Effect of preheating on the microstructure and properties of laser melting deposited 12CrNi2 alloy steel

In the case of preheating and non-preheating, the rapid prototyping system of high-power fiber laser and electromagnetic induction heating equipment was used to fabricate 12CrNi₂ alloy steel. The microstructure of the molded parts was observed under a scanning electron microscope, and the hardness test of different parts was conducted using the Victorinox hardness tester. The tensile properties in different directions were tested using the universal material testing machine. This study investigated the effect of preheating on the microstructure, hardness, and tensile properties of laser melting deposited 12CrNi2 alloy steel in different directions. The obtained results show that the microstructure of the single-channel molten pool without preheating is lath martensite, and the molten pool of block-shaped forming parts is tempered martensite and bainite mixed structure. No obvious structural difference between XOZ and YOZ sections is observed. However, the overall hardness of the YOZ cross section is larger than that of the XOZ cross section. Large-scale macroscopic crack defects appear in both sections, and the mechanical properties are poor. Under preheating conditions, bainite transformation occurs in the molten pool because of the decrease in the temperature gradient. The single-channel molten pool shows an excellent bainite structure. No tempered martensite transformation in the molten pool of block-shaped forming parts, mainly granular bainite, is observed. Cross-sectional hardness distribution is more uniform with preheating than without preheating. High tensile strength and low plasticity are detected in the tensile and overlap directions. The tensile strength is up to 1189 MPa, the yield strength is 951 MPa, and the elongation is only 2.8%. No obvious anisotropy in performance is observed. Preheating can reduce the temperature gradient in the molten pool, reduce the thermal stress, effectively control the crack defects, promote the homogenization of the microstructure, reduce the anisotropy of the microstructure and properties, and improve the mechanical properties of the alloy steel forming parts.