Stress Relief of nitrided gears and splines? 7

[724napier]

Is it generally best to stress relieve a gear or spline before final machining, but prior to nitriding? Or is it acceptable to stress relief and then go straight to nitriding? This is assuming the material has already been quenched and tempered to the appropriate hardness. I wasn't sure which way would minimize the amount of distortion during nitriding.

 

[mfgenggear]

724

It depends on a factors of how close tolerance & the geometry.
I generally choose to stress after the first finished semi finish turn.
prior to any spline or gear cutting. and especially if its finished prior to nitride.

some gears lend themselves to be ground after nitride for very precise tolerances.

plus if necessary do a rough turn prior to the core hardening. eliminating the stresses induced from machining the thick stock from the parts.

your enemy is the parts will stress relieve during nitriding. and which causes movement.
Nitride is a very stable process, such as the 2 stage gas nitride, & the ion nitride.

just be aware the gas nitride (2) stage will produce a light white layer. that is very brittle. most want it removed, by chemical, mechanical or by grinding.

Hope That Helps
Mfgenggear

 

[tbuelna]

I agree with the comments from mfgenggear about thermal stress relief of gears and splined shafts prior to nitride. You would normally want as little post-nitride distortion in your gear or spline tooth surfaces as possible, since the thin nitride case does not allow for much stock removal in a finish grinding operation. While most nitriding processes produce minimal distortion, they are not entirely free from distortion or dimensional changes. Nitriding also creates a residual compressive stress in the outer component surface, and finish machining the nitrided surface will relieve some of this residual stress which can result in some distortion. With gear and spline teeth that require very high levels of precision, the teeth are often carefully ground/shaved prior to nitriding to compensate for these small amounts of distortion produced by finish machine operations.

 

[724napier]

Thanks mfgenggear and tbuelna,

Most of this stuff is being outsourced since we do not have gear shaping or grinding equipment. The parts I am making are getting an internal spline put in them which is going to be nitrided. I am just trying to do as much as I can to prevent distortion. The parts have been rough machined and hardened, but have not yet been stress relieved. I am planning to have the parts stress relieved then have the final machining done (Getting the spline shaped into them). Then have them nitrided using the 2 stage process. I am hoping this will minimize the distortion and ensure the parts still fit together. It is a class 5 fit.

 

[tbuelna]

724napier-

What is the spline DP, number of teeth and length? If your internal spline has a small diameter, long length and a blind bore it can be difficult to get a uniform case along the tooth flanks with gas nitride.

Another process you might consider for case hardening your internal spline teeth is low temp liquid salt bath nitrocarburize (AMS 2753).

 

[724napier]

Tbuelna,

Internal spline
Ansi b92.1 fillet root side fit, tolerance Class 5
DP = 1.5
Diametral Pitch = 12/24
Number of teeth = 18
Spline length = 1 inch
Material: Vac-Melt Nitralloy 135M

Do you guys usually offset the spline profile at all to account for any growth during nitriding? Say 0.0005" to 0.001"?

 

[mfgenggear]

724

will the white layer be removed?
that's a fairly beefy spline. just remember a slight amount of stock will be removed for white layer. the easiest method is be chemical etch. the heat treat supplier can process it. leave the spline on the low limit of the tolerance. so that it does not go over size. after the chemical etch.
Please state the quantity of parts?

I generally cut the inter splines with no other adjustment.
if there are spline gauges available use them during int. spline cutting and during the white layer removal.
if no gauges it get harder but doable. use a three gear wires measurement with deltronic pins.
it will make it easier to verify. order cheap but very accurate deltronic pins.
, high & low M.B.W, use these as the go and no go for sizing.

if you use the ion, or as Terry suggested AMS2753 there will be no white layer. and will not require stock removal. the AMS2753 is a very small case depth. so be aware.
With experience I have seen parts change dimensional with that process. so it is a must to do a couple test parts. or the parts will go out of tolerance.
I have seen this and I have documented it over the years.

HTH
Mfgenggear

 

[724napier]

Mfgenggear,

Thank you for sharing all of your experience and knowledge. The white layer will be removed by the heat treater. It's an outside company and I am not exactly sure how they remove the white layer, but I can ask them. This is a low quantity job. Only 4 parts are being produced, so it's a small quantity job.

I am trying to learn as much as I can about this process, but I have nobody experienced to talk to about this subject. I am glad I that you and Terry are so generous in sharing you experience and knowledge.

 

[mfgenggear]

724

Thank You

Pass it on just like we did.if we all helped each other it would be a better world.

Mfgenggear

 

[tbuelna]

724napier-

Based on the description provided of your internal spline, gas nitride should work fine. Just be sure that prior to nitriding you temper your part at a level that is compatible with the specific nitriding process temps.

With a relatively small diameter internal spline the easiest way to remove the white layer is chem etch, and the amount of surface material removed will probably be .0002" at most. The amount of dimensional change on the tooth flanks during nitride will probably be .0005" at most, unless the case is exceptionally heavy. So it should not be necessary to apply any compensation when machining the spline teeth in order to achieve a cl.5 fit with a fillet root, side fit spline.

The only other advice is to make sure you round all of the sharp corners on your spline teeth prior to nitriding. The corner radius should be in proportion to the case depth. This will prevent fracturing of through hardened tooth edges.

Good luck to you.
Terry

 

[gearcutter]

All excellent advice given by both mfgenggear and tbuelna.

