrosion resistance are Cr, Mo, and N. W, although not commonly used, which are about half as effective on a weight percent basis as Mo. An empirical relationship called the Pitting Resistance Equivalent Number (PREN) has been developed to relate a stainless steel’s composition to its relative pitting resistance in chloride-containing solutions. The PREN relationship for austenitic and duplex stainless steels is given as follows:

* PREN = Pitting Resistance Equivalent Number = Cr + 3.3(Mo + 0.5W) + 16N

where Cr, Mo, W, and N represent the alloy’s chromium, molybdenum, tungsten, and nitrogen contents, respectively, in weight %.

Table 1 lists the chemical compositions and typical PREN range of the second-generation wrought and cast duplex stainless steels. The first-generation duplex grades and the most common austenitic stainless steels are included for comparison.

Note: Each stainless steel referenced by name or industry designation in the text may be found in Table 1 or Appendix 1.

Table 1: Chemical composition (weight %) and PREN range of wrought and cast duplex stainless steels* (austenitic grades shown for comparison)

Grade	UNS No.	EN No.	C	Cr	Ni	Mo	N	Mn	Cu	W	PREV
Wrought duplex stainless steel
First-generation duplex grades
329	S32900	1.446	0.08	23.0 – 28.0	2.5 – 5.0	1.0 – 2.0	–	1	–	–	30 – 31
	S31500	1.4424	0.03	18.0 – 19.0	4.3 – 5.2	2.5 – 3.0	0.05 – 0.10	–	–	–	28 – 29
	S32404		0.04	20.5 – 22.5	5.5 – 8.5	2.0 – 3.0	0.2	2	1.00 – 2.00	–	29 – 30
Second-generation duplex grades
Lean duplex
	S32001	1.4482	0.03	19.5 – 21.5	1.00 – 3.00	0.6	0.05 – 0.17	4.00 – 6.00	1	–	21 – 23
	S32101	1.4162	0.04	21.0 – 22.0	1.35 – 1.70	0.1 – 0.8	0.20 – 0.25	4.00 – 6.00	0.10 – 0.80	–	25 – 27
	S32202	1.4062	0.03	21.5 – 24.0	1.00 – 2.80	0.45	0.18 – 0.26	2	–	–	25 – 28
2304	S32304	1.4362	0.03	21.5 – 24.5	3.0 – 5.5	0.05 – 0.60	0.05 – 0.20	2.5	0.05 – 0.60	–	25 – 28
	S82011		0.03	20.5 – 23.5	1.0 – 2.0	0.1 – 1.0	0.15 – 0.27	2.00 – 3.00	0.5	–	25 – 27
	S82012	1.4635	0.05	19.0 – 20.5	0.8 – 1.5	0.10 – 0.60	0.16 – 0.26	2.00 – 4.00	1	–	24 – 26
	S82122		0.03	20.5 – 21.5	1.5 – 2.5	0.6	0.15 – 0.20	2.00 – 4.00	0.50 – 1.50	–	24 – 26
		1.4655	0.03	22.0 – 24.0	3.5 – 5.5	0.1 – 0.6	0.05 – 0.20	2	1.00 – 3.00	–	25 – 27
		1.4669	0.045	21.5 – 24.0	1.0 – 3.0	0.5	0.12 – 0.20	1.00 – 3.00	1.60 – 3.00	–	25 – 27
Molybdenum-containing lean duplex
	S32003		0.03	19.5 – 22.5	3.0 – 4.0	1.50 – 2.00	0.14 – 0.20	2	–	–	30 – 31
	S81921		0.03	19.0 – 22.0	2.0 – 4.0	1.00 – 2.00	0.14 – 0.20	2.00 – 4.00	–	–	27 – 28
	S82031	1.4637	0.05	19.0 – 22.0	2.0 – 4.0	0.60 – 1.40	0.14 – 0.24	2.5	1	–	27 – 28
