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.