Frictional effects on flow stress determination at high temperatures and strain rates

Abstract

It has been reported recently that the average die pressure as measured during the initial compression of non-lubricated cylinders was less than 5% greater than that for lubricated samples, when the final diameter-to-height ratio is less than 2. These tests were carried out at room temperature and low strain-rates. To determine whether these results occur at high temperature and high strain-rates, cylinders of hot-rolled low-carbon steel were upset under lubricated and non-lubricated conditions. The steel samples were deformed by a cam plastometer at constant true strain-rates of 2.0, 20.0 and 140.0 s −1. The results clearly show that the average die pressure for lubricated and non-lubricated samples are, within experimental accuracy, equal. Further testing demonstrated that the average die pressure does not depend on sample geometry for cylinders with initial aspect ratios of 0.5, 0.65 and 0.85, deformed to a true strain of 0.7. For the range of aspects ratios examined, it can be concluded that a correction for friction in the calculation of flow stress is unnecessary.