Understanding Case Hardening of Steel

What do gears, bearings, and shafts have in common? For one thing, they're often made out of steel. For another, they're subject to a failure mode known as rolling contact fatigue.

Rolling contact fatigue can occur in many kinds of rotating components. Tiny cracks start just below the surface of the component. As the cracks make their way to the surface, material begins to flake away, leaving pits. This process is called spalling. As this process continues, the rotating components become noisy, due to the debris that has been generated. If not stopped, the components will deteriorate, and eventually fail completely.

Parts used in this type of application are usually heat-treated to protect them against spalling. However, through-hardening treatments -- heat treatments that result in a uniform hardness throughout a part's thickness -- tend to make parts brittle. Therefore, components used in rolling contact applications are often case hardened.

Case hardening is a heat-treatment process that produces a hard and wear-resistant surface layer, while the inside of the part remains ductile. Think of an M&M candy: hard and crunchy on the outside, soft on the inside. The hardened layer is called the case. The thickness of the hardened layer is called the case depth.

There are several widely used case-hardening processes for steel. These include atmosphere carburizing, vacuum carburizing, and induction hardening, among others. The choice of a case-hardening process can affect the durability of the hardened part.

In atmosphere carburizing, steel parts are heated in a furnace with a mixture of carbon-containing gases (usually carbon monoxide, carbon dioxide, and methane). Carbon from these gases is absorbed by the steel, and diffuses into the metal from the surface inwards. The parts are then quenched -- usually in oil, but sometimes in water, brine, molten salt, or a polymer solution. The rapid cooling from the quench causes a change in the steel's crystal structure from austenite to martensite, increasing its hardness. The highest hardness is found near the surface, where the greatest amount of carbon has been absorbed. After quenching, the parts are tempered by placing them in a furnace at a temperature of 300F to 400F, in order to prevent brittleness and cracking.

In recent years, vacuum carburizing has become increasingly popular. In this process, parts are heated in a vacuum chamber. When the parts are at a high temperature, a carburizing gas (usually methane, propane, or acetylene) is injected into the chamber. This allows better control of carbon diffusion than is possible in atmosphere carburizing. Vacuum carburized parts can be quenched using traditional liquid quenchants, or they can be gas quenched, using an inert gas like nitrogen or argon. Gas quenching reduces dimensional distortion. As with atmosphere carburizing, vacuum carburized parts must be tempered after quenching.

Carburizing treatments (atmosphere or vacuum) are usually performed on steels with carbon contents in the range of 0.15% to 0.25%. After the carburizing process, the carbon content at the surface of the part is typically in the range of 0.8% to 1.0%, gradually tapering off with increasing distance from the surface. The higher the carbon content, the higher the hardness after quenching. (Of course, you can have too much of a good thing -- too high of a carbon content results in a brittle microstructure.)

Induction hardening is another case-hardening process. Parts are heated by electromagnetic induction, then rapidly quenched by a spray of water or polymer solution. Unlike carburizing, induction hardening does not increase the carbon content of the steel. Therefore, it is only done to steels that already have a carbon content of 0.4% to 0.6%. The main advantages of the induction hardening process are that it is quick (requiring minutes instead of hours) and easily automated.