Photothermal depth profilometry of heat-treated hardened 0.15%–0.2%C, 0.6%–0.9%Mn Steels

The thermal-diffusivity depth-profilometric properties of hardened AISI1018 steel samples (0.15%–0.2%C and 0.6%–0.9%Mn) are investigated from experimental data and compared to their microhardness depth profiles. To fully understand the effect of the individual steps of the heat-treating process on the thermal-diffusivity of the steels, the thermal-diffusivity depth profiles are reconstructed using laser infrared photothermal radiometry. The inverted depth profiles are compared to the results of microhardness testing after each step of carbonitriding and quenching. The comparison shows that there is a good to excellent anticorrelation between hardness and thermal-diffusivity profiles for both carbonitrided and quenched samples with 0.02in. case depth and gradually worsening anticorrelation trends for 0.04 and 0.06in. case depths. It is concluded that for this particular steel, both carbon diffusion and the complicated carbonitrided microstructure affect the absolute values of the thermal-diffusivity profiles as well as their depth distribution.