Determination of Dominant Parameter on Hardened Layer Depth of Laser-Hardened Steel using Transient Heat Conduction Theory

Abstract

In this study, a direct diode laser with a wavelength of 940 nm was uniaxially scanned on a carbon steel S45C surface at an incident angle δ=15 °. The effects of laser scanning conditions on the surface temperature and the hardened layer depth after quenching were investigated. The following conclusions are obtained using experimental results and transient heat conduction theory. At first, both surface temperature during laser scanning and the total hardened layer depth after quenching are found to be proportional to the laser energy density and, however, increase with an increase in the laser power even at constant energy density. These results indicate that the internal temperature of steel cannot be determined by one-dimensional steady-state heat conduction. Next, the internal temperature of steel is estimated by one-dimensional transient heat conduction. A penetration thickness, which is the depth where the laser energy affects the internal temperature of steel, is considered from the heat conduction theory. As a result, an increment of internal temperature of steel is found to be proportional to a new parameter, √vq₀. The surface temperature during laser scanning and total hardened layer depth after quenching are found to be also proportional to the new parameter, √vq₀, and be expressed as a single regression line of √vq₀, even if the laser output is varied. The dominant parameter that determines the hardened layer depth by laser hardening can be identified.