Response of laser power bed fusion manufactured austenitic stainless steel towards combined heat treatment and low-temperature thermochemical surface strengthening

In this work, a combination of heat treatment (HT) and surface strengthening is explored to adjust the residual stress distribution in 316L stainless steel manufactured by laser powder bed fusion (L-PBF). Different HT in the range 750 °C–950 °C were applied to relieve the overall residual stress, followed by low-temperature gaseous carburization (LTGC) to introduce compressive residual stress in the surface region. The results indicate that the hierarchical microstructure in as built 316L gradually disappeared during HT, accompanied by the relaxation of thermal residual stresses, a reduction in hardness, yield strength and ultimate tensile strength and an improvement of the elongation. HT at 900 °C for 1 h is sufficient to completely relax residual stresses in the built. At elevated temperatures, low angle grain boundaries (LAGBs) vanish, leading to a rapid change in mechanical properties. Based on these findings, the impact of different microstructures in HT specimens on the application of LTGC was investigated. It reveals that all HT specimens exhibited the formation of a uniform, approx. 30 μm thick, case after LTGC. This case possesses high hardness (∼12 GPa) and a carbon content at the surface of ∼3 wt%. Although interdependencies occur between composition, residual stress and hardness profiles over the thickness of the expanded austenite zone, the differences in hardness and composition profiles for different conditions are minor, albeit measurable. In contrast, the residual stress profiles over the expanded austenite zone are notably affected by the initial residual stress present in the starting material and the yield stress associated with the cellular structure.