	S82121		0.035	21.0 – 23.0	2.0 – 4.0	0.30 – 1.30	0.15 – 0.25	1.00 – 2.5	0.20 – 1.20	–	27 – 28
	S82441	1.4662	0.03	23.0 – 25.0	3.0 – 4.5	1.00 – 2.00	0.20 – 0.30	2.50 – 4.00	0.10 – 0.80	–	33 – 34
Standard duplex
2205	S31803	1.4462	0.03	21.0 – 23.0	4.5 – 6.5	2.5 – 3.5	0.08 – 0.20	2	–	–	33 – 35
2205	S32205	1.4462	0.03	22.0 – 23.0	4.5 – 6.5	3.0 – 3.5	0.14 – 0.20	2	–	–	35 – 36
	S32950		0.03	26.0 – 29.0	3.5 – 5.2	1.0 – 2.5	0.15 – 0.35	2	–	–	36 – 38
	S32808		0.03	27.0 – 27.9	7.0 – 8.2	0.8 – 1.2	0.30 – 0.40	1.1	–	2.1 – 2.5	36 – 38
Super Duplex
	S32506		0.03	24.0 – 26.0	5.5 – 7.2	3.0 – 3.5	0.08 – 0.20	1	–	0.05 – 0.30	40 – 42
	S32520	1.4507	0.03	24.0 – 26.0	5.5 – 8.0	3.0 – 4.0	0.20 – 0.35	1.5	0.50 – 2.00	–	40 – 43
255	S32550	1.4507	0.04	24.0 – 27.0	4.4 – 6.5	2.9 – 3.9	0.10 – 0.25	1.5	1.50 – 2.50	–	38 – 41
2507	S32750	1.441	0.03	24.0 – 26.0	6.0 – 8.0	3.0 – 5.0	0.24 – 0.32	1.2	0.5	–	40 – 43
	S32760	1.4501	0.03	24.0 – 26.0	6.0 – 8.0	3.0 – 4.0	0.20 – 0.30	1	0.50 – 1.00	0.5 – 1.0	40 – 43
	S32906	1.4477	0.03	28.0 – 30.0	5.8 – 7.5	1.5 – 2.6	0.30 – 0.40	0.80 – 1.50	0.8	–	41 – 43
	S39274		0.03	24.0 – 26.0	6.8 – 8.0	2.5 – 3.5	0.24 – 0.32	1	0.20 – 0.80	1.50 – 2.50	40 – 42
	S39277		0.025	24.0 – 26.0	6.5 – 8.0	3.0 – 4.0	0.23 – 0.33	0.8	1.20 – 2.00	0.8 – 1.2	40 – 42
Hyper duplex
	S32707		0.03	26.0 – 29.0	5.5 – 9.5	4.0 – 5.0	0.30 – 0.50	1.5	1	–	49 – 50
	S33207		0.03	29.0 – 33.0	6.0 – 9.0	3.0 – 5.0	0.40 – 0.60	1.5	1	–	52 – 53
Wrought austenitic stainless steel
304L	S30403	1.4307	0.03	17.5 – 19.5	8.0 – 12.0	–	0.1	2	–	–	18 – 19
316L	S31603	1.4404	0.03	16.0 – 18.0	10.0 – 14.0	2.0 – 3.0	0.1	2	–	–	24 – 25
Cast duplex stainless steel
CD4MCu/Grade 1A	J93370		0.04	24.5 – 26.5	4.75 – 6.0	1.75 – 2.25	–	1	2.75 – 3.25	–	32 – 33
CD4MCuN/Grade 1B	J93372		0.04	24.5 – 26.5	4.7 – 6.0	1.7 – 2.3	0.10 – 0.25	1	2.70 – 3.30	–	34 – 36
CD3MCuN/Grade 1C	J93373		0.03	24.0 – 26.7	5.6 – 6.7	2.9 – 3.8	0.22 – 0.33	1.2	1.40 – 1.90	–	40 – 42
CE8MN/Grade 2A	J93345		0.08	22.5 – 25.5	8.0 – 11.0	3.0 – 4.5	0.10 – 0.30	1	–	–	38 – 40
CD6MN/Grade 3A	J93371		0.06	24.0 – 27.0	4.0 – 6.0	1.75 – 2.5	0.15 – 0.25	1	–	–	35 – 37
CD3MN/Cast 2205/Grade 4A	J92205		0.03	21.0 – 23.5	4.5 – 6.5	2.5 – 3.5	0.10 – 0.30	1.5	–	–	35 – 37
CE3MN/Cast 2507/Grade 5A	J93404	1.4463	0.03	24.0 – 26.0	6.0 – 8.0	4.0 – 5.0	0.10 – 0.30	1.5	–	–	43 – 45
CD3MWCuN/Grade 6A	J93380		0.03	24.0 – 26.0	6.5 – 8.5	3.0 – 4.0	0.20 – 0.30	1	0.50 – 1.00	0.5 – 1.0	40 – 42
Cast austenitic stainless steel
CF3 (cast 304L)	J92500	1.4306	0.03	17.0 – 21.0	8.0 – 12.0	–	–	1.5	–	–	18 – 19
CF3M (cast 316L)	J92800	1.4404	0.03	17.0 – 21.0	9.0 – 13.0	2.0 – 3.0	–	1.5	–	–	24 – 25

