Bolt Grade Markings Explained: 4.6, 8.8, 10.9 & 12。Bolt Grade Markings Explained: 4.6, 8.8, 10.9 & 12の完全ガイド - 仕様、用途、アフリカ市場向けB2B輸出のサプライヤー選定。
Why Bolt Grades Matter: The Cost of Getting It Wrong
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How to Read Bolt Head Markings
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| Grade | Tensile (MPa) | Yield (MPa) | Proof Load (MPa) | Head Marking |
|---|---|---|---|---|
| 4.6 | 400 | 240 | 225 | 4.6 |
| 5.6 | 500 | 300 | 283 | 5.6 |
| 6.8 | 600 | 480 | 448 | 6.8 |
| 8.8 | 800 | 640 | 580 | 8.8 |
| 10.9 | 1000 | 900 | 830 | 10.9 |
| 12.9 | 1200 | 1080 | 970 | 12.9 |
Grade-by-Grade Selection Guide
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| Grade | Material | Heat Treatment | Typical Application | Relative Cost |
|---|---|---|---|---|
| 4.6 | Low carbon steel | None | General hardware, furniture | 1x |
| 8.8 | Medium carbon steel | Q&T | Structural steel, machinery | 1.8x |
| 10.9 | Alloy steel | Q&T | Heavy machinery, automotive | 2.5x |
| 12.9 | High alloy steel | Q&T | Critical connections, hydraulic | 3.5x |
Matching Nuts and Washers: A System Approach
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| Bolt Grade | Minimum Nut Class | Washer Hardness | Washer Standard |
|---|---|---|---|
| 4.6 | 4 | Soft (100 HV) | ISO 7089 |
| 8.8 | 8 | Hard (300 HV) | ISO 7090 |
| 10.9 | 10 | Hard (300 HV) | ISO 7090 |
| 12.9 | 12 | Hard (390 HV) | ISO 7093 |
Coating Interactions: What You Must Know
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| Grade | Hot-Dip Galv | Electroplate | Mechanical Galv | Sherardizing |
|---|---|---|---|---|
| 4.6 | ✅ Safe | ✅ Safe | ✅ Safe | ✅ Safe |
| 8.8 | ✅ Safe | ⚠️ Bake required | ✅ Safe | ✅ Safe |
| 10.9 | ⚠️ May reduce strength | ⚠️ Bake mandatory | ✅ Recommended | ✅ Recommended |
| 12.9 | ❌ Not recommended | ❌ Never | ✅ Recommended | ✅ Recommended |
5 Common Procurement Mistakes
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Frequently Asked Questions
See frequently asked questions below.
What does the dot in 8.8 mean?
The dot separates the two numbers. First number (8) = tensile strength / 100 = 800 MPa. Second number (8) = yield ratio × 10 = 0.8, so yield strength = 800 × 0.8 = 640 MPa.
Can I use Grade 8.8 instead of 10.9?
Only if the engineering design allows it. Grade 8.8 has 80% of the tensile strength but only 71% of the yield strength of Grade 10.9. Never substitute down without engineer approval.
Are stainless steel bolts as strong as Grade 8.8?
No. Standard A2-70 stainless bolts have a tensile strength of 700 MPa — less than Grade 8.8 (800 MPa). For equivalent strength, use A4-80 or A2-80 grade stainless, but these are specialty items with premium pricing.
How do I verify bolt grade on receipt?
Check the head marking matches the purchase order specification. For critical applications, request mill test certificates (MTC) per EN 10204 Type 3.1. Random hardness testing is also recommended for large orders.
Need certified bolts and nuts in bulk? TradeGo supplies all grades with mill test certificates.
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