Fastener Surface Defect Judgment: Practical Interpretation of the GB/T 5779 Series Standards
Executive Summary
Surface defect judgment is the inspection item that relies most on judgment skill: the same crack may be rejected in one case and accepted in another; the acceptance limit for the same fold changes with property class. Following the framework of the GB/T 5779 series, this article sorts out defect classification, acceptance principles and common inspection methods, and gives a field judgment workflow and recommendations for handling disputes.
Table of Contents
Same Bolt Batch, Two Inspectors, Two Conclusions
During incoming inspection of a batch of 10.9-class M12 bolts, Inspector A found a fine longitudinal line in the head transition fillet and rejected the batch; on re-inspection, Inspector B considered it a raw-material fold whose depth did not exceed the allowable value and passed the batch. Same shipment, two conclusions.
The root of the disagreement is not who is less professional, but that surface defect judgment requires answering three questions at the same time: what kind of defect is it? Where is it located? How deep is it? Only after the three answers are combined can a conclusion be reached.
GB 5779.2-2000 (Surface discontinuities on nuts) and GB 5779.3-2000 (Surface discontinuities on bolts, screws and studs — Special requirements) are precisely the standards that provide the judgment framework for these three questions. They do not answer "is there a defect or not"; they only give the criterion for "which defect, under which conditions, is acceptable."
The Standard Framework: From General to Specific
The GB/T 5779 series is divided by product type; the two most commonly used parts have different scopes:
| Standard | Applicable Product | Main Focus |
|---|---|---|
| GB 5779.1 | Bolts, screws and studs — General requirements | General classification of surface defects and general acceptance principles |
| GB 5779.2-2000 | Nuts | Judgment of defects on nut-specific features (bearing face, thread, washer face) |
| GB 5779.3-2000 | Bolts, screws and studs — Special requirements | Tighter limits by property class, especially for high-strength bolts |
The Four Defect Categories and Their Acceptance Principles
1. Cracks
Characteristics: The continuity of the metal is broken; the fracture surface shows fresh (unoxidized) metal color, has a sharp tip, and typically runs perpendicular or at an angle to the material flow lines.
Judgment: Cracks are the most harmful defect category. Cracks located at the thread root, the head transition fillet, or the bearing-face radius — regardless of size — must always be rejected; cracks in low-stress areas that are extremely shallow are judged by property class — the higher the class, the tighter the limit.
Key distinction: The fracture surface of a crack is fresh metal color, whereas the grooves of folds and laps usually contain oxidation color. This is the most direct field method for distinguishing cracks from folds.
2. Folds and Lap Seams
Characteristics: Strip-shaped grooves formed when material is pressed into the surface during forming; the groove bottom is rounded, and the groove interior shows oxidation color or a color different from the base material.
Judgment: The main hazard of folds is fatigue and decarburization, not direct fracture. The acceptance limit depends on location and depth: folds at the thread root and in transition zones should be judged strictly; shallow folds in the middle of the shank are acceptable. GB 5779.3 specifies the allowable depth of folds by property class.
3. Pits, Scratches and Mechanical Damage
Characteristics: Material is missing from the surface, with no metallurgical bonding features. Causes include die wear, handling bumps, and improper machining.
Judgment: The main criteria are depth, location, and whether pitch error is caused. A longitudinal scratch on the thread that causes the go thread ring gauge to fail must be rejected; one that only affects the thread profile but still passes the go gauge and the thread gap gauge is acceptable. A pit on the bearing face that affects bearing-face flatness or reduces the load-bearing area needs to be evaluated.
4. Heat-Treatment and Coating-Related Defects
- Decarburization: Complete decarburization is not permitted on the cross-section of an unengaged threaded section; the depth of partial decarburization is limited by property class (class 8.8 no greater than 1/2 H1, class 10.9 no greater than 1/3 H1, class 12.9 does not allow any complete decarburization).
- Coating blistering, peeling, local missing plating: These are coating adhesion issues and must be judged separately according to coating standards; they must not be lumped together with surface defects.
- Hydrogen-embrittlement micro-cracks: Usually invisible to the naked eye; must be judged by the pre-load test of GB/T 3098.17.
