The hydrogen attack of HSLA steels

Abstract

Three high strength low alloy steels with low carbon contents and varying alloy contents were exposed to 21 MPa (3000 psi) hydrogen pressure in the temperature range 350 to 510°C. The resulting sample expansion rate was measured using a highly sensitive capacitance dilatometer. In all three HSLA steels, the early expansion rate was found to be independent of exposure time, but a function of exposure temperature and pressure. The temperature dependence of the sample expansion rate was similar for all three HSLA and the reference carbon steels, and corresponded roughly to an activation energy (Q) of 190 KJ/mole. The later accelerating expansion rate fitted a Q of 160 KJ/mole. Though the sample expansion rates exhibited the same temperature dependence, they varied by as much as two orders of magnitude among the three HSLA and the carbon steels. This marked variation in rate was proportional to the estimated carbon activity in each steel. Comparison between the Nelson curves published by the Americal Petroleum Institute (API) for 1/2 pct Mo steel and the experimental curves constructed for HSLA steels indicated that these steels should provide comparable or better resistance to hydrogen attack than the 1/2 pct Mo steel.