Influence of surface nanocrystallization pretreatment on high-temperature vacuum carburizing behavior

The supersonic fine particles bombarding (SFPB) pretreatment is adopted to assist vacuum carburizing for refining the microstructure and improving the mechanical properties of carburized layer. The microstructures, micrhardness and tribology properties of SFPB carburized sample and vacuum carburized sample were studied. The results showed that SFPB pretreatment could form random oriented, ultrafine-grained microstructure and high density dislocations in AISI 9310 steel, leading to the increase of case depth of carburized layer, the refinement of grain size and the promotion of the formation of twin martensite and carbides in carburized layer. Besides, the surface microhardness, maximum residual compressive stress and wear rate of carburized layer were enhanced by SFPB pretreatment, which was attributed to the formation of twin martensite, grain refinement, dispersion strengthening of carbides and the increase of high misorientation angle grain boundary.