Comprehensive Introduction to Duplex Stainless Steel

Thread Content

Duplex stainless steel: Since the discovery of the duplex structure in 1927, three generations of duplex stainless steel have been developed. The first generation of duplex stainless steels is represented by AISI 329 steel; it contains high levels of chromium and molybdenum, offering good resistance to local corrosion, but it also has a relatively high carbon content ([0.1%]). The first generation of duplex stainless steels possesses good performance characteristics, but it has limitations in the welded state. The heat-affected zone of the weld has low toughness due to an excess of ferrite, and its corrosion resistance is significantly lower than that of the base metal; it is only used in certain applications where welding is not involved. With the invention of the argon-oxygen decarburization refining process, nitrogen-containing duplex stainless steels were referred to as second-generation stainless steels. Most of them are ultra-low carbon types and contain elements such as molybdenum, copper, or silicon that enhance corrosion resistance. The representative grade is SAF2205. The third generation of duplex stainless steels are super duplex stainless steels, with grades such as SAF2507, UR52N+, Zeron100, etc. These steels are characterized by a low carbon content, high levels of molybdenum and nitrogen, a ferrite content of 40–45%, and excellent pitting resistance. In a ferrous-based solid solution structure, the ferrite phase and the austenite phase each make up half of it; however, steels in which the proportion of the smallest phase is 30% or more are called duplex stainless steels. Austenitic joints possess good plasticity and toughness, but they have poor thermal conductivity, a high linear expansion coefficient, and significant welding stresses and deformations ； Ordinary ferritic stainless steels have lower thermal conductivity and linear expansion coefficients than austenitic stainless steels; they also possess higher strength and resistance to chloride stress corrosion. However, their plasticity is poor, and they are susceptible to embrittlement at 475°C, as well as embrittlement due to the formation of a δ-phase and grain coarsening at high temperatures. The development of duplex stainless steel aims to combine the advantages of austenite and ferrite while minimizing the disadvantages of both phases. The optimal composition for high-performing duplex stainless steels is one in which the ferrite content ranges from 60% to 40%, while the austenite content is between 40% and 60%; a significant reduction in either of these components leads to a decrease in the performance of the duplex steel. C Cr Ni Mo N Austenitic 308L 0.03 20 10 ---- ---- 20 316L 0.03 18 12 2 ---- 25 Duplex stainless steel 2205 0.03 22 5 3 0.15 34 255 0.03 25 6 3 0.20 38 2507 0.03 25 7 4 0.25 42 Room-temperature mechanical properties of typical duplex stainless steel grades Grade Standard Designation σ0.2/MPa σb/MPa δ5/% AK/J AISI304 UNS S30400 210 515–690 45 >300 AISI430 UNS S43000 205 450 20 SAF2304 UNS S32304 400 600–820 25 300 SAF2205 UNS S31803 450 680–880 25 250 SAF2507 UNS S32750 550 800–1000 25 Factors affecting the mechanical properties of duplex stainless steel include alloying elements, grain size, and phase proportions. The yield strength of duplex stainless steels is 2 to 3 times that of austenitic stainless steels. The reason why the yield strength of SAF2507 steel is higher than that of other duplex stainless steels is due to the strengthening effect of nitrogen. Moreover, there are more slip planes between the internal grains of austenitic stainless steel, so its elongation rate is significantly higher than that of duplex stainless steel. The main alloying elements in duplex steel, such as Cr, Ni, Mo, and N, play a very important role in various properties of the steel. The grain size of duplex stainless steel plays an important role in its yield strength and toughness. Fine grains endow the steel with higher yield strength and toughness. An increase in grain size also leads to an elevation of the brittle transition temperature. For example, in IN-744 steel, when the grain size increases from 2 μm to 25 μm, the brittle transition temperature rises from approximately -130°C to -45°C; thus, an increase in grain size reduces the low-temperature impact toughness of the steel. Corrosion resistance of duplex stainless steel joints: Duplex stainless steel not only possesses excellent mechanical properties, but also shows very good resistance to pitting corrosion, stress corrosion, and corrosion fatigue. Its joints also exhibit excellent resistance to pitting and chloride-induced stress corrosion cracking; their intergranular corrosion resistance is no lower than that of the base material. However, their resistance to H2S-induced stress corrosion cracking is relatively poor. The area with the worst corrosion resistance of the joint is the heat-affected zone (HAZ), mainly because the precipitation of a second phase (chromium nitride) in this zone forms a \"chromium-depleted\" layer. 