High temperature stability of laser additive manufactured stainless steel 316L structures

We have investigated the high-temperature behaviour of 316 L stainless steel manufactured using selective laser melting. The microstructure evolution and the corresponding microhardness variation were probed at temperatures from 350 degrees C up to 1200 degrees C. Scanning and transmission electron microscopy were used to characterise the grain size, morphology, dislocation structures and porosity in the as-built specimen and after annealing at selected temperatures. The melt pools in the as built specimen exhibited a fine cellular microstructure with average cell size similar to 500 nm. The cell structure starts to become unstable at temperatures >= 500 degrees C, leading to slow increase in the cell size as well as a reduction in grain boundary dislocations. At >= 800 degrees C the cell structure completely disappears, together with the local chemical segregations initially present at cell boundaries. Heat treating the material at 1200 degrees C results in a new equiaxed microstructure with an average grain size of similar to 63 mu m. The micro-hardness of the as-built specimen was H-V= 226, whereas annealing for three weeks at 350 degrees C reduced the micro-hardness slightly to H-V= 216. However, heat treating the sample for one hour at 1200 degrees C, the micro-hardness significantly decreased to H-V= 158. The trend curve for both microstructure and micro-hardness with temperature, exhibited a plateau region in the temperature up to 450 degrees C, and then sharp variations following the Hall-Petch relation. Heat treatments at higher temperatures resulted the evolution of small pores into larger size as well as extended porosity. This work provides novel insights into the potential of additive manufacturing structures for high-temperature applications.