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 H 2 S environment, the crack growth experiments were carried out under low frequency (f50?01 Hz) and three stress ratios (R50?7, 0?8 and 0?9). Results showed that there was obvious residual tensile stress in HAZ. The presence of H 2 S 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.

Prediction of Corrosive Fatigue Life of Submarine Pipelines of API 5L X56 Steel Materials

“…As the global sustainable development strategy becomes the mainstream, offshore natural gas, which is considered as a clean energy, has attracted the attention of various countries. Therefore, many researchers have studied the FCG rate and the cracking behavior of pipeline steels in different gas environments [9,10,11,12,13,14]. Wang et al [9] studied the FCG rate of the base metal and the heat affected zone of API 5L X70 pipeline steel under a hydrogen sulphide environment, and they found that the existence of H 2 S greatly accelerated the FCG rate.…”

Fatigue Testing of Pipeline Welds and Heat-Affected Zones in Pressurized Hydrogen Gas

“…For this reason, the fatigue properties of the welds, as well as those of the base metal, must be characterized. There is relatively little known about the FCGR of pipeline weld materials [15] and their associated heat-affected zones (HAZs) in the presence of hydrogen [16][17][18][19][20], and even less is known about their FCGRs under conditions of pressurized hydrogen gas [16,18]. All studies indicate that the FCGR of the weld fusion zone and/or HAZ is at least an order of magnitude higher in the presence of hydrogen than in air at values of the stress intensity range (ΔK) greater than 15 MPa•m ½ .…”

“…Differences among base metal, weld metal, and HAZs tested in the presence of hydrogen are more subtle. Wang et al [20] concluded that the X70 base metal exhibited a lower FCGR than the HAZ from a submerged arc weld of two plates for the same conditions, where the specimens were subjected to saturated H2S in solution. Thodla et al [19] concluded that the girth weld exhibited more fatigue resistance than either the base metal or the HAZ acquired from X65 welded pipe when tested in an H2S-saturated solution.…”