Analysis of temperature and surface hardening of low carbon thin steel sheets using Yb-fiber laser

Laser surface hardening of very low carbon steel of ~ 1 mm thickness containing 0.05% and 0.07% carbon was carried out with a high power fiber laser. The microhardness increased from 120 HV to 217 HV average in 0.05% C steel, and from 160 HV to 280 HV average in 0.07% C steel. Since the carbon content is very low, the improvement in laser hardening with permanent black ink coating and graphite coating on the surface was also investigated. A noticeable increase in the average microhardness was obtained with surface coating. The rise in temperature as a function of time and depth for different laser processing conditions was calculated using an analytical model which takes into account the finite sheet thickness and the cooling due to heat conduction along the depth as well as in the radial direction. The experimental results of depth of hardness compared well with the calculated depth up to which temperature rise was above the phase transformation temperature. The tensile test on laser hardened specimens revealed increase in tensile strength and reduction in percentage elongation in both grades of steel. SEM, EDX and XRD analyses were carried out to characterize the microstructure modified by the laser treatment. Wear test result showed relatively low mass loss of hardened specimen as compare to base material, and the scratch test confirmed improvement in scratch resistance of the hardened surface.