We uses the high-energy heat source of a high-power laser (20 kW) to address the problems of surface oxidation, decarburization behavior, uneven surface hardness, high costs, and environmental pollution in the conventional quenching process of 55 steel surface. By promoting the application of high-power lasers and technological innovation, the study aims to meet practical working conditions for high efficiency, low cost, energy conservation, and environmental protection. The single-variable principle is employed to obtain laser remelting and laser quenching process parameters with different laser power ranges, and the effects of both on the surface morphology, microstructure, and hardness of 55 steel are investigated. The goal is to enhance the working surface properties of 55 steel, thereby increasing its service life and safety, and reducing the likelihood of engineering accidents.

In the study of laser quenching on the surface of 55 steel, we found that neither an excessively high laser power nor an excessively quick scanning speed could improve the surface properties of the steel, particularly under high-power laser quenching with fast scanning. The hardened surface expanded with increased laser power, and despite remelting, the hardness value remained relatively constant. Consequently, a comparative study of laser remelting and laser quenching was proposed. First, to investigate the hardening effect of laser remelting and laser quenching on the working surface of 55 steel, the variation of surface hardness in specimens with increasing laser power at different laser ranges was investigated using the single variable principle, and the surface morphology and boundary thermal diffusion of each specimen were analyzed. Second, the effect of scanning speed on the surface hardness of 55 steel was investigated at medium laser power ranges to further characterize the hardening differences between laser remelting and laser quenching on that material. To characterize the intrinsic hardening difference between these processes, XRD analysis was performed on both low and high-laser-power specimens to study the variation of each physical phase with laser power and to obtain the physical phase difference between remelted and quenched specimens. Finally, the microstructure morphology and hardness changes of the cross-section of the high-laser-power specimens were analyzed, while the EDS line scan of the cross-section of the 2.1 kW specimen was performed to compare the diffusion of elements in the remelted and hardened layers, and based on the above analysis, the intrinsic hardening mechanisms of laser remelting and laser quenching were summarized and analyzed.

By comparing the laser-quenched and laser-remelted specimens within different laser power ranges, we found that the surface hardness has a similar variation pattern. The laser-remelted specimens exhibit a better hardening effect and a significant increase in the cross-sectional hardening layer depth with the increase in the laser power range (Figs. 9 and 10). In addition, the XRD analysis reveals that the laser-remelted specimens had fewer unfused carbide phases than the laser-quenched specimens at different laser power ranges (Fig. 8), and this conclusion is further verified by the microstructure morphology (Fig. 7). Moreover, in the XRD spectra of medium and low laser powers, the peak level of the strongest peak of martensite decreases first and then increases, which indicates that similar lattice distortion occurs with the increase of power in different laser power ranges. In the microstructure comparison, we found that the laser-remelted specimens exhibit more uniform and dense martensite (Fig. 6), and the carbon elements in the remelted layer show stronger diffusion ability than other alloying elements (Fig. 5).

In this study, the effects of different laser power and scanning speeds on the surface hardening effect of 55 steel are investigated. Additionally, the surface hardening differences between remelted and quenched specimens are compared, and the intrinsic hardening mechanism is also analyzed. We found that laser remelting exhibits a superior hardening effect than laser quenching. The surface hardness range of laser-quenched specimens is 446-520 HV, the depth range of the hardened layer is 621-709 μm, with a maximum cross-sectional hardness of 720 HV. For laser-remelted specimens, the surface hardness range is 480-613 HV, the depth range of the hardened layer is 709-813 μm, and the maximum cross-sectional hardness is 755 HV. Carbide particles in the remelted specimens gradually dissolve and diffused with increased power, forming a smoother grain boundary structure. Notably, the remelted layer contains higher carbon atom content and achieves higher hardness, while the quenched specimen still has visible unfused carbide particles in the microstructure of the hardened layer, limiting the acquisition of its high-hardness hardened layer. In addition, the microstructure of both quenched and remelted samples consists of lamellar and slate-like martensite, residual austenite, and some unmelted carbides, but the remelted sample has a more uniform and denser microstructure, a higher content of martensite, and fewer unmelted carbides, and the microstructure is free of pores, cracks, and other defects, which obtains a better hardening effect and provides guidance for the study of surface laser hardening of 55 steel.