* Maximum, unless range or minimum is indicated. – Not defined in the specifications.

Chemical composition and role of alloying elements in Duplex Stainless Steels

Chemical composition of duplex stainless steels

It is generally accepted that the favorable properties of duplex stainless steels can be achieved if ferrite and austenite phases are both in the 30 to 70% range, including in welded structures. However, duplex stainless steels are most commonly considered to have roughly equal amounts of ferrite and austenite, with current commercial production slightly favoring the austenite for best toughness and processing characteristics. The interactions of the major alloying elements, particularly chromium, molybdenum, nitrogen, and nickel, are pretty complex. Care must be taken to obtain the correct level of each of these elements to achieve a stable duplex structure that responds well to processing and fabrication.

Besides the phase balance, there is a second primary concern with duplex stainless steels and their chemical composition: the formation of detrimental intermetallic phases at elevated temperatures. Sigma and chi phases form in high-chromium, high-molybdenum stainless steels, precipitating preferentially in the ferrite. The addition of nitrogen significantly delays the formation of these phases. Therefore, sufficient nitrogen must be present in a solid solution. The importance of narrow composition limits has become apparent as experience with duplex stainless steel has increased. The composition range initially set for 2205 (UNS S31803, Table 1) was too broad. Experience has shown that for optimum corrosion resistance and to avoid intermetallic phases, chromium, molybdenum, and nitrogen levels should be kept in the higher half of the ranges for S31803. Therefore, a modified 2205 with a narrower composition range was introduced with the UNS number S32205 (Table 1). The composition of S32205 is typical of today’s commercial production of 2205. Unless otherwise stated in this publication, 2205 refers to the S32205 composition.

The role of alloying elements in duplex stainless steels

The following is a brief review of the effect of the most important alloying elements on the mechanical, physical, and corrosion properties of duplex stainless steel.

Chromium: A minimum of 10.5% is necessary to form a stable chromium passive film sufficient to protect steel against mild atmospheric corrosion. The corrosion resistance of stainless steel increases with increasing chromium content. Chromium is a ferrite former, meaning that the addition of chromium promotes the body-centered cubic structure of iron. More nickel is necessary to form an austenitic or duplex (austenitic-ferritic) structure at higher chromium contents. Higher chromium also promotes the formation of intermetallic phases. There is usually at least 16% Cr in austenitic stainless steels and at least 20% Cr in duplex grades. Chromium also increases the oxidation resistance at elevated temperatures. This chromium effect is significant because it influences the formation and removal of oxide scale or heat tint resulting from heat treatment or welding. Duplex stainless steels are more challenging to pickle, and heat tint removal is more complex than austenitic stainless steels.

