Hello,

I have a bit of a technical challenge, in that we have some geartrain designs that were converted from nitrided helical gears to carburized and ground some years ago. The same diametral shrink fits were carried forward (for example, .0015T/.003T on a 2.75 nominal), which calculates to 440F delta T for a drop-on assembly. We've always heated the gears to expand the bores and fit over room temperature shaft journals.

For those who don't already see the problem, our company standard practice is to keep carburized parts at 325F maximum and nitrided at 600F max. The literature still echoes those numbers. So these shrink fits are easily managed by soaking nitrided parts in our 525F oven, but this is too hot for carburized parts.

I don't think Engineering ever considered that the shrink fits of the nitrided parts will require different assembly methods in carburized material. So the shop has been keeping their ovens at 525F and having no issues with assembly. Most parts aren't in there more than an hour but I've come to learn that some have soaked at 525F for much longer. Fortunately, no performance issues to report, but a situation that we need to address.

One obvious answer is to keep dry ice on hand and shrink the shaft journals. It does add complexity, equipment, and safety hazards to the shop processes, so I owe the company at least another option.

Does anyone have data or practices for how to heat carburized gears to 450-500F and limit the damage? I'm considering taking a handful of heat treat coupons and testing microhardness traverse and grain structure before/after various amounts of time in the 525F oven.