Microstructural evolution during creep of 316LN stainless steel multi-pass weld joints

Creep rupture behaviour of 316LN austenitic stainless steel weld joint fabricated by multi-pass shielded metal arc welding process has been studied at 923 K over a stress range of 120-225 MPa. The weld joint exhibited inferior creep rupture strength than the base metal and extensive creep cavitation in weld metal led to the premature failure. Weld metal microstructure was found highly inhomogeneous having different morphologies of delta (δ)-ferrite. The δ-ferrite transformed into intermetallic phases on creep exposure and the creep cavitation was associated with the intermetallic phases. Extensive creep cavitation in weld metal was confined to regions containing δ-ferrite with vermicular morphology. The region near the weld pass interface having globular δ-ferrite was less susceptible to creep cavitation. The globular δ-ferrite region possessed higher hardness than the vermicular δ-ferrite region. High dislocation density was observed in the globular δ-ferrite region which is a consequence of the microstructural modification by the heat input from the successive weld passes. The strength inhomogeneity between the globular and vermicular δ-ferrite regions together with the transformation of δ-ferrite into intermetallic phases produced pronounced creep cavitation in the vermicular δ-ferrite region which led to premature failure of the stainless steel weld joint.