Stress Corrosion Cracking Behavior of Duplex Stainless Steel Weldments in Boiling MgCl2

Stress corrosion cracking (SCO experiments were conducted in 42% boiling MgCl2 on two Type 304L stainless steels which exhibited a duplex ferrite-austenite structure in the weld fusion zone. Results indicate that when weldments are transverse to the principal stress axis, carbon and nitrogen contents of the alloy can be important in determining SCC susceptibility. In the material containing higher C and N concentrations, which exhibited no delta ferrite in the HAZ, SCC occurred in the grain coarsened region adjacent to the weld fusion zone. In the material with less C and N, ferrite which formed in the HAZ prevented grain coarsening and SCC occurred in the unaffected base metal. If the ferrite-austenite interphase interfaces in ferrite containing weld metal were normal to the principal stress, stress corrosion cracks initiated and propagated along the interphase interfaces. This interfacial cracking can be ascribed to sensitization of the austenite-ferrite interface as a result of non-equilibrium segregation of Cr, C, and N both during solidification and subsequent phase transformation. Transgranular cracking in the austenite is inhibited by cathodic protection arising from preferentially corroding phase interface.