The high strength to weight ratios of ultra high strength boron steels (UHSS) make them attractive for automotive applications in view of increasing emphasis on improved crashworthiness and fuel efficiency. However, these characteristics of UHSS also introduce certain difficulties in forming complex shaped structural components and energy absorbing systems. Most UHSS components are therefore produced through hot forming processes. One positive aspect of this hot forming is the opportunity for simultaneous hardening through quenching directly after forming. The success of a hot forming operation relies on many different factors and the majority of problems may be attributed to friction and wear. High wear of the tools leads to increased costs as well as higher rejection rates for manufactured components. Friction affects the forming of the workpiece and too low or high friction can impair the quality of the end product. Despite the clear need for knowledge concerning the high temperature friction and wear behaviour of these materials, work carried out so far in this field is very limited. The work at LTU aims at enhancing knowledge in the field of high temperature tribology in general and for tool steel and ultra high strength boron steel pairs in particular. This paper presents in brief the high temperature tribological studies conducted by employing both a reciprocating sliding friction and wear tester as well as a unidirectional sliding pin on disc machine. The results have shown that the operating temperature influences the friction and that an increase in temperature leads to a decrease in friction. Wear of the tool steel increases when the temperature increases from 40 deg C to 400 deg C but a further increase in temperature to 800 deg C does not influence wear. The UHSS exhibited lowest wear at 400 deg C due to the formation of wear protective layers. A surface coating on the tool steel provided better protection against wear but increased friction. The UHSS sheets are sometimes provided with an Al-Si coating. The studies have shown that Al-Si coating of UHSS sheets reduces friction at elevated temperatures but induces higher wear on the tool steel. The Al-Si coated surface also undergoes significant morphological and compositional changes when exposed to elevated temperatures
Godkänd; 2008; Bibliografisk uppgift: CD-ROM; 20080625 (brap)