Nitriding splined section of Carburized Shaft

[BCpowertrain]

When dealing with integral pinion shafts with splined trasmission elements (e.g. integral pinion for a gearbox high speed shaft with a splined input connection - think input gear for an automotive manual transmission) I have noticed there seems to be a trade-off of what is commonly done for the heat treatment of the various shaft elements. The gear teeth require high case hardness with sufficient case depth to prevent crushing - which is easily achievable by carburizing and grinding, while the splined section would benefit from high surface hardness, but does not require the case depth, and is difficult / expensive to grind after carburizing, and often will not fit with its mate if left in the as-carburized condition.

A typical practice I have seen is to make the part from a carburizing steel grade and case carburize and grind the gear teeth and mask off the splined section to prevent distortion, but this leaves the splined section at the hardness of the base metal (typically 30-40 HRC) which is not sufficient for some splined connections.

To achieve a higher case hardness for the splined section without the distortion I have also seen alloy steels (eg 4340) used with the gear teeth induction heat treated and ground, and the splined section spin induction hardened. This has resulted for us in a hardened spline which will still fit with its mate, and a hardnened gear but is typically more expensive than a carburized part and has lower case hardness of the gear teeth than the equivalent carburized part.

When space permits, the most cost effective method is usually a steel shaft (alloy if required for strength), with a liquid nitrided spline, and then a shrink fitted, case carburized gear wheel - but often the size of the pinion gear does not permit it being a separate entity.

Now when design constraints (size, cost, etc) force a integral pinion design, as well as a highly loaded splined connection - what would be the optimal heat treatment process for such a component? (small quantities, 1-5 items)

Can the part be carburized to case harden the gear teeth, then dipped to liquid nitride the splined section? What effect would the heat from the liquid nitriding have on the carburized gear teeth?

Would a fully gas nitrided part be a competitive part - as I understand it gas / plasma nitriding would be an expensive procedure for a gear due to the time in the oven required to generate the case depth required for the gear teeth.

What do the automotive guys do for this sort of piece?

Thanks for any help and information you can supply.

Regards,

 

[CoryPad]

You cannot case carburize the part then do nitriding. The temperature (~ 500 [°]C will overtemper the case, and I don't think the carbon saturated martensitic structure will accept nitrogen into the crystal structure.

Nitriding (gas, liquid, plasma) can be competitive with carburizing depending on many factors.

Automotive guys use case carburizing exclusively for the entire piece (gears and splines). Distortion is controlled by part design, material selection, and process optimization.

 

[unclesyd]

Not mentioned above for making the part is Friction or Inertia Welding two different components together to make the shaft. I don't know if the small number of pieces would be worth the effort as we usually had around two hundred pieces to ma

This is one of several companies that do this type work. Look at the first example in the left column.

http://www.teamafw.com/afw_applications.htmke.

 

[tbuelna]

PrepGlobal,

As CoryPad noted, carburizing and nitriding are probably not compatible processes, primarily due to the temperatures needed for nitriding.

But since gear teeth and spline teeth are not subject to the same loading conditions, they usually don't benefit equally from case hardening. Gear teeth have high localized contact stress, so a fairly thick case with a high shear strength on the tooth profile surfaces gives a good fatigue life. Spline teeth, on the other hand, tend to have a lower contact stress that is well distributed between the spline's teeth. So having a heavy case is not so important.

Carburizing and finish grinding external spline teeth can be done. But you must be careful with the case depth, so that you do not thru harden the thin section spline tooth tips. They will become brittle and will fracture off. Talk to your heat treater and see if they can do a two step carburize process, where the gear teeth get a heavier case than the spline teeth.

Hope that helps.
Terry

 

[BCpowertrain]

Thanks for the responses everyone, this is what I was expecting would be the outcome of the nitriding after carburizing but was unable to get a solid answer anywhere.

With a two step carburizing procedure, what procedures, processes or design features can be used to minimize the distortion. We have a part that will soon be manufactured (QTY:12) which has an internal spline (2", 15T) with a helical gear (5.5 face, ~3.5 PD) at another part of the shaft. If one were to make a test blank with the spline features finished and run it thru the first part of the two-step carburizing process would the deformation be constant enough to measure the test part and adjust the tolerancing of the final 12 to ensure it will fit with its mate in the end?

Thanks again for your help.

 

[TVP]

Although I am not familiar with this being done, it may be possible to carburize the gear section and subsequently nitride the splined end using a liquid nitriding bath. Perhaps the part could be immersed into the salt bath only a short distance to the end of the splines, with the rest of the gear well outside of the salt bath. I'm not sure what the air temperature is immediately above a salt bath, but it will certainly be less than 500 C. The technical people at Nitromet are very responsive to these types of inquiries.

 

[CoryPad]

TVP,

Are you proposing to prevent carburization of the splined end ("stop-off"), then nitriding? Or carburize and then machine the splines? Or carburize the whole part and nitride the splines? I agree that the air temperature above the salt bath should not be a deal-breaker.

 

[BCpowertrain]

Yes - carburize only the gear tooth section of the part, with the rest of the item masked. And then dip the splined end to achieve the liquid nitriding.

I could see how this would work if there was sufficient distance between the gear and the spline - but in this current situation they are quite close to each other.

Unfortunately the proximity of the gear and splined section will not allow this, but for disucssions sake this would be a great idea when the splined end is separated from the gear by sufficient distance.

With partially nitrided shafts, i.e. when only one end is dipped into the salt bath, is there a concern with fatigue life / stress concentrations caused by the transition of the hardnened case to the soft parent material - i.e. when considering a pure torsional arrangement? Or does the shallow case depth of LN treatment make this an irrelevant concern?

 

[mfgenggear]

PrepGlobal

Option 1 : it is normal to apply a deeper carburize case for the gear then apply a shallow case for spline.
spline distort is corrected by spline lapping.

find a gear shop that knows how to do this.

option 2 : use nitralloy 135M steel nitride the gear teeth & the spline. apply ~ .015-.020 case

this will not effect the spline, lapping is still required.
due to movement from stress relieving of the part.

mixing of nitride & carburize is not standard manufacturing practice & is not advised.

 

[unclesyd]

One thing not mentioned in the last proposed process is on short pieces you can get a severe metallurgical notch. In some parts this can be extremely detrimental to part's longevity.