Distortion during Nitride hardening 4

[HD1]

We are experiencing growth of our ring gears during nitride. The gear teeth are fine, but at the top face where the gear is the thinest, the counterbores grow as much as .005". Our material is 4140 and nitride cycle time is currently 26 hrs.

 

[shooter45]

What is the prior heat treatment? Have you tried a stress relief after final machining, immediately prior to nitride?

 

[EnglishMuffin]

Is it really growth, or is it distortion? In my experience the deep cycle gas nitriding process itself usually produces something on the order of .001" per side, regardless of size, but you can get distortion of the part just due to the modest amount of heat applied, especially if the part is not stress relieved properly as shooter45 says. One other thing to check is the orientation of the part - ring plane should be horizontal. Shafts must be vertical.

 

[diamondjim]

You are nitriding for corrosion
resistance? Sounds like normal
growth as English Muffin suggests.
If you just want hardness and depth
consider carburizing unless you need
the greater surface hardness. I
assume the counterbore diameter is
decreasing by the .005 ie .0025 per
surface?

 

[carburized]

HD1

Distorsion of .005 during nitriding is too much. Usually nitriding will not distort a piece or so small, you do not have to worry about. We use nitrided gears in our gearboxes with no post treatment on these and have very good luck. You may want to check the temperature you are nitriding at. On the other hand, in carburizing you will get distorsion since the part is heated above critical temperature.

http://www.gearsandbearings.com

Ravi

 

[profengmen]

I've gone through hell and back for tight tolerance hardening. The best way to solve these problems is to call in your heat treat vendor and lay out all the information.

Shooter45 has a good question with the stress relief. If it is in fact growing, then the heat treating is providing the stress relief and changing the dimension.

Usually you find a post or pre machining operation necessary.

 

[involute]

we have experinced growth in the material when we use a case hardening steel .If the forgings are not normalised the growth can be excessive
Normal nitriding temperatures are fairly low and do not tend to generate a growth in the material surely you would need to stress releive the material if you are getting any growth in size.
are you using a forging? or are you using a rolled bar for your gear ? a forging will definatle have to be stress relieved

 

[MattRR]

Nitrided parts grow, but distortion is low. So account for it in processing. I remember .001 in/inch. I will look it up tonight in a book I have at home.

 

[gearman1234]

I have experienced no distortion after nitriding. We make ring gears from 4140 forging. We too have thin cross section at the top. But no change in diemsnion occurs. The forging is quenched and tempered to get desired hardness. We ensure that the tempering temperature at the end of heat treatment is more than nitriding temperature by about 100 deg F. After tempering the parts are cooled in still air.Because of this we do not need any special stress relieving cycle.

 

[HD1]

Great feedback, thanks to all!!
Here is more data for discussion.

1) We do core treat our forgings proir to machining to
BHN 285 avg. at 1180 dgrs. (F).

2) No stress relief proir to nitride has been tried, but will be soon!!!!

3) Our deep cycle gas nitride temp. is 1100 dgrs. (F)
with the part horizontal.

Looking forward to any opinions out there.

 

[GearmanPE]

If the teeth arn't moving, but the counterbore is, it is actually a distortion. The stress relive ideas are good, this should be done just prior to finishing the dimensions you wish to hold. But;

Nitrided parts can distort due to the internal stress created in the nitrided layer. If the part is uniform in section, and the part is nitrided all over, these stresses are quite balanced and little distortion occurs. Some odd shapes produce unbalanced stresses, hence the movement. (in gas nnitriding, there is also a uniform additional change of about .0007 in. added on each surface due to white layer).

You can try to mask either the inner or outer surface of the counterbore and see the results. You may be able to modify the movement this way. Masking for nitriding works well, and for production processes a masking ring may work. Whether you can control the process to your tolerance requirments is another issue. Hope this helps.