Welding of Dissimilar chrome-moly steel 2

[MetFreak]

Hi gent,
I've recently heard that there could be a problem in welding dissimilar chrome-moly steel when put in service. F22 with F9 for example.

Can anyone tell me more about it ? The problem seemed to be in the big difference in chromium content.

The question is not really clear, the reason is i'm not sure of what i'm looking for.

Thanks all

 

[metengr]

Yes, that is correct. There is a large difference in chromium content which results in carbon diffusion along the lower chromium content weld joint (T22). What you ultimately have is a region along the weld fusion zone of lower carbon material (T22) that is susceptible to creep damage.
There are several approaches to reduce the carbon diffusion or migration. One is to use a wider angle weld joint to reduce thermal/mechanical bending stresses and to select a filler metal that is either nickel-base or use a filler metal that is close to the chemical composition of the lower chromium content base metal or use an intermediate base metal as a transition piece (5% Cr – 1% Mo). The key is to prevent carbon diffusion to a higher chromium-containing weld metal and to reduce thermal/mechanical stresses. Carbon loves to combine with chromium.

 

[MetFreak]

Thank you very much for your quick answer Metengr, but there's something I don't get.

Why a chromium content difference takes us to : a carbon diffusion

 

[MetFreak]

Nevermind I get it ! Just had to do little brain gymnastic.

Thanks again !

 

[emxhps06]

Chromium carbides?