Problem
A tier-1 supplier saw a rash of head-off Class-10.9 fasteners on a safety-critical suspension component — days after a plating vendor swap.
Approach
Tensile + proof-load testing on retained lot samples; SEM fractography at the failure origin; microhardness surveys through the head-to-shank transition; hydrogen-content estimation by controlled-baking regressions.
Finding
Intergranular brittle fracture with hydrogen fingerprints — the new plating line was over-current on the acid clean, and post-plate bake was undersized. Textbook HE.
Outcome
Vendor requalified with a validated bake schedule; supplier contained the affected lots; zero further field failures in the following 12 months.
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