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 RestrictionsHDG 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 PlanASTM 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 TestMandatory 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 ThicknessISO 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 TestingThe 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 & RepairsThe 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 3269Hydrogen Test ASTM F606 (Wedge Test) ISO 15330 (Preloading test) Quenching Mandatory water quench Not explicitly mandatory; depends on agreement

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