Microstructure development of modified 9Cr-1Mo steel during laser powder bed fusion and heat treatment

Herein, the effect of laser powder bed fusion (LPBF) parameters on the microstructure development of modified 9Cr-1Mo steel and the relationship between the as-built and heat-treated microstructures were characterized using electron backscattered diffraction (EBSD). The optimal energy density for the steel was determined to be 100–150 J/mm3. The microstructure of the as-built steel was found to be mainly composed of coarse and columnar δ-ferrite grains and fine martensite grains. δ-ferrite was formed through rapid solidification, while martensite was formed in the heat-affected zone during LPBF. A higher energy density and scan strategy with a 67 deg. rotation in each layer were found to be suitable for obtaining a higher area fraction of martensite because they can provide a larger heat-affected zone compared with a lower energy density and a 90 deg. rotation. Tempering reduced the hardness via recovery of martensite and enhanced the precipitation of M23C6 carbide at martensite, while the microstructural morphology in the as-built condition was maintained. The typical as-built microstructure disappeared after the normalizing treatment, while the size of the prior austenite grains was affected by the as-built microstructure. In-situ EBSD observation of the phase transformation behavior during normalization revealed that martensite became fine prior austenite grains, while δ-ferrite became coarse prior austenite grains. From these results, the effects of energy density and scan strategy on microstructure development were identified not only for LPBF but also during the normalizing and tempering heat treatments.