Abstract: Based on the analysis of chemical composition, microstructure, mechanical properties, fracture and energy spectrum, this paper analyzes the fracture causes of S30409 wing valve diversion cone in an FCC unit. The results showed that a large amount of σ phase was formed in the matrix structure and weld during the long-term high temperature operation of the wing valve; precipitation of σ phase led to decrease of the plasticity and toughness of the material; when the device was switched to the fan operation, the fluctuation of the working condition resulted in the vibration of the cyclone system, which impacted the wing valve; the accumulated damage at the weld seam of the guide cone with large stress concentration formed micro-cracks; the vibration cycle accelerated the crack expansion and finally led to the fracture and fall-off of the guide cone of the wing valve.

Key words: S30409, wing valve, sigma phase, vibration, fracture