EN 24 (817M40 / 34CrNiMo6) is a high-tensile through-hardening nickel-chromium-molybdenum alloy steel, normally supplied hardened and tempered. It combines very high strength with excellent toughness, ductility and fatigue resistance, and its high hardenability lets it develop uniform properties even in large sections. It is a standard choice for heavily loaded, safety-critical components.
Composition limits for BS 970 817M40 (weight %). Balance iron.
| Element | Min % | Max % |
|---|---|---|
| Carbon (C) | 0.36 | 0.44 |
| Silicon (Si) | 0.10 | 0.35 |
| Manganese (Mn) | 0.45 | 0.70 |
| Phosphorus (P) | — | 0.035 |
| Sulphur (S) | — | 0.040 |
| Chromium (Cr) | 1.00 | 1.40 |
| Nickel (Ni) | 1.30 | 1.70 |
| Molybdenum (Mo) | 0.20 | 0.35 |
Hardened-and-tempered property levels by ruling section (EN 10083-3).
| Dimension (+QT) | Yield Rp0.2 (MPa) | Tensile Rm (MPa) | Elong. A5 (%) | Red. of area Z (%) | Hardness | Impact Kv |
|---|---|---|---|---|---|---|
| Ø up to 16 mm | 1000 min | 1200–1400 | 9 min | 40 min | 359–404 HB | — |
| Ø 16–40 mm | 900 min | 1100–1300 | 10 min | 45 min | 331–380 HB | 45 J |
| Ø 40–100 mm | 800 min | 1000–1200 | 11 min | 50 min | 298–359 HB | 45 J |
| Ø 100–160 mm | 700 min | 900–1100 | 12 min | 55 min | 271–331 HB | 45 J |
| Ø 160–250 mm | 600 min | 800–950 | 13 min | 55 min | 240–286 HB | 45 J |
| Soft annealed (+A) | — | — | — | — | max. 248 HB | — |
| Transformation point | Temperature (°C) |
|---|---|
| Ac1 | 715 |
| Ac3 | 770 |
| Ms | 320 |
| Mf | 100 |
| Treatment | Temperature cycle | Cooling / quenching |
|---|---|---|
| Hot forging | 900–1100 °C | In still air |
| Normalizing | 860–880 °C | In still air |
| Soft annealing | 650–680 °C | Furnace cool (HB max. 248) |
| Hardening | 830–860 °C | Quench in oil |
| Tempering | 540–660 °C | In air |
| Stress relieving | 550–650 °C | In still air |
Indicative curves from published band data (EN 10083-3), for a reference oil quench of 850 °C. EN 24 is a deep-hardening grade, so the Jominy band stays high across the bar.
Typical values at ambient temperature unless stated.
| Property | Typical value |
|---|---|
| Young’s modulus | 205 GPa |
| Poisson’s ratio | 0.30 |
| Shear modulus | 80 GPa |
| Density | 7,850 kg/m³ |
| Thermal expansion (20–300 °C) | 12.5 µm/m·K |
| Specific heat | 460 J/kg·K |
| Thermal conductivity | 38 W/m·K |
| Electrical resistivity | 0.19 µΩ·m |
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