Case hardening is a heat-treatment process that is used to prevent rotating components from becoming brittle and failing. Machines that use rotating components like ball bearings, gears, and shafts are subject to “rolling contact fatigue” or “spalling.” Rolling contact fatigue is caused by miniature cracks that occur just below the surface. The more a component is used, the more these cracks snake up to the surface. Eventually, once the cracks reach the surface of the component, they cause pits that result in flaking. The more material that is pitted, the more the component vibrates. Vibrating components are louder and often cause full equipment breakdowns.
It is possible to prevent rolling contact fatigue by heat-treating them. However, heat-treating also contains the potential to make the components brittle, which causes another set of problems. Case hardening is the process used to prevent brittleness after heat treatment.
Case hardening produces a hard and wear-resistant outer layer while preserving the ductile-strength of the interior. Case hardening a gearbox components creates a hard outer-shell and a pliable inner layer. The hardened layer is called the case. The thickness of the hardened layer is referred to as the case depth.
There are several methods of case hardening for gears, including vacuum carburizing, atmosphere carburizing, and induction hardening. Each process produces a variety of case depths and varying degrees of hardness. Whichever process you rely on depends on how much or how little you need to harden.
The two processes described below are carburizing processes, which mean that carbon is introduced into the steel to increase its strength. Carburizing treatments are useful for steels that have a carbon content between 0.15 to 0.25 percent. After the treatment, the carbon content at the surface of the steel should be raised to 0.8 to 1.0 percent.
In the atmosphere carburizing process, steel parts are heated in a furnace and mixed with carbon-containing gases such as methane, carbon monoxide, and carbon dioxide. The steel absorbs the carbon from these gases and diffuses from the surface of the steel inwards. Once they are mixed, the parts are then cooled, typically using oil, but brine, molten salt, water, or a polymer solution can also be used.
The rapid cooling from the quench causes the steel structure to change from austenite to martensite, which increases its hardness. Once quenched, the parts are tempered in a furnace between 300 to 400 degrees Fahrenheit to prevent cracking and brittleness.
Vacuum carburizing involves heating the steel in a vacuum chamber. Once the parts reach the necessary temperature, the carbon gases are injected into the chamber. This method offers several advantages over atmosphere carburizing, including better control of carbon diffusion, which minimizes dimensional distortions. The steel parts are then quenched in an inert gas, such as nitrogen, or in one of the liquids described above. Finally, the parts are tempered in a similar manner to atmosphere carburizing.
Induction case hardening involves heating the steel parts using electromagnetic induction. The parts are then rapidly cooled using a polymer solution or a spray of water. Unlike vacuum and atmosphere carburizing, induction hardening does not introduce new carbon into the steel. Because of this, induction hardening is only used for steel that already has a carbon content of around 0.4 to 0.6 percent. The advantage that this method offers is that it is fast and easily automated.
The method of case hardening you use depends on the needs of your company. It is useful to consult the technical requirements of your equipment based on its application. The amount of fatigue will determine which process to use, because you will require a certain thickness in the hardened case to prevent fatigue. In general, a carburizing process will produce thicker cases, so these are ideal for components that are under a lot of stress.