Delayed Brittle Fracture of Galvanized Grade 10.9 Bolts
A steel roof truss uses M24 grade 10.9 bolts, hot-dip galvanized after acid pickling. Six weeks after erection at a coastal site, 12 of 240 bolts have fractured at the thread root. The fractures show no necking, a flat brittle appearance, and slight rust staining. Measured hardness is 39 HRC on several intact bolts, against a maximum of 34 HRC for grade 10.9. Torque records show 800 N·m, within the specified range. The galvanizer's records show pickling for 40 minutes and galvanizing at 450 °C, with no post-galvanizing baking. The contractor wants to replace only the broken bolts and reopen the roof.
1. Set out a failure-analysis plan and rank the likely mechanisms.
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Plan.
- SEM fractography of the fracture surfaces to distinguish intergranular or quasi-cleavage from ductile dimples.
- Hydrogen content by inert gas fusion on fractured and intact bolts.
- Metallography for untempered martensite, decarburisation and coating microstructure.
- Review pickling time, acid concentration, galvanizing and baking records.
- Check torque, lubrication and whether bolts were tightened before or after any bake.
- Measure hardness on a statistically meaningful sample.
Ranking. Hydrogen embrittlement is the leading hypothesis: acid pickling introduces hydrogen, the high hardness of 39 HRC and the thread-root stress concentration trap it, and tightening applies sustained stress; delayed failure at six weeks fits. Stress corrosion cracking in the coastal atmosphere is a secondary possibility, but the process history makes hydrogen more likely. Over-torque is not supported by the records.
2. What is the root cause, and what immediate action is required?
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Root cause. Hydrogen embrittlement caused by pickling before galvanizing, compounded by hardness above the 34 HRC limit and the absence of a post-galvanizing bake. The thread root provides the stress concentration and the tightened bolt provides sustained stress.
Immediate action. Replace all 240 bolts, not only the 12 broken ones, because the remaining bolts have the same hydrogen and hardness condition and may fail progressively. Isolate or shore the affected truss while replacement is planned. Quarantine the remaining stock, review the fastener specification, and notify the designer and client in writing.
3. Recommend a permanent corrective specification for future bolts.
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Specify a fastening system that is not susceptible: either a lower-strength galvanized grade with hardness below the embrittlement threshold, or a corrosion-resistant stainless or weathering-steel fastener with the required strength. If galvanized grade 10.9 is retained, require baking within 4 h of pickling at 200–230 °C for at least 4 h, hardness verification on every batch, and a certified hydrogen bake record. Use direct tension indicators or calibrated torque with documented lubrication, and require the supplier to demonstrate compliance by sampling and hardness testing. Add a first-batch trial and a 48 h delayed-failure check before full installation.