Integration of Experimental Methods for the Correlation of Hardness with the Propagation Velocity of Ultrasonic Signals in Steel: A Proposal for an Undergraduate Material Science Lecture

This work provides the description of an experimental study for assessing the hardness of metallic test samples, subjected to the Jominy End-Quench Hardenability Test, by employing typical microhardness tests. Additionally, experimental measurements of the propagation velocity of ultrasound in the test samples were correlated with the microhardness results. A systematic protocol is presented for undergraduate mechanical engineering lectures, addressing the development of experiments, including the sorting and analysis of the resulting data; also, its impact on the expected competencies and abilities of the students is discussed. Several test samples of AISI 4140 steel were used for three experimental sets: heat-treated with the Jominy End-Quench Hardenability Test, fully tempered, and non-treated samples. All samples were evaluated by using typical microhardness tests and ultrasonic pulse-echo measurements to quantify their hardness at multiple locations. The proposed methodology not only strengthened some of the different required abilities of engineering students but also introduces a new procedure, using ultrasonic signals, to estimate hardness in undergraduate programs. The obtained results show a significant correlation between the experimental data from microhardness and ultrasonic measurements, with a maximum precision of -96.36% (inverse correlation) within the studied range. A discussion is provided regarding the means of improving the skills of undergraduate students in the course of Material Science, including experimental mechanical testing abilities through these systematic methods.