Microstructure evolution of carbide-free bainitic steels under abrasive wear conditions

Abstract

The idea of carbide-free bainitic (CFB) microstructure, a mixture of bainitic ferrite and retained austenite (RA), in high-silicon steels has been recently thoroughly investigated by numerous researchers. In this research, two medium-carbon steel grades were tested in a wear tumbling machine to investigate microstructural changes under wet sliding abrasive wear conditions and to benchmark their performance against conventional tempered martensitic steels. The nature and type of defects responsible for material removal were analysed using scanning electron microscopy. To observe microstructural evolution of heavily deformed top layers of test specimens, special layer-by-layer X-ray diffraction methodology was applied. This technique allows accurate quantification of the volume fraction of RA transformed into untempered martensite at different depths from the worn surface. The results suggest that better performance is achieved in specimens heat treated to carbide-free bainitic microstructure containing higher volume fraction of RA, however with blocks of RA thermally stable at room temperature. The improved wear resistance is due to the increased surface hardness caused by stress-induced transformation of RA into untempered martensite during wear, while maintaining good toughness in the subsurface zones, which prevent brittle cracking.