Roles of heat transfer modes on transient cooling by quenching process
In order to predict the metallurgical structure of the quenched part by numerical simulation, one needs the boundary condition at the part-bath interface. This last, generally of the third kind, is deduced from measurement of temperature and heat flux density of the surface of work piece. The main goal of this work is the understanding of the heat transfers mechanisms that control the cooling speed according to the size of the work piece. We developed an original device of measurement which allowed temperature and local heat flux estimating at the part-bath interface during quenching process. Experimental results have updated the prevalence of one heat transfer mode according to the more or less thermal resistive character of the quenched part. This prevalence is linked to the mean Biot number Bi m . When Bi m ? ?>?1, the part has a large thermal resistance such as the temperature of the bath is quickly imposed on its surface. Then, cooling is primarily ensured by convection. In this case, the part bulk is the seat of large thermal gradients which induce a strong distribution of cooling velocity. The latter is at the origin of some differences in metallurgical structure and of a residual stress field within the part. In the intermediate value range, 0.1?<?Bi m ?<?10, boiling and convection ensure successively the cooling, but in the boiling there is a prevalence of nucleate boiling mode which determines the reached maximum value of cooling speed located always in the close vicinity of the part-bath interface.
quenching cooling speed convection residual stress boiling modes heat transfer coefficient