Failure Analysis of a Four-Way Flange Cracking in a KQ65 Wellhead Christmas Tree

A Christmas tree is a key wellhead device consisting of valves and fittings used for controlling oil and gas well fluid. Generally, various combinations are applied on Christmas tree to meet the requirement of specific working conditions. In present work, fracture of a four-way flange of a KQ65 wellhead Christmas tree was investigated through mechanical properties testing, microstructure characterization and fractography analysis. It is revealed that the four-way flange possesses very low impact toughness, and its CVN impact energy was only 4 J at 243 K. Representative microstructure of the four-way flange consists of coarse polygonal ferrite and pearlite, and the size of pearlite cluster was up to 280 μm. Ferrite grains displayed networks along grain boundaries of pearlite colony. The brittle microstructure of the four-way flange was likely caused by inappropriate heat treatment process. In addition, fractography analysis showed that fracture crack initiated from the flange and four-way junction, where stress was concentrated and thus resulted in the dimensional and structural change of four-way flange. It is concluded that fracture failure of the KQ65 wellhead Christmas tree was caused by coupling effect of the brittle microstructure and the localized stress concentration.