Molybdenum: Molybdenum enhances the pitting corrosion resistance of stainless steel. When the chromium content of stainless steel is at least 18%, additions of molybdenum become about three times as effective as chromium additions in improving pitting and crevice corrosion resistance in chloride-containing environments. Molybdenum is a ferrite former that increases the tendency of stainless steel to form detrimental intermetallic phases. Therefore, it is usually restricted to less than 7% in austenitic stainless steel and 4% in duplex stainless steel.

Nitrogen: Nitrogen increases the pitting and crevice corrosion resistance of austenitic and duplex stainless steels. It also substantially increases their strength and is the most effective solid solution-strengthening element. It is a low-cost alloying element and a strong austenite former, able to replace some of the nickel content for austenite stabilization. The improved toughness of nitrogen-bearing duplex stainless steels is due to their excellent austenite content and reduced intermetallic content. Nitrogen does not prevent intermetallic phase precipitation but delays intermetallics formation enough to permit the processing and fabrication of the duplex grades. Nitrogen is added to highly corrosion-resistant austenitic and duplex stainless steels that contain high chromium and molybdenum contents to offset their tendency to form a sigma phase.

Nitrogen increases the strength of the austenite phase by strengthening solid solutions and also increases its work hardening rate. In duplex stainless steels, nitrogen is typically added, and the amount of nickel is adjusted to achieve the desired phase balance. The ferrite formers, chromium, and molybdenum are balanced by the austenite formers, nickel, and nitrogen to develop the duplex structure.

Nickel: Nickel is an austenite stabilizer, which promotes a change in the crystal structure of stainless steel from body-centered cubic (ferritic) to face-centered cubic (austenitic). Ferritic stainless steels contain little or no nickel, duplex stainless steels contain low to intermediate amount of nickel from 1.5 to 7%, and the 300-series austenitic stainless steels contain at least 6% nickel (see Figures 1, 2). The addition of nickel delays the formation of detrimental intermetallic phases in austenitic stainless steels but is far less effective than nitrogen in delaying their formation in duplex stainless steels. The face-centered cubic structure is responsible for the excellent toughness of the austenitic stainless steel. Its presence in about half of the microstructure of duplex grades dramatically increases its toughness compared to ferritic stainless steels.

 

Metallurgy of duplex stainless steels

Austenite-ferrite phase balance

The iron-chromium-nickel ternary phase diagram is a roadmap of the metallurgical behavior of the duplex stainless steels.
A section through the ternary diagram at 68% iron (Figure 3) illustrates that these alloys solidify as ferrite (α), which then partially transforms to austenite (γ) as the temperature falls, depending on the alloy composition. When water quenching from the solution annealing temperature, a microstructure of roughly 50% austenite and 50% ferrite can be achieved at room temperature. Increasing the nitrogen content increases the ferrite to austenite transformation start temperature (Ref. 1) and improves the structural stability of the grade, particularly in the HAZ.

The relative amounts of ferrite and austenite present in a mill product or fabrication of a given duplex grade depend on the steel’s chemical composition and thermal history. Minor changes in composition can have a significant effect on the relative volume fraction of these two phases, as the phase diagram indicates. Individual alloying elements tend to promote either the formation of austenite or ferrite.
The ferrite/austenite phase balance in the microstructure can be predicted with multivariable linear regression as follows:

Creq = Cr + 1.73 Si + 0.88 Mo

Nieq = Ni + 24.55 C + 21.75 N + 0.4 Cu

% Ferrite = -20.93 + 4.01 Creq – 5.6 Nieq + 0.016 T

where T (in °C) is the annealing temperature ranging from 1050–1150°C and the elemental compositions are in weight% (Ref. 2).

The goal of obtaining the desired phase balance of close to 45 to 50 % ferrite with the remainder austenite is achieved primarily by adjusting the chromium, molybdenum, nickel, and nitrogen contents and then by controlling the thermal history.

For mill products, solution annealing at an appropriate solution annealing temperature followed by immediate water quenching gives the best results. It is essential to keep the time between exiting the furnace and water quenching as short as possible. This minimizes the product’s heat loss, which could lead to detrimental phase precipitation before water quenching at room temperature.

