Fatigue behavior of 316 L stainless steel weldment up to very-high-cycle fatigue regime

In nuclear power plant, 316 L weldments are widely used in fabricating the primary coolant pipe and in-core components, which usually suffer from very-high-cycle fatigue. Therefore, in this study, the high-cycle fatigue behaviors of 316 L stainless steel with an argon-arc welding joint were studied up to very high-cycle regime by using an ultrasonic fatigue testing system. Microscopic examination indicated that both the base steel and welding seam showed austenitic equiaxed grains. The grain size and micro-hardness values of the welding seam were comparable to those of the base 316 L steel. And heat affect zone was not evidently observed at the weld joint. Fatigue tests showed that fatigue failure still occurred in 10(7)-10(9) cycles, indicating that the conventional fatigue limit does not exist for 316 L weldment. The fatigue strengths of 316 L weldments were close to those of the base 316 L steel. Scanning electron microscopic examinations revealed that multiple fatigue cracks initiated at specimen surface in the high cycle and very-high-cycle fatigue regime. A modified Murakami's model was suggested to take into account the effect of surface roughness and Vickers hardness on fatigue life/ strength of 316 L weldment. The fatigue strength predicted using the proposed model showed good agreement with the experimental results.