Dave Palmer asks, “What do gears, bearings, and shafts have in common?” The answer, “For one thing, they’re often made of steel. For another, they’re subject to a failure mode known as rolling contact fatigue.”
According to Palmer, many kinds of rotating components may fall prey to rolling contact fatigue. Just below the surface of the part, tiny cracks begin and as they work their way to the surface, those cracks lead to pits when material begins to flake away. If you’ve worked many hours with and around ball bearings, you’d recognize this process as spalling.
The more frequent this process continues, any rotating apparatus will start to vibrate, create noise, and quite possibly lead to equipment failure.
Palmer writes that machine parts that are used in those types of applications are protected against spalling by heat-treating them. Unfortunately, heat-treating may also make those machine parts brittle. To guard against becoming brittle, machine parts that are used in rolling contact applications are 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 300 °F to 400 °F, in order to prevent brittleness and cracking.
One process of carburizing has become popular as of late: vacuum carburizing. Palmer writes that parts are heated in a vacuum chamber and when they reach a certain high temperature, methane, propane, acetylene, or another carburizing gas is injected into the vacuum chamber. This method provides better control of carbon diffusion compared to atmosphere carburizing. To minimize dimensional distortion, the vacuum carburized parts are quenched either by using an inert gas, such as nitrogen or carbon, or with traditional liquid quenchants. One common step that atmosphere and vacuum carburizing have is the machine 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.
So, which hardening process provides the best resistance to spalling? Palmer explored that question as part of a research project. Palmer’s group studied four different steels: two were vacuum carburized, one was atmosphere carburized, and the fourth one was induction hardened. They measured the number of cycles to failure in a rolling contact fatigue test.
Usually, when engineers design case-hardened parts, they specify the surface hardness and the case depth. Case depth is traditionally defined as the depth to which the hardness is 50 Rockwell C or higher. However, simply specifying the surface hardness and the case depth doesn’t say anything about how the hardness varies with depth. To illustrate this, let’s consider two steels, both with a surface hardness of 60 Rockwell C and a case depth of .025 inches. The hardness vs. depth for the two steels is shown in Table 1, below. Steel A is representative of the vacuum carburizing process; Steel B is representative of the induction hardening process.
As you can see, both steels have the same surface hardness and case depth. However, Steel A maintains a higher hardness throughout the case. As a result, Steel A will have a greater resistance to spalling. There are two reasons for this. First, in rolling contact fatigue, the highest stress is not at the surface. Instead, it’s at a certain distance below the surface (in this case, about .005 inch). The hardness at this high-stress location is more important than the surface hardness. Vacuum carburizing provided the highest hardness at this depth. This explains why the vacuum carburized steels performed so well in the test.
Second, spalling occurs due to plastic deformation beneath the surface. Plastic deformation occurs when the stress exceeds the yield strength. The hardness is related to the yield strength. If the hardness drops off rapidly, the plastic deformation zone will be large. In contrast, if the steel has a high hardness throughout the case, the plastic deformation zone will be small. The smaller the plastic deformation zone is, the greater the resistance to spalling. The vacuum carburized steel had the smallest plastic deformation zone, due to its high hardness throughout the case depth. The result was excellent performance.
Palmer concludes by advising that when designing a case-hardened part, knowing the basics of rolling contact fatigue becomes important. If you specify only the surface hardness and case depth may not be enough to make a guarantee the machine part will last while it’s in service.
For example, you may also need to specify the hardness at a certain distance below the surface. Or you may want to specify that a certain percentage of the case depth must be above a given hardness. An accurate stress analysis is the best way to find these values. A well-designed part, made from the right material and hardened using the right process, should perform well.