Creep Properties of Nitrogen-Alloyed Type 316L Stainless Steel

Type 316L stainless steel (SS) alloyed with 0.06–0.08 wt% nitrogen is the principal structural material for the sodium circuit components of India’s Prototype Fast Breeder Reactor. Carbon in the range of 0.045–0.055 wt% and nitrogen in the range of 0.06–0.10 wt% have been specified for the welding consumable to provide weld joints with creep strength as close as possible to that of the base metal. Design of the components is based on RCC-MR fast reactor code. Creep properties of the plates and the welding consumables, which were produced by the Indian industry, have been studied at 873 and 923 K. Creep rupture strength of the weld joint was found to be comparable with that of the base metal, implying a weld strength reduction factor close to unity. Creep rupture strength of the weld metal was found to be lower than that of the weld joint at 923 K whereas it was comparable to that of the weld joint at 873 K. The creep failure location shifted from the base metal to the weld metal with increase in test temperature from 873 K to 923 K. The base metal and the weld joint satisfied the average strength requirements specified by RCC-MR code. Addition of nitrogen was found to increase rupture strength by about 35% as compared to that of 316 SS. Rupture elongation decreased in the order base metal > weld joint > weld metal. Phenomenological observations on creep behaviour have been rationalized based on the mechanistic aspects of deformation and damage and microstructural changes.