The Influence of Microstructure on the Sensitization Behavior of Duplex Stainless Steel Welds

The relative sensitization susceptibilities of autogenous type 308 (UNS⁽¹⁾ S30800) stainless steel (SS) tungsten inert gas (TIG) welds were investigated by isothermally heat treating welds for various lengths of time at 550 and 600°C and testing for intergranular corrosion resistance in ASTM⁽²⁾ A262E. The welds possessed three different microstructures: cellular (single-phase) austenite, duplex with skeletal ferrite, and duplex with lath ferrite. As expected, the fully austenitic welds were highly susceptible to sensitization. In contrast, those welds with skeletal ferrite exhibited only limited susceptibility to sensitization. In fact, heat treatments at 600°C longer than 1 h and at 550°C longer than 48 h did not sensitize welds with skeletal ferrite. Duplex welds with lath ferrite exhibited strong resistance to sensitization. However, because of the much greater amount of interfacial austenite–ferrite boundary area present in the lath ferrite welds, theoretical analysis predicted their immunity to sensitization. The discrepancy between theory and experiment appears to result from the former not accounting for the dependency of carbide precipitation at two-phase boundaries on the relative orientations of the austenite and ferrite grains. Because of the latter consideration, the effective amount of austenite–ferrite boundary area in welds with lath ferrite is significantly less than the actual amount.