A Comparison of Low-cycle Fatigue Properties in Stainless Steel Types 347N and 316N

Abstract

The low-cycle fatigue (LCF) properties of an Nb-stabilized grade, type 347N, and an unstabilized grade, type 316N, of austenitic stainless steels containing approximately 0.1 mass% nitrogen were evaluated at ambient temperature and at the operating temperature of a nuclear power plant (330 C). Type 347N exhibited a shorter fatigue life than type 316N. The difference in the fatigue lives of the two grades of steel was greater at ambient temperature than at 330 C. The inferior LCF resistance of type 347N compared to that of type 316N was largely attributed to the presence of carbo-nitride particles in type 347N, which exhibited a bimodal size distribution. Fine particles induced strong secondary cyclic hardening by pinning dislocation cell walls at ambient temperature, whereas no secondary hardening occurred at 330 C. The hardening caused a higher stress concentration at the tip of the fatigue crack at constant strain amplitude, which resulted in a lower fatigue life at ambient temperature. Coarse particles acted as material defects that deteriorated the fatigue resistance by creating voids on the surface of the propagating fatigue crack, regardless of the testing temperature. An observation of the outer surface showed that the two types of steel exhibited different crack initiation modes on the surface. In type 316N, the crack initiation mode was predominantly intergranular at a high strain amplitude but transgranular at lower strain amplitude. In type 347N, the crack initiation mode was always intergranular, regardless of the strain amplitude.

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