A New Methodology for Estimating Heat Transfer Boundary Conditions During Quenching of Steel Probes

Cooling curve analysis is a powerful method to characterize the cooling power of quench baths. From the cooling curve the heat transfer boundary condition may be estimated by solving an inverse heat conduction problem (IHCP). However, the fact that the rate of phase transformation and the rate of heat extraction are interrelated has prompted the use of probes fabricated with materials that do not transform upon quenching. Unfortunately, the thermophysical properties of such materials are not the same as those of heat treatable steels which hinders the use of the estimated boundary conditions for modeling purposes. To correct this deficiency, a series of quenches were conducted with instrumented AISI 4140 steel probes from temperatures below the austenitizing temperature in a fludized bed (alumina-air at room temperature); a probe was also quenched from the recommended austenitizing temperature.

Using the measured thermal responses the surface heat flux, as a function of surface temperature, was estimated for all experiments, generating a family of curves. In the region of the austenite-martensite transformation all curves have very similar values. Therefore, it is possible to build a composite curve that may be effectively used in mathematical models of quenching processes in a fludized bed. This methodology should be further explored to assess its applicability to quenching in vaporizable media.