Fracture Toughness and Fatigue Crack Growth Characteristics of X65 Linepipe and the Girth Welds Under Hydrogen Gas

Abstract Hydrogen is considered a crucial energy resource, with demand forecasted to rise for a carbon neutral society. Pipelines may be an efficient way to transport large quantities of gaseous hydrogen. However, carbon steel pipes, used for natural gas, are known to be susceptible to hydrogen embrittlement. Many researchers have investigated the mechanical properties of various carbon steels in pressurized gaseous hydrogen. Most of these studies have focused on base metals, with few specializing in welded joints, particularly on-site circumferential welds. In our previous research, we investigated tensile behavior of the API 5L X65 grade linepipe material and the girth weld under several pressurized gaseous hydrogen environments. Additionally, we evaluated the fracture toughness for both materials using JIC tests at 50 °C in accordance with ASTM E1820. In this paper, we continuously evaluated the influence of temperature on fracture toughness and fatigue crack growth characteristics of API 5L X65 under pressurized gaseous hydrogen. For evaluating fracture toughness, JIC tests were conducted under various hydrogen gas pressures and temperatures. As a result, fracture toughness decreased significantly with increasing hydrogen gas pressure up to 10 MPa, then remained almost constant at higher pressure. The tests also indicated that high temperature at 50 °C can lead to more severe hydrogen embrittlement compared to low temperature at −10 °C. In addition, the KJIH and KJQ values converted from the J values satisfied the specified values of 55 MPa · m1/2 in ASME B31.12 standard. Fatigue crack propagation tests were conducted under 10 MPa hydrogen gaseous environment at −10 °C. The results showed that the fatigue crack growth rate (FCGR) at 10 MPa hydrogen pressure became ten times faster compared with nitrogen gas at atmospheric pressure (0 MPa of partial hydrogen gas pressure). This FCGR is slightly slower than the design curve in ASME B31.12 standard. Moreover, fatigue crack growth analysis was conducted based on the FCGR curve. The fatigue life can be estimated to exceed one million cycles, providing a sufficient lifetime for pipelines.