Failure of a Coal-Pulverizer Shafts from a Generation Station

Two vertical coal-pulverizer shafts at a coal-fired generation station failed after four to five years in service. One shaft was completely broken, and the other was unbroken but cracked at both ends. shaft material was AISI type 4340 Ni-Cr- Mo alloy steel, with a uniform hardness of approximately HRC 27. Metallographic examination of transverse sections through the surface-damaged areas adjacent to the cracks also showed additional small cracks growing at an angle of approximately 60 deg to the surface. The crack propagation mode appeared to be wholly transgranular. SEM examination revealed finely spaced striations on the crack surfaces, supporting a diagnosis of fatigue cracking. Crack initiation in the pulverizer shafts started as a result of fretting fatigue. Greater attention to lubrication was suggested, combined with asking the manufacturer to consider nitriding the splined shaft. It was suggested that the surfaces be securely clamped together and that an in-service maintenance program be initiated to ensure that the tightness of the clamping bolts was verified regularly.