So which case hardening process gives steel parts the best spalling resistance? This was the topic of a presentation I gave earlier this year at the annual meeting of the Society of Tribology and Lubrication Engineers. The presentation was based on work I performed at BRP, together with Dr. Jane Wang of Northwestern University, Lechun Xie of Shanghai Jiao Tong University, Zhanjiang Wang of Chongqing University, and Fred Otto of Midwest Thermal Vac. The results will be published in an upcoming issue of the journal Tribology Transactions.
We studied four different steels (two vacuum carburized, one atmosphere carburized, and one induction hardened) at three different case depths. We measured the number of cycles to failure in a rolling contact fatigue test.
In our study, vacuum carburizing gave the best overall results. At the same case depth, the vacuum carburized steels lasted more than 100 times as long as the induction hardened steels. What accounts for the good performance of the vacuum carburized steels?
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.
When designing a case-hardened part, it’s important to understand the basics of rolling contact fatigue. Specifying only the surface hardness and case depth may not be enough to guarantee the part will survive 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.