Shrinkage of 13-8ph during Machining!!!

[will123123]

My company is turning a 500mm long length of 13/8PH steel bar, in condition A down from 38mm down to 12mm and the machinists have noticed that the length of the bar has reduced by 0.7mm. Note that the reason for using such large diamter bar is that the part in question has two bulbous end features and I confirm this is all before aging the material. The part will be aged to H950 hence the reason for machining before aging. This shrinkage is consistent for the three parts they have currently made and the material is from the same batch.
We have not experienced this before and I can only assume the reason for the shrinkage is due to residual stresses in the material. I assume the material towards the outer diameter of the bar was nominally compressive in nature, which held a central core in tension - as the bar was turned down the tensile stresses were relieved by reduction of the length.

Has anyone else experienced this before or are you able to draw any other conclusions?

Thank you in advanced for your help.

 

[Guest102023]

I can't come up with anything other than residual stresses - that is a lot of material removal. I presume the bar arrived annealed, so nothing metallurgical should be going on.

 

[IRstuff]

It almost seems insane to remove 90% of the bar to achieve the final product.

TTFN

FAQ731-376
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[EdStainless]

I think that you need to heat treat one of them and see what happens to the length when you relieve the stresses.

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Plymouth Tube

 

[Guest102023]

Maybe leave a little extra length to allow for this.

 

[arunmrao]

If you have another piece,need to reheattreat and machine after providing an additional allowance for the shrinkage(just for safety). Often,I see a whole lot of good material being removed and wasted because of shape/geometry considerations or application restrictions.

600mmdia and 500mm thick stock of 316 material is machined to provide a central bore or a step,(noncritical application).

 

[will123123]

This is a motorsport application, therefore we have time to manage the current situation and we are adjusting the lathe to compensate for the shrinkage. I have advised at this stage to use some other 13-8ph material we have in stock that is from a different source and hence different batch. If problems persist with this I will suggest a rough machine followed by re-solution treatment to relieve stress.

 

[EdStainless]

I would suggest machining enough to allow for the change in dimension in either direction, you may recover the length in heat treatment, or you may not.
If the length is really critical you will want to do the final aging treatment and then finish machine that dimension.

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Plymouth Tube

 

[unclesyd]

You can not recover size by heat treating as the material shrinks instead of growing.

http://www.hightempmetals.com/techdata/hitemp13-8MOdata.php

 

[EdStainless]

It may grow in the re-anneal, not knowing the actually heat treat condition of the center of the bar or the stress state. We often find that while the size change on aging is always accurate, guessing what will happen during the anneal or conditioning treatment is usually a crap shoot.
We have seen many cases where we made a cold drawn tube and after heat treat and age the pieces are exactly the size they were at draw.

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Plymouth Tube