Effect of Heat Treatment on Microstructure Evolution of X38CrMoV5-1 Hot-Work Tool Steel Produced by L-PBF

The X38CrMoV5-1 hot-work tool steel produced by laser powder bed fusion was investigated to assess the effect of quenching and tempering and direct tempering on the as-built microstructure. After the printing process, the material microstructure appeared to be characterized by a fine cellular network consisting of γ-Fe cell boundaries and α′-Fe cores. Scheil–Gulliver curves, X-ray diffraction patterns, and transmission electron microscopy images suggested a transformation of the inner core zone from δ-Fe to α′-Fe through γ-Fe. Air quenching promoted the transition of the solidification structure into a fully martensitic microstructure. Both as-built and quenched samples revealed the presence of manganese oxides and vanadium carbonitrides forming core-shell structures. After tempering, starting from as-built and from quenched condition, a dispersion of nano-sized V and Cr-rich second phases was formed in the microstructure, achieving hardness values comparable to those obtained by the same alloy produced by conventional methods. The specimen tempered directly after the laser powder bed fusion process showed a hardness peak shifted towards higher temperatures compared to the conventionally tempered sample.