Industry News
Why do high-strength bolts snap without warning? Hydrogen embrittlement and baking essentials
2026-09-20
Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.
Delayed fracture days after installation: how hydrogen embrittlement works
A bolt that torques up perfectly, then snaps days or weeks later with a flat, almost plastic-free fracture face — that is classic hydrogen-embrittlement delayed failure. Atomic hydrogen enters the steel during acid pickling and electroplating, migrates to highly stressed zones such as the first engaged thread and the head-shank fillet, and initiates intergranular cracking under sustained tensile stress. The harder the fastener, the greater the sensitivity: parts above roughly 320 HV (10.9 grade and up) are high-risk, especially 12.9 bolts, spring washers and hardened self-tapping screws. Fracture needs three factors together — hydrogen, tensile stress and a susceptible microstructure — which is why only some bolts in a batch actually fail.
Where hydrogen gets in, and how baking removes it
Hydrogen uptake concentrates in three steps: acid pickling, pre-plate activation and the plating bath itself, where hydrogen evolves on the cathode. The countermeasure is prompt post-plate baking: per ISO 4042 / GB/T 5267.1, hold at about 190–220 °C, from 4 hours up to 22 hours as hardness increases; for 12.9 electroplated bolts, no less than 20 hours, into the oven within 4 hours of plating. Where stronger corrosion protection is needed, zinc-flake coatings such as Dacromet and mechanical galvanising involve no electrolysis and introduce virtually no hydrogen — the reason Dacromet is the preferred finish for fasteners above grade 10.9. Never machine, straighten or re-heat-treat fasteners after plating, as this multiplies both hydrogen and residual-stress risk.
A procurement and acceptance checklist
Put four things in writing at order stage: (1) grade and hardness ceiling, with mandatory baking for 10.9 and above; (2) the coating process — electroplated zinc must come with baking records, and zinc-flake Dacromet preferred for high-strength parts; (3) the test basis — batch hydrogen-embrittlement sensitivity per the ISO 15330 / GB/T 3098.17 parallel-bearing test, or ASTM F1940 process-validation data from the supplier; (4) storage away from acidic environments on site, and installation to a torque value without over-yielding the bolt. If a failure has already occurred, keep the fracture face for metallographic analysis to separate hydrogen embrittlement from fatigue before taking corrective action. Need baking-compliant 10.9 / 12.9 bolts or zinc-flake coated supply? Contact JIALI Industrial (164.hk) with drawings and grades for matching specifications and certificates.
Related Reading
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- Stainless steel screw galling: causes, prevention and on-site fixes for seized threads
- JIALI launches Stainless Steel zone with full 304 / 316 range
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