ABSTRACT

Often material failures involve a synergy of mechanisms, environmental stressors, design constraints and loading scenarios, that work in congress to cause an unpredicted fracture or acceleration of degradation leading to failure. This work covers a root cause case study analyzing the combined influence of various factors resulting in the delayed cracking of welded transmission gear assemblies. The gear assembly was comprised of a single ring gear electron-beam (e-beam) 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 e-beam weld was ultrasonically 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. A combination of metallurgical events, storage conditions and orientation control of part grabbers ultimately led to the delayed cracking phenomenon.

INTRODUCTION

Root cause analysis involves a detailed process of evidence collection, investigation of the evidence, subsequent analysis, and identification of corrective actions based on conclusions drawn from the evidence. The process when diagrammed out seems simple, however paramount to the process is identifying the problem or failure accurately. Misunderstanding the failure leads to misidentification of the root cause which in turn begets mitigation efforts that may or may not impactfully remediate the original failure.

This root cause case study covers environmentally assisted cracking events in automotive transmission gear assemblies and highlights the need for open-minded, holistic approaches in failure analysis. Environmentally assisted cracking (EAC) is, in its essence, a synergistic failure mechanism that can take many forms and take myriad paths to that form. The more exposure the industry has to the variety of conditions that can lead to EAC, the better the understanding of the diversity of mechanism, thus allowing for a more informed set of corrective actions to be put in place to avoid further failures. This particular work covers a combination of steel processing effects on resultant metallurgy, detailed fractographic analysis, and corrosion mechanism characterization to arrive at a root cause linking the disparate factors that synergized to result in seemingly randomized gear assembly failures.

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