Heat treatment of medium carbon steel: Acoustic and mechanical characterization

Heat treatments are planned based on the pre-established mechanical requirements for an industrial component. The mechanical characterization of a component in service using conventional tests is not feasible due to its destructive nature, therefore, it is necessary to explore an alternative non-destructive method that allows predicting the mechanical properties of a material without affecting its service condition. The mechanical properties of a metal alloy have an intrinsic relationship with the microstructural characteristics, as well as with the acoustic properties of the material. For this reason, the objective of this research work was to obtain a quantitative relationship between acoustic and mechanical measurements for medium carbon steel subjected to heat treatments of quenching and tempering. For this purpose, nine experimental runs were planned by factorial interaction of the austenization temperature of 850, 900, and 950°C, and the tempering temperature of 25 (without tempering), 550, and 650°C. The samples were characterized by optical metallography, Vickers hardness, and ultrasound with longitudinal waves. The microstructural changes in the samples according to the imposed thermal cycle reported results that demonstrate a significant difference in the measurements of hardness, speed, and acoustic attenuation according to statistical tests. The experimental reports indicate a gradual decrease in hardness as the tempering temperature increases due to a greater presence of tempered martensite. The speed and acoustic attenuation measurements gradually decrease to a minimum value as hardness increases up to a maximum value obtained in the samples tempered without tempering; consequently, there is greater resistance to the propagation of the ultrasonic beam. Acoustic and mechanical correlations are presented as a non-destructive alternative for hardness characterization.