Influence of heating rate on the laser surface hardening of a medium carbon steel

The finite difference method was applied to analyse the influence of heating rate on the transformation phenomenon in laser surface hardening of a medium carbon steel. The implicit scheme of this method was adopted to improve the accuracy of the numerical analysis since the very high heating and cooling rate and the small hardened zone, which characterize the process investigated, need a very detailed mesh generation. The calculated cooling rate was high enough for all the material that undergoes transformation into austenite to be transformed into martensite. From the calculated temperature change the heating rate of the transformation zone boundary could be estimated to be of the order of 10⁴ °C s^-1, which causes a delay in the austenite transformation and consequently a shift in the transformation temperature. Considering a heating rate of 10⁴ °C s^-1, the A_s (start of austenitic transformation) temperature of the investigated medium carbon steel with 0.45 wt.% C should be approximated to be about 830 °C and the A_f (finish of austenitic transformation) temperature about 950 °C. Experimental results obtained by irradiating the test specimen with a 1 kW CO₂ laser showed better agreement with the hardened zone sizes predicted by the modified A_s temperature than with those predicted by the equilibrium A_s temperature. From simulations it appeared that the occurrence of surface melting and the size and shape of the hardened zone are strongly dependent on process parameters such as beam spot diameter and traverse speed.