High temperature tribology of high strength boron steel and tool steels

Abstract: There are many tribological interfaces that are exposed to elevated temperatures. Typical examples are the interfaces of various moving assemblies, for examples in aerospace industry, power generation and metalworking processes. The exposure of materials to elevated temperatures results in highly complex interfaces due to changes in morphology, microstructure and mechanical properties coupled with the occurrence of oxidation and diffusion. All of these changes will influence the tribological behaviour of materials at elevated temperatures. Another major concern is lubrication at elevated temperatures since conventional lubricants do not perform at temperatures above ~300ºC. High strength steels are commonly used as structural reinforcements or energy absorbing systems in automobile applications due to their favourable strength to weight ratios. The high strength of these steels leads to several problems during forming such as poor formability, increased spring back, and tendency to work-harden. In view of these difficulties, high strength steels are usually formed at elevated temperatures with a view to facilitating forming and simultaneous hardening by quenching of complex shaped parts. A review of published literature has revealed that only a few studies pertaining to high temperature tribology (including those of hot metalworking) have been carried out so far. The understanding of the high temperature tribological behaviour of high strength steels and tool steel pairs is also highly inadequate. The aim of this work is therefore to obtain a better understanding of the friction and wear mechanisms of tool steel and high strength boron steel tribological pairs at elevated temperatures. The experimental studies were carried out by using a high temperature version of the Optimol SRV reciprocating friction and wear test machine. The tribological studies were performed at temperatures ranging from 40ºC to 800ºC. The experimental materials were tool steels of three different alloying compositions (with and withoutnitriding) and high strength boron steel (unhardened, hardened, with and without Al-Si coating). The results have shown that both friction and wear of tool steel and high strength steel pairs are temperature dependant. An increase in temperature has resulted in lower friction for all the material pairs. Tool wear increased when the temperature increased from 40 to 400ºC during sliding against uncoated high strength steel but remained unchanged when the temperature increased further to 800ºC. When sliding against Al-Si coated high strength steel, tool wear increased with increasing temperature. Plasma nitriding of tool steels has been effective in reducing friction as well as in providing protection against severe adhesive wear. The Al-Si coating on the high strength steel has resulted in high friction at low temperatures and low friction at elevated temperatures. It has also shown an increased wear resistance at elevated temperatures. The coating undergoes significant surface morphological changes when exposed to elevated temperatures which are likely to influence its tribological behaviour. Hardening of the high strength steel has resulted in decreased friction at all temperatures. It led to higher tool wear at low temperatures and lower tool wear at elevated temperatures