Dissimilar chrome welding

[powerplantcwi]

I am in the fossil fuel power generation industry. When welding chrome-moly alloys of different composition, I have always suggested the use of the higher of the two for the filler metal (ex. welding T22 to T9 with ER80S-B8 or E 8018-B8). Recently some clients are requesting the us to use the lesser of the two joined metals (ex. T22 to T9 with ER90S-B3 or E9018-B3. This to me would seam to be a safety concern for the long term. Either pratice will require PWHT of the higher chrome content, so I feel like these clients can only be trying to save money on the filler materials. Can anyone help me with the short and long term affects of each?

 

[metengr]

There is no safety concern because the strength of the dissimilar metal weld joint for low alloy steels will be governed by the weaker of the two base materials. Filler metal selection can be based on either matching the higher alloy base material or the lower alloy base material – the end result will be failure in the lower alloy base material. For conventional Cr-Mo alloys, I have seen either higher or lower strength filler metals, where a lower alloy filler metal is used to match the lower alloy base material and vice versa.

For creep strength enhanced ferritic steels joined to conventional Cr-Mo alloys, the selection of filler metal is usually toward matching the creep strength enhanced ferritic steel.

 

[Guest102023]

I have also heard the same advice, but never a rationale for recommending the higher alloy filler.

metengr,
What is your opinion of using conventional B3 filler between a P22 header and T23 tube?

 

[metengr]

brimstoner'
I would actually use the B3 filler metal for a T23 tube to Grade P22 header attachment weld, which would be a partial penetration groove weld by design. For this weld joint design, I don't need strength comparable to the T23 tube material because the tube will be thinner and therefore able to better accommodate thermal strains because of the higher allowable stresses for T23 in comparison to T22.

For Grade 91 to T23, I would use the matching filler metal for T23. I have two units that have T23 terminal tubes welded to a Grade P91 header that was safe ended and PWHT in the shop under controlled conditions. No problems.

 

[Duwe6]

There is a slight safety risk, but opposite of what you are thinking. When you move up from 80-series to 90 or 100 series fillers, the probability of cracking increases quite a bit at every step. Never match the stronger of your dissimilar metals. Just increases the chances of having weld cracking, possibly underbead cracks, which are undetectable by X-ray or dye-penn or magnetic particle Try to use something halfway between the two metals.

As metengr said, the weakest material will limit the strength of the weld joint, regardless of using a high-strength filler.

 

[stanweld]

powerplantcwi,
The high side filler metal approach was common to the power industry, the low side approach was common in the petrochem industry per Duwe6 comments. The low side approach has been gaining greater traction in the power industry.

We weld T23 to T91/P91 with T23 matching filler metal as metengr recommends. We similarly weld T23 to T22 and T-11, primarily for logistical reasons in the field - not metallurgical. The lower PWHT temp permitted by Code and the carbide stabilizing elements in the T23 filler mitigate carbon migration to the P91 side of the fusion line during PWHT. I would similarly recommend a T23 like filler metal to be used in welding the 1Cr-1Mo-.25V casting alloy used for control valves and steam turbine casings to P91 pipe.

 

[Guest102023]

metengr,
The T23-P22/B3 joint is intended to be full penetration.