Welding 304SS to P91

[RobsVette]

Greetings everyone,

I am looking to make a WPS and PQR to weld 304 stainless steel to P91. Before I embark on this process I was hoping to get some feedback and input from forum members who have experience in with these metals. I have just read Replicas thread on the subject and not wanting to hijack his threaded decided to started my own.

My plan for this WPS was to perform the weld overlay (or buttering) on the P91and weld to the stainless as follows.

- begin process with preheat of P91
- using GTAW process and ERNiCr-3 filler weld about 5/16" of material to the face of the P91. Then grind down about 1/16" to have 1/4" of overlay
- Then PWHT the P91 as would typically be done
- Then weld the P91 to the SS using GTAW process and ERNiCr-3 filler rod. Before welding preheat the P91 as required by Section I.

My questions are as follows;
- in the other thread, it was discussed that this type of arrangement had failed even after a number of thermal cycling and that a filler piece is better to be used in between the 2 metals. Is that meaning that a thicker piece of P22 should be place between the stainless and the P91? We have been doing this successfully in the past and have qualified procedures for both.
- I assume all of this should be using GTAW process, but was wondering if there were any SMAW options for work like this? I assume not since I cannot seem to find any appropriate wires.
- In the event that a weld were to crack at the joint between the P91 and the SS, how would you go about performing a crack repair on such a joint? Given the complexity of the welding procedure, it would not seem to lend itself to crack repairs.

Any feedback from past experiences or knowledge of these metals on this would be greatly appreciated. I am looking to try and avoid the pitfalls that those more experienced than myself may have already overcome.

If any other information is needed please let me know and thanks in advance for any help.

Rob

 

[metengr]

To answer your first question, a transition piece is commonly used to reduce the thermal coefficient of expansion mismatch and creep strength differences between 304H and CSEF steel (Grade 91). The transition piece could be Grade 23 or Incoloy 800.

Either GTAW or SMAW processes would work.

The second question regarding repairs, it would not be that difficult. The failure would be on the ferritic side (Grade 91) of the DMW from carbon migration. The repair would require local crack excavation, followed by weld repair using the same filler metal ERNiCr-3, and local PWHT for the Grade 91 material.

 

[weldstan]

Agree with metengr; Alloy 800 has worked the best for us.

 

[RobsVette]

Thank you guys very much for the responses. As usual, you guys are an excellent source of information.

I have acquired the electrodes needed (Lincoln Tech-Alloy 606 for the GTAW and Tech-Rod Weld-A for the SMAW work) and the purchased a 1/2" test plate. I will let you guys know how we make out.

Thank you very much again.

 

[davefitz]

As the ferritic was described as P91 and not T91 I assume the actual item is piping outside of the boiler and in the workspace. For such a weld , we have been successful in replacing this high risk weld with a modern "compact flange" with a seal ring in lieu of gaskets. For example, for oringal pipng welds between P91 and 316 H using incponel filler operating in daily cyclic mode from ambient to 1060 F, a 6 yr weld life was a typical result prior to failure, while replacement with a compact flange has been successful for the last 6 yrs and counting.
The replacement flanges designed as per asme section VIII using section 2 approved bolting and seal ring material .

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick