Through-Hardened, High-Carbon Components
For through-hardening bearing steels (high-carbon), annealing after forging is important to prepare the microstructure (spheroidize anneal for grades like 52100). The process then involves heating to austenite for enough time to fully transform followed by an oil or salt quench, taking care to keep distortion low. There are fixture-quenching devices for thin-wall or larger parts such as Gleason presses or custom-tooling systems. It is important to temper soon after reaching close-to room temperature to prevent cracking. Special tempers are used to respond to customer needs for higher operating-temperature stability so that the microstructure does not break down and change the size of the bearing when exposed to temperature extremes.
It is important to know there is an international system for dimensional stability. If the print calls for SN, S0, S1, S2, etc., then special heat treatments are required for dimensional stability. Each code corresponds to an increasing operating temperature, from the normal (SN) at 120°C (248°F) up to more than 200°C (392°F). As you reach higher levels, hardness above 58 HRC can no longer be maintained, and bearing life suffers.
Through-hardening microstructures are generally martensitic with the goal of very low retained-austenite levels and fine carbides. Hardness standards are usually 58-64 HRC. Some companies try to enhance performance with nitriding, carbonitriding, ferritic nitriding or a bainitic microstructure. Nitriding has multiple effects, but distortion must be tightly controlled or the benefits will be ground off.
One added consideration: Large inclusions will create a stress riser that can crack a through-hardened part upon quenching. I have seen multiple examples over the years where inclusions in parts, especially larger rollers, will crack or even split apart after quench. Stress risers like sharp corners, sharp holes or special features can also act as crack initiation points. Careful review of incoming turned components will identify these potential concerns before heat treat.
Carburized or Surface-Treated Components
Case-carburized bearings don’t usually need the spheroidizing step and go from forging/machining to the carburizing step. Of course, the key here is cleaning the surface to remove cutting fluids and contaminants that will impede carburizing or result in wavy case. Avoiding atmospheres that generate large or network carbides is mandatory. Some manufacturers single quench by lowering the carburizing temperature before quenching to achieve a reasonable microstructure. Others choose to reharden the parts in a second austenitizing step to refine the microstructure and possibly fixture quench to control distortion. Again, tempering soon after cooling is important to prevent cracking, and the dimensional stability requirements described in the through-hardening section may apply.
The main benefits of case carburizing are the compressive residual stresses at the surface and the relatively soft, tough core. If a crack does occur, it typically does not through-fracture like a through-hardening bearing. The microstructure is also martensitic, and more retained austenite is permitted due to the dual microstructure (case and core). This dual microstructure does not have the dimensional movement that a similar through-hardening bearing has because the volume of high-carbon martensite is only in the carburized zone. Again, hardness (case) is typically 58 to 64 HRC, and core hardness is now a concern and must be specified for crushing-load resistance. This varies by application. The material grade and heat treatment can be engineered to result in satisfactory core hardness each time.