PWHT and Ferrite number 1

[Nashanas]

Hello,

According to API RP 582, the ferrite test should be performed prior to PWHT. But, I was reading a research article where it was shown that a large percentage of delta ferrite desolves during PWHT (in the research findings it went down from 8% to 1.2%) if the PWHT temperature range is 950 - 1050 °C. I am talking about 304H here. So, to me it seems that if I am performing PWHT in this temperature range, I should logically perform ferrite test after PWHT, as my concern should be the ferrite remaining in the final structure.

Why then is the API code insisting on perfroming ferrite test before PWHT?

 

[EdStainless]

I see both required. The one after weld is part of the weld process QA, the one after is to confirm response.
In a true solution anneal the delta ferrite should drop below 0.5%.
Why that temp range?
The lower end is still in the stabilization range and the upper end is nearly a solution anneal. You will get very different response (and properties) at 950 vs 1050.

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[Nashanas]

You are right, I will refine my question. The client has asked to perfrom 960° for 90 minutes. I found in literature the graph attached. PWHT at 1050 for 4 hours reduces it to 1.2%. Interpolating from the graph, PWHT at 960° for 90 minutes the reduction should be to around 4.7%.

But in any case since the total ferrite content is reducing after PWHT what is the point of ferrite test before PWHT?

And what is the remedy in case before PWHT the ferrite number is higher than expected? Refusal of component? Or modification of PWHT?

 

[0707]

In case before PWHT the ferrite number is higher than expected? Refuse the component, becouse is an exigence of the sepec.

luis

 

[EdStainless]

You need to check before to make sure that the weld procedure was followed.
If you are expecting to hit some value after there is a big difference between starting at 6 vs 12 FN.
960C is not and anneal, it is too low especially for an "H" alloy.
If the intention is to re-anneal you need 1020C or so in order to get it done.
Did the paper that you got the figure from have any micros in it? Did they talk about the changes in carbide size and distribution?
Strictly speaking reduction in delta ferrite is a side effect of annealing SS, you are really trying to eliminate grain boundary carbides. The problem isn't the carbides but the fact that the Cr in them has been sucked from the material along the boundaries leaving these areas depleted in Cr and with much lower corrosion resistance. This is the sensitization that people talk about.

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[gtaw]

Ferrite, a small amount, is desirable during welding to mitigate the potential for solidification cracking.

Performing a PWHT at the higher temperatures, cited in another response, would as permit the chromium carbides to dissolve and go back into solution thereby eliminating sensitization and mitigate the potential for intergranular stress corrosion and intergranular stress corrosion cracking.

At least that’s what I read in a book once upon a time.


Best regards - Al

 

What is the equipment and why are welds being annealed?

Ferrite in austenitic fillers is only needed to avoid hot cracking; after that we usually don't care, although ferrite cannot be tolerated at all in some corrosive environments.

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

 

[EdStainless]

It also cannot be tolerated in high field magnetic applications.

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