Feasibility study of monitoring the steel quenching process using acoustic emission technology

- An acoustic emission technique is proposed to monitor the steel quenching process.
- Heat transfer stages in different quenching media exhibit an acoustic signature.
- The acoustic signature can be correlated with mechanical properties.

This paper presents a non-destructive method of monitoring and evaluating the steel immersion quenching process using AE (acoustic emission) technology. The immersion quenching process is used in the heat treatment industry in order to harden metals and to achieve the required mechanical properties. During quenching, several stages of heat transfer occur, each stage with its own thermo-fluid dynamic phenomena and an AE signature. In order to ensure high quality of quenched workpieces and repeatability of the production process, a method of quality control is required. Several cylindrical steel specimens are quenched in water or in aqueous polymer solutions with 10% or 20% concentration. AE signature of the quenching process is recorded using a piezoelectric hydrophone. AE signal characteristics are considered in correlation with video recordings. After quenching, mechanical properties of specimens such as hardness and tensile strength are determined. A correlation between AE signal characteristics and mechanical properties is established. Results show a non-linear correlation between AE signal peak amplitude, signal RMS (root mean square) value and mechanical properties, and a nearly linear correlation between AE signal duration or number of signal packets and mechanical properties. The total elastic energy correlation is omitted due to high sample standard deviations. Based on the results, it is concluded that AE is a reasonable way of monitoring the steel quenching process and mechanical properties of quenched workpieces.