Inhomogeneous microstructure and its evolution of laser melting deposited 24CrNiMo steel: From single-track to bulk sample

Abstract

The microstructure of 24CrNiMo steel samples fabricated via laser melting deposition (LMD) is inhomogeneous, and has not received enough attention. To study the inhomogeneous microstructure and its evolution rule during LMD, single-track, single-layer, and multi-layer samples were fabricated via LMD. The microstructure, grain size, and texture intensity of the samples were analysed by optical microscopy, scanning electron microscopy, and electron backscatter diffraction. In addition, the hardness and strength were measured, and the evolution of the temperature field was simulated using the finite element method. The results indicate that heat accumulation during the LMD process caused the substrate temperature to rise. The higher the substrate temperature, the lower the cooling rate of the molten pool. Thermal accumulation not only altered the rules of microstructure change, but also caused grain refinement and weakened the texture. In addition, the thermal cycle caused phase transformation of the previously formed microstructure, resulting in grain refinement and weakening the texture. Thermal cycle and thermal accumulation caused the microstructure of multi-track samples to become inhomogeneous. The uneven microstructure made the hardness distribution irregular, and caused the macro-mechanical properties of the LMD 24CrNiMo sample to fluctuate in a relatively large range. In this study, the microstructure evolution of the LMD process was comprehensively traced, and the formation process of inhomogeneous microstructure was revealed.