9 Chrome

I recently performed a weld repair on a 16" Sa335 Gr.91 1.4 wall weld on an HP steam system. After Post weld Heat Treatment we performed HB hardness testing and found that the hardness was as follows:
Pipe 200 HB
Weld 225 HB
Fitting (elbow) 150 HB

We preped multiple areas on the elbow to verify the numbers and they all ranged around 150 HB.
Any ideas on why the fitting is so low? The heat treatment was 200 degree F ramp rates per hr up to 1375 degrees F, hold at 1375 degrees F for 2 hrs, and 200 degree F ranmp rates to 600 degrees F. We cooled under insulation after 600 degrees F. Preheat was 400 degrees F with a max interpass temp. of 600 degrees F.

 

[metengr]

Did you obtain hardness of the fitting before welding? This is critical because I suspect the fitting was soft from day 1. I have seen "soft" Grade 91 material because of improper heat treatment during original fabrication. You probably have ferrite in addition to tempered martensite. There is little you can do other than consider replacement of the fitting because you cannot reheat treat the fitting in place.

 

[Randyatryan]

No We did not, but will in the future!! Thank you and I figured it would be that , but thought I would post

 

[weldstan]

The fitting also may not be P91. Or have you confirmed that it is? It was not uncommon for P91 fittings to have been erroneously annealed with P22 fittings by the manufacturer. In one case, all of the fittings on one size line were found to have been improperly heat treated by the manufacturer. Because they were not yet installed, we normalized and tempered each of the pipe spools. There was one project (not ours) where over 30% of the P91 fitting were improperly heat treated.

If the fitting is indeed P91, you can remove the fitting and normalize and temper it and reuse or scrap it and buy another one. Do check the hardness and PMI the new fitting before installing.

 

[weldstan]

One further thought, if the fitting has been in use for a considerable time, Creep damage may well have occurred. In which case scrap it and replace.

 

[Randyatryan]

Thank You for all your help!!!!