Understanding the limits of acceptance for general application fasteners.
At Ever Power, we believe that quality assurance goes beyond simple dimensions. The integrity of a fastener is often determined by the surface anomalies introduced during manufacturing. This guide details the GB/T 5779.1-2000 standard (equivalent to ISO 6157-1), providing engineers and procurement specialists with the technical criteria to distinguish between cosmetic imperfections and critical failures.
Table of Contents
1. Cracks: The Most Critical Defect
A crack is a clean, crystalline fracture passing through or across grain boundaries. They can occur due to excessive stress during forging, heat treatment, or exist inherently in the raw material.
1.1 Quench Cracks (Heat Treatment)
Cause: High thermal stress and strain during the phase transformation process (quenching).
Appearance: Irregular, erratic lines often crossing each other on the surface.
1.2 Forging Cracks
Cause: Occurs during the cut-off or heading (forging) operations due to material flow limits.
| Defect Type | Acceptance Limit (d = nominal diameter) |
|---|---|
| Length (l) | l ≤ 1.0d |
| Width/Depth (b) | b ≤ 0.04d |
2. Forging and Shear Bursts
Bursts are surface separations resulting from the material’s inability to withstand the deformation forces.
2.1 Forging Bursts
Typically found on the wrenching flats of hex heads or the periphery of flanged products.
- Hex Head/Flange: Must not extend into the crown (chamfer circle) or the under-head bearing surface.
- Diagonal: Must not reduce the width across corners below the minimum standard.
- Recessed Heads: Width ≤ 0.06d.
- Round Heads: Limit ≤ 0.08dc (if only one burst) or ≤ 0.04dc (if two or more).
2.2 Shear Bursts
Found on the periphery of round or flanged products, usually at a 45° angle to the product axis. These occur during the trimming process.
Criteria: Similar to forging bursts, they must not compromise the bearing surface or the head integrity.
(d = nominal diameter, dc = head/flange diameter).
3. Raw Material Defects: Seams and Laps
These defects originate in the steel mill. A seam is a longitudinal surface defect that is long, narrow, and often straight. It is an inherent flaw in the wire rod.
| Defect | Description | Acceptance Limit |
|---|---|---|
| Seams & Laps | Longitudinal lines along the thread, shank, or head. | Depth ≤ 0.03d. If extending into the head, forging burst limits apply. |
4. Manufacturing Anomalies
4.1 Voids
Shallow depressions caused by lack of material fill or trapped rust/scale.
Limit: Depth (h) ≤ 0.02d (max 0.25mm). Total area on bearing surface must not exceed 10%.
4.2 Folds
Material doubling over itself during forging.
Limit: Not permitted at the interior corner of the bearing surface (stress riser). Allowed at exterior corners.
4.3 Tool Marks
Grooves caused by machining or tooling movement.
Limit: Surface roughness Ra ≤ 3.2μm on shank, fillet, or bearing surface.
5. Physical Damages (Nicks & Gouges)
These are not manufacturing defects but occur during handling, transport, or bulk packaging. They include dents, scrapes, and nicks.
Acceptance Criteria: Generally accepted unless functionality is impaired.
Critical Rule: Damage on the first 3 threads must not prevent the GO-gauge from passing. Torque required to pass the gauge should not exceed 0.001d³ N·m.
6. Inspection and Sampling Procedures
GB/T 5779.1 outlines a specific protocol for accepting or rejecting batches based on surface defects. Note: Surface coatings must be removed for accurate inspection if they hide defects.
Step 1: Non-Destructive Inspection (Visual)
Random sampling from the lot. Use visual checks, magnetic particle, or eddy current testing.
| Batch Size (N) | Sample Size (n) |
|---|---|
| N ≤ 1,200 | 20 |
| 1,201 ≤ N ≤ 10,000 | 32 |
| 10,001 ≤ N ≤ 35,000 | 50 |
| N ≥ 35,001 | 80 |
Based on GB/T 15239, Level S-4.
Step 2: Destructive Inspection (Arbitration)
If defects are found in Step 1, a second sample set (the most serious defects) is selected for sectioning (microscopic evaluation).
| Defective Parts Found in Step 1 (N) | Secondary Sample Size (n) |
|---|---|
| N ≤ 8 | 2 |
| 9 ≤ N ≤ 15 | 3 |
| 16 ≤ N ≤ 25 | 5 |
| 26 ≤ N ≤ 50 | 8 |
| 51 ≤ N ≤ 80 | 13 |
Based on GB/T 2828, Level II.
Final Verdict
Visual Fail: Any quench cracks, folds at inner corners, or severe wrinkles.
Destructive Fail: Any defects exceeding the dimensional limits (depth/width) specified in Sections 1-4.
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