The Research Of Metallic Surface Nanocrystallization By Ultrasonic Rolling Extrusion Processing

In our research, the TJUH-I ultrasonic surface rolling extrusion (USRE) processing treatment equipment that was invented by ourselves was applied to the treatments of 40Cr steel and the other kinds of metal. The microstructures of the sample surface and sub-surface were studied by HRTEM. The mean grain size and residual stress of the treated sample were calculated from XRD. A scanning electron microscope (SEM) (XL30ESEM) was used to examine the micro-structural development along the section perpendicular to the treated surface of the specimen. Roughness of the surface was measured by high-resolution surface roughness measuring instrument. Micro-hardness tester was used to determine micro-hardness variation along depth. Front fasten tip was improved by sintered-carbide ball that can roll freely so that the wear of tip could diminish obviously.Our work draws those following conclusions:1) A nano-crystallised layer which is 150μm thick is realized on quenched and tempered 40Cr by ultrasonic surface rolling extrusion processing (USRE). After USRE treatment, the initial coarse-grained structure in the surface layer was refined into equiaxed ultrafine grains(4.65-7.39nm) with random crystallographic orientations.2) After USRE treatment, the surface roughness can reduce to nano-scale of 50nm. Meanwhile, Micro-hardness tester measurements indicate that hardness is enhanced in the nano-crystallised surface layer relative to the matrix. As the depth increases, the micro-hardness decreases dramatically.3) This process induces large residual compressive stress and the thickness affected by USRE is about 1300-1400μm. The magnitude of compressive residual even reaches the material yield strength at some depth, which is considered to have great relationship with material mechanical property and stress state.