Local microstructural evolution and the role of residual stresses in the phase stability of a laser powder bed fused cold-work tool steel

Economic benefits for the tooling industry can be realized through laser powder bed fusion (LPBF) by the implementation of internal cooling channels or usage of near-net-shaping in additively manufactured tool steels. As the microstructural evolution of the latter has not been fully illuminated yet, this work intends to shed light on the influence of in-situ tempering processes on microstructure development and to clarify the influence of residual stresses on phase stability. Hence, a carbon-bearing cold-work tool steel was processed via LPBF without base plate preheating. Besides well-established techniques such as light optical and scanning electron microscopy, X-ray diffraction phase analysis showed significant differences depending on whether weld bead layers were in-situ tempered during LPBF or not. These tempered layers yielded higher austenite and lower carbide contents than the non-tempered top layers. To distinguish between different phases within the matrix, which is surrounded by a eutectic carbide network, correlative energy-dispersive X-ray spectroscopy, electron backscatter diffraction analysis and atom probe tomography (APT) were carried out. Neither the atomic resolution in APT delivered conclusive differences in chemical composition between martensite and austenite. Therefore, another austenite stabilization mechanism has to prevail for the investigated alloy, i.e., stress-related stabilization. This phenomenon was addressed by the evaluation of strain profile measurements in dependence of the part height. These experiments were performed by cross-sectional synchrotron micro-diffraction. Results showed that sample preparation has a great influence on the determined austenite amounts. Material removing processes, such as cutting, grinding, polishing, focused ion beam milling or ion slicing were made responsible for attenuating respectively extinguishing austenite phase stability.