Observations of the micromechanisms affecting the fracture path for thermal fatigue-creep loading of a 316L stainless steel

Abstract

This paper examines the effects of hold time on the microstructure of 316L stainless steel cylindrical components subjected to repeated thermal shock loading with a hold time at the maximum temperature (Tmax = 600°C or 650°C) during the cycle. Due to gradients in stress and temperature across the wall thickness the observed microstructural effects are confined to a region at the external surface; within this region precipitation of large M23C6 carbide particles is observed at the grain boundaries. Subsequently, intergranular creep voids develop from these grain boundary precipitates, where there is sufficient tensile stress. The observed creep voids are related to changes in the fracture path for a crack which initiates and grows from the external surface of the cylinder. The initiated defect is observed to follow a transgranular fracture path initially, where a thermal fatigue crack growth mechanism dominates. Subsequent cycling allows sufficient time for creep effects to become significant resulting in an intergranular fracture path, where a thermal fatigue-creep crack growth mechanism dominates.