1 Pitting and crevice corrosion: The resistance of duplex stainless steel to pitting and crevice corrosion is primarily determined by the contents of Cr, Mo, and Ni elements. The index used to measure this corrosion resistance is the PREN value (pitting resistance equivalent), where PREN = Cr% + 3.3Mo% + 16N%. The formula for calculating the pitting resistance equivalent PREN quantitatively describes the influence of Cr, Mo, and N elements on it. The formula takes into account only the effects of Cr, Mo, and N, without considering the influence of microstructural heterogeneity and precipitate phases. Selecting an appropriate solution treatment temperature to ensure that the two phases have comparable PREN values is necessary to achieve the best pitting resistance for the steel. 2 Stress corrosion: The yield strength of duplex stainless steel is higher than that of ordinary stainless steel; therefore, the critical stress value for stress corrosion cracking (SCC) is also higher ； The presence of second phases in steel acts as a mechanical barrier to crack propagation, prolonging the crack growth period ； In a neutral chloride medium, pitting usually serves as the initiation point for SCC in stainless steel, whereas the composition and microstructural characteristics of duplex stainless steel make it difficult for pitting to occur, thereby prolonging the incubation period for pitting. Therefore, duplex stainless steel has a strong resistance to SCC. The SCC sensitivity of various stainless steels and duplex steels is also related to temperature and chloride concentration; whether SCC occurs over a wide range of chloride concentrations depends on temperature. 3 Corrosion fatigue: Corrosion fatigue is a form of corrosion caused by the combined effect of corrosion and cyclic stress, with the stress involved being primarily cyclic in nature. When the cyclic stress is relatively low while the corrosive environment is strong, the source of corrosion fatigue often lies in the non-metallic inclusions in the surface layer, which are also the sites where localized corrosion occurs. As for duplex stainless steels, especially those with high chromium content, they possess both high resistance to localized corrosion and high strength; therefore, they also have strong resistance to corrosion fatigue. Due to its excellent corrosion resistance and high hardness, duplex stainless steel also boasts good wear and corrosion resistance. Numerous practical examples have shown that duplex stainless steel exhibits excellent properties in terms of resistance to intergranular corrosion and uniform corrosion. Image: Limitations of the use of duplex stainless steel. Compared to austenitic stainless steel, duplex stainless steel has the following disadvantages: 1 Its applicability and versatility are lower than those of ordinary stainless steel, and its operating temperature must be kept between -50°C and 250°C. 2 The plasticity of austenitic stainless steel is lower, and its performance in cold and hot working processes as well as its formability are slightly inferior. 3 There is a medium-temperature brittleness zone, requiring strict control of heat treatment and welding procedures to prevent the formation of harmful phases, otherwise performance will be compromised. Welding of duplex stainless steel: Duplex stainless steel exhibits good weldability, with a low tendency to develop thermal cracks. Preheating is not required during welding, nor is heat treatment necessary after welding. Compared to austenitic stainless steel, it has a lower tendency for thermal cracks in the welds ； Compared to welding ferritic stainless steels, the degree of embrittlement in the welded joint after welding is lower ； Moreover, the degree of coarsening of the single-phase ferrite phase in the welding heat-affected zone (HAZ) is also low. The key to welding duplex stainless steel is to maintain an appropriate amount of ferrite and austenite in both the weld metal and the heat-affected zone (HAZ). All data related to the welding procedure qualification for 2205 (ASTM A240-00 UNS S31803) utilize wire ER2209 (SANDVIK).