Gaseous Hydrogen Embrittlement in API X65 Pipeline Steel: Effects of Environmental and Mechanical Factors

Hydrogen transportation is a critical aspect of hydrogen technology, particularly the delivery of large quantities to end-use locations. This is often achieved by utilizing existing gas pipeline infrastructure and blending hydrogen with natural gas. However, this approach increases the risk of hydrogen embrittlement (HE), leading to early material failure and fractures. To mitigate these risks and ensure safe pipeline operation, it is essential to understand the environmental and mechanical factors influencing embrittlement. The primary objective of this thesis is to experimentally investigate gaseous HE in API X65 pipeline steel under varying test conditions. A slow strain rate testing (SSRT) setup for hollow samples was designed to facilitate these experiments. The material was tested at strain rates of 10−5, 10−6 s−1 under hydrogen pressures of 1, 5, and 10 MPa. To better simulate blended gas environments, additional tests were conducted at hydrogen blending ratios of 10%, 15%, and 100% at a strain rate of 10−6 s-1. The findings, presented in Chapter Four, explore the effects of pressure on tensile properties and fractographic behavior, revealing that higher pressures exacerbate hydrogen embrittlement, particularly at slower strain rates. Additionally, the influence of strain rate on embrittlement mechanisms and plasticity is examined, with slower strain rates amplifying embrittlement and microscopic analyses—such as scanning electron microscopy, electron backscatter diffraction, and X-ray diffraction—showing reduced plasticity at higher pressures. Finally, the impact of blending ratio on hydrogen embrittlement behavior is assessed, demonstrating that increased hydrogen content in methane mixtures correlates with heightened embrittlement and plasticity reduction, with the effect most pronounced in pure hydrogen environments. Overall, the results demonstrate that hydrogen embrittlement intensifies with higher gas pressure, slower strain rates, and increased hydrogen blending, providing valuable insights into safe pipeline operation for hydrogen transportation.