Failure Analyses of Six Cylinder Aircraft Engine Crankshafts
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Results of failure analyses of two aircraft crankshafts are described. These crankshafts were forged from AMS 6414 (similar composition to AISI 4340) vacuum arc remelted steels with sulfur contents of 0.003% (low sulfur) and 0.0005% (ultra-low sulfur). A grain boundary sulfide precipitate was caused by overheat of the low sulfur steel, and an incipient melting of grain boundary junctions was caused by overheat of the ultra-low sulfur steel. The precipitates and incipient melting in these two failed crankshafts were observed during the examination. As expected, impact fractures from the low sulfur steel crankshaft contained planar dimpled facets (PDF) along separated grain boundaries with a small spherical manganese sulfide precipitates within each dimple. In contrast, PDFs along separated grain boundaries of impact fractures from the ultra-low sulfur crankshaft steel contained a majority of small spherical particles consisting of nitrogen, boron, iron, carbon, and a small amount of oxygen. Some other dimples contained manganese sulfide precipitates. Fatigue samples machined from the ultra-low sulfur steel crankshaft failed internally at planar grain boundary facets. Some of the facets were covered with nitrogen, boron, iron, and carbon film, while other facets were relatively free of such coverage. Results of experimental forging studies defined the times and temperatures required to produce incipient melting overheat and facets at grain boundary junctions of ultra-low sulfur AMS 6414 steels.
KeywordsAircraft crankshaft Overheat Low sulfur steel Fracture facet
The mechanical tests and Auger analyses contained herein were provided to R. W. Hinton by the following testing facilities: Ohio State University/Welding Research Institute (Gleeble), Laboratory Testing, Inc. (Impact Tests), Westmoreland Mechanical Testing and Research (Fatigue Tests), Evans East (Physical Electronics 680 Scanning Auger Nanoprobe). Some of the macrographs and SEM micrographs of failed crankshafts LF5 and LF14 were graciously provided by Dr. Gary Fowler of Fowler, Inc. Erie Tool and Forge, Inc. provided the slot preheat furnace and forge facilities for the forging studies.
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