Effects of Decarburization in Aircraft Components

A connecting rod from a failed engine ruptured in fatigue without evidence of excessive stresses, detonation, overheating, or oil starvation. The origin of the fatigue failure was completely mutilated but decarburization was observed. Significant amounts of decarburization (0.010 to 0.015 in.) were found also in other forgings, such as exhaust rocker arms, main rotor drag brace clevises, bolts of carriage diagonal struts, and spring legs of main landing gears. The failure mode was low-stress, high-cycle fatigue involving tension and bending loads. The main cause was a manufacturing deficiency. The usual way to eliminate decarburization is to machine off the soft skin or employ better quality control when making them. Many aircraft manufacturers employ forged parts with machined surfaces or with shot-peened as-forged surfaces without excessive decarburization.