Effects of Al, Ti, B and N on hydrogen attack were examined by using simulated HAZ specimens of Cr-Mo steel. The hydrogen attack resistance was evaluated by the ratio of Charpy absorbed energy measured after exposure to hydrogen atmosphere and that measured after aging in air. The hydrogen attack resistance increased with sol.Al content more than 0.01% for simulated HAZ of 10 ppm B bearing Cr-Mo steels. The addition of Ti and B also improved the resistance to hydrogen attack. It was found that M23C6 type carbide precipitated preferentially in the simulated HAZ of B bearing Cr-Mo steel. On the other hand, only the carbide of M7C3 type was observed when B was not added. The carbide of M23C6 type precipitation was promoted by free B in solution when N was fixed by Al and Ti.
It was recognized that M23C6 was more stable than M7C3 at high temperature and high pressure in hydrogen atmosphere. The carbide of M7C3 type reacts with hydrogen to form methane and metal. The reason why the additions of Al and B improve the resistance to hydrogen attack is that the precipitation of M23C6 type carbide is accelerated by free B in solution which is more stable at high temperature and high pressure of hydrogen.
