Stress corrosion cracking of gas pipeline steels of different strength

With the development of the natural gas industry, gas transmission pipelines have been developed rapidly in terms of safety, economy and efficiency. Our recent studies have shown that an important factor of main pipelines serviceability loss under their long-term service is the in-bulk metal degradation of the pipe wall. This leads to the loss of the initial mechanical properties, primarily, resistance to brittle fracture, which were set in engineering calculations at the pipeline design stage. At the same time stress corrosion cracking has been identified as one of the predominant failures in pipeline steels in humid environments, which causes rupture of high-pressure gas transmission pipes as well as serious economic losses and disasters.In the present work the low-carbon pipeline steels with different strength levels from the point of view of their susceptibility to stress corrosion cracking in the as-received state and after in-laboratory accelerated degradation under environmental conditions similar to those of an acidic soil were investigated. The main objectives of this study were to determine whether the development of higher strength materials led to greater susceptibility to stress corrosion cracking and whether degraded pipeline steels became more susceptible to stress corrosion cracking than in the as-received state. The procedure of accelerated degradation of pipeline steels was developed and introduced in laboratory under the combined action of axial loading and hydrogen charging. It proved to be reliable and useful to performed laboratory simulation of in-service degradation of pipeline steels with different strength. The in-laboratory degraded 17H1S and X60 pipeline steels tested in the NS4 solution saturated with CO2 under open circuit potential revealed the susceptibility to stress corrosion cracking, reflected in the degradation of mechanical properties, and at the same time the degraded X60 steel showed higher resistance to stress corrosion cracking than the degraded 17H1S steel. Fractographic observation confirmed the pipeline steels hydrogen embrittlement caused by the permeated hydrogen.