Evolution of sub-surface microstructure under linear reciprocating wear of nanostructured bainitic steel

Nanostructured bainitic steels have high strength as well as a combination of significant ductility, fracture toughness and sub-critical fatigue crack growth life. Hence, they can be potentially useful in structures suffering rolling and/or sliding fatigue e.g. rails, mining components, bearings and shafts. However, such applications require high wear resistance which is not solely dependent on the initial strength since repetitive sliding often leads to sub-surface microstructural evolution that alters the tribological response with time. In the current study, three nanostructured bainitic blocks with lath thickness of 45, 54 and 77 nm have been made from steel of composition: Fe-0.86C-1.45Si-1.8Mn-0.47Al-1.88Co-0.23Mo (wt%) by austempering at 250, 300 and 350 degrees C respectively. We have investigated the friction and microstructural evolution under reciprocating sliding motion of a 10 mm spherical tungsten carbide indenter at 10 kN load and correlated the findings with the initial morphology of bainite. All specimens exhibited hardening after multiple cycles of deformation. However, the steel austempered at the lowest temperature showed the highest sub-surface hardness and lowest friction coefficient and was the most resistant to wear when tested under sliding fatigue for up to 500 cycles. Repeated sliding resulted in formation of two kinds of layers: white etching layer (WEL) and severely deformed layer (SDL). WEL exhibited very fine microstructure whereas the SDL showed alignment of retained austenite and bainitic laths along the direction of highest shear stress. There was a predominant presence of WEL in the specimens austempered at the lowest temperature and SDL in the specimens transformed at the highest temperature. Atomic probe tomography (APT) was used to investigate the distribution of carbon in the deformed layers just below the wear track after 500 sliding cycles for 250 and 350 degrees C austempered condition. No significant change in carbon has been detected in any of the austempered specimens after wear.

Automated summary

Nanostructured bainitic steels exhibit high strength, ductility, fracture toughness, and fatigue life, making them suitable for structures under rolling and/or sliding fatigue, but they require high wear resistance; in sliding fatigue tests, steel austempered at lower temperature demonstrates the highest wear resistance, with the formation of two distinct layers.