Welding Metallurgy of Duplex Austenitic-Ferritic Stainless Steels

Abstract

Common austenitic chromium-nickel stainless steels show only a limited resistance to stress corrosion cracking in chloride containing media when compared with ferritic chromium stainless steels with more than 18% chromium. However on the other hand ferritic chromium stainless steels—including the socalled “Super Ferritics”—are relatively difficult to weld (see chapter 6). During welding the predominantly ferritic structure shows a tendency towards coarse grain formation, particularly in the HAZ. With the carbon content of the common ferritic chromium steels in the range of 0.10% and above, an additional amount of brittle martensite is formed in the structure. Both phenomena cause a drop in toughness and an increased cold cracking sensitivity in the welded joint. The older types of austenitic-ferritic duplex steels containing 0.1–0.2% carbon were also unsatisfactory in terms of todays demands on the toughness and corrosion resistance of weldments, mainly because of their high susceptibility to intergranular corrosion (IC).

The term duplex steel is generally used to define steels with a two-phase structure. With stainless steels it is common usage to employ this term for austenitic-ferritic steels with a high delta ferrite content in the range of 30–60%.