EFFECT OF MULTIPLE ULTRASONIC NANOCRYSTAL SURFACE MODIFICATION ON SURFACE INTEGRITY OF 304 STAINLESS STEEL

The ultrasonic nanocrystal surface modification(UNSM)technique can build a strengthened layer with good surface integrity on the metal surface,and this layer improves the mechanical properties of metals.The effect of multiple UNSM on the surface integrity of 304 stainless steel was investigated,and a good strengthened layer was obtained.The surface of 304 stainless steel sample was strengthened by multiple UNSM technique,and the strengthened layers with different depths were prepared on the surface of the material.The hardness gradient,residual stress gradient,microstructure gradient and surface morphology of the sample were measured by using the microhardness tester,X⁃ray residual stress analyzer,3D morphometer and metallographic microscope.The results show that the grain size of 304 stainless steel is obviously refined after multiple UNSM treatment,and the thickness of the strengthened layer increases with the increase of treatment times.Comparing with the untreated sample,after two times of UNSM treatment,the residual stress on the surface of the sample increased from-156 MPa to-425 MPa,the depth of residual compressive stress reached 0.7 mm,and the surface microhardness increased from 185 HV0.2 to 350 HV0.2.The results show that the UNSM technology can build a deeper strengthening layer,but it needs to be optimized.