🔩 Interpretation of Fastener Performance Grade (Strength) – Differences between Grade 8.8/10.9/12.9

1、 What does performance level labeling mean?

In the national standard (GB/T 3098.1), “X.Y grade” refers to the mechanical performance grade of carbon steel/alloy steel bolts:

-X · Y level
The first X (such as 8)=tensile strength σ b ÷ 100 MPa → nominal tensile strength=X × 100 MPa

The product X × Y=yield ratio (yield strength ratio) × 100 ≈ yield strength ÷ 10

Example of Level 8.8:
• Tensile strength=8 × 100=800 MPa

Yield strength ≈ 800 × 0.8=640 MPa

2、 Comparison of 8.8, 10.9, and 12.9 levels

Performance grade Tensile strength (MPa) Yield strength (MPa) Typical materials Common uses

4.8 grade 400 ≈ 240 Q235 low carbon steel furniture, non stress ordinary connection

8.8 grade 800 ≈ 640 medium carbon steel quenched and tempered (35 #/45 #) for mechanical equipment, steel structures, and general automotive parts (most commonly used)

Grade 10.9 1000 ≈ 900 alloy steel quenched and tempered (40Cr, etc.) heavy-duty, bridge, key stress nodes

12.9 grade 1200 ≈ 1080 high-strength alloy steel quenched and tempered mold, engine cylinder block, extremely important stress bearing parts

✅ The larger the number → the higher the strength → the material undergoes quenching and tempering heat treatment → the higher the price

3、 How do you look at the physical object?

The top surface of the outer hexagonal bolt head will be stamped with numerical markings:

• 8.8 10.9 12.9

-Level 4.8 usually has no markings or only factory labels
The side of the nut also has grade markings (such as 8, 10, 12), which should be used in conjunction with the bolt (nut grade ≥ bolt grade)

4、 Key precautions ⚠️

1. Prohibition of downgrading and substitution: The drawing requires a grade of 10.9 and cannot be replaced with a grade of 8.8. Insufficient pre tightening force may cause loosening or breakage
2. Equip with nuts of the same grade/higher grade: for example, 8.8 bolts with grade 8 nuts, 10.9 bolts with grade 10 nuts
3. Use with caution for soft/thin connectors at level 12.9: Excessive strength can easily crush the clamped component
4. Hydrogen embrittlement risk: After electroplating at levels 10.9/12.9, dehydrogenation annealing is required, especially for stressed parts
5. The torque system is determined based on the level: for the same specification M12, the recommended torque for level 12.9 is significantly higher than that for level 8.8