Characteristics of surface and subsurface of formed thread parts by axial-infeed thread rolling process

The thread rolling process has been widely applied to manufacture high-performance thread parts. In this process, the evolutions of surface and subsurface are frequently introduced, which affect the working performance of manufactured parts. In this study, an axial-infeed thread rolling process (ATRP) is employed, and the macro-meso surface characteristics under different lubrications and operating conditions are investigated. Moreover, the distributions of microstructure and hardness on the subsurface of formed tooth are analyzed in detail, along with the study of stress state and yield strength change. It is found that the MoS₂ grease is more effective in reducing the surface roughness and defects than the lubrication oil and water-base graphite during the ATRP process. Increasing rolling speed improves the quality of surface morphology and can reduce the surface roughness. On the subsurface of bottom and flank, intensive shear stress occurs in a narrow region, resulting in the elongation and refinement of the grains and increasing the low angle grain boundary fraction. Based on the grain size and plastic strain, the yield strength is predicted. The maximum yield strength and hardness on the bottom of formed tooth are improved by 41.2% and 39.4%, respectively.