The Effect of die Temperature on Metal Flow and die Wear During High-Speed Hot Forging
This paper describes the results of tests carried out on a Mk IIA Petro-Forge high-speed hammer in order to determine the effects of the bulk die temperature on the die-wear rate during simple upsetting of hot steel (EN8) billets. Variations in the bulk die temperature are achieved by changes in cycle time and dwell time. Each test consists of upsetting 1000 billets to a reduction of 75 per cent, both with a colloidal graphite lubricant and dry, after which the dies are measured for wear with the aid of a Talysurf.
The frictional conditions at the die-billet interface are studied with the aid of the ring test, in which annular billets are upset between lubricated dies to various levels of deformation from 0 to 65 per cent. The amount of lubrication applied is the same as in the wear tests. Four die-temperature levels are used in order to simulate the interface temperature conditions appertaining in the wear tests.
The results of the wear test indicate that within the ranges tested cycle time has only a marginal influence on die temperature and wear, whereas dwell time has a significant effect on both temperature and wear. It is deduced from these results that the real independent parameter influencing wear is die temperature, dwell time and cycle time affecting wear only through changes in die temperature. The main conclusions are that under lubricated conditions wear is inversely proportional to bulk die temperature (and dwell time), whereas under dry conditions the reverse is true.
The evidence of the flow pattern of etched billet sections and of the appearance of the dies after testing indicate that an increase of die temperature causes a decrease in the amount of billet sliding over the die surface. This is further supported by the results of the ring test which show that as the die temperature is increased so is the interfacial coefficient of friction.
Using the values of the coefficient of friction obtained from the ring test, predictions are made of the interfacial metal flow during simple upsetting for various die temperatures. From these predictions theoretical wear profiles at each die temperature are constructed in arbitary units. These profiles show a good qualitative agreement with experimentally obtained profiles. Finally, the wear volume (in arbitary units) is obtained from the theoretical profiles at each die temperature. It is shown that the theoretical rate of change of wear volume with die temperature is very close to the experimentally derived ratio. Therefore it is concluded that the reduction in wear with increase of die temperature (and of dwell time) is mainly attributable to the change in the interfacial flow pattern.
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