Dissimilar Cr-Mo Weld Joint Failure (continued)
Dissimilar Cr-Mo Weld Joint Failure (continued)
(OP)
Has anyone encountered DMW failures in P91-T23 joints welded with B9 filler? Because I think that is what I am seeing in my 1.5Ø x 1/4" T23 SH tube joined to a 2" thick P91 header ('joined' indicating the past tense). The hardness drop is 300 to 200 HV500. The tube is at an 'extreme' location at the end of the header, nearest the sidewall, in the first SH harp facing the CT.
Four metallographic sections were examined. Fracture in the tube HAZ follows the fusion line very closely in the outer 50~75% of the thickness, despite a very low fusion line angle, typically 45° but approaching 0° at a few points. Fracture is square in the inner 25~50%, which I believe is terminal fatigue. Unfortunately the failure is old and oxidation has obliterated the surface details.
Four metallographic sections were examined. Fracture in the tube HAZ follows the fusion line very closely in the outer 50~75% of the thickness, despite a very low fusion line angle, typically 45° but approaching 0° at a few points. Fracture is square in the inner 25~50%, which I believe is terminal fatigue. Unfortunately the failure is old and oxidation has obliterated the surface details.





RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
Tube HAZ hardness is low, and remains fairly flat all the way out into unaffected base metal, so it appears PWHT was indeed on the high side. Hardness is below 200 everywhere in the tube, with 170 just 0.5mm from the fusion line, with several results in the 170s in the base (the lowest 167). Grain boundaries are well decorated with carbides.
(1) I think I have the evidence, but what detailed examination to confirm carbon migration would you recommend?
(2) Tell-tale orange carbonitride particles are evident in the matrix; is this expected or another possible indicator of incorrect processing?
p.s., the tube exhibits surface decarb, so it did not have a happy start in life. You might even say it was not born into a good family (like V&M).
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
Chemical etching and microhardness testing. The local region of decarburization caused by carbon migration is obvious in this DMW.
No. In my experience optical metallography alone is not a reliable predictor for service performance with creep strength enhance ferritic steels.
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
(1) That work is complete; my question is more of an academic nature. The hardness results certainly seem consistent with HAZ decarb.
(2) Not sure I was clear enough. I am asking whether the presence of these particles is normal or whether it indicates possible processing deficiencies. I would not attempt to draw any conclusions about creep strength from the microstructure. This was more of a 'BTW' question.
Further to a previous discussion re: etchants, I switched to Vilella's for this job, with far better results. It revealed more detail in the T23 and weld, whereas nital barely touched the B9 weld.
Thanks again.
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
I would highly recommend you obtain the publication below from EPRI regarding Grade 91 steels. It was made available to the public domain because of safety concerns;
ERPI Report 1023199. This is one of the best reference documents to date to keep owner/user/manufacturers out of trouble with Grade 91 steels.
In this report, the desired microstructure for Grade 91 consists of M23C6 carbides and MX-type carbonitrides that precipitate on lath boundaries and defect sites. Typical photomicrographs will not display the precipitates. So, if you are seeing large MX-type carbo-nitrides, there is a processing problem.
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
My carbonitrides are clearly visible at medium magnification, ~400x, although they would not be as obvious in a micrograph at that that magnification. At 1000x, no problem.
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
I expect x-ray would be difficult; perhaps reflected shear wave UT or advanced UT method. Is there an accepted industry practice?
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
Does HAZ decarburization happen strictly during PWHT, or can it continue at service temperatures? I would think not very much, but I'd like it confirmed.
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
Yes.
No. It is elevated temperature exposure in service for many thousands of hours.
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
What is the actual fracture mechanism in this narrow zone?
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
I'll be interested to learn how similar joints behave in some recent coal fired plants. While I know how we (as the Assembler) made the welds in the field, I'm not so well aware of how the welds were made in the boiler manufacturer's shop since I did not have any contractual controls over the manufacturer.
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
As much as cycling plants are abused by operators, most of the failures I have investigated had pre-existing issues from the factory. With old-fashioned boilers this is rarely the case. The gas plant building boom happened with great haste, designing on the fly, and using new and under-appreciated exotic alloys. Among many other factors.
Much of the component fabrication is done out of sight and far away, and on-site surveillance (something I grew up with) is a favourite place for EPCs to slash their costs. Surveillance these days seems to be just an exercise in faxing. We've all seen what happens when regulators fail.
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
One would think that after the millions of dollars that we have had to spend to rectify shop screw-ups, there might be some lessons learned. Oh well!!!
RE: Dissimilar Cr-Mo Weld Joint Failure (continued)
We could consume multiple lagers together discussing the problems of modern manufacturing... ;)