Subsurface characteristics of a Fe–0.4 wt%C martenstic steel abraded with nanoindentation and cross-sectional TEM techniques

Abstract

The present investigation examined the unidirectional abraded surfaces of a martensitic (0.4wt%C) steel with silicon (1.5wt%Si), chromium (1.5wt%Cr) and molybdenum (1.9wt%Mo) additions in order to elucidate the work hardening and softening near the surface layer caused by abrasion, particularly its relation to wear behavior. The abrasion testing was performed on a pin-abrasion apparatus, and a small pin of the specimen was ground on silica abrasive paper at an applied load of 2.1N and a sliding speed of 0.66m/s. The abraded surfaces were examined on a nanometer scale with a nanoindentation apparatus to evaluate the changes in nanohardness as the sliding time progressed. A cross-sectional transmission electron microscope (TEM) technique was also employed to clarify the structural changes in the region close to the abraded surface. The abrasion-induced work hardening with sliding time was observed in the case of chromium and molybdenum addition steels. A fine dispersion of molybdenum carbide (Mo2C) was observed in the surface of the molybdenum steel after abrasion. Mo2C precipitates at approximately 550°C, indicating that surface and near-surface temperatures exceeded the carbide formation temperature after abrasion-induced frictional heating. In contrast, the silicon addition steel softened, which seems to be caused by abrasion heating that leads to some tempering effects. Work hardening and softening, which are caused by abrasion-induced subsurface deformation and frictional heating, respectively, seem to take place simultaneously and thus counteract each other's effect. Metallurgical reaction, such as precipitation and temper by frictional heating, has been found to play an important role in controlling the wear characteristics of steels. The present article discusses the influence of the alloying element addition on the wear response of the martensitic steel.