Creep Properties of Low-Carbon Nitrogen-Alloyed Modified 316 Stainless Steel

Abstract

Effects of carbon, nitrogen and tungsten on the tensile and creep properties of a composition-controlled modified 316 stainless steel for high-temperature use, have been investigated. This steel has a lower carbon and higher nitrogen content than conventional 316 stainless steel (SUS316). The results obtained are as follows.
(1) Modified 316 exhibits high creep ductility, which changes little with increasing rupture time. It also has a generally higher creep strength than SUS316, and this advantage increases with longer rupture time.
(2) The improved creep strength in the modified 316 is attributable mainly to a lower creep rate in the tertiary creep range, and this reduced creep rate is thought to be explicable in terms of stable solution hardening by nitrogen and suppressed grain boundary embrittlement resulting from the low carbon content.
(3) Addition of tungsten increases the creep strength of the modified 316 with little change in creep ductility. A high tungsten content, however, is observed to enhance Fe₂(Mo, W) formation. It is thought that the resultant reduction in Mo and W contents in the grain matrix lowers the creep strength as the rupture life increases.