A STUDY OF THE HEAT-TRANSFER COEFFICIENT AS A FUNCTION OF TEMPERATUREAND PRESSURE

The accuracy of thermal-mechanical models of the hot/warm forging proc ess depends on the proper description of the boundary conditions. At t he hot workpiece-cold die interface this requires knowledge of the hea t-transfer coefficient. A method for the determination of the heat-tra nsfer coefficients in bulk metalforming processes is presented. The te chnique consists of two steps. The first involves measuring the temper ature distributions within two dies, one of which simulates the cold f orming tool and the other the hot workpiece. The dies are brought into contact under closely controlled conditions. The second step makes us e of the finite-element simulation of the resulting heat-transfer prob lem. The interfacial heat-transfer coefficient is treated as an unknow n variable and is determined using a non-linear optimization technique , forcing the measured and the computed temperature distributions to b e as close as possible. Examples of the interfacial heat-transfer coef ficients as functions of the time of contact are given for different s urface temperatures and interfacial pressures for 303 stainless steel. An empirical relationship is then developed, giving the coefficient o f heat transfer as a function of time, temperature and interfacial pre ssure. The predictive capability of the relationship is substantiated by comparing its output to data obtained with the original values of t he coefficient. The empirical relationships may be of use in the plann ing of hot-forming processes when the contact times are in excess of h alf of a second.