TENSILE AND LCF PROPERTIES OF AISI 316LN SS AT 300 AND 77 K

Abstract

Tensile and low‐cycle fatigue tests were performed on a 316LN austenitic stainless steel at 300 and 77 K. The tensile and low‐cycle fatigue properties were obtained and analysed in terms of the influence of temperature on the plastic deformation process and the formation of strain‐induced martensite. The martensite content was evaluated using measurements of magnetic saturation. No α′‐martensite was detected at 300 K under either monotonic or cyclic straining. On the contrary, at 77 K, strain‐induced martensitic transformation is responsible for the higher elongation in tension and the secondary hardening observed on hardening/softening curves in low‐cycle fatigue. The induced martensite content in tensile tests is a function of strain which deviates from Angel’s model. In low‐cycle fatigue, it is a function of the strain level and the accumulated plastic strain. At a given total strain amplitude, the decrease of temperature from 300 to 77 K results in the decrease of plastic strain amplitude and homogenization of plastic strain distribution, and thus in the prolongation of fatigue life. The cyclic over‐stress at 77 K, due to an intermediate ageing at 300 K, is related to pinning of initially free dislocations resulting from nitrogen diffusion during isothermal holding at room temperature. This results in a reduced fatigue life.