A multi-scale experimental investigation on fatigue crack propagation rate behavior of powder bed fusion-laser beam 316L stainless steel subjected to various heat treatments

In this study, three heat treatments referred to as heat treatment 1 (HT1), HT2, and HT3 were applied to tune the microstructure of powder bed fusion-laser beam (PBF-LB) 316L stainless steel. Furthermore, the fatigue crack propagation rate (FCPR) of as-built PBF-LB 316L specimens and specimens subjected to the three aforementioned heat treatments was thoroughly investigated. To this end, the FCPR was evaluated by a fatigue testing machine combined with an infrared thermal camera, and the Paris law was established to quantitatively assess the FCPR of PBF-LB 316L. In addition, a systematic investigation of microstructural evolution, residual stress, tensile properties, and fatigue crack propagation (FCP) fracture surfaces as well as their effect on the FCPR in function of various heat treatments were carried out. The results indicate that the release of residual stresses, the annihilation of the cellular structure, and the occurrence of recrystallization are obtained through the application of HT1, HT2, and HT3 respectively. Furthermore, it is shown that although all three heat treatments can improve the FCPR behavior of PBF-LB 316L to some extent, HT3 PBF-LB 316L presents the lowest FCPR mainly due to its most pronounced plasticity-induced crack closure effect, indicating its highest resistance to FCP even though the recrystallization obtained by HT3 results in the lowest yield strength (YS). Finally, some recommendations about adequate heat treatments concerning fatigue life are suggested.