Transverse Hardness Photothermal Phase Imaging and Depth-profilometry of Heat Treated Steels

A method to image near-surface hardness profiles of heat-treated case-hardened steels using laser infrared photothermal radiometric phase imaging is described. It is shown that thermophysical and mechanical transverse inhomogeneity profiles in industrial case hardened steel samples are well correlated. Phase surface scanning imaging leads to a practical criterion for assessing transverse hardness homogeneity. A simple method based on phase imaging is proposed as a quantitative criterion to determine which steel samples should be rejected for thermal-wave depth-profilometric reconstruction of thermal diffusivity. Having developed this lateral hardness homogeneity criterion, we selected laterally homogeneous case-hardened industrial steels for thermal-wave frequency scans in order to perform depth profilometry of their thermal diffusivity. The reconstructed thermal diffusivity depth profiles from the samples were compared to the results of microhardness testing after each step of heat treatment: carbonitriding and quenching. The comparison showed that there is a good to excellent anti-correlation between hardness and thermal diffusivity profiles for both carbonitrided and quenched samples with 0.02 " case depth and gradually worsening anti-correlation trends for 0.04 " and 0.06 " case depths. Introduction: Thermophysical depth profilometry is a thermal-wave inverse-problem technique where thermal diffusivity profiles of a material are reconstructed from experimental surface data. Specifically for steels, since the thermal diffusivity depends, among other things, on their microstructural properties, monitoring this parameter indirectly gives information on changes that take place as a result of surface or bulk modification processes. In particular, thermophysical depth profilometry as implemented by laser photothermal means, has shown promise as a non-destructive alternative to existing costly, time-consuming and destructive techniques to determine metallurgical properties of case-treated steels. Various independent groups of researchers [1-6] have studied the by now well-established anti-correlation between thermal diffusivity and microhardness. Only very recently have attempts been made to offer physical interpretations of photothermally reconstructed hardness depth profiles in processed steels. Nicolaides et al. [7] sought to understand the mechanism by which the thermal diffusivity profiles in carburized and hardened AISI 8620 steel arises. In the work of Nicolaides et al., a set of 8620 steel samples were studied after carburizing, followed by quenching. These two steps are normally performed sequentially to produce a quenched, hardened steel but were studied independently so that the origins of the thermal diffusivity profiles could be understood in detail. That study concluded that the depth distribution of the thermal diffusivity profile is dominated by carbon diffusion during carburization, while the absolute thermal …