Effects of process parameters on the microstructure and mechanical properties of 24CrNiMo steel fabricated by selective laser melting

In this paper, the effects of selective laser melting (SLM) process parameters on the microstructure and mechanical properties of 24CrNiMo high-strength low-alloy steel were investigated. The optimal parameter combinations and corresponding action rules were obtained by relative density index method which directly affects the microstructure and mechanical properties of materials. As a result, the relative densities of as-received 24CrNiMo steel increased rapidly with the growth of energy densities in the range of 38-50 J/mm(3), then increased slowly between 50 and 100 J/mm(3) and ultimately stabilized at maximum 99.6%. The hardness and tensile strength showed an initial upward trend and afterward a fall with increasing the energy densities. In addition, high energy densities led to an increase in the sub-micron cellular and columnar grain size produced by the rapid solidification during SLM, while the grain evolved from columnar grains into cellular grains as the ratio of temperature gradient to solidification rate decreased from the bottom to the top of the molten pool. The granular bainite and the meta bainite including carbon-rich retained austenite films were obtained in as-fabricated samples. The quantity of retained austenite phase decreased due to austenite decomposition with increasing energy densities. Thus, 24CrNiMo steel exhibited a high microhardness of 374.4 HV10 and a high tensile strength of 1249.5 MPa respectively at the most optimized energy density of 85 J/mm(3) because of low porosity, fine grain size, and an appropriate fraction of stable retained austenite.