Introduction
Hydrogen embrittlement (HE) is one of the most serious and unpredictable failure modes in high-strength fasteners. Unlike fatigue or overload failures that show progressive warning signs, hydrogen embrittlement causes sudden, catastrophic brittle fracture — often at stress levels well below the nominal yield strength of the material, and sometimes during storage rather than in service.
Understanding hydrogen embrittlement is essential for anyone specifying, manufacturing, or using high-strength fasteners, particularly electroplated fasteners of Grade 10.9, 12.9, or equivalent hardness levels.
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1. How Hydrogen Embrittlement Occurs
During manufacturing and service, hydrogen atoms can enter the steel matrix through various processes:
- Manufacturing processes: Quenching and tempering (Q&T), cyaniding, carburizing, acid pickling, phosphating, electroplating, thread rolling, machining (especially with inadequate lubrication causing burning)
- Service environments: Cathodic protection side reactions, corrosion reactions in acidic or hydrogen-rich environments
Once absorbed, hydrogen atoms diffuse into the steel lattice and concentrate at grain boundaries, inclusions, and stress concentration points. Under tensile stress — even below yield strength — this leads to:
- Reduction or loss of ductility and load-bearing capacity
- Crack initiation (often sub-microscopic)
- Sudden brittle fracture during service or storage
Electroplating is one of the most critical hydrogen introduction steps in fastener manufacturing. High-strength fasteners that undergo cold drawing, cold forming, thread rolling, machining, grinding, followed by hardening heat treatment and electroplating are particularly vulnerable.
2. Conditions That Increase Hydrogen Embrittlement Risk
Three conditions must be present simultaneously for hydrogen embrittlement failure:
- High tensile strength or hardness — surface-hardened or through-hardened material
- Absorbed hydrogen — from manufacturing processes or service environment
- Tensile stress state — applied or residual stress
Risk factors that increase HE susceptibility:
- Higher hardness and carbon content — hydrogen solubility increases with hardness
- Greater degree of cold work hardening
- Acid pickling and electroplating exposure duration
- Smaller diameter fasteners are more susceptible than larger diameter fasteners of the same grade
3. Prevention Measures for Electroplated Fasteners
A. Stress Relief Before Cleaning (Hardness ≥ 320 HV)
For electroplated fasteners with hardness ≥ 320 HV, a stress relief process must be performed before the cleaning step. During cleaning, use inhibited acid, alkaline, or mechanical methods. Immersion time in inhibited acid should be minimized.
B. Heat Treatment Compliance (Hardness > 320 HV)
Fasteners with hardness > 320 HV that undergo cold drawing, cold forming, machining, or grinding before heat treatment must comply with ISO 9587 requirements.
C. Minimize Residual Stress
Avoid intentionally introducing residual stress. Best practice: roll threads after heat treatment (not before) for bolts and screws.
D. Avoid Acid Pickling for Very High Hardness (Hardness > 385 HV or Grade 12.9+)
Fasteners with hardness > 385 HV or Grade 12.9 and above must not be acid pickled. Use acid-free cleaning methods instead: alkaline cleaning, shot blasting, or abrasive blasting.
E. High-Cathode-Power Plating for Hardness > 365 HV
Fasteners with hardness > 365 HV (heat-treated or cold-work hardened) should use high cathode power electroplating solutions to minimize hydrogen absorption during plating.
F. Minimize Acid Exposure Time
Steel fasteners should be cleaned with the minimum possible immersion time before electroplating.
G. Control Coating Thickness
Thicker coatings make subsequent hydrogen release (de-embrittlement baking) more difficult. Select the minimum coating thickness that meets corrosion protection requirements.
H. Mandatory De-embrittlement After Plating
The following fastener types must undergo de-embrittlement treatment after electroplating:
- Bolts, screws, and studs of Grade 10.9 and above
- Spring washers or spring washer assemblies with hardness ≥ 372 HV
- Nuts of Grade 12 and above
- Surface-hardened fasteners: self-tapping screws, self-drilling screws, self-tapping locking screws
- Fasteners with tensile strength ≥ 1000 MPa or hardness ≥ 365 HV (e.g., metal spring clips)
4. De-embrittlement Treatment (Hydrogen Bake-Out)
De-embrittlement is essentially a controlled baking process — heating the plated fasteners at a specified temperature for a specified time to drive out absorbed hydrogen. The baking process causes hydrogen atoms to diffuse out of the steel matrix and be released.
The specific baking procedure varies based on part type, geometry, material, performance grade or hardness, cleaning process, coating type, and plating process. Key guidelines:
| Parameter | Requirement |
|---|---|
| Baking temperature | 200–230 °C (must not exceed the tempering temperature of the part) |
| Baking duration | 2–24 hours; 8 hours is the typical standard |
| Timing | Must be performed immediately after plating (ideally within 1 hour), before chromate passivation treatment |
| Temperature limit | Must not exceed the part’s original tempering temperature — exceeding this would reduce hardness and mechanical properties |
General principle: Lower baking temperature + longer baking time is preferred over higher temperature + shorter time, as it reduces the risk of over-tempering while still achieving effective hydrogen removal.
Summary: Hydrogen Embrittlement Risk by Fastener Grade
| Fastener Grade / Hardness | HE Risk Level | Key Requirements |
|---|---|---|
| Grade 8.8 / < 320 HV | Low | Standard cleaning and plating procedures |
| Grade 9.8–10.8 / 320–365 HV | Medium | Stress relief before cleaning; inhibited acid; controlled plating |
| Grade 10.9 / ≥ 320 HV | High | Mandatory de-embrittlement baking after plating |
| Grade 12.9 / > 385 HV | Very High | No acid pickling; alkaline/mechanical cleaning only; mandatory baking |
| Surface-hardened (self-tapping, etc.) | High | Mandatory de-embrittlement baking after plating |
Related Guides
- Bolt Breaking Torque Reference Tables — Stainless & Carbon Steel
- 10B21 Boron Steel — Cold Heading Wire Rod for Grade 8.8/9.8 Fasteners
- 40Cr Alloy Steel — For Grade 10.9/12.9 Fasteners
- Why Are Stainless Steel Fasteners Magnetic?
Standards reference: ISO 9587 (pre-treatment of metals before electroplating) | ISO 9588 (de-embrittlement of iron and steel) | GB/T 3098.17
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