Comparison of creep rupture behaviour of type 316L(N) austenitic stainless steel joints welded by TIG and activated TIG welding processes

Abstract

► In the present study, Creep rupture behaviour of 316L(N) base metal, and its weld joints were investigated at 923 K. ► Both the weld joints possessed lower creep rupture strength than the base metal. ► A-TIG joint had higher creep rupture strength than the MP-TIG joint. ► Minimum steady state creep rates of the base metal, A-TIG and MP-TIG weld joints were comparable. ► Reduced cavitation in A-TIG joint has been attributed to lower δ-ferrite content and lesser extent of strength heterogeneity. ► The columnar grains along the applied stress direction in weld metal leading to improvement in creep strength of A-TIG joint. Creep rupture behaviour of type 316L(N) austenitic stainless steel base metal and its weld joints fabricated both by single-pass activated TIG (A-TIG) and multi-pass conventional TIG (MP-TIG) welding processes were studied at 923 K over a stress range of 160–280 MPa. Both the weld joints possessed lower creep rupture lives than the base metal. The A-TIG weld joint displayed higher rupture lives than the MP-TIG weld joint. Failure in the weld joints occurred in the weld metal. Progressive localization of creep deformation in the weld metal of both the joints led to the premature failure. Accumulation of creep deformation at higher rate was observed in the weld metal of the MP-TIG joint than in the A-TIG joint. Finer microstructural features and higher amount of δ-ferrite was observed in the weld metal of MP-TIG joint than in the A-TIG weld joint. Orientation of the columnar grains and δ-ferrite was nearly transverse to the welding direction in the A-TIG joint, whereas it was towards short transverse to the welding direction in the MP-TIG weld joint. TEM investigation of creep exposed weld metal showed the extensive formation of M 23 C 6 carbides, σ-phase and Laves phase along the boundaries in MP-TIG joint, which were less prevalent in the A-TIG joint. With creep exposure, the δ-ferrite transformed to σ and Laves phases, and creep cavitation was found to be associated with the intermetallic phases. Creep cavitation was more pronounced in the MP-TIG weld joint than in the A-TIG weld joint.