Laser transformation hardening of various steel grades using different laser types

Abstract

Laser surface hardening has become a promising replacement technique for conventional heat treatment. The laser is a heat source, deployed with optics to modify the surface along with the desired geometry. Laser density and its interaction time can be controlled and varied depending upon the final requirement at the surface. Increased hardness, diffusion-less phase transformation, self-quenching, uniform case depth over the entire scanned area are the reasons for laser-assisted hardening to become very popular and successful. Another reason for its success is that there is no bulk transformation of the material; only selected regions are subjected to laser irradiation for better surface improvements. This paper is a precis of various investigations done with a gas type CO2 laser, a solid-state Nd: YAG laser, high-power diode laser and fiber laser (only Continuous Wave mode) over a variety of steel grades. Laser wavelength, power intensity, transverse/scanning speed, focal plane distance, surrounding atmosphere, pre-treated surface and surface absorptivity are parameters involved in laser surface hardening technique. By varying these independent variables, the desired surface property is achieved on the substrate subjected. A significant analysis of various experimental investigations done in this area is provided. Particular reference is made to surface hardening using different lasers as mentioned above. Although laser hardening is a potential technique for enhancing the surface properties, multi-track hardening/back tempering of the large surface area is still a concern and needs some attention. The main reason is a decrease in hardness in the overlap region. Various studies indicate that a single-track laser hardened region results in 3- or 4-times higher hardness than that of the original or base metal hardness. In contrast, in the overlapped region, the hardness value decreases, i.e., the difference between the hardness values at the single-track hardened region and an overlapped region is around 100–400 HV. This article reviews the chronological development of laser transformation hardening method explicitly by outlining the effect of process parameters on hardened depth and width, hardening influence in wear and corrosion resistance, numerical simulation and experimental validation techniques. Particular reference is made to CW (continuous wave) type laser hardening.