Abstract
The current work investigates the effect of the H2 partial pressure in a NG (natural gas) blend on the mechanical behaviour of A37, X52 and L485 ferritic-perlitic steels, widely used on existing pipeline networks. The NG composition was chosen as close as possible to the service conditions and hence includes CH4, gaseous hydrocarbons, CO2, H2S, THT, etc. The influence of those constituents in the gas blend, on the hydrogen embrittlement sensitivity of the three alloys is considered. Three H2 contents have been examined: NG + 25% H2, NG + 10% H2 and NG + 2% H2. The results are compared with testing performed in NG environment.
It is established that the material resistance to hydrogen embrittlement is dependent on the type of mechanical loading and of the presence of crack. This work addressed this aspect by considering three types of mechanical loading: low cycle fatigue, fatigue crack growth and fracture toughness testing.
The analysis of the resulting mechanical behaviour and the fracture surfaces leads to a discussion on the susceptibility to hydrogen embrittlement of the three alloys in a natural gas mixture with a given partial pressure of hydrogen. The resistance to hydrogen embrittlement, with and without the presence of a crack, in NG + 2% H2 was higher than for the other gas blend. The results are discussed based on the material microstructural features (i.e. grain size, inclusions,) and gas blend content, including impurities. Finally, the fatigue crack growth properties are compared to the master curves recently proposed in the ASME standards.
The results provide quantified data necessary to optimise the operating conditions (working pressure range, preventive maintenance) of a gas grid dedicated to the transport of a NG/H2 blend.