We’ve all heard the term, but have you ever wondered what that heat treatment actually looks like, or how it’s done? Well, you are in luck.
Heat-treating a camshaft ensures the lobes will hold up under the demands of the valve train. Pushrods can apply as much as 2,000- to 6,000-lb loads between the lobe and roller wheels in a roller camshaft system, and as much as 10,000-lbs in Pro Stock engines.
Most roller cams are made from either SAE 8620, SAE 5160, SAE 5150 steel or some grade of tool steel. Note that a tool steel cam can be ground to any finish profile without cutting through the hardened surface. Tool steel is through-hardened and has no hardness variation when cut, so a cam of that material isn’t shown in this article’s photography. The heat-treatment process is performed on the cam cores before the lobe profiles are ground.
However, an SAE 8620 cam has a hardened heat treatment layer that is easy to see. Cams made from 8620 are globally heat treated in a furnace through a process called carburizing. Since only the lobes need treating, everything else is masked off with a copper coating that preserves the core’s lower hardness and ductility.
Ductility is a metal’s ability to plastically deform and adapt while being stressed with a tensile load. If the entire SAE 8620 camshaft was heat-treated, it would become brittle and snap under operating conditions. Since only the surface of the lobe faces is needed to carry the workload, they exclusively are carburized.
During the carburizing process, the cams are heated in a furnace in the presence of carbon dioxide. Once heated, the atoms of carbon find their way into the spaces between the metal’s web-like crystalline lattice work of iron atoms. They work the outer layer into a tailored depth of surface protection.
For SAE 5150 cams, COMP Cams always starts with certified bearing-quality steel with less than .02% of sulfides, phosphates or oxides. These impurities could make the cam easier to machine, but they can also perforate the structure and cause failure. The 5150 cams are induction hardened, with the induction coils wrapped around each of the lobes and journals, selectively hardening them and not the entire core.
Induction-hardening is a no-contact process that uses a low-voltage, high-current, high-frequency electromagnetic current induction. This produces an intense, localized and controllable heat inside the surface layer. It uses a series of “pulses” or electromagnetic frequencies that basically flow back and forth upon themselves in alternating pulls to create intense heat inside the metal. The temperature, frequency and localized hardening of specific areas can all be controlled. The heated area is quickly quenched and the process, or heat cycle, can be repeated as needed for desired results.
The hardened layer on the SAE 5150 lobes shows up as a smooth, non-grainy layer of metal that looks polished. It’s deep enough to withstand a wide range of profile grinding on the lobes and strong enough to withstand the rigors of the valve train. This process is preferred in high-volume camshaft runs, especially for extreme-length applications like endurance racing.
Without these heat-treating procedures, or if they are done incorrectly, camshafts would not last very long in any engine.