(V) The Complex Synergy of Delayed Cracking Failures in Transmission Gear Assemblies

Often failure analyses involve the identification of a synergy of mechanisms, various environmental stressors, design constraints and loading scenarios, that work in congress to lead to an unpredicted fracture or failure. This work covers a causal analysis of a delayed cracking failure observed in transmission gear assemblies. The gear assembly was comprised of a single ring gear electron beam (EB) welded around its interior circumference onto a mated shaft. Both components were composed of a proprietary high strength steel. Mated surfaces were cleaned prior to assembly. The surface-tempered, low-vacuum EB weld was UT inspected after assembly, and in all cases no solidification cracks were detected in the failed components. The delayed crack feature was seemingly presenting itself at random with no correlation to time of production, lot of material, or process change. Combining experience in steel process metallurgy, fractographic analysis and corrosion mechanism characterization, a root cause was elucidated that tied together many disparate factors that synergized in select assemblies to cause the failure.