Abstract
The embrittlement behavior of the welded joints of 1/2 Mo steels exposed to high pressure hydrogen at high temperature has been investigated. And, the effects of carbon content and microstructure on the hydrogen attack and comparison between Nelson diagram and the critical embrittling temperature have been discussed. The results are summarized as follows:
(1) The hydrogen attack is easy to occur in the coarsed-grained heat-affected zone (HAZ), especially as welded, comparing with the base metal.
(2) The critical embrittling temperature is lowered as increasing the carbon content, and the drop of the temperature is remarkable in the coarsed-grained HAZ as welded.
(3) Nelson's curve can be applied to the base metal, but not be applied to the welded joint of the steel containing high carbon content, because even if in the safety range of Nelson diagram, it may be suffered by the hydrogen attack. In order to apply to the welded joint, carbon content of the steel should be less than 0.20% and the welded joint should be sufficiently post weld heat treated to be less than 225 in Vickers hardness of HAZ.
(4) It is considered that the coarsed-grained HAZ is easy to become brittle by hydrogen attack because carbide dissolution and carbon supply to the grain boundary is more easy than in the base metal.
