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FULLERKREG GB70-85 / ISO 4762 Grade 8.8 Black Oxide Socket Cap Screws are medium-strength hexagon socket head cap screws manufactured from 10B21 boron steel or 35# carbon steel with a black oxide surface treatment. Conforming to both the Chinese national standard GB 70-85 and the international ISO 4762 standard, these screws offer reliable Grade 8.8 performance with a professional matte black appearance. They are a cost-effective, widely stocked solution for general industrial, machinery, and structural applications.
Black oxide (also known as blackening or chemical blacking) is a conversion coating applied to steel fasteners through a controlled chemical reaction. It produces a uniform matte black finish that provides:
FULLERKREG GB70-85 Grade 8.8 black socket cap screws are manufactured from 10B21 boron-alloyed steel or 35# medium carbon steel. All values are maximum unless a range is indicated.
| Grade | C | Si (max) | Mn | P (max) | S (max) | B |
|---|---|---|---|---|---|---|
| 10B21 | 0.18–0.23 | ≤0.30 | 0.80–1.10 | ≤0.030 | ≤0.030 | 0.0005–0.003 |
| 35# Steel | 0.32–0.39 | 0.17–0.37 | 0.50–0.80 | ≤0.035 | ≤0.035 | — |
| Property | Requirement | Unit |
|---|---|---|
| Tensile Strength (Rm) | Min. 800 | MPa |
| Yield Strength / Proof Load Stress (Rp0.2) | Min. 640 | MPa |
| Core Hardness | 22–32 | HRC |
| Surface Hardness (max) | ≤36 | HRC |
| Elongation after fracture (A) | Min. 12 | % |
| Reduction of area (Z) | Min. 52 | % |
| Thread Tolerance Class | 6g | — |
| Surface Treatment | Black oxide (blackening) | — |
GB 70-85 is the Chinese national standard for hexagon socket head cap screws issued in 1985. It is dimensionally compatible with ISO 4762 and DIN 912 for most common sizes, making these screws interchangeable in most applications.
| Aspect | GB 70-85 | ISO 4762 / DIN 912 |
|---|---|---|
| Issuing body | China National Standard (GB) | ISO / DIN (International / German) |
| Year | 1985 | ISO 4762: 2004 / DIN 912: 2014 |
| Head dimensions | Compatible for M3–M48 | Compatible for M3–M48 |
| Thread standard | GB/T 197 metric thread | ISO 261 metric thread |
| Mechanical requirements | GB/T 3098.1 | ISO 898-1 |
| Interchangeability | Generally interchangeable for standard sizes | |
| Property | Black Oxide | Zinc CR3 |
|---|---|---|
| Appearance | Matte black | Bright silver |
| Coating thickness | 1–2 μm | 5–12 μm |
| Corrosion resistance | Moderate (with oil) | Good (72–120 hrs salt spray) |
| Dimensional impact | Minimal | Minor |
| RoHS compliance | ✅ Compliant | ✅ Compliant (CR3) |
| Best for | Indoor, precision, aesthetic applications | Outdoor, corrosive environments |
| Price | 🟢 Economy | 🔵 Slightly higher |
FULLERKREG GB70-85 / ISO 4762 Grade 8.8 black oxide socket cap screws are widely used in:
| Thread (d) | Pitch (p) | Head Dia. dk Max. | Head Dia. dk Min. | Head Height k Max. | Head Height k Min. | Key Size s | Key Depth t Min. | Body Dia. ds Max. | Body Dia. ds Min. |
|---|---|---|---|---|---|---|---|---|---|
| M3 | 0.5 | 5.50 | 5.32 | 3.00 | 2.86 | 2.5 | 1.3 | 3.00 | 2.86 |
| M4 | 0.7 | 7.00 | 6.78 | 4.00 | 3.82 | 3.0 | 2.0 | 4.00 | 3.82 |
| M5 | 0.8 | 8.50 | 8.28 | 5.00 | 4.82 | 4.0 | 2.5 | 5.00 | 4.82 |
| M6 | 1.0 | 10.00 | 9.78 | 6.00 | 5.70 | 5.0 | 3.0 | 6.00 | 5.82 |
| M8 | 1.25 | 13.00 | 12.73 | 8.00 | 7.64 | 6.0 | 4.0 | 8.00 | 7.78 |
| M10 | 1.5 | 16.00 | 15.73 | 10.00 | 9.64 | 8.0 | 5.0 | 10.00 | 9.78 |
| M12 | 1.75 | 18.00 | 17.73 | 12.00 | 11.57 | 10.0 | 6.0 | 12.00 | 11.73 |
| M14 | 2.0 | 21.00 | 20.67 | 14.00 | 13.57 | 12.0 | 7.0 | 14.00 | 13.73 |
| M16 | 2.0 | 24.00 | 23.67 | 16.00 | 15.57 | 14.0 | 8.0 | 16.00 | 15.73 |
| M20 | 2.5 | 30.00 | 29.67 | 20.00 | 19.48 | 17.0 | 10.0 | 20.00 | 19.67 |
| M24 | 3.0 | 36.00 | 35.61 | 24.00 | 23.48 | 19.0 | 12.0 | 24.00 | 23.67 |
| M30 | 3.5 | 45.00 | 44.61 | 30.00 | 29.48 | 22.0 | 15.5 | 30.00 | 29.67 |
| M36 | 4.0 | 54.00 | 53.54 | 36.00 | 35.38 | 27.0 | 19.0 | 36.00 | 35.61 |
| M42 | 4.5 | 63.00 | 62.54 | 42.00 | 41.38 | 32.0 | 24.0 | 42.00 | 41.61 |
| M48 | 5.0 | 72.00 | 71.54 | 48.00 | 47.38 | 36.0 | 28.0 | 48.00 | 47.61 |
Q: Is GB 70-85 the same as ISO 4762 / DIN 912?
GB 70-85 is the Chinese national standard issued in 1985 and is dimensionally compatible with ISO 4762 and DIN 912 for standard sizes M3–M48. In most applications, GB 70-85 and ISO 4762 screws are interchangeable.
Q: How durable is the black oxide finish?
Black oxide provides moderate corrosion resistance suitable for indoor and dry environments. For enhanced protection, screws are typically treated with oil or wax after blackening. For outdoor or humid environments, we recommend Zinc CR3 plating instead.
Q: Will black oxide affect the thread dimensions?
No. Black oxide coating is only 1–2 μm thick and has virtually no impact on thread dimensions or fit. This makes it ideal for precision assemblies where tight tolerances must be maintained.
Q: Can I get material certificates and test reports?
Yes. PPAP reports, material certificates, hardness test reports, and tensile test reports are available upon request for all FULLERKREG products.
Q: What is the minimum order quantity?
We supply both small quantities and large bulk orders. Please contact our sales team for pricing and MOQ details.
All FULLERKREG GB70-85 / ISO 4762 Grade 8.8 black oxide socket cap screws are manufactured under strict quality control procedures including:
For wholesale pricing, bulk orders, or technical inquiries, please contact our sales team.
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