51313-02119-Fatigue Fracture of a Stainless Steel Rotary Valve Shaft

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Product Number: 51313-02119-SG

ISBN: 02119 2013 CP

Author: Jeffrey Xie

Publication Date: 2013

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The shaft of a rotatory valve for polyethylene pellet service fractured. In order to understand the fracture mechanism and mitigate future fracture failures the fracture was investigated in term 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 the formation of multiple ratchet 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 an excessive amount of manganese sulfide (MnS) stringers was present along the axial direction of the shaft across the entire thickness. Combined with the microstructure and composition the shaft was determined to be made of AISI 304L stainless steel with an 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 stress concentration thus promoting fatigue failure of the shaft.