For welded structures, the heat input has to be optimized for each grade and weld configuration so that the cooling rate
will be quick enough to avoid detrimental phases but not so fast that excessive ferrite remains near the fusion line. This situation may occur when welding widely differing section sizes or heavy sections with very low heat inputs during fabrication. In these cases, the thick metal section can quench the thin weld so rapidly that there is insufficient time for enough ferrite to transform into austenite, leading to excessive amounts of ferrite, particularly in the HAZ.

Because nitrogen increases the temperature at which the austenite begins to form from the ferrite, as illustrated in Figure 3, it also increases the rate of the ferrite-to-austenite transformation. Therefore, even at relatively rapid cooling rates, the equilibrium level of austenite can nearly be reached if the grade has sufficient nitrogen. In the second-generation duplex stainless steels, this effect reduces the potential of excess ferrite in the HAZ.

Precipitates

Detrimental phases can form in minutes at the critical temperature, as seen in the isothermal precipitation diagram for 2507 and 2205 duplex stainless steels in Figure 4 (Ref. 4, 5, 6, 7). They can reduce corrosion resistance and toughness significantly. Therefore, the cumulative time in the temperature range where they can form, especially during welding and cooling after annealing, has to be minimized. Modern duplex grades have been developed to maximize corrosion resistance and retard precipitation of these phases, allowing successful fabrication. However, they can only be removed once formed by full solution annealing and subsequent water quenching.

Sigma phase (Figure 5) and other intermetallic phases, such as chi, can precipitate from the ferrite at temperatures below austenite formation on cooling too slowly through the temperature range of 700– 1000˚C (1300–1830˚F). Sigma phase formation in mill products can, therefore, be avoided by water quenching the steel as rapidly as possible from the solution annealing temperature and preventing the sigma phase field (Figure 6).

 

 

 

The driving force for sigma phase formation increases with increasing molybdenum and chromium content. Therefore, the more highly alloyed grades are the most affected from 2205 on up. Precipitates tend to form quicker with increasing alloy content, as shown in Figure 4, where the start curve 2507 is to the left (shorter time) of the one for 2205. Lean duplex grades such as 2304 do not readily form intermetallic phases, and nitride precipitation is more likely, as shown in Figure 4.

The sigma phase decreases the pitting resistance of duplex stainless steels due to the depletion of chromium and molybdenum in surrounding areas. This depletion leads to a local reduction of the corrosion resistance next to the sediments. Toughness and flexibility are also sharply reduced when intermetallic phase precipitation occurs.

For some grades, chromium nitride precipitation can start in only 1–2 minutes at the critical temperature. It can occur in the grain or phase boundaries due to too slow cooling through the temperature range of 600– 900°C (1100– 1650˚F).
Nitride formation is not very common in most duplex grades. Still, it can be an issue with some lean duplex stainless steels due to relatively high nitrogen content and reduced nitrogen solubility compared to higher alloyed grades. Similar to the sigma phase, water quenching from an adequate solution annealing temperature can largely be avoided in the steel mill.

Chromium nitride can, however, also precipitate in the HAZ and weld metal in welded fabrications. Due to very rapid cooling in this area, a high ferrite content in the vicinity of the fusion line can lead to nitrogen oversaturation. Ferrite, generally, has very low solubility for nitrogen, which decreases further as the temperature decreases. So, if nitrogen is ‘caught’ in the ferrite phase, it might precipitate as chromium nitride upon cooling. A slower cooling rate will result in competition between nitride precipitation and an increase in austenite retransformation. More austenite allows more nitrogen to dissolve in the austenite grains, reducing the nitrogen oversaturation of the ferritic grains and the amount of chromium nitride. The precipitation of chromium nitrides in welds can, therefore, be decreased by increasing the austenite level through higher heat input (slower cooling) or additions of austenite-promoting elements such as nickel in the weld metal or nitrogen in the shielding gas.

Chromium nitrides may adversely affect corrosion resistance and toughness properties if formed in large volumes.

