Subsurface Fatigue Initiation in a Steel Driveshaft Involving a Weld Repair

Abstract

A steel shaft used in the separation and milling operations of raw cement materials failed during service at a diameter reduction. Standard metallurgical, optical microscopy, and scanning electron microscopy techniques were used to analyze the failure. This case history highlights several important points regarding metallurgical failure analysis: (1) visual inspection cannot necessarily determine the root cause of fractures. Initial inspection of the fracture surface without magnification appeared to indicate a simple stress-concentration/rotating bending fatigue failure; however, the fatigue initiation was actually primarily caused by sub-surface weld HAZ cracking and other metallurgical issues. (2) Maintenance records are critical and can often simplify determination of a failure cause. In this case, even though the fracture was short term, the customer was not aware of the prior weld repair. (3) Qualified welding procedures and practices are required when operating on medium or high carbon steels. This weld build-up was post-machined and was entirely acceptable by visual examination. If any non-destructive testing had been conducted on the weld repair, it would have been unlikely to detect the small transverse HAZ cold cracks.