The aim of the control and appropriate optimisation of the hardening process by selecting the most suitable quenching agent is to ensure required variations of hardness and residual stresses, particularly in the surface layer of a machine part. The paper describes the development of a system designed to capture acoustic emission and including the assessment of hardening efficiency during the quenching process itself. The sutdy of acoustic emission in the course of the quenching process was carried out on cylindrical specimens made of 42CrMo4 heat-treatment steel. The cylindrical specimens of the same size were quenched in two recommended quenching agents, i.e., quenching oil Olmaka Rapid 90 and 15% polymeric water solution Aquatensid - BW . The variations of sound pressure of correctly selected steel quenching conditions permitted to show the timedependent amplitude of sound pressure in diagrams and frequency spectrum in 3D diagrams. The analyses of the signals of acoustic emission captured in the course of the quenching process confirmed useful information and also significant differences existing among the quenching conditions selected. A comparison of the results of cross-sectional hardness measurements at the specimens and the data obtained in the analyses of the sound pressure during quenching permitted a new approach to the elaboration of hardening technology for 42CrMo4 steel.
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