Photothermal, Structural, and Microstructural Characterization of SAE4320H Automotive Steel

SAE4320H steel was characterized by photothermal radiometry spectroscopy, scanning electron microscopy, x-ray diffraction, and Vickers microhardness, to study metallurgical property changes as a result of the annealing heat treatment. Photothermal radiometry (PTR) images of hot forged and annealed SAE4320H steel were obtained to study the thermal changes, as a consequence of metallurgical microstructure changes that were produced by the heat treatment conditions. X-ray diffraction showed that the annealing process improves the crystalline quality of the SAE4320H steel and releases of any thermal stress. Widmanstatten microstructure was identified as a typical structure after the forging process. The Widmanstatten is a metallographic microstructure transformation to ferrite and pearlite which affected SAE4320H steel hardness and thermal properties. Vickers test showed that the hardness decreases as a result of the annealing process. A positive correlation between Vickers microhardness and PTR amplitude images was found, indicating that the annealing process increases the PTR signal. This methodology allows the determination of the changes in the Vickers microhardness from a non-contact and remote method as in PTR.