Surface modification of 40Cr steel subjected to composite strengthening grinding process

The microstructure state of machined superficial layer materials has a crucial influence on the in-service functionality, longevity and overall performance of mechanical components. Grind-strengthening technology coupled with the precision machining and surface quenching heat treatment process can be known to produce a gradient microstructure modification with grain refinement and phase transformation under strong thermal mechanical coupling. An in-depth understanding of the strengthening mechanism is a significant aspect for balance the strength and toughness of the transmission part surface. In the present work, the grinding temperature and strain distribution of the ground strengthening layer (GSL) were predicted and characterized based on the analytical models. Moreover, the microstructure of the ground-modified layer was characterized by an optical microscope (OM) and scanning electron microscope (SEM). Gradient distributions of prior austenite grains and martensite structures are recognized. The penetration depth profiles of the phase transformation white layer were obtained. Furthermore, the grinding phase transformation strengthening mechanism was explained. The gradient microstructures of GSL are quantitatively analyzed, which is composed of the grain refinement layer caused by dynamic recrystallization and severe plastic deformation (SPD). Consequently, the findings show that the grind-strengthening technology utilizes grain refinement and phase transformation mechanisms to create a strengthened layer of graded martensite structure with high micro-hardness and residual compressive stress, which has the potential to provide solutions for mismatch of strength and toughness and improve the wear resistance and dynamic static contact performance of the parts.