Hydrogen is known to cause premature failure in various steel infrastructures due to effects of embrittlement, which is particularly detrimental to ferritic steel structures such as pipelines and pressure vessels. Therefore, understanding the susceptibility of these steels to hydrogen embrittlement and the effect of various microstructures found in welds and heat-affected zones (HAZs) is critical for material selection. Here, we report results of mechanical measurements performed in air and in hydrogen of girth welds used in pipeline steels. Fracture toughness was found to be significantly reduced in base, weld and HAZ when measured in gaseous hydrogen. Charpy tests reveal a lower upper shelf energy (USE) of welds compared to base metal, despite exhibiting lower average hardness values for the weld regions, which may be caused by complex microstructure resulting from the welding process.
Conventional fossil fuels are not only a finite resource for the world's ever-growing energy needs, but they are also known to have a negative impact on the climate, which has presented new challenges among energy sectors to identify environmentally conscious solutions (Baykara, 2018). Hydrogen has long been considered a viable carbon-free option for ever-increasing societal desires to transform our energy infrastructure towards more renewable and alternative technologies (Mazloomi and Gomes, 2012). However, the effects of hydrogen-assisted damage mechanisms that result in the embrittlement of metals, particularly ferritic steels, significantly reduces their lifetime and is a persistent obstacle in designing and manufacturing reliable structural materials for use in energy storage and transportation applications. Ferritic steel pipelines used to transport natural gas are relatively cost effective and easy to manufacture, but are susceptible to embrittlement and subsequent structural failure due to fatigue, reduction in ductility, and fracture when exposed to hydrogen (Martin, et al., 2020).
Predicting lifetimes of pipelines can be difficult due to many variables including geography (terranean vs. offshore), composition of the gas being transported, and steel grades and welding processes involved in the manufacturing and joining pipelines in the field. Previous measurements on several pipeline steel grades in the presence of gaseous hydrogen have been performed to elucidate the behavior of the base metal fracture resistance (San Marchi, et al., 2010, 2011). However, it has not been until more recently that the effects of gaseous hydrogen on the fracture resistance of pipeline welds have been closely investigated (Ronevich, et al., 2021). Despite recent efforts to characterize the fracture resistance of modern and vintage pipeline welds, some inconsistencies remain, which may result from the complicated weld microstructure and processing. The welding processes, in particular, can result in a variety of local chemistries and microstructures, some of which are more susceptible to embrittlement than others. To maintain safe operation of pipelines for hydrogen and/or blended gas mixtures, it is critical to assess the weld qualification requirements when considering new pipeline materials and weld processes, particularly as it relates to fracture properties.
