Dependence of the local heat transfer coefficient on temperature and surface roughness in quenching steel parts in high efficiency quenching oil

摘要

The heat transfer coefficient in immersion quenching has a high influence on the development of distortion and residual stresses in metal parts. Therefore, it is essential to investigate the heat transfer coefficient thoroughly to enable a realistic modeling of directly the case hardening process of a shift collar. Unfortunately, the heat transfer coefficient is no measurable quantity, but it can be calculated based on the surface temperature and the fraction of volume to surface of the sample as well as on the specific heat capacity and density of the material used. Therefore, temperatures in the sample have to be measured and extrapolated to the surface or calculated by using the finite difference method. In order to determine the heat transfer coefficient a diversity of parameters has to be taken into account. In this work the local heat transfer coefficient was investigated by quenching notched samples, as an abstraction of a shifting collar, using high efficiency quenching oil. In order to investigate the influence of latent heat resulting from phase transformations during quenching in addition to the case hardening steel SAE 5120 (20MnCr5) an austenitic steel AISI 316L (X2CrNiMo17-12-2) was evaluated. Simultaneously the surface roughness of the samples was varied to investigate the influence on the cooling curve.