Case hardening Steel
Tough core and a hard case are the target properties of components made of case hardened steel. That combination of wear resistance and fatigue strength in the surface and impact strength in the core zone is achieved by carburizing the surface layer of the component, which is subsequently quenched and tempered.
Components produced that way with optimized properties between core and case include gear components of all kind, camshafts, cardan joints, driving pinions, link components, axles and arbors.
Applications include:
- Transportation: Case hardened components are needed in any engine driven vehicle, whether it's a small car, a race car, a truck or an ocean vessel.
- Energy generation: Gear wheels and components in large dimensions have to withstand both stress and wear in equipment such as hydroelectric power stations, wind turbine generators, propeller drives of drilling rigs or steam turbine gears of power stations.
- General mechanical engineering: forging presses, steel rolling equipment, machine tools; drivelines of mining equipment and heavy duty transmissions; earth moving equipment and heavy duty construction cranes. The combination of wear resiastance and fatigue strength is always a key characteristic of the case hardened steels used for these applications.
For carburisation the steel is heated in a carbon releasing medium to a temperature where the base material is completely transformed into austenite (here the solubility for carbon is much higher than in the ferritic structure).
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Case hardened gears for everything that moves
This way the surface layer is carburised up to 0.7% carbon, while the carbon content of the core material is limited to about 0.25%.
Quenching and tempering following the carburisation produces a high carbon martensitic structure near the surface, with great hardness and wear resistance, while the core retains its original strength and toughness properties.
| DIN - EN | SAE/ASTM | % Alloy content | |||
|---|---|---|---|---|---|
| C | Cr | Mo | Other | ||
| MnCr Steel | |||||
| 20MnCr5 | 5120 | 0.2 | 1.2 | 1.3 Mn | |
| CrMo Steel | |||||
| 20MoCr4 | 0.2 | 0.4 | 0.5 | ||
| 20CrMo5 | 8620 | 0.2 | 1.2 | 0.25 | |
| NiCrMo Steel | |||||
| 20NiCrMo2-2 | 0.2 | 0.5 | 0.25 | 0.5 Ni | |
| 18CrNiMo7-6 | 0.18 | 1.7 | 0.3 | 1.5 Ni | |
Table 1: Standard case hardening steels
Molybdenum (0.15 - 0.50%) is used in carburising steels to simultaneously increase the hardenability of the low carbon core and toughen the high carbon case. It is especially effective in large cross sections, such as in gears. Molybdenum is not oxidised during carburisation, making it an effective hardening agent which does not cause increased surface cracking and spalling.