Susceptibility of a X80 steel to hydrogen embrittlement

The susceptibility to hydrogen of a X80 grade steel produced by a thermo-mechanical control process (TMCP) has been investigated by keeping straining notched specimens under continuous charging conditions. Hydrogen charging was carried out either in synthetic seawater under potentiostatic control at
1000 mV vs. SCE or in sulphuric acid with an absorption promoter under galvanostatic control at
5 mA/cm2. Results reported in terms of hydrogen effect on the ductility of the steel as a function of both cross head speed and root radius of the notch indicate that under the combined effect of cathodic charging, notch severity and very low strain rates the ductility of the TMCP X80 steel can be greatly affected by the presence of hydrogen. With notched specimens strained in air increasing loss of ductility in terms of reduction in area is observed as the notch severity increases. Notched specimens are fairly “more brittle” than smooth ones. As notched specimens are strained under cathodic charging at
1000 mV vs. SCE in the synthetic seawater, considerable decrease of reduction in area (RA) is observed. The same trend is observed for displacement and load at fracture both being connected with ductility even if a definite tendency is not always obtained. As the notched specimens are strained under cathodic charging in seawater the fracture morphology shows regions of mixed ductile and brittle fracture and zones where intergranular and/or transgranular fracture path are prevailing. Area of intergranular and transgranular fracture path, that can be more strictly associated with the presence of hydrogen, tends to increase as the strain rate decreases, which suggests a fracture behaviour influenced by hydrogen diffusion. Several mechanisms were involved in the rupture process in sulphuric acid depending on the notch geometry and, especially, on the cross head speed. Apparently, transgranular (quasi-cleavage) rupture tends to prevail as the displacement becomes lower and lower. No evidence of intergranular fracture was observed. High strength low alloy steels (>X70) are widely used for long distance gas transmission pipeline applications where high internal pressures are expected to operate (up to 75 bar and higher). However these steels can be susceptible to hydrogen-induced crack growth. Thus, understanding the effects of hydrogen on crack growth susceptibility is critical for a safe selection and use of steels in hydrogen and hydrogenproducing environments. Among the several hydrogenating environments, those originated by sour “transported” fluid of medium aggressiveness and by localised cathodic over protection covering the range of situations in which the steel is expected to operate are of main concern. Hydrogen damaging mechanisms have been extensively studied for H2S containing (sour) environments and several metallurgical requirements are quite well established [1, 2]. A more complex pattern concerns the mechanisms of failure involved in hydrogen embrittlement (HE) phenomena because of the combined interactions of several factors such as the amount of hydrogen inside the steel, the state of stress, the