Assessment of temperature on the die surface in laboratory hot metal forming

The temperature in the die surface layer during hot forming is difficult to measure accurately with a thermocouple and is still the subject of experimental and numerical modelling studies. In this paper a method of embedding the thermocouple just below the die surface, the application of such an embedded thermocouple in measurements of the die temperature during cyclic hot compression, and quantitative estimation of the temperature on the die surface is described, based on numerical extrapolation of measured data which were obtained by the use thermocouples embedded in the die. The results show that the applied “thermocouple just below the die surface–numerical extrapolation” method demands a precise knowledge of the distance of the thermocouple from the ideal die surface. The temperature on the die surface can be more easily assessed if it is slightly above the tempering temperature of the die material and if the method of “multiple contact” is applied. It is essential that no wear occurs. Thus, measurements of microhardness values can indicate to what depth of the embedded thermocouple the measured values are related. Exact calibration of the surface temperature demands additional indirect methods.