Influence of the ultrasonic surface rolling process on stress corrosion cracking susceptibility of high strength pipeline steel in neutral pH environment

A newly developed surface enhancement technique, ultrasonic surface rolling processing (USRP), was applied on X80 pipeline steel. Results from scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and surface roughness testing indicated that the processing induced a plastic flow (120 mu m deep) and a nano-structured layer, with a 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. 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. The effects of grain size and surface roughness as well as the stress field on stress corrosion cracking susceptibility of X80 pipeline steel treated by USRP were investigated.