Carbon steel is the most common material for industrial fasteners. The strength depends on carbon content and heat treatment. Low carbon (grades 4.6, 4.8, SAE Grade 2) is unhardened and used for light-duty work. Medium carbon with quench-and-temper heat treatment (grades 8.8, SAE Grade 5) is the standard for structural and automotive applications.
The TorqueSpec database covers all standard carbon steel property classes from 4.6 through 8.8 (metric) and Grade 2 and Grade 5 (SAE/imperial). These represent the vast majority of bolts in everyday use.
| Grade | Proof Stress | Heat Treatment | Application |
|---|---|---|---|
| 4.6 | 225 MPa | None | Light fixtures, furniture |
| 4.8 | 310 MPa | None | General hardware |
| 5.6 | 280 MPa | None | Non-structural |
| 5.8 | 380 MPa | None | Medium-duty |
| 6.8 | 440 MPa | None | Mid-range structural |
| 8.8 | 580 MPa | Quenched & tempered | Structural, automotive |
| SAE Gr.2 | 33 ksi | None | Light-duty, hardware |
| SAE Gr.5 | 85 ksi | Quenched & tempered | Automotive, machinery |
Carbon steel bolts are almost always coated to prevent rust. The coating type significantly affects the friction coefficient and therefore the correct torque. A zinc-plated M10×8.8 bolt needs less torque than a plain/dry one because zinc reduces friction (K = 0.18 vs K = 0.20). A hot-dip galvanized bolt needs more (K = 0.25) because the thick, rough coating increases friction.
Common coatings include zinc electroplating (most common), hot-dip galvanizing (outdoor/infrastructure), black oxide (appearance, minimal protection), and phosphate-and-oil (automotive OEM). TorqueSpec provides torque values for all six standard surface conditions on every page.