The effect of post-processing heat treatment on the microstructure, residual stress and mechanical properties of selective laser melted 316L stainless steel

Additively manufactured 316L austenitic stainless steel typically displays a hierarchical microstructure consisting of fine columnar grains, cellular dislocation tangles and nano-inclusions, which provides a combination of exceptional strength and ductility. However, the rapidly solidified microstructure often contains significant residual stress and various post-processing heat treatments are generally used to relieve the residual stress and to alter the microstructure and properties. In this work, a 316L austenitic stainless steel additively manufactured by a laser-based powder bed fusion process (selective laser melting, SLM) was for the first time subjected to various heat treatments to systematically study the evolution of residual stress, microstructure and mechanical properties. Significant compressive residual stress was revealed in the core volume of the as-built condition, whilst moderate to full stress relief of 24%, 65% and ~90% was achieved upon 2 h post-processing annealing at 400 and 650 °C and solution annealing at 1100 °C for 5 min, respectively. The extent of stress-relieving is closely associated with the evolution of substructure (i.e., dislocation tangles), which also affects the yield strength. Marked alteration from the as-built metastable microstructure was seen except for the low-temperature treatment at 400 °C. This includes the precipitation of embrittling σ phase or its precursors at 650–800 °C which results in a reduction in ductility. Unlike conventional wrought 316L, no carbide formation was seen in the treatment temperature regime. Recrystallization of columnar grains and coarsening of nano-inclusions took place over time upon solution annealing at 1100 °C, causing softening and unexpected reductions in ductility. This work led to the establishment of heat treatment-property relationships and corresponding microstructural changes, which are of great significance for the component design and structural application of SLM 316L.