Fabrication and mechanical properties of micro/nano-crystalline layers in M50NiL carburized steel

Abstract

To enhance the wear resistance of carburized M50NiL steel, this study introduces a method for refining the grains in carburized layer to a micro/nano-scale. The method involves extending the carburization time to ensure the formation of large M7C3 carbides at grain boundaries, and adjusting the quenching temperature to facilitate the particle-stimulated nucleation (PSN). The process promoted the recrystallization of the residual austenite (RA), together with the subgrains unexpectedly discovered, successfully achieving the grain refinement. In this paper, the formation mechanism of recrystallization and subgrains are discussed based on interfacial energy model and first principles calculation. The results indicate that recrystallization is driven by the release of energy at the interface between M7C3 carbides and the matrix during quenching, while the formation of subgrain boundaries is due to the reduction of fault energy in the RA, which caused by the increase of C content, promotes the dislocation movement and entanglement. Furthermore, the effects on hardness and wear resistance of micro/nano-crystalline region (MNCR) were investigated. The results show that MNCR after complete carburizing heat treatment has abundant dispersed carbides and refined martensite, which led to an approximate 15HV1 increase in the hardness, and exhibited nearly a 50% reduction in wear loss.

Keywords

• Particle stimulated nucleation
• Micro nanocrystalline
• Wear resistance
• Interfacial energy
• Stacking fault energy