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Heat treating AISI 321 stainless steel
3

Heat treating AISI 321 stainless steel

Heat treating AISI 321 stainless steel

(OP)
According to ASME Code, section II,  SA 240, heat treatment of material shall be in accordance with SA 480.
In the case of austenitic stainless steels, the material shall be solution annealed.
For Type 321 Ti-stabilized stainless steel, section 15.2.3 of SA 480 states that a stabilization anneal may be used in case solution annealing temperature is higher than 1950°F and the steel is subsequently exposed to sensitizing conditions.
If this is not the case, it appears that AISI 321 stainless steel can be purchased in the solution annealed condition (no stabilization).
According to ASM Handbooks Type 321 stainless steel is to be given stabilization HT only if in contact with highly corrosive media.  
Is this correct?
How would the steel be stabilized and resist to intergranular corrosion if it solution annealing isn't followed by stabilization, i.e. no specific heat treatment for Ti-carbides precipitation is applied?
Does TiC precipitation occur during solution annealing?

RE: Heat treating AISI 321 stainless steel

2
The addition of Ti results in stabilization of the AISI Type 321 ss without the need for a separate stabilization heat treatment, which is separate from solution annealing. TiC precipitation should not occur during solution annealing - all of the carbon at this temperature should be dissolved in the austenite matrix.

Regarding the information in ASM Handbooks for Type 321, refer to Thread330-119356.

RE: Heat treating AISI 321 stainless steel

Practically speaking, you often don't need to worry about stabalizing 321.
Either because your service temp is high enough to do it or because there is so little C that it doesn't really matter. (the alloy wouldn't sensitize anyway)
The one time that you really should care is when you are operating at the very lower range of sensitization.  If you willb e running between 900-1100F then prior treatment may be very helpful.  This is too cold for the material to naturally stabalize on its own and yet some sensitization could occur.
Just remember if you have material stabalized then the original flatness/straightness specifications do not apply.  Make sure that you can live with that.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Heat treating AISI 321 stainless steel

(OP)
Thanks Metengr, just another question.
If all of the carbon at annealing temperature is dissolved into the austenitic matrix (this is what originated my question), then Type 321 ss, after annealing, would be free from TiC.
The result would be a kind of 'type 304' steel with addition of Ti and no stabilisation since dissolved carbon could form Cr-carbides when heated in the precipitation range, this is a nonsense to me (I must be misunderstanding something).
Are Ti-C present before annealing and do they remain after this HT, since Ti-C dissolve at higher temperatures?

RE: Heat treating AISI 321 stainless steel

The addition of Ti serves two purposes in austenitic stainless steel; besides forming TiC or Ti(C,N), it also increases mechanical properties at elevated temperatures, so your statement regarding the formation of a type 304 ss is not correct.

RE: Heat treating AISI 321 stainless steel

(OP)
Thanks! This is something I wouldn't have found in books!

RE: Heat treating AISI 321 stainless steel

Ti has higher affinity for carbon than chromium. Hence always TiC will be first formed to tie up any undissolved carbon.

RE: Heat treating AISI 321 stainless steel

Not exactly always.
If you are at the lower edge of the sensitization range there is not enough diffusion to allow Ti to tie up all of the C.  The result is that some of the carbon will form CrCabides along the grain boundaries and result in some sensitization.

I have done it in the lab.  I have never seen this happen in real life.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Heat treating AISI 321 stainless steel

One reason for using 321SS is to aid in staving off polythionic stress corrosion cracking.  If you're using it for that, or in environments susceptible to that, wouldn't you definitely want it to be annealed/stabilized prior to installation?

~NiM

RE: Heat treating AISI 321 stainless steel

Another item to think about:  Stabilization annealing of 347 stainless (Adds Columbium/Niobium instead of Titanium) apparently significantly reduces the creep properties.  I would expect the same thing happen with 321.

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