Fei Weng

Low carbon structural steel, valued in industry for its low cost and adequate strength, exhibits limited hardness and poor wear resistance. In this study, NbC‐reinforced Ni35 coatings are fabricated on low‐carbon structural steel by laser cladding. The influences of NbC content on microstructure and properties are analyzed. The large NbC particles are effectively retained within the coatings, while fine NbC particles precipitate between these large particles. In addition, various solid solutions, carbides, and borides are also present in the coatings. Higher NbC additions increase the volume fraction of precipitates and the dilution ratio. The coating achieves a maximum hardness of 783 HV _0.1 at a 50 vol% NbC content. A further increase to 70 vol% NbC lowers hardness because excessive dilution enlarges the heat‐affected zone. The friction coefficients of the coatings are lower than those of the steel, exhibiting a minimum value of 0.52. Steel substrate experiences severe plowing and brittle spalling. Elevated NbC contents progressively improve the wear resistance of coatings by suppressing plowing, but the cracks caused by fatigue and the subsequent brittle spalling remain quite serious. At 70 vol% NbC, the wear volume is only 4.55% of that of the substrate.

This work investigated laser aided additive manufacturing (LAAM) high-strength steel by leveraging the intrinsic tempering effect to facilitate the formation of high-fraction of metal carbides (e.g. M₂₃C₆ and M₇C₃) in the as-built samples. The intrinsic tempering effect contributes to a superior mechanical property than traditional manufacturing methods in as-built condition, promoting subsequent heat treatments (HTs) for excellent mechanical properties. The influence of HTs on the microstructures and mechanical properties were characterised in multi-scales. A large number of carbides are intrinsically formed due to the tempering effect during deposition. The high-density dislocations in the as-built sample facilitate the formation of massive nano-twins and carbides during HT. The HTed sample achieves a true tensile stress of about 1.81 GPa together with a true strain of about 21%, achieving an excellent strength-ductility combination compared to wide-range high-strength steels processed by additive manufacturing and conventional methods. The grain and twin boundaries strengthening, precipitation strengthening and dislocation strengthening contribute to the high strength, while the good ductility originates from twinning induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effects, and high work-hardening rate, during deformation. The findings imply a potential way to develop AM-customised materials by fully understanding and utilising the IHT effect.