Residual Stress - Heat treatment of 316L

[MildJoe]

I experience some deformation in the steel while machining (rough milling), and suspect that there are some residual stress in the metal as a result of the fabrication of the stock.
The stock are forged 316L steel, with dimensions approx 20cm x 5cm x 4cm. Forged in temperature range 955-1175C (1750-2150F), and quenched in water afterwards.

Do anyone know if there would be smart to try to stress relieve the material, and if so how? what temperature, for how long, and how to cool it down.
Will such a treatment affect the properties of the steel?

Thank You in advance!

 

[Metalguy]

It "should be" real soft, so the amount of res. stress would be small. Perhaps they continued forging it when the temp. got too low.

Since there is no structure transformation involved, there shouldn't be much in the way of res. stresses from that cause.

 

[EdStainless]

Are they soft? And is that uniform?
If not then you can rough machine and re-anneal. Since this is 316L I would heat to 1800F just long enought to get the entire part hot and air cool. You will get distortion on cooling so you might want to play with this a bit (fan/no fan and such).

If they are soft and uniform then I would just straighten them with force and go on my way.

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

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[Goahead]

Also rough milling could introduce surface stresses.
Could you try to change the milling procedure to one less aggressive, removing less material in more passes?
And possibly in a symmetric way, depending on design.

http://www.welding-advisers.com/

 

[EdStainless]

Be careful, SS work hardens a lot and you really need to cut deap enough with each pass to get under any existing surface layer. In stainless you are usually better off taking as much cut in each pass as you have the power for. But don't rush the feed.

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

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

 

[DDRSS10]

SS In my experience has a ton of internal stress. I have machined a lot of it and any time I've removed more matl from one side than the other I've found warpage. It can be very trying with asymetrical parts. What I've found is best to do is rough machine all large features then go back and finish machine to size. Seems like a lot of work, but you get good parts.

Nick

"Speed costs money boys, how fast do you want to go?"

 

[MildJoe]

Thank you for excellent response.
However i'm not too sure of what to do yet.

We have done a test on another kind of 316L-steel, and we see that there is no problem with that. The problem kind is 316L UGIMA, produced by Ugine-Savoie Imphy. It's an improved machining grade, equal to every 316L steel apart from a special melting process. The other steel we tried with no problem is PRODEC in 40mm plate, made by Avesta Sheffield (now OutoKumpu in Finland). This is also improved for machinability.
Our machine operator says that the forged UGIMA steel is softer and easier to machine than the PRODEC.

Our product is custommade, and we have no option but placing it out of symmetry in the stock. No two pieces are equal, and we produce in small batches. So there is no option to re-anneal between rough and fine machining. And there is no option to straighten things out with force, as the tolerance is quite rigid.

As long as there is asymmetrical machining, and the problem is residual stress, there will probably be of no use to reduce the feed\speed.

But i think i will try two cases.
1: Stress relieve one stock, and then machine in our normal way.

2. Change feed\speed, and see if there is an effect.

I would be gratefull for feedback from someone with experience (i'm pretty fresh out of school) on what temperature to use, and for how long. The volume of the stock is about 370ccm (22 cubic inches).
I've heard of two alternatives: ca 1050 C and quench in water, or ca 400 C and cool in air.
What would be the preffered process here, and what effects could i expect?
What i want is improved stability in the material, but they say that this steel is not hardenable by heat-treatment.

Could you also help me with cut depth and feed\speed rates, in [mm], [RPM] and [mm/min] We now use a Coromill 300 for the rough machining.

Thank you so much once more!!

 

[arunmrao]

After rough machining can you use soft jaws for holding the job. I have experienced this problem while machining 600 OD x 580 ID x 500 L. The job was bulky and was being finish machined on a Vertical Boring machine.You measure the dimensions on the machine,get it cleared by the inspector and then release the jaws,ID would be distorted by 0.5-0.8mm.

Try the second heat treatment option thereby lesser thermal stresses are introduced .

 

[Metalguy]

I wouldn't think that 400C/~750F would relieve much stress in 316L.

I think Ed is correct with the 1800F and air cool, but I'd use a fan to cool it off faster.

Do you know what the C amount is?

 

[DDRSS10]

Mildjoe,
Speeds and feeds are very part, machine, part, cutter, insert, ect specific. There are a million variables that go into them. The best thing you could do is talk with one of the older experienced machinists on the job, someone who can look at the print and you can explain what operations you are doing. I have been in the machining field for 13 years and have run many, many diferent machines, I am still surprised and learn things everyday. If you can't get it figured out try calling one of the application guys at sanvic they could probably be of some help. Good luck, keep up updated.

Nick

"Speed costs money boys, how fast do you want to go?"

 

[EdStainless]

In my exerience, I don't think that you will see any impact of using 400C. That means going clear to annealing temp. You could probably get away with 950C. Using "L" material you will not need to water quench, air cool should work.

I doubt that the Ugima is softer than Prodec, it is probably loaded with more 'stuff'. Prodec is well know for good properties, no loss in corrosion resistance, and (for a stainless) excellent machinability. Most 'machinability enhanced' grades have poorer corrosion resistance than standard 316L.

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

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

 

[billy8581]

i work on forging of 316l and they are approx350*358*280 and i usually use the formula



recommended cutting speed

------------- = revs

pie * dia



then for feed: revs * number of teeth* 0.1 = feedrate

try working with the cutter reps who supply the tooling/tips maybe that can help if it is a speed/feed problem

 

[MildJoe]

Thank you everyone!
I think you have increased my knowledge so i'm ready to do some testing now. I'll try the heating to about 950C and also some feed\speed alterations.
I'll let you know how it goes.

METALGUY: the C-amount is 0.03.
And in case you wonder:
Chromium 16.5-17.5,
Nickel 10-11,
Molybdenum 2-2.50 ,
Manganese 1.25-2 ,
Silicone 1.0,
Phosphorous 0.04,
Sulfur 0.015-0.030,
Copper 0.7,
Nitrogen 0.1