Investigation of hydrogen induced cracking susceptibility of API 5L X65 pipeline steels

Abstract

The service environment of oil and gas pipelines often subject them to harsh conditions, such as, very low temperatures, microbial attack, acidic or sour environments etc. These conditions can trigger different failure mechanisms such as corrosion, embrittlement and cracking. The hydrogen induced cracking (HIC) phenomenon often affects pipelines exposed to sour and acidic environments. It involves the ingress of hydrogen atoms into the steel, which leads to the degradation of its mechanical properties such as, strength and ductility as well as embrittlement and cracking of the steel.

This study aims at relating the microstructural and hydrogen diffusion characteristics of steels to their cracking behaviour, and identifying the important microstructural characteristics that influence their hydrogen assisted cracking behaviour. Two X65 pipeline steels were used for this investigation. Firstly, the steels were characterized using optical microscopy, electron backscatter diffraction (EBSD) and mechanical tests. Then, the diffusion and trapping parameters of the steels were evaluated using hydrogen permeation experiments, while the hydrogen microprint technique was used to ascertain the important pathways for hydrogen transport through the steels. Finally, electrochemical hydrogen charging was used to generate cracks in the steels. This facilitated the HIC analysis of the as-received and heat-treated steels, in relation to their microstructural characteristics.

After hydrogen charging, microstructural evaluation revealed cracks at the mid-thickness (segregation zone) of both steels. The cracks initiated and propagated mainly through Si-enriched inclusions and complex precipitates, which are believed to have facilitated crack propagation. However, more severe cracks were observed in plate 1, which had a more strained microstructure, while plate 2 which had a less deformed microstructure and a higher fraction of recrystallized grains, was less severely cracked. The cracking behaviour observed in the steels was related to their hydrogen diffusion and trapping characteristics. The results confirmed that a higher ratio of reversible to irreversible hydrogen trapping sites contributes to increasing HIC severity in steels. Additionally, the hydrogen transport through the investigated steels was most prominent along the grain boundaries, thus, indicating the importance of grain boundary character and distribution to HIC. Finally, the change in microstructure of the heat-treated steels clearly influenced their cracking behaviour. The result confirmed that important factors such as the dislocation density, fraction of deformed and recrystallized grains and grain size distribution are directly related to the resistance of steels to HIC failure.