Influence of Ultrasonic Nanocrystal Surface Modification Treatment Temperature on Microstructure and Tribological Behavior of Heat-treated and Non-heat Treated Bearing Steel

A nanostructured surface layer was successfully produced in heat-treated and non-heat treated AISI 52100 bearing steel for bearings in rotating machinery using an ultrasonic nanocrystalline surface modification (UNSM) technique at high temperatures (up to 500 0C). It is known that the structure of unhardened bearing steel is in the pearlite state. When bearing steel is heat-treated its structure transforms from pearlite to austenite. Only after quenching its structure then transforms from austenite to martensite which is very hard to be used for bearings. Hence, the aim of this study is to investigate the microstructural, tribological and mechanical properties of the heat-treated (martensite state) and non-heat treated (pearlite state) specimens and to evaluate if the non-heat treated specimens subjected to high-temperature UNSM treatment can be used for bearings without heat treatment. In this study, the tribological properties of heat-treated and non-heat treated bearing steel specimens were assessed using a ball-on-disk tribo tester against bearing steel ball. The mechanical properties, in particular hardness and compressive residual stress with respect to depth from the top surface were measured. Scanning electron microscopy (SEM) characterization revealed that the severe plastic deformation introduced by UNSM technique resulted in nano-sized grains in the topmost surface layer. The findings from this preliminary study are expected to be implemented to the bearing industry to increase the efficiency and performance of bearings by improving the tribological properties and modifying the microstructure of bearing steel.