8620 Getting longer after heat treating

[camhead420]

We are turning some shafts 2.5" od and 12.5" long. I turned these for years with no problem, now when they return from heat treat (case hardened 58-62, .030" deep) the are .005" - .015" longer?
This was never a problem for over a decade. I am just searching for answers. Everyone is telling me its the same material?? I think before they were getting 86L20. I'd like some opinions.

 

[007Rob]

That sounds about right/expected. 8620 will grow a bit during heat treat. We sometimes make mold bases from 8620 that get case hardened we usually apply a scaling factor of .999-.9995 (depending on part size) to the model beforehand to account for this to make sure everything cleans up nice after heat treat.

 

[nateacox]

I'm having the opposite problem right now LOL. We are doing some 5" x 25" 4140 prehard shafts, and when roughing them off, they are shrinking .020" in length.
Its fairly normal for material to grow in length at heat treat.

 

[wesg]

5" 4140 ...

Cold Finished? I wouldn't think at 5".

I had some 17-4 CF round that warped badly when milled into rectangular shapes. Running it to H-1150 made no difference. Just a sight rail for a pistol. Gave up and used 7075.

Compressive stress in the 'skin' from drawing. Might make your bar 'longer' than it wants to be, and it relaxes to normal once you turn it down.

Or not....

 

[nateacox]

It's hot rolled. It's not warping best I can tell, and we are using a tail stock. It shrinks about .002 in length each roughing pass. It's odd, never had it happen before.

 

[camhead420]

That sounds about right/expected. 8620 will grow a bit during heat treat. We sometimes make mold bases from 8620 that get case hardened we usually apply a scaling factor of .999-.9995 (depending on part size) to the model beforehand to account for this to make sure everything cleans up nice after heat treat.

I wish we could apply a scaling factor, it is causing the grinding operation to take a bit longer due to the size variance. .006-.015 long

 

[Mtndew]

I wish we could apply a scaling factor, it is causing the grinding operation to take a bit longer due to the size variance. .006-.015 long

I think we allow .001" per inch on the length of shafts for growth on 8620 when carbeurizing.

You could always make a dummy shaft and slap some OD grooves in at 1" intervals then send it to carb and see the changes.