Alpha prime can form in the ferrite phase of duplex stainless steels below about 525°C (950°F). It takes significantly longer to create than the other phases discussed above and is first noticed as an increase in hardness and only later as a loss in toughness (Figure 4). In ferritic stainless steels, alpha prime causes the loss of ambient temperature toughness after extended exposure to temperatures around 475°C (885°F); this behavior is known as 475°C/885°F embrittlement. Fortunately, because duplex stainless steels contain 50% austenite, this hardening and embrittling effect is not nearly as detrimental as fully ferritic steels. It does affect all duplex stainless steel grades but is most pronounced in the molybdenum-containing grades and much less in the lean duplex grades.

Alpha prime embrittlement is rarely a concern during fabrication because of the long time required for embrittlement to occur. One exception that must be carefully evaluated is the stress relief treatment of duplex-clad carbon steel constructions. Any heat treatment in the critical temperature range for alpha prime formation of 300–525°C (575–980°F) (or for intermetallic phase formation of 700–950°C (1300–2515°F), for 2205) has to be avoided. If a stress relief treatment is required, it is best to consult the clad plate producer for advice.

However, the upper-temperature limit for duplex stainless steel service is controlled by alpha prime formation. Therefore, pressure vessel design codes have established upper-temperature limits for the maximum allowable design stresses. The German TüV code distinguishes between welded and unwelded constructions and is more conservative in its upper-temperature limits than the ASME Boiler and Pressure Vessel Code. The temperature limits for these pressure vessel design codes for various duplex stainless steels are summarized in Table 2.

Second-generation duplex stainless steels are produced with very low carbon content, so detrimental carbide formation is typically not a concern.

Table 3 summarizes several important precipitation reactions and temperature limitations for duplex stainless steels.

Table 2: Upper temperature limits for duplex stainless steels for maximum allowable stress values in pressure vessel design codes

Grade	Condition	ASME		TüV
		°C	°F	°C	°F
2304	Unwelded	315	600	300	570
2304	Welded, matching filler	315	600	300	570
2304	Welded with 2205/2209	315	600	250	480
2205	Unwelded	315	600	280	535
2205	Welded	315	600	250	480
2507	Seamless tubes	315	600	250	480
Alloy 255	Welded or unwelded	315	600

Table 3: Typical temperatures for precipitation reactions and other characteristic reactions in duplex stainless steels

	2205		2507
	°C	°F	°C	°F
Solidification range	1470 – 1380	2680 – 2515	1450 – 1350	2640 – 2460
Scaling temperature in air	1000	1830	1000	1830
Sigma phase formation	700 – 950	1300 – 1740	700 – 1000	1300 – 1830
Nitride, carbide precipitation	450 – 800	840 – 1470	450 – 800	840 – 1470
475°C/885°F embrittlement	300 – 525	575 – 980	300 – 525	575 – 980

Corrosion resistance

Duplex stainless steels exhibit high corrosion resistance in most environments where the standard austenitic grades are used. However, there are some notable exceptions where they are decidedly superior. This results from their high chromium content, which is beneficial in oxidizing acids, and sufficient molybdenum and nickel to resist mildly reducing acid environments. The relatively high chromium, molybdenum, and nitrogen levels also resist chloride-induced pitting and crevice corrosion. The duplex structure is an advantage in potential chloride stress corrosion cracking environments. If the microstructure contains at least thirty percent ferrite, duplex stainless steels are far more resistant to chloride stress corrosion cracking than austenitic stainless steel Types 304 or 316. Ferrite is, however, susceptible to hydrogen embrittlement. Thus, the duplex stainless steels do not resist highly in environments or applications where hydrogen may be charged into the metal and cause hydrogen embrittlement.

Resistance to acids

To illustrate the corrosion resistance of duplex stainless steels in solid acids, Figure 7 provides corrosion data for sulfuric acid solutions. This environment ranges from mildly reducing at low acid concentrations to oxidizing at high concentrations, with a strongly reducing middle composition range in warm and hot solutions. Both 2205 and 2507 duplex stainless steels outperform many high nickel austenitic stainless steels in solutions containing up to 15% acid. They are better than Types 316 or 317 through at least 40% acid. The duplex grades can also be very useful in oxidizing acids containing chlorides. The duplex stainless steels do not have sufficient nickel to resist the strong reducing conditions of mid-concentration sulfuric acid solutions or hydrochloric acid. At wet/dry interfaces in reducing environments with a concentration of acid, corrosion, especially of the ferrite, may be activated and proceed rapidly.

