Heat treatment Stainless Steel 316L 2

[MildJoe]

Hi,
I've posted here before, asking for help with residual stress in forged 316L Ugima steel. I experience change in the part during machining, that i don't see in 316L Prodec plate material.
We have tried with less aggresive machining without positive result. The producer of the steel told us to try stressrelieving at 315C for 2-6 hours. This didnt help either. Then they proposed to anneal at 1100C for 1 hour and quench in water, this to increase the grainsize so it will resist the cold working better. We havent tried this, because we dont have the equipment for it.

Before we send it away for this treatment i would like to try stress relieving at a higher temp than 315C.
The information on the internet is very varying concerning this, so i would appreciate some help from you guys.

As far as i see i could heat it up to 600-650C and air cool.

But for how long? The piece is 200mm x 40mm x 50mm.

And could i expect any change in material properties at 600C? Will this affect the milling afterwards? And how about corrosion resistance? ( what will the result be after annealing at 1100C? )

This steel isn't supposed to have any residual stress, but it might have been forged in a way that resulted in this anyway.

Thanks!

 

[EdStainless]

What is your carbon level? This will limit how hot and how long you can heat treat before you sensitize the material.
I could see you trying 500C for 1 hr, but I am pretty sure that it won't help you at all.
Is the material well annealed now? Clean microstructure and all of that? If so then I would suggest re-annealing at 1,000C for just long enough to get the part fully heated. The cooling is the trick. If the part cools unevenly you will end up with even higher stress levels.
The lower annealing temp will prevent excessive grain growth. You will need to have the parts pickled if the anneal isn't done in hydrogen.

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Corrosion, every where, all the time.
Manage it or it will manage you.

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[MildJoe]

Thank You Ed,

You can find the product sheet here:

http://www.uginestainless.com/pdfs/ugima316.pdf

As far as I know, the material is annealed. I will contact the people who forged it now, to get all the details.

Why do you say 500C? what do you expect will happen if we heat to 600C for 2 hours, and air cool?

If we heat to 1100C, you say the cooling is the trick. But shouldnt we just drop it into water?

 

[arunmrao]

Ed
The weight of each component is about 3.1kgs,also it is quite thick,to resist any warpage. If they can resolutionize at 1050C and water quench there should be no problem of residual stresses.

 

[EdStainless]

If the part doesn't cool evenly the differential thermal expansion (contraction in this case) will create great stresses. Take a piece of plate 4" x 12" x 1/2" and heat it red hot. Now hold it with one of the long edges down and only water qunech the lower half of it. As it cools it will bow.
Your part may not bow, but it could after you start machinging. If your part has uniform thickness (constatn cross section) then you may be able to water quench. Unsually air cooling is fast enough and you can prevent too much variation in cooling rate.

If you heat to the wrong temperature range for too long you will form chromium carbides in the grain boundaries. This is sensitization. It will reduce toughness and make the part succeptable to corosion along the grain bondaries (inter granular atack). This material will also be much less resistant to choride stress cracking.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[MildJoe]

Thats exactly my problem now; it bows during machining.

So what your saying is that i should heat it at 1100C for 1 hour, and then just let it lie there to cool off afterwards for some hours.
Then there will be no residual stress, and it will probably behave better during machining?

I think i've heard that i have to waterquench it to keep the corrosion resistance intact. (?)

 

[kenvlach]

MildJoe,

Is this the same problem as before?
Residual Stress - Heat treatment of 316L
thread1135-141805

EdStainless & others gave good info. I suggest your problem arises because there are 2 different types of annealing w.r.t. austenitic stainless steels.

A 'solution anneal' is heating at high temperature to dissolve any carbides, then rapid cooling typically by water quenching. Drawback is residual stresses are created by quenching. Asymetric parts may warp immediately, and machining releases (unbalances) stresses to cause warping.

a 'full anneal' is solutionizing followed by slow cooling (furnace cooling for at least most of the way). Drawback is possible carbide formation known as 'sensitization' and possible brittle sigma phase formation. Usually not a problem for 316L.

For stress-relieving 316L, some recommended options are
A) anneal at 1065-1120 C, slow cool.
B) stress relieve at 900 C, slow cool.
C) anneal at 1065-1120 C, rapid cool.
F,G) stress relieve < 480 C, slow cool. [many hours, only partial relief]
-- Metals Handbook, vol. 4 Heat Treating, 9th edn., p. 647-649.

Bottom line: 'Annealed' can be ambiguous. For 316L, you prefer 'full annealing' (furnace cooling).

 

[MildJoe]

Hi again!

We've now run som tests at 1100 C at 2hours, and slow furnace cooling. This gave very good results regarding the machining. It doesnt bow anymore, but we observed that the machine have to work a little harder. And the surface of the metal is harder.
I've been told that this isn't actually stress-relieving, but what we've done is to create larger grains in the microstructure. And that this helps prevent stress inducing during machining.
Any comments on this?

We also tried 12 hours at 500 C. This didn't give any results.

And some more questions:
is the material still 316L after this treatment?
What kind of changes in the properties could we expect?

Thank you all so much for helping me on this!

 

[EdStainless]

You don't need to go clear to 1100C. I would expect 1020C to work fine. And you don't need two hours. Enough time for the entire part to reach uniform temp and then another 15 min should be all that you need.

At high temperatures the stresses are relieved. You want to minimize the grain growth if you can. It is mroe difficult to get good surface finishes in coarse grain material.
You have more difficulty machining because the material is tougher. It wants to smear or tear instead of cut.

You should do a tensile test and look at the microstructure. Those will tell you if you have altered the material.

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Rust never sleeps
Neither should your protection

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[TEV]

We have had similar problems with barstock, and as I recall, it was also with ugine material. When we drilled a hole in one end of the bar, then unchucked, rechucked and drilled the other end, the first end became oval. We initially thought it was due to chucking pressures, but further investigation revealed that it was due to radially non-symmetric residual stresses. We brought the problem to the attention of Ugine, and I believe they solved it by different rolling, drawing and heat treat processes. Have you discussed this with them?

Yes, your material will still be 316L after your heat treatment. EdStainless' comments about doing a tensile and metallography are important. With the metallography, I would suggest doing the Practice A of ASTM A262 to detect sensitization (carbide precipitation).

Also, after the heat treatment you state that machining became more difficult. It is necessary to use fairly heavy tool feeds with annealed austenitic stainlesses, as it tends to work harden in front of the tool if you allow the tool to dwell without feed or feed too slowly.