Abstract: Metallographic structure analysis, crack propagation analysis, fracture analysis and mechanical property tests were carried out to find the cause of the sudden failure of a natural gas transmission bend that had been in service for many years. The results show that the bend pipe is unevenly heated during the induction process, the heating temperature of the failure part is too high, and the lath martensite and Widmanstatten structure are formed at a high cooling rate, resulting in an increase in the strength of the part, a decrease in plasticity and fatigue resistance, and fatigue failure occurs under long-term alternating stress.