In our industrialized society, bearings are literally what makes the world go ‘round. Without bearings, modern-day machines could not exist. This includes things we take for granted such as planes, trains and automobiles. Without heat treatment, bearings could not do their job. So, by extension, heat treatment keeps the world’s wheels turning.
Bearings take a variety of forms for different types of jobs. Although we can’t discuss them all, here are just a few: babbit, air, magnetic, ball and roller. Ball and roller bearings are two types of “rolling-element bearings,” and further discussions will focus on this type of bearing.
One of the earliest and best-known rolling-element bearings dates back to ancient Egypt during the construction of the Pyramids of Giza. A set of logs was laid on the ground with a large stone block on top. As the stone was pulled, logs rolled along the ground with little sliding friction. Logs needed to be continually moved from the back to the front to keep the stone rolling on. Another early example of a wooden ball bearing supporting a rotating table was retrieved from the remains of a Roman shipwreck circa 40 B.C.
Much later in our history Leonardo da Vinci (circa 1500 A.D.) spent much time analyzing bearings, linkages, gears and other mechanical transmission modes. The first practical caged roller bearing was invented in 1760 A.D. Caged ball and roller bearings prevent the bearing elements from rubbing against each other, causing additional friction. Steel developments in the 1800s transformed the manufacture and use of rolling-element bearings.
Anti-friction rolling-element bearings are manufactured from two basic types of steels: through hardened and case hardened. The designations are more descriptive of the heat treatment versus the steel itself. Through-hardened components, as the name implies, have the same hardness all the way through the part. Case hardened, on the other hand, denotes a hard surface but a softer interior (core). Bearings are typically case hardened by carburizing the surface followed by a conventional austenitize, quench and temper-hardening process.
Similarly, although the chemical composition of through-hardened bearing steels varies somewhat, most have carbon concentrations in excess of 0.8%. The hardening process typically involves the following:
- Austenitizing (hardening)
- Subcooling (deep-freezing)
Preheating prior to hardening serves several purposes. Preheating provides a controlled rise in the material temperature. This prevents thermal shock, which can cause shape distortion. Preheating also stabilizes the material at a temperature closer to the hardening temperature, thereby reducing the time necessary at the higher hardening temperature. Minimizing the time at hardening temperature is important because as the carbon is put into solution in the austenite, grain growth can occur with longer hold times at the hardening temperature.
After hardening and quenching to martensite for high hardness, another important stage in the hardening process is subcooling, particularly for high-precision bearings. The martensite finish temperature is often quite low with these materials, and austenite will be retained at room temperature if a subcool is not performed. Retained austenite will reduce the final hardness of the bearing and will also result in dimensional instability. Many times, multiple deep-freeze/temper cycles are performed to further reduce the retained austenite.
While bearings play a key role in making our industrialized world go ‘round, without bearings could not keep on rolling heat treatment.