Influence of Processing Parameters, Feedstock and Heat Treatment on the Microstructure of PBF-LB 316L Stainless Steel

Sammanfattning

The increasing demand for reliable and rapidly deployable spare parts in highlyregulated industries such as nuclear power has accelerated the interest in additivemanufacturing (AM) of stainless steels. In this work, the influence of heat treatment,process parameters, powder type and part geometry on the microstructureof powder bed fusion-laser beam (PBF-LB) produced 316L stainless steel was investigated.Samples using both vacuum induction gas atomized (VIGA) and gasatomized (GA) were subjected to stress relieving (SR) and solution annealing (SA)and a combination of these treatments. Microstructural evolution was characterizedprimarily using light optical microscopy (LOM) and quantitative image analysis,with selected samples additionally characterized by EBSD.The results showed that stress relieving preserved the characteristic anisotropic meltpool and columnar grain structures formed during printing, while SA at 1200 ◦Cpromoted varying degrees of recrystallization depending on processing route andpowder feedstock. Samples produced using GA powder exhibited extensive recrystallizationand annealing twin formation after SA, whereas VIGA samples largelyretained the original grain morphology despite identical HT schemes. Variations inlaser power influenced melt pool geometry and porosity formation, but only limitedeffects on recrystallization behavior were observed. Part geometry showed minimalinfluence on recrystallization, although thin wall specimens were more susceptibleto distortion during quenching. A custom image analysis method based on Frangifiltering was also developed to quantify grain morphology from etched LOM images.The findings demonstrate that recrystallization behavior in PBF-LB 316L SS isstrongly dependent on the combined effects of feedstock condition, thermal historyand processing route. This study highlights the complexity of grain boundaryengineering in the anisotropic, non-equilibrium microstructure of additively manufacturedstainless steels and contributes to the understanding of post-processingstrategies for the tailoring microstructure of AM components intended to replaceconventional 316L parts in nuclear power plants.