Warm and Hot Upsetting Sliding Test: Tribology of Metal Processes at High Temperature

Abstract

Identification of friction data at high temperatures is still a critical point in the optimization of forming processes. In the present paper the WHUST (Warm and Hot Upsetting Sliding Test) dedicated to the tribology of metal forming processes at high temperature is presented. The friction test is able to operate according to a large range of plastic strains, contact pressures and temperatures. The methodology to use the friction test to respect contact conditions encountered at the tool/workpiece interface of various industrial forming processes is discussed. Then three designs of experiments are achieved. The first set of experiments is performed to study the effects of the composition of graphite based lubricants on friction of hot forging tool. A particular attention is paid on the volume of lubricant used, the size of graphite particles and the amount of binding agents. The second set of experiments is performed to quantify the ability of two white lubricants to reduce friction and protect tool surface. The first lubricant is a mineral salt; the second one is an organic salt. Tests are performed at 1200°C on AISI 4820 steel specimens, with AISI H11 nitrided steel contactors. Results show the tested white lubricants lead to coefficient of friction in the same range of the graphite lubricant one, but white lubricants lose their ability to reduce friction as soon as the sliding lengths becomes greater to 10mm, where graphite lubricants can undergo sliding length greater than 30mm. The last experiments are performed to qualify a new sol-gel coating deposited on a common tool steel AISI H13. Tests are operated at 1100°C on AISI 4820 steel specimens. After each test, the ability of the coating to reduce friction and prevent metal transfer from specimens to the coated tool surfaces is established.