A tough core and a hard case are the desired attributes of case-hardened steel components. This combination of properties provides wear resistance and fatigue strength at the surface, and impact strength in the core. It is achieved by carburizing the component’s surface, then quenching and tempering the part. Carburized components include gears of all kind, camshafts, universal joints, driving pinions, link components, axles and arbours. All these components must resist wear and fatigue, have inherent toughness, and still be machinable.
Typical 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 large components have to withstand cyclic stress and wear in hydroelectric power stations, wind-turbine generators, propeller drives of drilling rigs and steam-turbine gears of power stations.
- General mechanical engineering: Applications in this area include forging presses, metal rolling equipment, machine tools; drivelines of mining equipment and heavy-duty transmissions; earthmoving equipment and heavy-duty construction cranes. Wear resistance and good fatigue strength are always key characteristics of the case-hardened steels used for these applications.
Everything that moves needs case-hardened gears
During carburisation, the component is heated in a carbon-releasing medium to a temperature where the steel is completely austenitic. Carbon’s solubility is much higher in austenite than in ferrite, which allows carbon to pass through the steel surface and diffuse into the component. Carburization can increase the surface carbon content up to 0.7%. Controlling the time at temperature allows control of the depth to which the carbon diffuses, and thus the thickness of the “case.” It also allows the carbon content of the core to remain at about 0.25%. An important microstructural goal during carburisation is a stable, uniformly fine-grained austenite. A uniform austenite grain size results in low distortion after heat treatment, while a fine austenite grain size improves fatigue resistance and toughness.
Quenching from the carburising temperature and subsequent tempering of the component produces a high-carbon martensite having great hardness and wear resistance near the surface. The uncarburised core retains its original good strength and toughness properties.
The selection of appropriate alloying elements permits precise control of hardenability from the surface to the core. (See Figure 1 for an example of a Jominy curve used to assess hardenability.) The appropriate steel depends on the size of the part to be treated, since it is a goal to produce a strong, tough, tempered martensite structure in the core.