A STUDY OF THE HEAT-TRANSFER COEFFICIENT AS A FUNCTION OF TEMPERATURE AND PRESSURE

The accuracy of thermal-mechanical models of the hot/warm forging process depends on the proper description of the boundary conditions. At the hot workpiece-cold die interface this requires knowledge of the heat-transfer coefficient. A method for the determination of the heat-transfer coefficients in bulk metalforming processes is presented. The technique consists of two steps. The first involves measuring the temperature distributions within two dies, one of which simulates the cold forming tool and the other the hot workpiece. The dies are brought into contact under closely controlled conditions. The second step makes use of the finite-element simulation of the resulting heat-transfer problem. The interfacial heat-transfer coefficient is treated as an unknown variable and is determined using a non-linear optimization technique, forcing the measured and the computed temperature distributions to be as close as possible. Examples of the interfacial heat-transfer coefficients as functions of the time of contact are given for different surface temperatures and interfacial pressures for 303 stainless steel. An empirical relationship is then developed, giving the coefficient of heat transfer as a function of time, temperature and interfacial pressure. The predictive capability of the relationship is substantiated by comparing its output to data obtained with the original values of the coefficient. The empirical relationships may be of use in the planning of hot-forming processes when the contact times are in excess of half of a second.