Influence of hydrogen and low temperature on mechanical behaviour of two pipeline steels

In the presence of H2S, metallic materials, such as carbon and low alloy steels, may suffer hydrogen damage and hydrogen embrittlement. Sour service materials must be used in compliance with international standards when fluids are classified as sour. Gas transporting pipes in low temperature environment, during the shutdown and the subsequent re-starting operations, are exposed to very low temperatures (T=-40°C). In presence of high H2S content in the gas the risk of brittle failure can be increased due to the effect of hydrogen on steel toughness. In this paper the influence of hydrogen and low temperature on mechanical properties of two pipeline materials, X65 micro-alloyed and F22 low alloy steels, is studied. An electrochemical hydrogen charging method has been setup, avoiding any critical conditions from the point of view of preparation, safety and disposal. Diffusible hydrogen content of steels is in the range 0.6 to 2 ppm. Charpy and J-R curves tests were carried out in the range from room temperature to T=-120°C. Hydrogen affects mechanical properties of the tested materials, mainly reducing fracture toughness in J integral tests, while little influence has been observed in CV tests. Fracture surface examination confirms the results of mechanical testing.