Abstract
Medium and high carbon steels (carbon ≥ 0.25 wt.%) are considered ‘non-weldable’ because of their high susceptibility to cracking during martensitic transformation. Therefore, in order to produce reliable, high-performance steel parts using laser powder bed fusion (LPBF), it is critical to optimize the process parameters. In line with this, an increased energy input is believed to lead to higher heat accumulation and subsequent tempering effects, thereby mitigating cracking.
To investigate this, non-spherical water-atomized (WA) low-alloy steel powders with 0.59 wt.% carbon were LPBF processed in three operational regimes – conduction mode, transition mode, and keyhole mode. The effects of changing the melting mode on the microstructure and mechanical properties were then systematically studied using optical microscopy, microhardness mapping, tensile testing, X-ray diffraction (XRD), field emission-secondary electron microscopy (FE-SEM), and X-ray computed tomography (XCT). In addition, the softening (tempering) response of the experimental steel was studied through a series of ex-situ furnace heat treatments and thermodynamic calculations.
Results showed that the in-situ heat treatment of steel during LPBF can be described as a sequential process of quenching and tempering, where a carbide-free martensitic structure forms initially during layer solidification and is subsequently tempered through heat conduction and heat accumulation. As the effective input energy increases, the extent of in-situ tempering increases as well, as evidenced by the coarsening of nano-sized carbides, dissolution of transition ε-carbides, reduction in lattice distortion, and decomposition of retained austenite. Notably, the keyhole specimen, which underwent the highest level of tempering, displayed a relatively high microhardness of 476.5 ± 32.9 Hv, close to that of the conduction specimen (489.9 ± 26.3 Hv). This was ascribed to the density of melt pool boundaries. Specifically, the keyhole sample, characterized by a lower density of melt pool boundaries, was less affected by the localized soft zones caused by cyclic heating and cooling in the vicinity of the melt pool boundaries.