PWHT Dissimilar Metals/9cr-2.25cr 1

[rmillercwi]

In process of writing a spec for welding 9cr-2.25cr. Section 1 HRSG Superheater. MAWP about 2500psi, steam temp about 900f. Field install.

The header itself is going to be 9cr. (header pipe, endplates, nozzle, etc). There will be 2" od 2.25cr stubs welded to the 9cr header to transition into 2.25cr in the field. Field supply is header with stubs. Boiler tubes installed in the field one tube at a time.

The question is regarding PWHT. I have typically chosen the PWHT temp for matching 9cr materials to be 1375 to 1425. I have typically spec'd 9cr-2.25cr PWHT @ 1300-1350.

The rub here is this is this is a 9cr header with 2.25 stubs that will be fabricated as a whole and stressed as a whole. The majority of the materials are 9cr.

Not sure to go with the 1375 to 1425 stress temp or the 1300 to 1350. I have read the relevant code and see the requirement for P15E dissimilar metals is min 1300. In this case to do you favor the 9cr stress temps or the 2.25cr?


Perhaps the answer is obvious. Stress the entire header at the 1375 to 1425 and be done with it.

 

[weldstan]

If the header has the Grade 91 end caps welded to it and you PWHT the entire header assembly, you must comply with the P91 requirement of 1350 min. If not and you make the dissimilar weld with "B3" welding filler metal, choose the lower temp range to mitigate carbon migration from the weld metal into the P91 header.

 

[metengr]

First, is the 9 Cr material you reference a standard 9Cr-1Mo, or is this Grade 91 material? If this is simply 9Cr-1Mo, a P-No 5B Gr 1 material, you can select 1300-1350 deg F to accomplish PWHT for both materials.

If the material is a P-No 15E Gr 1, Grade 91, this type of weld joint should have been designed as a dissimilar metal weld joint and safe ended using Grade T23 tube material or Grade T5 material, so the field welding would be T23 or T5 to T22 tube material. If you are stuck with welding Grade 91 to T22, you should use E9013/ER90S B3 filler metal and PWHT at 1380-1425 deg F. The lower critical transformation temperature for T22 material is 1480 deg F, so you have some margin.

 

[davefitz]

I am not sure of the optimum PWHT as it would depend on whether you are also making a major header butt weld and/or are using matching p91 filler at the stub welds. If you are also including a major P91 butt weld in header assy or are using matching p91 weld filler on the stub welds then the highest PWHT temperature that can be tolerated by P22 ( lower critical temp - margin of safety for thermocouple error)would be selected and I would use the "tempering parameter " published by NEM ( using the LMP) to ensure the final hardness reading near the major P91 welds are below 270 Hv.

If there is no major P91 butt weld included in the instant header assy and the stub weld filler will match the 2.25 Cr 1 Mo tubes then the lower PWHT temperature above 1300 F as per table PW-39-5 notes (2) and (3), and not use the NEM correlation if the P91 header ass'y was previously PWHT'd.

"Whom the gods would destroy, they first make mad "

 

[rmillercwi]

You bring up a valid point regarding filler metal of the stub to the header. I hadn't thought much about it and was intending to use B9 filler throughout the header assembly but now re-thinking it. It appears the consensus that a B-3 filler would be appropriate (undermatching).

Regarding base materials, I am not understanding the question. Header pipe/nozzle SA 335 P-91, tube stubs are SA-213 T-22,SA 387-91 end plates....

 

[weldstan]

The SA-387-91 end plate to P91 header pipe weld governs and you must use the 1350 F Min hold temp. If you use the "B3" filler metal, PWHT at the permitted lower temp range to mitigate carbon migration from the "B3" deposited weld metal into the header is recommended. Carbon migration will increase with increasing temperature. The effect is to denude the deposited weld metal of carbon adjacent to the fusion line, which creates a weak link in the chain dissimilar metals chain.

 

[rmillercwi]

Thank you. I understand the need for using the 1350 min hold temp. If I am reading your latest response correctly, "if" I choose to use the B3 filler at the tube to header weld (using B9 for all other matching materials) the recommendation is to keep the soak temp as close to 1350 as possible.

My apologies for dragging this out. I see that you said "if" which make me think there is a choice here .Would you not recommend B9 filler for the tube to header welds? If so, what would you recommend for soak temp?



From a fabrication standpoint, keeping the filler rod, pre-heat etc. the same does have some benefit and is what I predict the vendor is going to come up with. Maybe not. Just trying to be informed of possibilities.

I do understand the recommendation above of using T-23 tube stubs. It would simplify things but have had some bad experiences with it in the past.

 

[weldstan]

The dissimilar metals weld in question, has been made with "B3" and "B9" weld filler metals. The choice is up to the Boiler Manufacturer unless the Purchaser/Owner specifies which one is to be used.

 

[rmillercwi]

I agree both fillers have been used in the past. Many times. Welded probably a couple thousand similar joints myself back in the day.

The decisions mentioned above is where I am at with this now. The client is looking to us for guidance. We have a well known oem doing the work but history has shown that oem suggestions alone is no sure thing in all cases. I like to do my own homework before accepting any unorthadox weld procedure.

If advice I receive here jibes with others/the oem suggests.....I can typically feel good about it. If not.....I keep digging.

 

[weldstan]

From what I read today, the general consensus appears to be to use "B3" filler metal for the joint and keep the PWHT temp low. The HAZ of the P91 responds well at a temp. of 1325F. I require a temp range from 1325F to 1350F when using "B3" or the Owner's requirements when greater minimum temps are specified.