Second Paper: An Investigation into the Effects of Pre-Heat Temperature on Ductility in a Warm Heading Process

Abstract

Two-stage warm heading operations were carried out on a mild steel and three high tensile bolt materials (types 1, 2 and 4 from B.S. 3111:1959) at pre-heat temperatures in the range from ambient up to 600°C. The tests were carried out in the form of a transfer process (approximately 2 seconds delay between stages) and also as an interrupted process (approximately 30 minutes delay between stages). Artificial surface defects of various depths were machined on the heading specimens to aid measurements of the change in ductility with increase in pre-heat temperature. The method consisted of estimating the critical depth of groove which would be just on the point of fracture at the end of a particular heading operation. The steels tested were found to have low ductility at pre-heat temperatures in the range from 200°C to 300°C; at a pre-heat temperature of 600°C the steels exhibited extremely high ductility. The results suggest that rolled bar of relatively poor surface quality could be warm headed at 600°C without fracturing. A solid film of colloidal graphite and a high chlorine oil were both found to be satisfactory lubricants at all specimen pre-heat temperatures up to 600°C.