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 (≤M16) 600 MPa (M18–M64) | Quenched & tempered | Structural, automotive |
| SAE Gr.2 | 55 ksi (1/4″–3/4″) 33 ksi (7/8″–1-1/2″) | None | Light-duty, hardware |
| SAE Gr.5 | 85 ksi (≤1″) 74 ksi (1-1/8″–1-1/2″) | Quenched & tempered | Automotive, machinery |
Proof stress is tiered by diameter in both standards, so a single figure per grade would be wrong for part of the range. ISO 898-1 rates Class 8.8 at 580 MPa up to and including M16 and at 600 MPa from M18 up. SAE J429 rates Grade 2 at 55,000 psi for 1/4″–3/4″ and 33,000 psi for 7/8″–1-1/2″, and Grade 5 at 85,000 psi up to 1″ and 74,000 psi above it. Grade 8 is 120,000 psi across the whole range. Every torque value on this site is calculated from the tier that applies to that specific size.
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.