Effect of microstructure on creep mechanical behavior and damage mechanisms of AISI 316L (N) austenitic stainless steels

The designation AISI 316L (N) regroups different materials that, depending on slight modifications of their chemical composition or on their manufacturing route, may develop different creep mechanical behavior and damage mechanisms. In this article, two AISI 316L (N) alloys are compared: a rolled plate and a niobium (Nb) rich hot forged thick plate. Both materials are submitted to the same creep conditions, 575 °C and 310 MPa. Due to a lower yield stress, the tensile strain at the end of the loading phase is higher for the Nb-rich alloy than for the rolled material. On the contrary, the subsequent creep strain rate and creep ductility are significantly lower for the Nb containing steel. The creep lifetime of the Nb-rich material is far longer than the one of the rolled Nb-free plate. Multiple internal crack initiations occur preferentially at the grain boundaries for both materials, but crack propagation eventually becomes mostly transgranular in the highly deformed rolled plate material while staying intergranular in the Nb-containing thick plate. Niobium carbonitrides and subgrain boundaries are revealed at the as-received state of the Nb-rich alloy. After this creep test, microstructural evolutions are only noticed in the Nb-free alloy.