Crack-free in situ heat-treated high-alloy tool steel processed via laser powder bed fusion: microstructure and mechanical properties

In this study, high-alloy tool steel S390 was processed crack-free and dense for the first time using laser powderbed fusion (LPBF). The resulting mechanical properties and microstructure of the LPBF steel parts were investigated.High-alloy tool steels, such as high-performance high-speed Boehler S390 steel (containing 1.64 wt% C andW, Mo, V, Co, and Cr in the ratio 10:2:5:8:5 wt%), are prone to cracking when processed using LPBF because thesesteels have high carbon and carbide-forming alloying elements content. Cracks are induced by thermal stressesand solid-phase transformation, combined with weak grain boundaries caused by segregated primary carbides.Substrate plate heating reduces thermal stresses and enables in situ heat treatment, thus modulating solid-phasetransformation and carbide precipitation and preventing cracking during cooling. The resulting microstructure,precipitations, and mechanical properties of the as-built LPBF specimens, which were in situ heat-treated at 800C, and the conventionally post-heat-treated specimens were assessed using optical microscopy, scanning electronmicroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatterdiffraction, X-ray diffraction, hardness testing, bending testing, and density measurement. In situ heat treatmentimpacts microstructure, precipitation behavior, and solid-phase transformation, causing a change in the microstructureof the material along the build direction due to different thermal histories. The as-built specimensexhibit a hardness gradient along the build direction of 500 HV1 to 800 HV1 in the top layer. The average bending