A Study of Micro-cracks in Overheated Forgings of Ultra-Low Sulfur Steels for Aircraft Engine Crankshafts

摘要

Several crankshafts of aircraft engines fractured in service. Consequently, the present investigation of twenty two (22) broken and recalled crankshafts was conducted. The investigation included hundreds of metallurgical evaluations, numerous fatigue and fracture tests to determine the cause of the failures. The crankshafts were forged from AMS 6414H (AISI 4340) vacuum arc remelted steels with ultra-low sulfur content of less than 0.001%. The investigation revealed that the crankshafts broke because small cracks created in the forging process became large when the crankshafts were subjected to expected operating loads in aircraft engines. The small cracks were caused by incipient melting overheat at grain boundaries. These cracks continued to grow as the engine accumulated more operating time until they reached a critical size causing a sudden failure.