Grade 8.8 vs 10.9 vs 12.9 Bolts: Strength, Hardness, Toughness and Price
Comparison of the three carbon steel property classes 8.8, 10.9 and 12.9 per GB/T 3098.1 / ISO 898-1: nominal tensile 800/1000/1200 MPa, minimum yield 640/940/1100 MPa, hardness 22-32/32-39/39-44 HRC, plus quench-and-temper duty, toughness and relative price (1 / 1.2-1.4 / 1.5-1.8).
Overview
The property class number decides how much load a bolt carries and how high it may be torqued. 8.8 is the general-purpose high-strength class, 10.9 serves alternating loads and vibration, and 12.9 is reserved for dies and hydraulics under high static load - but the higher the grade, the more brittle and the more sensitive to hydrogen-delayed fracture. This comparison quantifies eight key parameters and closes with a selection conclusion.
Parameter Comparison Table
| Property | Class 8.8 | Class 10.9 | Class 12.9 |
|---|---|---|---|
| Nominal tensile strength | 800 MPa | 1000 MPa | 1200 MPa |
| Minimum yield strength | 640 MPa | 940 MPa | 1100 MPa |
| Hardness range | 22-32 HRC | 32-39 HRC | 39-44 HRC |
| Typical material | 35#/45# C-steel | 35CrMo | 42CrMo |
| Quench & temper mandatory | — | ✓ | ✓ |
| Fracture toughness | Good | Moderate | Brittle - no impact |
| M10 reference torque (mu ~0.14) | 50 N·m | 70 N·m | — |
| Relative price | ×1.0 | ×1.2-1.4 | ×1.5-1.8 |
Selection Conclusion
Use 8.8 for ordinary static structures; move to 10.9 under alternating loads, vibration or high reliability demands - insufficient preload is the number-one cause of fatigue fracture, and 10.9 lets you clamp the joint much harder. Use 12.9 with care where impact exists: higher strength means greater brittleness, and plated quenched-and-tempered parts must be de-embrittled. Never mix 8.8 and 10.9 in one joint - the weaker bolts fail first. Prices are common domestic ranges, moving with size and finish.
FAQ
For 8.8: the first digit x 100 = nominal tensile strength in MPa; the digit after the point x the first digit = the yield value, i.e. 0.8 x 800 = 640 MPa yield. The rule comes from GB/T 3098.1 (equivalent ISO 898-1).
Typically 20-40% more (about 1.2-1.4x); 12.9 sits around 1.5-1.8x. Prices move with size and finish - higher grades also tend to come with Dacromet-type coating and de-embrittlement costs.
Higher strength means lower toughness: 12.9 runs 39-44 HRC, is brittle under shock loads, and quenched-and-tempered alloy parts are more sensitive to hydrogen-delayed fracture - de-embrittlement at 190-220°C is mandatory after plating.
No. Mixed grades mean the weaker bolts overload and fail first, then the rest follow. Keep one grade and one batch per joint, torqued to the matching chart.
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