Location Determines Severity
A defect of the same depth and form can have completely different consequences depending on where it lies. The reason lies in the stress distribution:
| Defect Location | Stress State | Judgment Tendency |
|---|---|---|
| Thread root (near the first load-bearing thread) | Most severe stress concentration; preferred crack-initiation site | Strictest, usually rejected |
| Head transition fillet (under-head radius) | Stress-concentration zone, superimposed bending and tension | Strict; longitudinal defects usually rejected |
| Bearing face (bolt head or nut pressure face) | High contact pressure; affects friction and crushing | Check whether it affects load-bearing area and flatness |
| Middle of the shank | Mainly under tension, no significant concentration | Shallow defects acceptable |
| Thread runout end | Lower stress, but may affect engagement | Check whether it affects thread function and engagement length |
| Nut bearing face and thread flanks | Shear and contact stress | Judged per GB 5779.2 |
Common Inspection Methods and Their Limitations
| Method | What It Can Detect | Limitations |
|---|---|---|
| Visual inspection (with 5–10× magnifier) | Obvious cracks, folds, pits, coating defects | Relies on inspector experience; no quantitative depth measurement |
| Magnetic particle testing (MT) | Surface and near-surface cracks in ferromagnetic materials | Ferromagnetic materials only; demagnetization required; insensitive to folds |
| Dye penetrant testing (PT) | Surface-open defects on non-magnetic materials (stainless steel, aluminum) | Only detects open defects; sensitivity drops on rough surfaces |
| Metallographic sectioning | Defect depth, whether decarburized, whether it is a fold | Destructive, sampling only; section position determines whether the defect is hit |
| Thread gauges (go gauge + no-go gauge) | Whether thread size and form are affected | Judges thread function only, not the defect itself |
| Eddy current testing | Surface and near-surface defects | Poor adaptability to complex shapes (thread roots) |
Note: Magnetic particle testing is not sensitive to groove-type defects such as folds (because the oxides inside the groove do not produce magnetic indications), whereas penetrant testing is effective for open folds; conversely, penetrant testing may miss closed cracks. Therefore, when the defect type is uncertain, the two methods should be used together.
Judgment Workflow and Dispute Resolution
Five-Step Field Judgment Method
- Location: Determine where the defect is on the part (thread root / transition zone / bearing face / shank).
- Classification: Decide which category it belongs to (crack / fold / mechanical damage / coating defect). Checking for oxidation color inside the groove is the key to distinguishing cracks from folds.
- Measurement: Measure defect length and depth. Depth can be estimated by metallographic sectioning or a depth gauge; do not estimate depth by eye.
- Standard lookup: Look up GB 5779.2 or 5779.3 by product type, and consult the limits corresponding to defect type, location and property class.
- Conclusion: Comprehensively judge as acceptable / unacceptable / conditionally released (e.g., downgraded use or used on non-load-bearing positions).
Three Recommendations for Dispute Resolution
- Unify the standard version and criteria first: Supplier and purchaser should state in the technical requirements in advance the standard number, version and specific clauses being used, rather than each citing its own version when a dispute arises.
- Use metallographic sectioning for arbitration: For defects whose form and depth are disputed, the most effective arbitration method is to take a section sample and use metallographic photos to determine depth, whether decarburized, and whether there is metallurgical bonding with the base material. In the face of data, the dispute can end.
- Build a defect sample library: Make historical typical defects into physical samples or photo sets, as a common reference for both parties. This is far more effective than textual description and can also significantly shorten the training cycle for new inspectors.
Closing
The three core questions of surface defect judgment are always: what is it, where is it, and how deep is it. The GB 5779 series provides the criteria, but answering these three questions relies on inspection methods and accumulated experience.
The most effective path in practice is: visual 100% screening; when suspicious parts are found, use magnetic particle or penetrant testing to confirm the form; when necessary, use metallographic sectioning to measure depth and determine whether decarburized; finally judge according to the corresponding clause of GB 5779.2 or 5779.3. When a dispute arises, use sectioning data for arbitration and use a physical sample library to align understanding.
What to remember is that the standard will not judge for you; it only tells you where the criteria are. Which location the defect appears at, what the property class is, and how many micrometres deep it is — obtaining these specific pieces of information is the true dividing line of inspection capability.
