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 similar to 90% was achieved upon 2 h post-processing annealing at 400 and 650 degrees C and solution annealing at 1100 degrees 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 degrees C. This includes the precipitation of embrittling a phase or its precursors at 650-800 degrees 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 degrees 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.

Automated summary

Additively manufactured 316L austenitic stainless steel was subjected to various heat treatments to study the evolution of residual stress, microstructure, and mechanical properties. Significant compressive residual stress was revealed in the as-built condition, with stress relief achieved at different temperatures. Microstructural changes, such as recrystallization, coarsening of nano-inclusions, and precipitation of embrittling phases, were observed during the heat treatment process, impacting the material's ductility and strength. The study established important relationships between heat treatment, properties, and microstructural changes for the structural application of SLM 316L.