Abstract
The martensitic stainless steel pipe flange used for the 138 MPa Christmas tree failed to leak during ultra-deep high-pressure, high-temperature gas well operations. In order to study the cause of leakage, non-destructive testing, chemical composition analysis, mechanical property analysis, metallographic structure testing, surface characterization, cracking characterization and finite element analysis were performed in the present research. The results show that the leakage failure is related to the crack in the ring groove. The cracks initiated from the edge of the corrosion pits and propagated along the delta ferrite/martensite interfaces. The Cr-rich carbides in the matrix reduced the pitting resistance, while delta ferrite deteriorated the crack resistance, which was the main factor contributing to the failure. In addition, the stress level in the sealing structure of the Type 6BX flange further influenced the crack propagation behavior.
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