One of the first questions that must be answered in any failure analysis is "Was the part made from the specified material?" If the answer is "yes", the analyst may be tempted to go off in another direction in search of the cause of a failure. Our recent analysis of a failed axle shaft demonstrates that a more careful evaluation is prudent even when the chemical analysis is "in spec".

By any criteria the shaft was within specification. However, the carbon is at the maximum specified value, manganese is near the maximum specified value and chromium is in the upper third of the specified range. The maximum or near maximum values of this combination of elements, although within specification, result in a higher than typical hardenability when this material is heat treated. The result of this higher than typical hardenability was an unacceptable percentage of quench cracks during a heat treating procedure that had been used successfully in the past on axle shafts with more typical SAE 4140 chemistries. Many of these cracked axle shafts were placed in service, resulting in short term failures.