Fatigue Fracture of an Electron Beam Weld in the Web of a Steel Gear Because of Incomplete Fusion

A 9310 steel gear was found to be defective after a period of engine service. A linear crack approximately was discovered by routine magnetic-particle inspection of an electron beam welded joint that attached a hollow stub shaft to the web of the gear. The welding procedure had a cosmetic weld pass on top of the initial full-penetration weld. There were no other known service failures of gears were welded by this method. One zone of the welded joint showed incomplete fusion, surrounded by two zones containing fatigue beach marks This indicated that the incomplete-fusion zone was the site at which primary fracture originated. The possible causes of incomplete-fusion include localized magnetic deflection of the electron beam, a momentary arc-out of the electron beam, and eccentricity in the small weld diam. The failure was attributed to fatigue originating at the local unfused interface of the electron beam weld, which had been the result of a deviation in the welding procedure. Examination of the possible causes of failure gave no evidence that a recurrence of the defect had ever occurred. Thus, there was no basis on which to recommend a change in design, material, or welding procedure.