The Laser Powder Bed Fusion (LPBF) is a promising technology for the production of tools with complex geometry and integrated functions. However, tool steels normally present poor laser processability and tend to crack due to the thermal stresses during the LPBF process. The increase of powder bed temperature is one solution to avoid large thermal gradients. In this study, the influence of the powder bed temperature on microstructure and resulting hardness was systematically investigated to understand the phase evolution during the LPBF process.

AISI M50 samples were produced by LPBF from room temperature to a preheating temperature of 650 °C. The microstructure was characterized by optical microscopy, scanning electron microscopy employing energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis. A cellular/dendritic microstructure formed during the rapid solidification with retained austenite located at the interdendritic regions. Moreover, a high preheating temperature reduces the retained austenite fraction.