The only thing that I can add is that I would recommend that you perform a hardness check after all of the pre-nitriding machining & heat treatment processes have been completed. This is done to help ensure that the hardness of the part falls within the range required to achieve the best result post-nitriding. Case depth and hardness required should have already been determined by you so speak to the vendor performing the nitriding telling them what you're hoping to achieve and they should then give you the level of pre-nitriding hardness that will be required.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[FennLane]

It is quite important to define the nitriding process that you are going to use and the depth of the 'case' you are specifying.

If you are using conventional gas nitriding then as has been stated the 'white layer' developed will need to be removed and the schedule of heat treatment needs to be well specified.

As you correctly state it is important to nitride and hardened and tempered material in order to produce the correct residual stress in the diffusion layer.

Clearly hardening and tempering will cause some residual stress to be present and subsequent machining will add to this and will almost certainly produce distortion.

Any component you are planning to nitride will need an intermediate heat treatment following machining and prior to nitriding.

A small allowance should be left on the part so the distortion produced can be corrected.

Traditionally this heat treatment has been known as 'stabilising' and it is custom and practice to use a temperature about 10 degC above the maximum nitriding temperature.

This should ensure that no further movement takes place during nitriding other than the small amount of growth caused by the diffusion of nitrogen.

As has been stated this growth is limited and relatively predictable.

I would also agree that that ion or plasma nitriding produce little or no white layer but again stabilising prior to final machining is needed.

The final process that is commonly used is Salt Bath Nitriding and this used to be known as Soft Nitriding.

Commercially these processes are called Tuftriding, Tenifer, QPC, Nitrotec or Melonite and they are all variations on ferritic nitrocarburising.

Some of these process also produce a white layer and it is important that if this is the process you are using that this layer is NOT removed or the majority of the benefits obtained by using this process will be lost.

 

[tbuelna]

FennLane- Nice post. I recently had some 4340 steel parts treated with liquid salt bath ferritic nitrocarburize (AMS 2753) and was quite pleased with the result. There was a tiny amount of dimensional growth (<.0002" per side). I specified a QPQ process and the finished surface was extremely smooth. The vendor said they used a tumble polish to clean the part surface between treatments. An added benefit of this treatment is the part surface has fairly good corrosion resistance. The process involves temps up to 1075degF and the vendor recommended tempering the material accordingly prior to nitrocarburizing.

 

[spigor]

If the parts were to be carburized, hardened and tempered to 60 HRC on the surface- would such stabilizing treatment also help to reduce the HT distortions?

 

[tbuelna]

With conventional carburizing most of the HT distortion occurs during the initial quench. Normally carburizing is only used when a heavy case is required. With most spline joints, the thinner case produced by nitride processes is usually more than adequate. Nitriding has the benefit of not requiring a quench along with the distortions it creates. The outer surface hardness of a nitrided part is usually harder than the outer surface hardness of a carburized part. But the nitrided case is typically much more shallow than a carburized case, and the hardness thru a thin nitride case drops away far more quickly than with a thicker carburized case.

Here's the basic decision you need to make between nitriding and carburizing- If you need to perform any finish grind operation on your case hardened spline teeth, while nitriding will produce much less HT distortion than carburizing, the nitrided case will have far less stock available for any finish grind operation. If you need to remove more than about .005" stock during finish grind, then the carburized case will probably provide a harder and thicker finished case surface than the nitrided part.

 

[724napier]

tbuelna,

Would it be wise to build in an additional .0005" of clearance to account for growth during nitriding? I managed to find a graph of distortion vs. wall thickness for nitriding in the ASM heat treating handbook. From the graph it looks like there is about .001" of distortion for a wall thickness of 0.5". I just want to have a high level of confidence that the spline will fit after nitriding.

 

[mfgenggear]

724napier

if all the above has been followed there should be be very little distortion.
but it would be wise to tighten the the tolerance of the spline circ. tooth thickness, or space width. if it's an internal or external. just be aware like my previous discussion there will be slight stock removal from the white layer removal
and glass bead clean prior to and after nitriding. pinch the tolerance from the top and bottom of the tolerances.
I like to hold the slightly above the Min actual of the external to at the max actual CTT of externals. by using gauges
it actually gives more tolerance by using the max effective for final buy off after the parts are completed.. and vise a verse for Internals.

Good designers will use procedures as mentioned by like as mentioned by Fennlane. AMS2759/7 & /8
are very good specification. and is use very much in the parts I process out.

Mfgenggear

 

[724napier]

mfgenggear,

I have followed all of the AMS procedures, plus what you guys have mentioned as close as I possibly can. I have been tracking the growth after every heat treat procedure and so far have found there to be very little. However, I am just worried that since this is an internal spline, that there may be growth/distortion which would cause fitment errors.

Since this is an internal spline would I tighten the the circ. tooth thickness or increase it? I was thinking if it's an internal spline it would grow therefore, making the tolerance tighter? For example if it was just an internal diameter, say 2 inches, wouldn't it end up being 1.9xx after nitriding?

Thanks

 

[mfgenggear]

724napier

there may be a slight increase growth but by removing the white layer it will increase the circular tooth space.
no worries. it will all be good. hold the MBW maximum and minimum. except pinch the tolerance.
then after Nitride use the gauges to buy off the parts. if the go Gage fits. parts will assemble Guaranteed.
no worries go make your parts. if it is tight to the go gauge which does happen a light glass bead blast will take care of it.

Mfgenggear