Correlation of Hydrogen Diffusion and Trapping Behaviour with Hydrogen Embrittlement Resistance in Line Pipe Steels

摘要

In this study the hydrogen diffusion and trapping characteristics of four API line pipe grade steels were investigated during electrochemical charging. In addition, the slow strain rate (SSRT) method was used to determine the steels resistance to hydrogen embrittlement in terms of ductility ratio and time to failure. Permeation tests were used to characterise the hydrogen diffusivity and apparent solubility, whilst complimentary thermal desorption tests were used to assess the hydrogen solubility and to characterise the nature of the trapped hydrogen. From the results obtained, it was shown that irreversibly trapped hydrogen was present in all steels in the absence of hydrogen charging (as-received state). Furthermore, hydrogen introduced during electrochemical charging was reversible in nature for all steels. Comparison of hydrogen embrittlement resistance with the determined diffusion and trapping characteristics revealed improved performance for steels with lower diffusivity and higher hydrogen solubility. The trend with diffusivity was explained in terms of a critical hydrogen concentration for failure, which was shown to be higher in steels with lower diffusivity. This critical concentration was reached much later in the steels with lower diffusivities, thus giving improved ductility ratios and longer failure times in the SSRT tests.