Their resistance to oxidizing conditions makes duplex stainless steel a good candidate for nitric acid service and organic solid acids. This is illustrated in Figure 8 for solutions containing 50% acetic acid and varying amounts of formic acid at their boiling temperatures. Although Types 304 and 316 will handle these strong organic acids at ambient and moderate temperatures, 2205 and other duplex grades
are superior in many processes involving organic acids at high temperatures. The duplex stainless steels are also used in processes involving halogenated hydrocarbons because of their resistance to pitting and stress corrosion.

Resistance to caustics

The high chromium content and ferrite presence perform well for duplex stainless steels in acidic environments. At moderate temperatures, corrosion rates are lower than those of the standard austenitic grades.

Pitting and crevice corrosion resistance

To discuss the pitting and crevice corrosion resistance of stainless steel, it is helpful to introduce the concept of critical temperatures for pitting corrosion. For a particular chloride environment, each stainless steel can be characterized by a temperature above which pitting corrosion will initiate and propagate to a visibly detectable extent within 24 hours. Below this temperature, pitting initiation will not occur. This temperature is known as the critical pitting temperature (CPT). It is a characteristic of the particular stainless steel grade and the specific environment. Because pitting initiation is statistically random, and because of the sensitivity of the CPT to minor within-grade variations or within-product variations, the CPT is typically expressed for various grades as a range of temperatures.

However, the research tool described in ASTM G150 makes it possible to determine the CPT accurately and reliably through electrochemical measurements.

There is a similar temperature for crevice corrosion, which occurs in gasket joints, under deposits, and bolted joints where an aperture is formed in fabricated products. The critical crevice temperature (CCT) depends on the individual sample of stainless steel, the chloride environment, and the aperture’s nature (tightness, length, etc.). Because of the dependence on the geometry of the aperture and the difficulty of achieving reproducible crevices in practice, there is more scatter for the measurement of CCT than for the CPT. Typically, the CCT will be 15 to 20°C (27 to 36°F) lower than the CPT for the same steel and same corrosion environment.

Duplex grades’ high chromium, molybdenum, and nitrogen contents provide very good resistance to chloride-induced localized corrosion in aqueous environments. Depending on the alloy content, some duplex grades are among the best-performing stainless steels.
Because they contain relatively high chromium content, duplex stainless steels very economically provide a high corrosion resistance. A comparison of pitting and crevice corrosion resistance for several stainless steels in the solution annealed condition as measured by the ASTM G48 procedures (6% ferric chloride) is given in Figure 9. Critical temperatures for materials in the as-welded condition would be expected to be somewhat lower. Higher critical pitting or crevice corrosion temperatures indicate more excellent resistance to initiating these forms of corrosion. The CPT and CCT of 2205 are well above those of Type 316. This makes 2205 a versatile material in applications where evaporation concentrates chlorides, as in the vapor spaces of heat exchangers or beneath the insulation. The CPT of 2205 indicates that it can handle many brackish waters and deaerated brines. It has been successfully used in deaerated seawater applications where the surface has been maintained free of deposits through high flow rates or other means. 2205 does not have enough crevice corrosion resistance to withstand seawater in critical applications such as thin wall heat exchanger tubes or where deposits or crevices exist. However, the more highly alloyed duplex stainless steels with higher CCT than 2205, such as the super duplex and hyper duplex grades, have been used in critical seawater handling situations where strength and chloride resistance are needed. Although the super duplex grades do not corrode in lower-temperature seawater, they have limits in higher-temperature service. The improved corrosion resistance of hyper duplex stainless steels extends their use to aggressive chloride environments, such as in hot tropical seawater, mainly when crevices exist.

