Difference between duplex steel UNS S32205 and UNS S31803

Duplex stainless steels account for more than 80% of the consumption of duplex, super duplex and super duplex steels. Duplex alloys were developed in the 1930s for paper and pulp manufacturing. Duplex alloys are based on a 22% Cr composition and a mixed austenitic:ferritic microstructure that provides desirable mechanical properties.

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UNS S31803 and UNS S32205 are commonly referred to as 2205 stainless steels, and their two-phase names consist of an arrangement of a mixture of ferrite and austenite containing approximately equal parts.

The duplex stainless steel series is typically twice as strong and offers significantly improved corrosion resistance compared to normal 304/316 austenitic stainless steels. Increasing the chromium content of the stainless steel will improve its resistance to pitting. However, the pitting resistance equivalent number (PREN), which infers the pitting resistance of an alloy, also includes many other elements in its formula. This subtlety can be used to explain how the difference between UNS S31803 and UNS S32205 is formed and whether it matters.

With the development of duplex stainless steels, the initial specification was UNS S31803. However, some leading manufacturers have been producing the grade to the upper limit of the allowed specification. This reflects their desire to maximize the corrosion performance of the alloy, which has been facilitated by the development of the AOD steelmaking process, which allows for tighter control of composition. In addition, it allows the level of nitrogen addition to be influenced, rather than just being present as a background element. As a result, the highest performance dual grades seek to maximize levels of chromium (Cr), molybdenum (Mo) and nitrogen (N). The difference between a duplex alloy whose composition meets the bottom of this specification and one that reaches the top of this specification can be based on several points of the equation PREN = %Cr + 3.3% Mo + 16% N.

Corrosion resistance of duplex steels

The pore corrosion resistance equivalent PRE = % Cr + 3 , 3x % Mo + 1 6x % N. This value provides an easy way to identify the corrosion resistance of a material, with higher values representing better corrosion resistance. Duplex stainless steels offer a wide range of corrosion resistance in a variety of environments. A corrosion level below 0.1 mm/year is generally considered to have good corrosion resistance. Because of its high chromium content, duplex stainless steel has excellent corrosion resistance in many corrosive catalysts.

In order to distinguish between duplex stainless steels produced at the top of the composition range, another specification was introduced, UNS S32205. duplex stainless steels produced under the heading UNS S32205 (F60) will fully satisfy the requirements of the heading UNS S31803 (F51), and vice versa. Therefore, UNS S32205 can be dual certified as S31803.

What is the difference between UNS S31803 and S32205 duplex steel?

Flanges commonly used duplex steel grades are 2205 and UNS S31803, their chemical composition is very similar, strength, elongation, hardness index is also basically the same, sometimes we just substitute between them. So what is the main difference between them? Which material has better overall performance?

Difference in chemical composition between UNS S31803 and UNS S32205 dual phase steel

UNS C Si Mn P S Cr Ni Mo N
S31803(F51) ≤0.030 ≤1.00 ≤2.00 ≤0.030 ≤0.020 21.0-23.0 4.5-6.5 2.5-3.5 0.08-0.20
S32205(F60) ≤0.030 ≤1.00 ≤2.00 ≤0.030 ≤0.020 22.0-23.0 4.5-6.5 3.0-3.5 0.14-0.20

Mechanical Properties of UNS S31803 and UNS S32205 Duplex Steel (ASTM A182)

UNS The yield strength Tensile strength Elongation R.A. % Hardness
S31803(F51) ≥450MPa ≥620MPa ≥25% ≥45%
S32205(F60) ≥450MPa ≥655MPa ≥25% ≥45%

Characteristics of UNS S31803 and UNS S32205 duplex steel

UNS S31803, UNS S32205 commonly referred to as duplex 2205, contains a mixture of ferrite and austenite in approximately equal proportions. It is the origin of the name “duplex” stainless steel. Developed in Sweden more than 70 years ago in the sulfite paper industry, it is used to overcome corrosion problems caused by bearing cooling water and other corrosive chemicals. After that, nitrogen was deliberately used as an alloying agent, which quickly became the mainstream stainless steel alloy composition of the duplex.

When it comes to 2205 duplex steel, specify UNS S31803/F51 steel grades. For UNS S32205/F60, it meets the 2205 but takes the higher corrosion resistance range. That is, UNS S32205 requires a higher chromium, nitrogen, and molybdenum content range to ensure better corrosion resistance. S32205 is usually produced by the factory to match its composition to two grades of UNS S31803/S32205.

In fact, before 2000, 2205 chemical composition requirements and UNS S31803 exactly the same, and then the American Society for Materials found that this chemical composition has some problems, so in the 2000 version of the ASTM standard 1.03 volume of ASTM A240/A240M-99A to 2205 chemical composition within the composition of UNS S32205. The biggest change is to change the mass fraction of N in the alloy from 0.08 to 0.20% to 0.14 to 0.20%, raising the lower limit value of N content.
So why this change? We know that the ideal room temperature organization of duplex steel is 50% ferrite and 50% austenite, and its high temperature stage during the heating period (greater than 1100 ℃) austenite is gradually transformed to ferrite, when higher than the ferrite solid solution line temperature (generally in the range of 1250 – 1350 ℃) will all turn into ferrite, and then in the subsequent cooling process, ferrite and then partially transformed into austenite, but The amount of this transformation is closely related to the composition of the material and the cooling rate. The various stages of the heat affected zone of a duplex steel are shown in Figure 1.
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Figure.1 The dual-phase steel heat-affected zone of the various stages of organizational change
Duplex steel welding heat-affected zone from the fusion line closer, the higher the ferrite content, in the European standard EN13445-4 clause 7.3 b on the heat-affected zone of duplex steel regulations: in the fusion line from the two grain size wide range of ferrite content can not exceed 85%. Welding heat-affected zone in the cooling, all by the austenitizing elements (Ni, C, N, Mn) diffusion of ferrite transformation into austenite. Due to Ni, Mn atomic radius, is a replacement type alloying elements, in the rapid cooling of its diffusion rate is very low; on the contrary, C, N is a small gap-type elements, in 1040 ℃ to the ferrite solid solution line temperature of this zone are very fast diffusion rate, and C due to the negative impact on the corrosion resistance of the general content are very low, so that N becomes the key element in the welding cooling conditions to control the phase equilibrium. When the mass fraction of N is increased from 0.12% to 0.18%, austenite can be nucleated in the ferrite grain at higher temperatures, thus transforming into more austenite.
In addition, in the duplex steel N is mainly solid solution in austenite, and the mass fraction of N in austenite is generally 0.25 – 0.60%, and in ferrite is generally 0.00~0.05% as measured by the test. Figure 2 shows the solid solution degree of N in ferrite and austenite, comparing the two curves, the solid solution degree of N in austenite phase is much higher. So if the mass fraction of N in the base material is less than 0.14%, then the ferrite in the welded heat-affected zone can not be faster and more transformation to austenite during cooling, it will cause the N content exceeds the solid solution limit in the ferrite, when a strong nitride precipitation reaction will occur, mainly Cr2N, the result will cause heat-affected plasticity, toughness and corrosion resistance decline. Therefore, we should prefer S32205 to the obsolete UNS S31803 in the selection of duplex steel grades.
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Figure.2 Solid solution of nitrogen in ferrite and austenite

The biggest difference between UNS S32205 and UNS S31803 is that the lower limit of elemental N content is different: the mass fraction of N of UNS S31803 is 0.08-0.20%, and the mass fraction of N of UNS S32205 is 0.14-0.20%.

Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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