Coupling surface vibratory extrusion and carburizing heat treatment to improve the surface microstructure and mechanical properties of powder metallurgy steel

Surface integrity is critical to the reliability of powder metallurgy steels in service environments. However, surface densification, although necessary for structural support, reduces the pore-assisted pathways available for carbon transport during carburizing, making it difficult to achieve both a dense surface layer and effective thermochemical strengthening. To address this issue, a coupled processing route consisting of surface vibratory extrusion and carburizing heat treatment was proposed for Fe-Mo-Cu-C powder metallurgy steel. Surface vibratory extrusion produced a 620 μm densified layer and promoted continuous dynamic recrystallization, generating a high density of grain boundaries and dislocations. These microstructural features increased the effective diffusion coefficient by about 51 % compared with steady-state extrusion. The treated surface developed high-carbon fine-grained martensite and fine dispersed carbides, increasing the surface peak hardness to 882 HV 1.0 , corresponding to a 3.95-fold increase over the untreated state, and extending the effective case depth to 1265 μm. The carbon concentration gradient also contributed to a surface compressive residual stress of 277 MPa. Tribological investigations showed improved friction and wear behavior through a transition from severe pore-induced spalling to mild abrasive wear with frictional oxidation. These results provide useful guidance for designing surface strengthening processes for high-performance powder metallurgy components.