Case-hardening steel

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.

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.

Table 1: Standard case-hardening steels

Molybdenum (0.15 - 0.50%) is used in carburising steels to increase the hardenability of the low-carbon core and toughen the high-carbon case at the same time. It is especially effective in large cross sections like those of wind-turbine gears. Molybdenum is not oxidised during carburisation, so it does not cause increased surface cracking and spalling. This also means it is not lost by reaction, but remains present in the alloy to provide effective hardening.

Wind power – a major driver for carburising steel development

Gears used in large wind turbines are subject to extreme loads at the flanks and toes of their teeth, especially when sudden changes in wind speed or hard stops occur. A hard case and tough core result in a more wear-resistant gear capable of handling high impact loads. Wind-turbine gearboxes are designed to minimize mechanical noise for quiet operation, but gear noise increases during life due to abrasion of gear tooth surfaces. Increasing the surface hardness and abrasion resistance of gears will thus decrease gearbox noise. The hard case/tough core combination possessed by carburized gears is of advantage in this regard. The low-alloy steels generally used for case-hardening processes (e.g. 20MnCr5) are not applicable when long fatigue life and high toughness are required. High-performance NiCrMo case-carburizing steels provide deep hardening ability and possess high fatigue resistance. Currently, the grade 18CrNiMo7-6 is the standard gear steel for windmill gearboxes. With respect to further optimizing carburizing steels for large and heavily loaded gears, the following priorities can be defined:

• Increased core tensile strength and toughness
• Higher fatigue strength in both core and case
• Improved hardenability
• Low distortion upon quenching
• Improved properties at elevated service temperatures.

Several sources of property degradation must be attacked to accomplish these goals. Intergranular oxidation in the carburized layer can initiate fatigue fracture, reducing the fatigue strength of the tooth. It also causes a soft zone near the surface of the carburized layer. Eliminating anomalies in surface structure is thus an important goal in the development of gears with high fatigue strength. Raising the tempering temperature improves toughness, but requires increased tempering resistance in order not to lose strength. The initial approach to implement these improvements is to adjust the steel’s chemical composition, using the following guidelines:

• Prevent intergranular oxidation → reduce Si, Mn, and Cr.
• Improve hardenability → increase Mo.
• Improve toughness → increase Ni and Mo.
• Refine and homogenize grain size → balance Nb, Ti, Al and N microalloying additions.
• Strengthen grain boundaries → reduce P and S.

The case hardness can be increased further by forming a dispersion of ultra-hard Mo and Nb carbides. This will offer a better mechanical support to the carburized case or a potential hard surface coating. Simply raising the bulk carbon content would of course also raise the hardenability, but this approach sacrifices toughness.