Contact stress and temperature during air-stamp hammer upsetting of a circular cylinder

Air-stamp hammer forging is a traditional hot-forging method. This method is mostly used based on the operator's skill and not on the measurement of the contact stress condition. In this study, we measured the contact pressure and friction on the tool surface during the upsetting of a circular cylinder of hot steel using an air-stamp hammer. For the upsetting process, we installed friction sensors that could detect the pressure and friction stress on the tool surface on the flat die. A tool surface temperature sensor with a thin thermocouple was also installed 0.3 mm under the tool surface. During the upsetting process, the pressure near the center increased with the increase in the ratio between the diameter and the height. The friction was higher at greater distances from the center than it was near the center. We investigated the relationship between the pressure and friction distributions at the end of the stamping when the material had a high diameter-to-height ratio; this relationship was compared using elemental analysis. The results show that the contact-stress behavior observed during hot forging was similar to that observed in cold forging without a lubricant. The tool surface temperature rapidly increased during stamping but also increased when the tool surface had barely touched the workpiece. The data obtained about the pressure and friction behavior will be useful for designing new forging processes, and the information about the effects of stress and temperature on the tool surface will be useful for investigating tool wear.