Sound Emission Analysis During The Quenching of 42CrMO4 Steel in Various Specimen Shapes

F. Ravnik, J. Grum:
New challenges in heat treatment and surface engineering – conference in honour of prof. Božidar Liščić (09 – 12 June 2009, Dubrovnik – Cavtat, Croatia)

The hardenability of steel through the whole workpiece depends on several facts: chemical composition, the type of steel, the size and shape representing the mass of the workpiece, microstructure prior to heat treatment, the type of quenching medium applied, the quenching method and the quenching process itself.
Most common difficulties encountered are caused by deviations due to a vapour film formation around the workpiece. The vapour film formed around the workpiece produces an uneven removal of heat and consequently, a different microstructure than the one expected.
In order to control the hardening process, one should be able to monitor the quenching process in real time. The paper treats an experimental setup comprising detection of sound emission together with some results obtained in the course of quenching. Due to heat transfer from a specimen to a quenching medium, film boiling and nucleate boiling occur around a heated specimen, which strongly affects sound-pressure signals emitted from the surface during quenching. Different amplitudes of emitted sound-pressure signals at different frequencies therefore mostly depend on the wetting behaviour around the surface of the specimen.
It has turned out that an analysis of sound emission signals can provide useful information that confirms differences occurring in quenching specimens with different shapes in different quenching media under different quenching conditions. Analyses of sound emission demonstrated that sound emission during the quenching process can be used for monitoring the hardening process.

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