Photothermal, Structural, and Microstructural Characterization of SAE4320H Automotive Steel Alberto Lara-Guevara, Ignacio Rojas-Rodrı ´guez, Ruben Velazquez-Hernandez, David Jaramillo-Vigueras, Karina del A ´ ngel-Sa ´ nchez, and Mario E. Rodrı ´guez-Garcı ´a (Submitted June 29, 2015; in revised form October 6, 2016; published online March 29, 2017) SAE4320H steel was characterized by photothermal radiometry spectroscopy, scanning electron micro- scopy, 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. Keywords annealing, gear manufacturing, metallurgical microstructure, microhardness, photothermal radiom- etry 1. Introduction SAE4320H steel is commonly used for gear manufacturing, a process in which pieces are forged and annealed before machining operations. Then, these have to be carburized and hardened. The automotive industry, among others, due to the production volume attempts to simplify the quality assurance inspection steps. Brinell hardness testing and microstructural analyses are typically used to evaluate forged and annealed steel at the production area. However, these techniques are destructive; this fact, unfortunately, delays the production flow. Recently, photothermal radiometry (PTR) based on the measurement of blackbody radiation has been used successfully to characterize semiconductors, as well as metals (Ref 1, 2). In the same manner as PTR, infrared photocarrier radiometry (PCR) has been applied in semiconductor characterization (Ref 3). Garcia et al. (Ref 4) characterized thermo-physical proper- ties of sprayed tungsten carbide coatings on aluminum-killed steel and stainless steel substrates. By using a one-dimensional model of a three-layered system in the backscattering mode and frequencies from 10 Hz to 10 kHz, these researchers also showed that PTR could be used to determine the thickness of tungsten carbide coatings. Liu et al. (Ref 5) found that, in the case of steel samples, the signal from thermal processes (infrared emissions) was affected by the optical and the steel structural properties (heat treatment and hardness). Rojas-Rodriguez et al. (Ref 6) characterized silver alloys by using PTR to correlate photothermal signal with silver content. It was found that if the silver content increased the PTA, amplitude signal decreased and an opposite trend was found for the phase signal. On the other hand, Rojas-Rodriguez et al. (Ref 7) used PTR in the characterization of Cu steel bonded by impact welding to determine the impact effect on the microhardness and thermal properties behavior of Cu steel samples with different surface finishing. They found that the steel structure suffered a stronger damage because steel has a lower ductility than copper. The development of nondestructive and non-contact tech- niques to evaluate metallic materials is crucial for the metal characterization. The field of nondestructive evaluation with thermal techniques has been reviewed extensively by Callister and Retwish (Ref 8). Some photothermal applications to measure the hardness of metals have also been reported in the literature (Ref 6-12). Wang and Mandelis (Ref 13) studied surface hardness properties of steel and developed a numerical technique which is based on the solution of the thermal wave equation using a two-dimensional finite difference grid. Qu et al. (Ref 14) reconstructed depth profiles of thermal conductivity of case-hardened steels using a three-dimensional PTR. The studies showed that the depth variation of thermal diffusivity is dominated by the carbon concentration profile while the thermal diffusivity values are dominated by microstructural properties (Ref 15-18). In the works above, Alberto Lara-Guevara, Facultad de Informa ´tica, Universidad Auto ´noma de Quere ´taro, C.P. 76230 Quere ´taro, QRO, Mexico; Ignacio Rojas-Rodrı ´guez and Ruben Velazquez-Hernandez, Divisio ´n Industrial, Universidad Tecnolo ´ gica de Quere ´taro, C.P. 76148 Quere ´taro, QRO, Mexico; David Jaramillo-Vigueras, Centro de Investigacio ´n e Innovacio ´n Tecnolo ´ gica, Instituto Polite ´cnico Nacional, C.P. 02250 Mexico, DF, Mexico; Karina del A ´ ngel- Sa ´ nchez, Universidad Tecnolo ´gica Gral. Mariano Escobedo, C.P. 66050Libramiento Noreste Km. 33.5, Escobedo, NL, Mexico; and Mario E. Rodrı ´guez-Garcı ´a, Departamento de Nanotecnologı ´a, Universidad Nacional Auto ´noma de Me ´xico, C.P. 76226 Quere ´taro, QRO, Mexico. Contact e-mail: irojasmx@yahoo.com.mx. JMEPEG (2017) 26:2040–2046 ÓASM International DOI: 10.1007/s11665-017-2633-7 1059-9495/$19.00 2040—Volume 26(5) May 2017 Journal of Materials Engineering and Performance