Because the CPT is a function of the material and the particular environment, studying the effect of individual elements is possible. Using the CPT as determined by ASTM G 48 Practice A, statistical regression analysis was applied to the compositions of the steels (each element considered as an independent variable) and the measured CPT (the dependent variable). The result was that only chromium, molybdenum, tungsten, and nitrogen showed consistent, measurable effects on the CPT according to the relationship:

CPT = constant + Cr + 3.3 (Mo + 0.5W) + 16N.

In this relationship, the sum of the four alloy element variables multiplied by their regression constants is commonly called the Pitting Resistance Equivalent Number (PREN). The coefficient for nitrogen varies among investigators, and 16, 22, and 30 are widely used (Ref. 8). The PREN is helpful for ranking grades within a single family of steels. However, care must be taken to avoid inappropriate over-reliance on this relationship. The ‘independent variables’ were not truly independent because the steels tested were balanced compositions. The relationships are not linear, and cross relationships, such as the synergies of chromium and molybdenum, were ignored. The relationship assumes an ideally processed material but does not address the effect of intermetallic phases, non-metallic phases, or improper heat treatment that can adversely affect corrosion resistance.

Stress corrosion cracking resistance

Some of the earliest uses of duplex stainless steels were based on their resistance to chloride SCC. Compared with austenitic stainless steels with similar chloride pitting and crevice corrosion- resistance, the duplex stainless steels exhibit significantly better SCC resistance. Many of the uses of duplex stainless steels in the chemical process industries are replacements for austenitic grades in applications with a significant risk of SCC. However, as with many alloys, the duplex stainless steel may be susceptible to SCC under certain conditions. This may occur in high temperatures, chloride-containing environments, or when conditions favor hydrogen-induced cracking. Examples of environments in which SCC of duplex stainless steels may be expected include the boiling 42% magnesium chloride test, drop evaporation when the metal temperature is high, and exposure to pressurized aqueous chloride systems in which the temperature is higher than what is possible at ambient pressure.

An illustration of relative chloride SCC resistance for several mill annealed duplex and austenitic stainless steels in a severe chloride environment is given in Figure 10 (Ref. 9). The drop evaporation test used to generate these data is very aggressive because it is conducted at
a high temperature of 120°C (248°F). The chloride solution is concentrated by evaporation. The three duplex steels shown, UNS S32101, 2205, and 2507, will eventually crack at some fraction of their yield strength in this test, but that fraction is much higher than that of Type 316 stainless steel. Because of their resistance to SCC in aqueous chloride environments at ambient pressure, for example, under insulation, the duplex stainless steels may be considered in chloride cracking environments where Types 304 and 316 have been known to crack. Table 4
summarizes chloride SCC behavior of different stainless steels in various test environments with multiple severities. The environments listed near the top of the table are severe because of their acid salts, while those near the bottom are severe because of high temperatures. The environments in the center are less severe. The standard austenitic stainless steels with less than 4% Mo undergo chloride SCC in
all these environments. In contrast, the duplex stainless steels are resistant throughout the mid-range, moderate testing conditions.

 

Resistance to hydrogen-induced SCC is a complex function of ferrite content and strength, temperature, charging conditions, and applied stress. Despite their susceptibility to hydrogen cracking, the strength advantages of duplex stainless steels can be used in hydrogen-containing environments, provided the operating conditions are carefully evaluated and controlled. The most notable of these applications is high-strength tubular handling mixtures of slightly sour gas and brine. Figure 11 (Ref. 10) illustrates regimes of immunity and susceptibility for 2205 in sour environments containing sodium chloride.

Conclusion

In conclusion, duplex stainless steels offer exceptional strength, corrosion resistance, and durability, making them ideal for demanding applications in various industries. Fabricators can maintain the high performance of duplex alloys by following proper fabrication guidelines, such as selecting appropriate filler metals, using controlled heat input, and ensuring proper post-weld treatments. Adhering to these practices ensures optimal mechanical properties and corrosion resistance, extending the lifespan of fabricated components. Duplex stainless steels perform superiorly in challenging environments with careful attention to these guidelines.