In order to extend the life of petroleum pressure vessels operated in long term, it is needed to establish the reliable repair welding technique. Weld cold cracking sometimes occurred in long-term operated petroleum pressure vessels due to hydrogen embrittlement by thermal stress and diffusible hydrogen after repair welding. The cracking was caused by the hydrogen concentration at the base meal of 2.25Cr-1Mo steel/overlaying metal of austenitic stainless steels interface during the service with high temperature and hydrogen partial pressure. The tendency was accelerated by carbide precipitation at the interface due to the post weld heat treatment (PWHT) and the operation with high temperature. That is, the crack susceptibility at the interface became markedly higher owing to the hydrogen embrittlement with metallurgical degradation by thermal embrittlement. To make clear the effect of weld thermal cycles during repair welding on the hydrogen content and weld cold cracking at the interface in the structural material of petroleum pressure vessels, the crack susceptibility was estimated by y-groove weld cracking test with varying overlay thickness and hydrogen exposure conditions. In addition, the hydrogen distribution in the material was calculated by the theoretical analysis using the diffusion equation based on activity. The crack susceptibility was raised with increase in the hydrogen content at the interface. It was concluded that the cracking could be prevented by controlling the repair welding process to reduce the hydrogen content at the interface.

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