V.S. Heat Treatment
Heat treatment & metal finishing experts
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V.S. Heat Treatment
Heat treatment & metal finishing experts
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A heat treater’s view: a bolt’s property class comes from material plus quench & temper — not just the number stamped on the head.
Grade 8.8 vs Grade 10.9
Class 8.8 has a nominal minimum tensile strength of 800 MPa (about 22–32 HRC); class 10.9 reaches about 1,040 MPa (32–39 HRC). Both are quenched and tempered, but 10.9 needs alloy or boron steel with higher hardenability, pairs with class 10 nuts, and must be hydrogen de-embrittlement baked if zinc plated afterwards.
| Decision factor | Class 8.8 | Class 10.9 |
|---|---|---|
| Minimum tensile (nominal) | 800 MPa (830 MPa above M16) | 1,040 MPa |
| Minimum yield (nominal) | About 640 MPa | About 940 MPa |
| Hardness range | About 22–32 HRC | About 32–39 HRC |
| Typical material | Medium-carbon or boron steel: S45C, 10B21 | Alloy/boron steel: SCM435, SCM440, 10B33 |
| Process | Quench & temper (min 425°C temper) | Quench & temper (min 425°C, tighter control) |
| Matching nut | Class 8 | Class 10 |
| Hydrogen embrittlement after plating | Lower risk | Baking required per ISO 4042 |
| Typical use | General structural and machine joints | High-preload joints, automotive, cyclic loading |
Choose 8.8 for general fastening where the design load has margin — cheaper, easier material sourcing and lower embrittlement risk after plating.
Choose 10.9 when the joint needs high preload or fewer/smaller bolts — automotive and cyclically loaded structures — accepting the stricter baking and process-control requirements.