Stainless steels depend on chromium for their basic corrosion protection.  When welding, the chromium can become tied up with carbon and lose some of this protective ability.  One way around this is to use low-carbon stainless steels such as 304L or to use specially stabilized ones such as 321 or 347.

In general, austenitic stainless steels do not require any heat treatment. For most applications, SS grade is selected so as not to warrant any heat treatment subsequent to welding. A good guideline is provided by davem&p above. If u wish to get further clarificatins/answers  me know.

Dear All,
Do we need solution annealing for duplex steel, UNS S31803?
The thickness of my plate is 12.7mm. Please advise. Thank you.

Regards,
Lee
hockchong.lee@akerkvaerner.com

Solution treating of stainless steels is primarily a base materials specification requirement - to be performed by the stainless steel product (tube, plate, forging, etc) manufacturer. It is not normally required of a fabricator of equipment/components made from stainless steel unless mandated by the Engineer/Designer or Customer.

We've done some solution heat treatment of austentic stainless in the field and on some quite thickn sections. It's not for the faint hearted - because of the low thermal conductivity heating rates need to be quite slow in order to avoid inducing through wall stresses. Similarly HT of branch geometries is quite complex.

As Stanweld points out it is not usually a requirement and would normally only be specified for some particular reason. Ours was removal of residual stresses.  

Sometimes our direct resistance heating is non uniform when solution annealing 304 and 316 stainless, I understand the set point temperature is 1900 - 2000 F.  Is their a maximum temperature that should not be exceeded?  

Grain size increases with temperature, as you know. What is less obvious is that the tendency to sensitize increases with increasing grain size. So, if you don't have a low carbon chemistry, you'll invite problems.
Also less known is the fact that austenitic and duplex welds have significantly poorer corrosion resistance than the wrought base metal. A long anneal can reduce this problem. Patented versions of 304L and 316L have titanium additions to prevent this phenomena by gettering oxygen and sulfur before they can cause de-chromization.

When you know that you can't postweld anneal you need to plan ahead.
You should always use low carbon grades to minimize sensitization.
You should also weld with filler metals that are alloyed to help improve the corrosion resistance of the welds.  In austenitic alloys this usually means higher Cr and Mo, along with higher Ni.  In duplex alloys the weld fillers have a different phase balance so that the un-annealed welds end up with the correct microstructure.
In a 3XX stainless I don't like annealing temps above 2150F.  But to be honest more damage is done during slow heating and cooling.  If you are annealing parts that must be heated and cooled slowly you need to really know what you are doing or you can end up worse off.

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Corrosion never sleeps, but it can be slowed down.