Thread Content
The common duplex steel grades used for pressure vessels are 2205 and S31803; their chemical compositions are very similar, and their strength, elongation, and hardness values are also roughly the same. Sometimes we use one of these grades as a substitute for the other. So what are their main differences? Which material has better overall performance? In fact, before the year 2000, the chemical composition requirements for 2205 were exactly the same as those for S31803. Later, the American Society for Testing and Materials identified certain issues with this chemical composition; as a result, in ASTM Standard Version 2000, Volume 1.03, ASTM A240/A240M-99a, the chemical composition of 2205 was specified to fall within the range of UNS S32205. The biggest change is that the mass fraction of medium N in the alloy has been changed from 0.08–0.20% to 0.14–0.20%, raising the lower limit of the N content. Then why make such a change? We know that the ideal microstructure of duplex steel at room temperature consists of 50% ferrite and 50% austenite. During the high-temperature phase when the temperature is above 1100°C, austenite gradually transforms into ferrite; when the temperature exceeds the ferrite solidus temperature (usually in the range of 1250–1350°C), it all converts into ferrite. During the subsequent cooling process, part of the ferrite transforms back into austenite, but the extent of this transformation is closely related to the material’s composition and the cooling rate. The reaction stages in the heat-affected zone of duplex steel are shown in Figure 1. Figure 1: Microstructural changes in different stages of the heat-affected zone of duplex steel. The closer the welding heat-affected zone is to the weld line, the higher the ferrite content. Clause 7.3b of the European standard EN13445-4 specifies that in a range of two grain sizes away from the weld line, the ferrite content must not exceed 85%. During the cooling of the welding heat-affected zone, it is the diffusion of austenitizing elements (Ni, C, N, Mn) that causes ferrite to transform into austenite. Due to their large atomic radii, Ni and Mn are substitutional alloying elements, and their diffusion rate is very low during rapid cooling ； In contrast, C and N are small interstitial elements that exhibit rapid diffusion rates in the temperature range from 1040°C to the ferrite solidus temperature. Since C generally has a very low content due to its negative impact on corrosion resistance, N becomes the key element for controlling phase equilibrium under welding cooling conditions. As the mass fraction of N increases from 0.12% to 0.18%, austenite can nucleate within the ferrite grains at higher temperatures, thereby resulting in more austenite formation. Furthermore, in duplex steel, N is primarily dissolved in austenite; tests have shown that the mass fraction of N in austenite is generally 0.25~0.60%, while the mass fraction of N in ferrite is generally 0.00~0.05%. Figure 2 shows the solubility of N in ferrite and austenite. Comparing the two curves, the solubility of N in the austenite phase is much higher. Therefore, if the mass fraction of N in the base material is below 0.14%, the ferrite in the welding heat-affected zone cannot transform into austenite more rapidly or to a greater extent during cooling. This results in the N content exceeding the solubility limit in ferrite, leading to intense nitride precipitation reactions, primarily of Cr2N. As a consequence, the plasticity, toughness, and corrosion resistance of the heat-affected zone are reduced. Therefore, when selecting duplex steel grades, we should give priority to S32205 rather than the outdated S31803. Figure 2 Solubility of nitrogen in ferrite and austenite