Stress relieving on SS316

[dogbural]

Hi,

If the extruded SS 316 tube is stress relieved at the following condition, can we expect to have any hardness change?
- furnace @980 deg Celsius for 20 hours
- cool by air

 

[weldstan]

Yes, but why for 20 hrs?

 

[EdStainless]

From that temp you would need to heat quickly and cool quickly.
My guess is that 20min would be long enough.
And yes, you may see a change in properties.
It depends on how the original material was cooled and handled.
If it was quenched from above 960C then I would expect no change.
If it slow cooled, you are actually hurting the material by not going hotter (1060C min) for full solution anneal.
If it had much cold work done to it, then you will recover ductility and reduce the yield strength.

 

[dogbural]

Supplier actually confirmed the process as below

Placing the SS tubes for 3 hours (this is a stage where the temperature ramps up to 980 deg)
Open the furnace and cool it down for 17 hours

I have not checked with raw material supplier as to how they do quenching.

It seems believe or not, the hardness was measured higher after stress relieving. Please note that we just compared different samples as we did not measure the hardness before and after the relieving.

Is it worthwhile to do mechanical property testing to see the samples are actually still okay?

 

[EdStainless]

If they slow cooled enough to form carbides and/or intermetallics in the structure, then the hardness should go up.
And corrosion resistance and ductility both go down.

 

[dogbural]

What would be your recommendation to maintain the same properties?

Should use oil? For how long?

 

[EdStainless]

How heavy is the wall of the tube?
Is the material 316L?
If so, then charge into a hot furnace, once everything stabilizes give it another 15min, pull it out and force cool it with fans.
You want to be under 800F (~450C) within a few minutes.
If it was well annealed to start with then this is all that you need.
If it was 'process annealed' originally then I would raise the temp and do a real anneal at 1075-1100C.