Question:
We are looking for guidance to better understand the contribution of heat treating to stresses present in gear wheels so we can continue to make improvements in our products.

Answer:
Let’s continue our discussion of how variations in heat-treatment parameters can influence the development of stress in gear wheels. We now consider the influence of part geometry and other factors.

As far as some surfaces growing and some getting smaller, that can be directly related to the geometry of the part. This is true assuming this is not a new condition. If you are consistent with what happens lot-to-lot over time, you've done all you can. You can then adjust size to compensate later.

If you want to minimize distortion from lot to lot you have to control the Jominy or DI of the material to a tighter range. Again, many automotive companies specify specific Jominy-range steels that were used for specific parts. More distortion-prone parts required using a very tightly held material hardenability range.

The quality of the heat treatment (equipment & process variables) is the last contributor. To that end, here are a couple of excerpts from technical papers in our possession that you might find useful:

“Size change (growth or shrinkage from original machined dimensions) can be controlled if the material can be purchased to a close DI range (-0.0 to +0.3). The closer DI does not prevent it from changing size but should make it very repeatable. An example would be holding a ±0.001-inch (0.025-mm) tolerance after carburizing. Unfortunately, smaller companies find it harder to purchase steel to this kind of DI control. Some companies purchase steel to restricted hardenability and adjust stock for finish machining or grinding after carburizing when dealing with tolerances closer then ±0.002 inch (0.050 mm).”

“Other factors that can influence the results are fixturing. Are your trays flat?”

“By far the largest source of problems for heat treaters is distortion of parts after heat treatment. Distortion causes excessive noise in the gear drivetrain and potentially early failure due to high residual stresses. It can be seen that many of the sources of residual stress and distortion occur before heat treatment and quenching, yet it is often the heat treater that gets the blame for a distorted part.”

Here are some other factors to consider:

Material – The alloy chosen can play an important role in how sensitive a part is to distortion during quenching. If the equivalent carbon Ceq is greater than 0.52, it is prone to high residual stresses from transformation stresses and is prone to cracking. If it has low hardenability, fast quench rates are required to meet properties. This can also cause residual stresses because of the development of thermal gradients during quenching that can cause some areas to transform to martensite earlier than other areas. Segregation in the raw material can cause local areas of high hardenability, which can also cause localized early transformation to martensite, creating metallurgical notches that are prone to cracking.

Design – The design of a part is often responsible for cracking or distortion. For instance, if sharp radii are used or there is little transition between thick and thin sections, a high level of constraint forms and distortion or cracking can result. It is always a good idea to use generous transitions between sections, and minimize the use of thick and thin sections.

More to follow…