Stepwise martensitic transformation in tool steel during laser powder bed fusion revealed by operando X-ray diffraction

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A mechanistic understanding of phase transformation behaviour under extreme thermal conditions is essential for controlling microstructure and performance of additively manufactured metallic components. However, direct experimental evidence capturing phase evolution during rapid solidification remains limited. In this study, phase transformations in M50 tool steel processed by laser powder bed fusion (LPBF) were investigated using operando X-ray diffraction in reflection mode to monitor phase evolution during laser melting and subsequent cooling processes. Under cooling rates in the order of 10 5 ℃/s, austenite formed directly from the liquid while δ-ferrite formation was suppressed. Martensitic transformation proceeded in a stepwise manner, with significant changes in phase fraction occurring at different cooling stages, highlighting the influence of rapid solidification on martensite nucleation and growth. Distinct transformation stages were identified, consisting of early abrupt transformation stages followed by later stages characterised by more gradual transformation. The carbon content in martensite increased from 0.58±0.04 wt.%to 0.68±0.02 wt.%, as determined from the evolution of local tetragonality. While variations in LPBF volume energy density had a limited effect on the cooling rate, lower energy densities were associated with higher martensite tetragonality and carbon enrichment. These findings provide valuable insights into phase evolution in rapid solidification processes for tailoring microstructure through manufacturing process control in additively manufactured steels.