Effect Of Ultrasonic Surface Rolling Process On Mechanical Characteristics Of Stress Corrosion Cracks In Pipeline Steels

As a new surface-treatment method for surface nanocrystallization,USRP has been widely applied to various fields.It has been found that USRP can synthesize nanostructured surface layer,improve the mechanical property,increase the fatigue strength and wear resistance of different metals.Theoretically,the USRP technology can be used to treat the weld joints of X80 pipeline steel to increase the comprehensive properties and decrease the stress corrosion crack susceptibility,which is of great importance for the application of X80 pipeline steel.Firstly,A newly developed surface enhancement technique,ultrasonic surface rolling processing(USRP),was applied on X80 pipeline steel.Scanning electron microscopy(SEM),transmission electron microscope(TEM),X-ray diffraction(XRD)and surface roughness experiment were used to test the microstructure,surface state and residual stress field.Meanwhile,four point bending(FPB)experiment was also used to compare the stress corrosion susceptibility of X80 steel.The effect of grain size,surface roughness and residual stress on the SCC susceptibility of X80 steel.Besides,the coupling model of USRP and welding process was built by ABAQUS software.The effect of USRP parameters(pressure,displacement,friction coefficient,diameter of balls,etc)on residual stress and PEEQ in the surface of specimen was studied.The effect regularity of USRP on the surface residual stress and PEEQ of X80 welded joint was also analyzed.At last,the coupling model of USRP and FPB specimen with crack was built to simulate the FPB and USRP.The crack tip opening angle,crack tip opening displacement,residual stress field and J-integral of FPB specimen before and after USRP were compared and the mechanism of crack propagation depressed by USRP was discussed.Experimental results showed that the processing induced a plastic flow(60 ?m deep)and an nano-structured layer,with grain size of 40-100 nm.Surface roughness was reduced to 15%of the original specimen.Meanwhile,residual compression stress was achieved in the surface layer and the maximum value is about-296.07 MPa.The bent-beam stress-corrosion test showed that corrosion product scales of the USRP specimen were more compact together with a reduction in scale thickness.Compared with the original specimen,width of the stress corrosion cracks,roughness of the scale/steel interface and corrosion rate decreased after USRP.Result of numerical simulation showed that USRP on the surface of material leaded to plastic deformation and residual compressive stresses.Meanwhile,the residual compressive stresses decreased gradually to zero with the depth and transformed into tensile stresses.Peak value of residual compressive stresses and the thickness of compressive stresses layer increased with the displacement.The crack in the surface layer of FPB specimen closed after USRP.In addition,compared with the original specimen,the crack tip opening angle,crack tip opening displacement,residual stress field and J-integral decreased.