Very important: T22 and T91 Coating curing at 700 after welding

[kiperest]

Hello,
I have a question regarding stability of welding joints between T91 and T22.
The part should be coated and cured at 700C (1292F), my question is regarding the welding joints, can the curing cause any problems or future defects in those areas, or does it decrease the corrosion resistance or mechanical properties?
Thanks in advance

 

[stanweld]

Assuming that this represents a materials spec break in the superheater, why are you employing a high temp. diffusion coating?
What welding filler metal are you using for the dissimilar joints?

 

[EdStainless]

Does this exceed the original temper temperatures for either of the parts?
If so then don't do it.
Can you look at this under guidelines for PWHT?

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Plymouth Tube

 

[stanweld]

Also, what is the hold time at the 700 C temperature during the coating process?

 

[metengr]

If you are coating a finished assembly of T22 and T91 with all weld joints completed and PWHT'd in accordance with Code of Construction (ASME) requirements, technically there should be no issue.

The 700 deg C exposure of the T22 material and the T91 will not adversely effect bulk or weld region properties because the temper requirements are minimum not maximum in material specifications.
What I would do is to have a coupon of T22 to T91 material with a finished welded joint and subject it to the above coating curing time and temperature and evaluate the properties (even hardness testing could be performed for evaluation). This is easy enough to have done and can be defended regarding effects on original properties and of the weld region.