PWHT in stainless steel 2

[purode333]

Dear sir,

Why PWHT is not allowed/done in austenitic stainless steel.
Example 316.

Regards
Prasad

 

[metengr]

Because the 316 stainless steel, being an austenitic stainless steel, is not hardenable by heat treatment or from welding.

 

[weldstan]

PWHT is often done on stainless steel when required by Owner or Owners' Engineers; e.g.,. solution annealing, stabilization treatments, primarily where corrosion service dictates.

 

[Chumpes]

yo
PWHT usually stand for "post weld heat treatment" of carbon steels and low alloy steels. it is performed at 600-700°C.
as per ASME code, austenitic stainless steel should not normally be post weld heat treated at 600-700°C because this can lead to dimensionnal variations and loss of mechanical properties and corrosion resistance (precipitation of harmful intermetallic phases, sigma phase embrittlement, 475C embrittlement, sensitization by chromium carbide precipitatation, ...). refer to ASME section dedicated to PWHT.
other heat treatments exist for austenitic stainless steels but these heat treatments (annealing, stabilizing, ...) should not be confused with PWHT of ferritic materials weldements.

regards

 

[arunmrao]

@ weldstan,

I agree with your comments, but,I am stumped by this term "Owners' Engineers;", can you please explain!.


"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[DVWE]

thick sections of ASS

Hmmm..

Shirley, we're going to have to come up with a better acronym for Austenitic Stainless Steel.

 

[EdStainless]

and don't call me shirley

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P.E. Metallurgy, Plymouth Tube

 

[pradipgoswami]

Hi Prasad,
In short there could a few types of PWHT scenarios for austenitc stainless steel.
-Solution Anneal-restores the complete corrosion resistance after any welding fabrication, especially in thick sections of Austenitic Stainless Steels. The problem is heavy distortion. Hence none would pursue this unless mandatory.

-PWHT below AC1 temperatures , usually hastens Carbide precipitation & Sigma phase(in specific prolonged PWHT cases) in unstabilized grades e.g 304, 304H, 316, 321, 321H, 347...

- Stabilizing anneal is a lower temperature prolonged soak heat treatment applied primarily for stabilized and non stabilized grades. Look at NACE RP/SP-0170-2015, for details. Helps to restore post fabrication corrosion resistances in polythionic acid/refinery corrosion media.

One has to look trough all the scenario and use the PWHT very judiciously for Austenitic Stainless Steels.In many instances the requirements are mandated by the design/service related specifications.

Thanks.


Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299

 

[Chumpes]

@pradipgoswami
stabilized grades are : 321, 321H, 347...
unstabilized grades are : 304, 304H, 316...

only stabilized grades (stabilization by chemical addition) may be subjected to stabilization heat treatment at 900-950°C (321, 347). other grade (316, 304) should not be normally stabilization heat treated

 

[weldstan]

arunmrao
I was just using the the plural of Owner and Engineer because a number of different Owners or their Engineers, with whom I have had direct experience, have required these PWHTs for specific services.

 

[arunmrao]

Thanks weldstan. I have had to contend with some engineers and their bosses, who felt that CF8 grade castings are not weldable.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[weldstan]

Ignorance is sometimes bliss; however, we must confront the mistaken ideas of Owners and their Engineers when our provable knowledge dictates and when harm may come to others if their mistaken ideas are implemented.

 

[gtaw]

"Are we talking about that shiny silver stuff that doesn't rust?"
I can't believe how many times I've heard that from engineers and owners alike.

Best regards - Al

 

[purode333]

Dear all,

pls clear the concept of the subject.

I think it is due the carbide precipitation.
But my question is that when this precipitation starts.

Regards
Prasad rode

 

[arunmrao]

purode333,

There are very clear pointers provided in the discussion. For more material input, do your own research and come back, if any doubt persists.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[EdStainless]

In most grades Cr carbides will start to form at about 800F and they form most rapidly at 1200F.
at these temperature not only do Cr carbides form but you also get severe levels of Cr depletion along the grain boundaries (because the Cr is being taken up in the carbides). This is what is called sensitization.
At higher temps the Cr carbides still form (say up to about 1750F depending C levels). But above 1500F (roughly) the temp is high enough that Cr diffusion 'heals' the depletion along the grain boundaries. You still have carbides, and this can still cause problems with mechanical properties and corrosion, but you don't have severe Cr depletion.

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P.E. Metallurgy, Plymouth Tube

 

[sahsanb]

We had a shell & tube heat exchanger with SS 316 tubes and CS material tube sheet and shell. Tubesheet to shell joint type and thickness demanded PWHT be carried out to comply all ASME U stamp requirements. Manufacturer developed a tubesheet test piece with SS tubes and tubesheet with same thickness as original. PWHT was carried out on tubesheet, we carried out metallurgical analysis to confirm no chromium carbide formation had occurred after PWHT and hence proceeded with PWHT on the exchanger.

 

[EdStainless]

sahsanb, people usually design those exchangers to that the final shell closure weld is made in a collar of material that does not require PWHT.
I have seen many SS exchangers fail from exposure to PWHT, and they thought that they were safe.

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P.E. Metallurgy, Plymouth Tube

 

[weldstan]

Agree with Ed. Big mistake also the thickness of the weld would no doubt govern PWHT.