Effect of Bath Agitation on the Transient Cooling by Quenching Process

Abstract

During the quenching process, the liquid bath is usually agitated to homogenize the temperature and to enhance convective heat transfer. The purpose of this paper is to characterize on the one hand the agitation of a water bath due to the movement of a three-blade turbine and on the other, the cooling of an Inconel 718 part being quenched in a stirred water bath. Velocity measurements were taken by particle image velocimetry (PIV) with and without the metallic part. We found that the velocity field became purely axial when we were far enough away from the turbine. Moreover, a high turbulent mixing level was shown for this type of jet. Velocity measurements were carried out for two agitation intensities. The axial velocity amplitude, as well as the turbulent kinetic energy, decreased dramatically as the rotational speed of the propeller decreased from 410 to 100 rpm. This caused the thermal behavior of the part to differ during quenching. Indeed, we found that the part cooled faster under stronger agitation. During the film boiling and transition phases, no appreciable effect of agitation could be observed. However, from the middle of the nucleate boiling phase, the part-bath heat transfer coefficient was found to decrease much less rapidly with the surface temperature if agitation was strong than if it was weak or if the bath was completely calm. In such a case of strong agitation, both nucleate boiling and convection concomitantly ensure part cooling.