High-strength bolts and nuts' inspection standards for hot-dip galvanizing (HDG)

Here is the process flow and inspection standards for hot-dip galvanizing (HDG) of high-strength bolts and nuts according to American (ASTM) and German/DIN/ISO standards.

1. American Standard (ASTM)

The primary specification for centrifuged hot-dip galvanizing of fasteners is ASTM F2329 / F2329M. This standard references ASTM A153 for processing details but imposes stricter quality control and mechanical property safeguards.

Process Flow (ASTM F2329)

1. Stress Relief (Pre-Galvanizing): Fasteners that have undergone severe work hardening must be stress-relieved by the fastener manufacturer prior to galvanizing to prevent hydrogen embrittlement or distortion .
2. Cleaning (Pickling): Acid pickling removes scale and rust. However, for high-strength fasteners (≥33 HRC), mechanical descaling (e.g., blasting) with a flash pickle is recommended over prolonged acid immersion to minimize hydrogen absorption .
3. Fluxing: Application of a flux solution to prevent oxidation before dipping.
4. Hot-Dip Galvanizing: Immersion in molten zinc bath (typically 840–860°F (449–460°C) ). The bath temperature must be monitored because if it exceeds the fastener's tempering temperature, mechanical testing is required post-galvanizing .
5. Centrifuging (Spinning): Immediately after withdrawal from the bath, fasteners are spun at high speeds to remove excess zinc from the threads, ensuring proper fit .
6. Quenching: Parts are quenched in water immediately after centrifuging to cool the coating, prevent zinc oxide formation, and harden the surface .
7. Post-Treatment (Optional): Chromating or phosphating may be applied to prevent white rust (wet storage stain) or to provide a base for painting .

Inspection & Testing Standards (ASTM F2329)

1. Applicability & Grade Restrictions
HDG is NOT recommended or is prohibited for grades with a hardness of 33 HRC and above (approx. 1100 MPa tensile strength) due to hydrogen embrittlement risk.

· Prohibited/Not Recommended: ASTM A490, ASTM A354 Grade BD, SAE J429 Grade 8 .
· Permitted: ASTM A325, ASTM A449, SAE J429 Grade 5, ASTM F1554 Grade 55/105 .

2. Coating Thickness (ASTM F2329 Table 2)

· For fasteners > 3/8 in [M10]: Average thickness shall be 50 μm (0.0020 in) , with individual specimens measuring min 43 μm (0.0017 in) .
· For fasteners ≤ 3/8 in [M10]: Average thickness shall be 43 μm (0.0017 in) , with individual specimens measuring min 38 μm (0.0015 in) .

3. Sampling Plan
ASTM F2329 requires a strict "Zero Defects" acceptance criterion. Sampling differs based on process type (Batch Lot vs. Production Lot).

· Batch Lots: For every batch, sample sizes vary by lot size (e.g., for 151–500 pcs: 5 samples for thickness, 5 for appearance, 5 for adhesion) .
· Production Lots: If a "Prevention Process" is validated, sampling frequency is reduced .

4. Adhesion Test
Mandatory for every batch lot (unlike ASTM A153). Conducted via the "Stout Knife Test" per ASTM A153 Annex; the coating must not peel or flake from the base metal at the interface .

5. Hydrogen Embrittlement Relief

· Baking: If required by the purchaser or if the fastener hardness is ≥33 HRC, baking must be performed after pickling but before galvanizing (typically 375–450°F for several hours) to diffuse absorbed hydrogen .
· Testing: If embrittlement testing is specified, it is performed per ASTM F606 (Wedge Test) or ISO 15330 .

---

2. German / European Standard (DIN / ISO)

The European standard is DIN EN ISO 10684 (adopted from ISO 10684). It is the equivalent standard to ASTM F2329 for hot-dip galvanized fasteners in the metric system.

Process Flow (ISO 10684)

The process is largely identical in metallurgical principle:

1. Pre-treatment: Degreasing and pickling.
2. Fluxing: Usually a zinc ammonium chloride flux.
3. Galvanizing: Immersion in molten zinc (approx. 450 °C).
4. Centrifuging: Mandatory for threaded fasteners to ensure thread clearance (spun galvanizing) .
5. Cooling: Air or water cooling.

Inspection & Testing Standards (ISO 10684)

1. Applicability & Grade Restrictions

· Sizes: Applicable for coarse thread fasteners from M8 up to M64. Not recommended for diameters smaller than M8 or pitches below 1.25 mm .
· Strength Restrictions: According to ISO 10684, hot-dip galvanizing is not recommended for property classes 12.9 and above (or fasteners with hardness ≥ 40 HRC), as well as surface-hardened steel fasteners, due to the risk of hydrogen embrittlement .
  · Note: High-strength structural bolts (e.g., 10.9 grade for steel structures per EN14399) are commonly galvanized, but strict process control is required .

2. Coating Thickness
ISO 10684 generally requires a minimum average coating thickness that varies by thread diameter, similar to ASTM but with metric values:

· Thread Diameter M8 to M20: Minimum average thickness 45 μm.
· Thread Diameter M22 to M36: Minimum average thickness 55 μm.
· Thread Diameter M40 to M64: Minimum average thickness 70 μm.
· Local thickness (individual measurement) must not fall below 80% of the specified minimum average .

3. Thread Fit (Overtapping)
A critical requirement for ISO 10684 is that nuts must be overtapped (tapped with a larger thread tolerance) before galvanizing to accommodate the thickness of the zinc coating on the bolt threads, ensuring assembly capability .

4. Hydrogen Embrittlement Testing
The standard references ISO 15330 (Preloading test for the detection of hydrogen embrittlement). Fasteners are loaded to a specific stress level and held for a period (usually 48 hours) to ensure no sudden failure occurs .

5. Appearance & Repairs
The coating must be smooth and uniform. Minor damage or bare spots (less than 0.5% or 1% of the surface area, depending on the specification) may be repaired with zinc-rich paint containing 65-92% zinc dust in the dried film . 

---

Summary of Key Differences

Feature ASTM F2329 (US) ISO 10684 (DIN/DE)
Hardness Limit 33 HRC (Risk threshold) / 40 HRC (Prohibited)  40 HRC (Not recommended) 
Coating Thickness 50 μm (for > M10) 45 μm (M8–M20) / 55 μm (M22–M36)
Sampling Strict "Batch Lot" plan; zero defects allowed  Typically based on production lot or statistical sampling per ISO 3269
Hydrogen Test ASTM F606 (Wedge Test)  ISO 15330 (Preloading test) 
Quenching Mandatory water quench  Not explicitly mandatory; depends on agreement