Effects of heat treatment on microstructure and tensile properties of laser melting deposited AISI 431 martensitic stainless steel

As a martensitic stainless steel, AISI 431 steel is widely used as a key material in marine and steam turbine applications. In this research, a plate of AISI 431 stainless steel was first fabricated using laser melting deposition process, and the microstructure evolutions during the deposition process, the phase transformation under different heat treatment conditions and the influences of microstructures on tensile properties of the steel were investigated. The results show that the laser deposited AISI 431 steel had a fine well-aligned directional microstructure, which consists of dendritic ferrite phases, inter-dendritic ferrite phases and (Cr, Fe)₂₃C₆ carbides. Due to the low bonding force between ferrite and carbides, cracks can easily generate and propagate along the interface, resulting in unsatisfied mechanical properties. After solution treatments at temperatures from 1000 to 1100 °C, the interlayer heat affected zones of the steel were eliminated, and the carbides gradually dissolved when the solution temperatures were raised. The amount of ferrite did not change until reaching 1100 °C, and the steel had the optimal mechanical properties after heat treatment at 1050 °C with a strength of 1283 MPa.