Fatigue Fracture of a Stainless Steel Rotary Valve Shaft

摘要

The shaft of a rotatory valve (RV) for polyethylene pellet service fractured. In order to understand the fracture mechanism and mitigate future fracture failures, the fractured shaft was investigated in terms of metallurgical, compositional and morphological characteristics on the fracture surface and cross-sections of the shaft. The characterization determined that the fracture of the rotatory valve shaft was the result of rotational bending fatigue. Fatigue cracks were initiated from multiple locations, leading to formation of multiple ratchat marks around the shaft circumference. The torsional stress caused the ratchet marks to be roughly 45° to the fracture plane. The stress was high at the time of the fracture, inducing an overload zone to be roughly 60% of the entire fracture surface. The concave curvature of the fracture surface indicated stress concentration at the location of fracture. Microstructural analysis revealed that excessive amounts of manganese sulfide (MnS) stringers were present across the entire thickness along the axial direction of the shaft. Combined with the microstructure and composition, the shaft was determined to be made of UNS S30400 stainless steel with excessive amount of MnS inclusions. As a result, the fatigue resistance was greatly compromised. The fatigue cracks were determined to initiate from the MnS inclusions, acting as a stress riser, on the outer diameter. The selected stainless steel with MnS inclusions and the particular geometry of the rotatory valve shaft induced weak load-bearing capacity and excessive stress concentration, thus promoting fatigue failure of the shaft.