Microstructure and Mechanical Properties of Laser Additive Repaired 20CrNi4Mo Piercing Plug with and without Preoxidized Layer

The 20CrNi4Mo alloy with excellent high‐temperature stability is often used to manufacture low‐alloy steel piercing plugs, which are subject to harsh service conditions and usually fail due to severe wear. Repairing and remanufacturing failed piercing plugs not only extends the product's lifecycle but also has significant resource and environmental benefits. In this work, the feasibility of laser additive repair of failed piercing plugs using 20CrNi4Mo powder by selective laser melting technology is explored and the effect of preoxidation on the surface microstructure and mechanical properties of the repaired layer is investigated. The results show that the repaired layer has good metallurgical bonding with the substrate, and the densification reaches 99.8%. Compared with the substrate, the ultimate tensile strength and microhardness of the repaired layer increase by 88.97% and 70.68% to 1287 MPa and 380 HV, respectively. The oxide film exhibits a clear two‐layer, with the repair layer exhibiting a thicker oxide film. The average friction coefficient of the oxidized repair layer (0.263) is reduced compared with that of the oxidized substrate (0.382). This work provides a feasible and effective way to realize the repairing and remanufacturing of failed piercing plugs with enhanced surface mechanical properties.