Hydrogen Embrittlement in High-Strength Bolts: Mechanism, Detection and Baking Control

Hydrogen Embrittlement in High-Strength bolts: Mechanism, Detection and Baking Control

Hydrogen embrittlement is the most insidious failure mode of high-strength fasteners. It does not happen on the assembly line but hours or days after tightening, when the bolt suddenly snaps with a flat fracture.

Hydrogen atoms diffuse under stress into triaxial tensile zones, weakening grain boundaries until the steel fails below yield. Three conditions must coincide: a susceptible material (strength above 1000 MPa, hardness above 32 HRC), hydrogen inside the steel, and sustained tensile stress. Remove any one and the risk drops sharply.

Where the hydrogen comes from

Electroplating and acid pickling dominate. During cathodic plating, hydrogen ions reduce at the steel surface and a fraction enters the substrate; pickling does the same, aggravated by high acid concentration or long immersion. Processes without electrolysis, such as mechanical zinc and zinc-aluminium flake coatings, carry clearly lower risk.

How to detect it

A tensile test will not find it, because embrittlement is delayed failure needing both time and stress. The accepted method is a preload test: per GB/T 3098.17 (equivalent to ISO 15330) bolts are assembled in a parallel-bearing jig, loaded to a specified value and held 48 hours. Intergranular fracture, failure within 24 hours of assembly and cracks at the thread-to-head fillet point indicate hydrogen.

Key baking parameters

Baking is the most effective remedy but must follow plating within four hours, since delay makes clustering harder to reverse. Temperature is typically 190 to 230 degrees Celsius; grade 12.9 parts commonly see 200 degrees for four hours or more, extended for thick coatings.

A prevention checklist

Avoid cathodic plating on high-strength parts in favour of flake coatings; control pickling time and temperature with inhibitors; bake immediately after plating; avoid over-tightening; and add hardness checks, watching the upper limit more than the lower limit.

Conclusion

Embrittlement runs through material selection, surface treatment, baking and assembly. Zhuhai Jiali Hardware (FASTKING) applies time-limited post-plating baking with retained temperature curves and can supply process descriptions and inspection records. Torque tools are in our screw calculator directory; more articles in the screw knowledge section.