Tempering of an additively manufactured microsegregated hot-work tool steel: A high-temperature synchrotron X-ray diffraction study

Rapidly solidified microstructures, produced by concentrated heat sources, have gained increasing attention due to the widespread expansion of additive manufacturing techniques. In laser powder bed fusion of tool steels, the as-built microstructure consists of a microsegregated network of cell-like walls decorated with retained austenite (RA). This inhomogeneous elemental distribution leads to phase transformation sequences dictated by local – instead of global – equilibrium. This work uses synchrotron X-ray diffraction to study the kinetics of retained austenite decomposition during tempering of a microsegregated as-built AISI H13 tool steel. The RA decomposition products are also characterized by hardness tests, scanning electron microscopy, and atom probe tomography. Present results evidence the formation of high temperature para-equilibrium (T < 650 °C) or equilibrium (T ≥ 650 °C) microstructures. The para-equilibrium microstructural path evidences isothermally stable RA, which transforms into fresh martensite upon cooling due to carbon depletion. However, the cell-like microsegregation arrangement is retained due to insufficient homogenization of substitutional elements. ... mehrThe equilibrium microstructural path results in isothermal transformation of RA into ferrite + carbides, as well as dissolution of the cell-like microsegregation structure. These results provide insights towards the understanding and development of optimized heat treatment cycles appropriate for such increasingly common inhomogeneous microstructures. The results are relevant not only to laser powder bed fusion, but also to other localized melting/remelting processes with associated high cooling rates.