Microstructure evolution and gradient performance of 24CrNiMo steel prepared via laser melting deposition

Abstract

Gradient 24CrNiMo steel was successfully fabricated via laser melting deposition (LMD). To explain the gradient properties of the sample, the microstructures of single-track and bulk samples were examined. Additionally, the texture characteristics and mechanical properties were investigated. The results indicated that as the molten pool cooled from a high temperature, the microstructure of the molten pool experienced a transformation of liquid → austenite → martensite because of the high cooling rate. The microstructure of the heat-affected zone (HAZ) transformed into austenite when heated, and the austenite transformed into martensite when cooled; thus, the microstructure in the HAZ was identical to that in the molten pool after cooling. Consequently, the surface of the bulk sample mainly consisted of martensite and a small quantity of lower bainite owing to the heat accumulation, while the microstructure in the interior of the bulk sample transformed from martensite into tempered martensite owing to the thermal cycling. Thus, the surface of the sample exhibited high strength and hardness, while the interior had better plasticity. The fracture mechanism exhibited dimple rupture features on both the surface and in the interior of the bulk sample; however, the interior had better toughness than the surface. A <100> texture was observed on the surface of the LMD sample, but the orientation of the grains was random in the interior of the sample owing to the multiple nucleation cycles of the grains.