Failure Analysis of Gas Well Casing Threaded Connection

N80 grade steel casing which used in gas wells failed after being run in well. The macroscopic observation and mechanical properties and chemical composition analysis of the failure joints were done. Thread parameters and appearance of the same batch unused casing connection were inspected. According to well site condition, it was known that failure reason of threaded connection was defects of connection and exceeded torque, and the safety proposal was provided at last.