Alloy steel fasteners contain additional elements — typically chromium, molybdenum, vanadium, or nickel — beyond the carbon and manganese found in plain carbon steel. These alloying additions allow the steel to achieve higher hardness and strength through quenching and tempering.
In the ISO system, alloy steel is used for property classes 10.9 and 12.9. In the SAE system, Grade 8 bolts are quenched and tempered medium carbon alloy steel. Socket head cap screws (SHCS) per ASTM A574 are also alloy steel, typically at class 12.9 strength levels.
| Grade | Proof Stress | Tensile | Typical Alloy |
|---|---|---|---|
| ISO 10.9 | 830 MPa | 1040 MPa | Cr-Mo or Mn-Cr steel |
| ISO 12.9 | 970 MPa | 1220 MPa | Cr-Mo-V steel |
| SAE Grade 8 | 120,000 psi | 150,000 psi | Medium carbon alloy |
| ASTM A574 | 140,000 psi | 180,000 psi | Alloy steel SHCS |
Hydrogen embrittlement: High-strength alloy steel bolts (HRC 39+, which includes class 12.9) are susceptible to hydrogen-induced delayed fracture. Electroplating processes introduce hydrogen, which must be baked out within 4 hours of plating. This is a leading cause of unexpected bolt failures in service.
Temperature limits: Standard alloy steel bolts should not be used above 300°C (570°F) continuously. Elevated temperatures reduce proof strength and can cause stress relaxation, leading to clamp load loss over time.
Corrosion: Alloy steel has no inherent corrosion resistance. Protective coatings (zinc plating, black oxide, phosphate) are required for all but dry indoor environments. The coating type affects the K-factor and therefore the torque specification — always verify which surface condition the torque spec assumes.