Abstract
In petroleum pressure vessels, some thermal degradation problems occurred in the post weld heat treatment (PWHT) such as the sigma phase transformation in the overlay weld of austenitic stainless steel and the carbide precipitation at the interface between the base metal of 2. 25Cr-1Mo steel and the overlay weld. In addition, hydrogen embrittlement occurred at operating conditions, such as elevated temperature and high pressure hydrogen environment. In this study, the degree of thermal and hydrogen embrittlement in the pressure vessel steel was evaluated by the notch tensile test. The tensile strength and rupture ductility decreased by increasing of the tempering parameter in PWHT and severer hydrogen exposure conditions. The sigma phase transformation in the overlay welds after PWHT was verified by optical microscopic observation of cross section for the tensile tested specimen. The carbide precipitation at the interface and hydrogen embrittlement was verified by the micro structure and fractographic observation of the specimen. It was confirmed that the crack propagated through the austenite grain boundary in the overlay weld near the carbide layer at the interface due to hydrogen embrittlement.
