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

Creep rupture behaviour of type 316L(N)austenitic stainless steel base metal and its weld joints fabricated both by single-pass activated TIC (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-TIC joint. Finer microstructural features and higher amount of delta-ferrite was observed in the weld metal of MP-TIG joint than in the A-TIC weld joint. Orientation of the columnar grains and delta-ferrite was nearly transverse to the welding direction in the A-TIC 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 M23C6 carbides, sigma-phase and Laves phase along the boundaries in MP-TIG joint, which were less prevalent in the A-TIC joint. With creep exposure, the delta-ferrite transformed to a and Laves phases, and creep cavitation was found to be associated with the intermetallic phases. Creep cavitation was more pronounced in the MP-TIC weld joint than in the A-TIC weld joint. (C) 2011 Elsevier B.V. All rights reserved.