Corrosion fatigue crack growth rates of X70 base metal and heat affected zone in H2S corrosive environment

Abstract

Fatigue crack growth rates (FCGRs) of X70 pipeline steel base metal (BM) and heat affected zone (HAZ) were studied in the hydrogen sulphide environment. X-ray diffraction method was used to measure the residual stress of BM and HAZ. Modified wedge open loaded specimen was selected for the test in the simulated actual working conditions. In the saturated H2S environment, the crack growth experiments were carried out under low frequency (f = 0·01 Hz) and three stress ratios (R = 0·7, 0·8 and 0·9). Results showed that there was obvious residual tensile stress in HAZ. The presence of H2S greatly accelerated the FCGR. With rising stress ratio, crack closure effect weakened, and then the increasing degree of hydrogen embrittlement of the crack tip leads to FCGR acceleration. Microstructure defects, inclusions and high residual tensile stress of HAZ made the FCGR of this area higher than the FCGR of BM. Scanning electron microscopy scans of the fracture suggested that the fracture mode of the corrosion fatigue of the X70 material belonged to brittle fracture. Cleavage fracture was the dominant form. The differences between corrosion fatigue fracture morphologies of BM and HAZ were not very large, and the relief of the fracture of HAZ was larger and the cracks longer.