8640 vs 8740
These two grades are listed as equivalents of each other: their chemistry windows overlap and they serve the same applications. They are not always identical — check the differences below before substituting, especially in certified designs.
8640
Alloy & Engineering Steel
8640 is the 0.40% carbon triple-alloy Q&T steel: 4140-class strength with nickel for extra toughness, common in aircraft-quality bar specifications. It bridges general engineering and aerospace practice.
Standards: AISI 8640 · JIS SNCM240 (near)
8740
Alloy & Engineering Steel
8740 is chemically 8640 with a higher molybdenum floor, specified where a guaranteed minimum hardenability response is required. In practice the two grades are frequently dual-certified.
Standards: AISI 8740
Chemical composition compared (%)
| Element | 8640 | 8740 |
|---|---|---|
| C | 0.38–0.43 | 0.38–0.43 |
| Si | 0.15–0.35 | 0.15–0.35 |
| Mn | 0.75–1.00 | 0.75–1.00 |
| P | ≤0.035 | ≤0.035 |
| S | ≤0.040 | ≤0.040 |
| Cr | 0.40–0.60 | 0.40–0.60 |
| Ni | 0.40–0.70 | 0.40–0.70 |
| Mo | 0.15–0.25 | 0.20–0.30 |
Properties compared
| Property | 8640 | 8740 |
|---|---|---|
| Hardness (Q&T) | 30–42 HRC typical | 30–42 HRC typical |
| Tensile strength | 950–1200 MPa (Q&T, typ.) | 950–1200 MPa (Q&T, typ.) |
| Density | 7.85 g/cm³ | 7.85 g/cm³ |
Sourcing either grade?
Nasteel stocks both families from Shenzhen with EN 10204 3.1 mill certificates by heat number.
Ask for a quotationSee full data sheets: 8640 · 8740. Substitution decisions for certified parts stay with the design authority.