GB/Z 41117-2021
Fasteners—Fundamentals of hydrogen embrittlement in steel fasteners (English Version)
- Standard No.
- GB/Z 41117-2021
- Language
- Chinese, English version preview
- Release Date
- 2021
- Published By
- General Administration of Quality Supervision, Inspection and Quarantine of the People‘s Republic of China
- Latest
- GB/Z 41117-2021
- Scope
- This document provides a brief but complete introduction to the latest knowledge on hydrogen embrittlement from a technical perspective. This document applies to steel fasteners.
- Introduction
Core Failure Mechanisms of Hydrogen Embrittlement
Hydrogen embrittlement (HE) is defined as the loss of ductility of metals caused by the combined action of atomic hydrogen and stress. GB/Z41117 standard clearly distinguishes two types of hydrogen sources:
- Intrinsic hydrogen embrittlement (IHE): caused by residual hydrogen in the manufacturing process
- Environmental hydrogen embrittlement (EHE): caused by hydrogen evolution in the service environment
Key threshold of material sensitivity
Hardness is the primary indicator for measuring sensitivity. When hardness＞390HV, a ductile-brittle transition occurs:
Hardness range (HV) Sensitivity level Typical applications <390 Low risk Grade 10.9 standard parts 390-435 High risk 12.9 grade fasteners >500 Extremely high risk Surface hardened screws Abnormal metallographic structure can reduce the threshold stress by more than 50%, and the material quality needs to be verified by impact test.
Interaction of three elements of hydrogen embrittlement
Typical case: After 72 hours of installation, an engine bolt broke at the bottom corner of the head, and the fracture surface showed 100% intergranular morphology. Test results:
- Hardness 420HV (exceeding the standard limit)
- Exceeding the pickling time resulted in a hydrogen content of 8ppm
- Prestressing reached 85% of the yield strength
The synergistic effect of the three factors caused hydrogen atoms to gather in the high stress area, causing delayed fracture.
Special process risk control
Surface hardened fasteners
The core hardness should be <370HV, and the depth of the hardened layer should be strictly limited to avoid full cross-section hardening in thin-walled areas.
Hot-dip galvanizing process
The thermal shock of the molten zinc pool will release trapped hydrogen, and the zinc layer will hinder the escape of hydrogen. Fasteners below grade 10.9 can be used safely, but it is necessary to ensure that the material is free of defects.
Post-plating baking specifications
For >390HV fasteners, bake at 190-220℃ for at least 8-10 hours, and the zinc coating must be placed in the furnace within 4 hours. Zinc-nickel alloy coating can reduce the risk.
Comparison of Hydrogen Embrittlement Test Methods
Method Standard Features Applicable Scenarios Sustained Load ISO15330 Qualitative/Failure Time Judgment Production Inspection Slow Strain Rate ISO7539-7 Quantitative/Measurement of Threshold Stress R&D Analysis Step Loading ASTM F1624 Simulation of Actual Stress State Failure Analysis
Implementation Recommendations
- 12.9 grade fasteners are required to adopt the thread rolling process after heat treatment
- Avoid cathode electrolytic cleaning before electroplating, and the pickling time should be ≤3 minutes
- Establish a linkage control chart of hardness, hydrogen content and stress
- Zinc-nickel alloy plating is preferred for key parts
GB/Z 41117-2021 history
- 2021 GB/Z 41117-2021 Fasteners—Fundamentals of hydrogen embrittlement in steel fasteners