A comparison of conventional and additively manufactured 316L under thermomechanical fatigue

Abstract

The present study compares the thermomechanical fatigue (TMF) behaviour of 316L stainless steel manufactured conventionally by hot rolling and additively by laser powder bed fusion (L-PBF). Machined cylindrical specimens were tested under strain-controlled in-phase and out-of-phase TMF loading at a temperature range of 550–750 °C and total mechanical strain amplitudes of εamech = 0.2–0.6 %. While the conventional 316L significantly outperforms the L-PBF 316L under in-phase TMF, their lifetimes are comparable under out-of-phase TMF. Under in-phase TMF, creep damage in the form of intergranular crack networks occurs which is significantly more pronounced for the L-PBF 316L due to the higher amount of interfaces. Under out-of-phase TMF, the damage is mostly due to stress-assisted oxide cracking and the crack propagation is fatigue-dominated. TEM inspection revealed that L-PBF 316L exhibits a cellular dislocation structure in the initial state, which rearranges only slightly during cycling. For conventional 316L similar dislocation cells form during TMF cycling indicating that they represent a stable dislocation arrangement in 316L under TMF loading. This is evidenced by the rather stable cyclic stress response of the L-PBF material when compared to the conventional material.