buttering to minimize sensitizing of stainless

[kevlar49]

I have an urgent need to repair a catalytic reformer 304 shroud to 1.25Cr reactor shell weld. The shroud has buckled and needs to be reattached to the shell. I think that I have to join it with Inco 182 to increase thermal fatigue resistance. I am recommending PWHT of the 1.25Cr at 1300-1350 F. Preheat at 300 F. I read that in order to avoid sensitizing the stainless component, some companies butter the alloy steel then PWHT that and weld to the butter without subsequent stress relief. I want to avoid sensitization to prevent chloride scc which I am told has happened in a major refiner's 300-series reactor internals.

The question I have is how does this prevent sensitization of the 304 (non-L grade)? Is this procedure only valid if the 304 is L grade or does the fact that you minimize time at temperature minimize but not eliminate sensitization of 304 as welded?

Thanks very much.

 

[kevlar49]

Sorry, should clarify. The reason I ask this question is because I alwasy thought that the non-L grade would sensitize anyway as-welded.

 

[kevlar49]

Something else that I forgot to mention: the reactor internals are exposed to 970 F so they are likely sensitized already. Could a PWHT at 1300 F make it any worse?

 

[metengr]

I read that in order to avoid sensitizing the stainless component, some companies butter the alloy steel then PWHT that and weld to the butter without subsequent stress relief.

Yes, buttering is commonly done to avoid having to PWHT the entire weld region. Use Inconel to butter against the Grade 11 plate material. PWHT as you stated and finish the weld with Inconel against the 304L shroud with no PWHT. You will need two weld procedures. one with Inconel on P-No 4 base material with PWHT, and the second procedure will be Inconel on P-No 8 base material.

 

[metengr]

kevlar49;
In answer to your second question, the technique I described above is to avoid exposure of any austenitic stainless steel base material to sensitization from welding. The fact that you have 304L is good because it will sensitize less in comparison to the higher carbon grades. However, the peak for sensitization occurs between 1050-1300 deg F. So, anything you can do to avoid exposure to this temperature range helps.

 

[stanweld]

The use of the buttering method will also help with the 304 S.S. since its exposure to the PWHT is eliminated. You must cool rapidly during welding to mitigate sensitization of the non-low carbon 304 S.S. Suggest interpass temperature < 200 F and applying chills. Assuming that the 304 shroud is relatively thin, it is doubtful that the HAZ will be free from sensitization but it can be minimized.

If shroud replacement is an option, I suggest 304L or 321 be used.

 

[EdStainless]

Kevlar, I am not sure that the process is clear.
You take the steel parts and butter with 182.
Stress relieve just these parts.
Now weld the 304L to the 182. No perheat and no PWHT. You have choice of filler. most would use either 182 again or 309L.
You are finished.

304L at 970F? Yes you are sensitized. In 304L the amount of Cr carbide formed is rather low. Nothing you can do to fix this. In the future either use 321 or go to very low carbon 304L.

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