Characterization of Quenching and Tempering of a Martensitic Steel Based on Precise Internal Friction Measurement Using Electromechanical Impedance

Thermal treatment is very important for steel as it can provide products with varied mechanical properties. In practice, the effect of thermal treatment is typically examined using hardness testing, which is a local method and less accurate. In this work, a nondestructive method for studying the quenching and tempering of martensitic steels (65 Mn) is proposed, based on a quantitative electromechanical impedance (Q‐EMI) internal friction (IF) measurement. The measured IF of water‐quenched, oil‐quenched, and fully annealed steel samples are more sensitive to thermal treatment than hardness. Then, the isothermal aging process at 40 °C is monitored by successive IF and Young's modulus measurement for 48 h. It is found that after aging, the IF of water‐quenched and oil‐quenched samples drops by 15.4% and 11.3%, while Young's modulus only increases by 0.27% and 0.20%. The results of the tempering samples between 100 and 600 °C show that IF drops quickly with the increasing tempering temperature below 400 °C but drops slowly above 400 °C. In comparison, the HRA hardness drops steadily with the increasing tempering temperature from 100 to 600 °C. The IF measurement using Q‐EMI provides an effective approach for characterization of thermal treatment on metals and alloys.