Clarification SS316/316L 4

[pnrngv]

Hi,
In Client general Piping Class standard it is mentioned as SS 316/SS 316L. It is not specified as Dual Grade.

However, End user/Client deliverable (ISO, MTO and other documents) specified as SS 316L only.

Reviewer is insisting on the Dual Grade requirement.

Need some clarification on the interpretation. When specified as SS 316(Stroke)SS 316L, either of the grade or Dual grade can be selected based on the application.
Since the SS 316L is higher grade material and more corrosion resistant, the same can be accepted?

Please advice.

 

[XL83NL]

im: love that curve of sensitisation. Used it quite often for instructing IWP/IWS course students in metallurgy, and for process engineers in our company to explain why we should jump to 321 once we pass a certain temperature.
Does anyone know why the lines ends at 1000h and don’t go through 10000h?

Huub

 

[EdStainless]

Somewhere in the 10,000-50,000hr range the top line and bottom line converge.
The issue with sensitization isn't really the formation of Cr carbides, it is the local depletion of Cr around them. It is this Cr lean region that corrodes easily.
At temps over 1050-1100F 0.02% C material never sensitizes. First very little Cr carbide will form in the low C material, and secondly it forms so slowly that the Cr can diffuse and 'heal' the effected region. The same is true at higher temps for higher C materials.
With 321 you have to be careful, if the service temp is near the lower end of the range and you didn't pre-stabalize the material then the Cr carbides will form faster than the Ti carbides and you will still get sensitization.
Most people that use 321 ignore the fact that it has worse corrosion resistance than 304L because of all of the TiC (and oxides and nitrides) trash in it.
I'll try to find an original reference, but each of these curves is based on some extent of damage, so the shapes depend on what is being measured. Jim Fritz wrote a paper about measuring this electrochemically while he was at Allegheny Ludlum, I will look for a copy.
There are other issues also. In SS welds there will be some residual delta ferrite which corrodes very easily, and there are some other phases that can form.

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P.E. Metallurgy

 

[moltenmetal]

It merits mentioning the HIGH carbon grades 304H and 316H for instance, where the high carbon is required for creep resistance. They find considerable successful use in service despite the risk of sensitization.

The stabilized grades 321 and 347 are used when equipment will be operated hot, then brought down to colder temperatures and exposed to low temperature corrosion mechanisms. If the thing will spend most of its life hot, and won't be allowed to rot by exposure to corrodents when cold, you can get away with the H grades, as we have done many times in past. 347 is expensive and a waste when not truly required.

 

Designers should call for the stabilized grades only when absolutely necessary, given that processing and service exposure make them prone to instability and expensive failure.

Other monsters lurk at long high temperature service, such as sigma phase embrittlement. Welding has an impact on that phenomenon by virtue of ferrite that is normally present in deposits. Ferrite starts precipitating sigma phase far faster and sooner than austenite phase, one reason I tend to prefer the lower end of the recommended range. 347 filler metal used for both 321 and 347 contains Nb, a strong ferrite promoter that increases sigmatization tendency.

As I like to say about exotic T23 and P91 ferritic boiler steels, "live by the carbides, die by the carbides".


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[0707]

This question will be a never ending story. This is a question for avocats on the main contract small words.

luis