Welding 9% Chrome with ERNiCr variants without PWHT - NBIC Weld Method 6 1

[RandySavage]

Hello All,

Trying to qualify a procedure for welding 9% Chrome material (Grade 91) to itself with ERNiCr-3 N06082 filler metal - in accordance with the parameters laid out in the NBIC Section 3 book's Weld Method 6.
For any interested it can be found in section 2.5.3, 2.5.3.6.

This weld method gives guidelines that must be followed and various restrictions to variables in order to not PWHT the P91 material. I know that 9% Cr is somewhat taboo to bring up, but the reality is that it exists and people continue to struggle with it. Please note this is not for new builds and only pertains to repairs of Grade 91 boiler tube specifically, internal to the boiler, with a maximum size of NPS 5 in diameter and 1/2" MWT for which notch toughness testing was not required. Our normal procedures surrounding 9% Cr do use appropriate filler metal with PWHT, but just trying to stay on-top of certain things in the industry changing.

I was wondering if anyone else has tried coming up with a process for this, and if so if there were any tips or advice that could be offered.

The problem I'm currently facing is that when bending the roots there is some form of failure in the weld joint itself, from the first attempt of snapping it in half to more recent attempts with changing some of our methods that are still failing but at least not to the same degree. The only consistency is that the caps are bending quite well. Unsure if it is the heat cycling, heat distribution due to rig angle changing, etc.

All of this is being done entirely with the GTAW process.
Base material is SA 335 P91 Type 1, 2-1/2" Sch 80(0.276")
Filler Metal is ERNiCr-3, have 3/32" and 1/8"
Butt-welded, 3/32" Gap with 37.5 degree bevel is about average so far, trying both Argon purge and Solar Flux Type I.
Sorry for the length of this, but I figured some documentation of the process and changes we made may not only help determine errors here, but could be useful for others in the future.

1.) Started around 110 Amps with a preheat of 200 degrees F and never neared the interpass temp of 550 deg F. Tried Paste here, figured results may have been running too cold to burn out flux and that some may have been trapped. - Very bad results, but it was the initial trial.

2.) 110 Amp root again, but with 100% Argon purge. Slightly elevated preheat to 275 Deg F - hit 375 degrees F at completion of root. Hot pass hit 383 degrees, cap hit 430 degrees. Same results as first where root failed entirely, cap had some minor discontinuities but not terrible.

3.) Elevated Preheat to 300 degrees F - Wrapped with K-Wol in-between passes to reduce cooling rate. Rooted at 150 Amps with 3/32" wire, fill and cap done at 170 Amps with 3/32" as well. On cap we started to get close to the interpass temp reaching 515 Degrees. Root bend failed, but finally did not come apart, caps finally began to bend well and had no visible defects, but evidence of the HAZ of the base metal being strong and ductile is there. On these bends the failure was at the weld interface itself and not directly in the center of the weld. Seemed as though it wasn't strong enough to fully keep itself fused to the base metal.

4.) Preheat elevated to 400 Degrees, 1/8" gap however with a slight land this time of the bevels - near 1/32", 1/8" filler, 190 Amps for the root. Also wrapped to reduce cooling rate. Maximum temperature achieved was 515 Degrees. Cap again came out well, if anything the roots were worse than trial #3.

If you would like any additional info please just ask. We are continuing to try different methods, but if anyone has some additional tips or insight that would be much appreciated.

Thank you,

 

[EdStainless]

I normally wouldn't do this but considering the topic ....
I will pass along some second hand 'hear say' evidence.
I knew someone that had tried this procedure (or some variant of it).
His comment was that they could make it work with some heats of 91 and not others.
He also believed that the results were impacted by the thermal history of the material.
He said that he based this on trial that they made on samples removed from boilers (in previous repair work).
He felt that it often worked better on material that had been in service.
But his sample size was only a dozen or so trials.

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P.E. Metallurgy, consulting work welcomed

 

Not to be pedantic here but I'll be pedantic here: saying '9% Chrome material' is a bit careless, because there are several kinds.

EdS' friend might be an insider at EPRI, where they have done a ton of work on Grade P91 repair.

P91 has proved very problematic in practice. IMO if we knew in the 1990s what we know today, we would have killed this alloy in its cradle.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[davefitz]

As I recall the referenced method was developed ( by EPRI) to allow quick repair of small tube failures within the boiler setting without requiring PWHT- by avoiding PWHT a savings of 1-2 days outage can be had in some cases where access is tight , as in HRSG harps. Many combined cycle plants ( which use HRSG's) bid power on a firm basis ,and every day of an outage can cost millions in penalty payments. One issue found during development was that the room temp strength of the weld is weaker than the base metal, which contradicts other code requirements, but is waived because the operating temp strength is higher- but it leaves open the issue of RT hydrotest issues.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[RandySavage]

Thank you for the replies so far, still completing a lot more tests and finding some degrees of success, but not the big eureka moment yet.

Ed, while it may be hear say it is helpful to know that there may be a legitimate link between my thoughts on heat cycling with the material, will need to do some more research though to see if it would be possible outside of that in-service conditioning.

Ironic, the grade of 91 was mentioned within the post but if it is in reference to Type 1 or Type 2, we are dealing with type 1. I agree with your position on 9% chrome as a whole but, reality is what it is. Best I can do is try to ensure a satisfactory repair with this now accepted method.

Dave, you are absolutely right with the reasoning here. Those few days and the cost of stress relief together make for these new methods coming about.

The one consistent thing I can take away from each post here though is to find more information on these materials from EPRI.

 

[EdStainless]

This is important enough that most of the EPRI work is publicly available (maybe for a price).
The later variations of 91 (with some micro-alloy) are very good, but too little too late in this case.

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P.E. Metallurgy, consulting work welcomed

 

EPRI 3002018025 (2021) -- Guidelines and Specifications for High Reliability Fossil Power Plants: Best Practice Guideline for Manufacturing and Construction of Grade 91 Steel Components, 3rd Edition

EPRI considered this document so important for the public interest they released it at no cost in 2011.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[RandySavage]

I wanted to follow this up for anyone else that winds up looking for similar information. We did finally get some good results and with a bit more refinement got the procedure to pass. Thank you to all who replied for the information and help.

I used GTAW fully for this, so unfortunately I can't offer much help for the SMAW option.

The process is already limited to 1/2" by the NBBI, I think going with something like Sch 40 would've helped to make the bending process easier. I ran with NPS 2.5" Sch 80 to get the WPS qualified up to that 0.5" max.

Change the filler metal for the root to the approved ER80S-B8. Only use that for the root. While it will help with the root bend results, after putting enough heat and passes into the material the 80 does not like to bend well on the cap.
Switch over to the ERNiCr-3 for your fill/cap. Ran at around 150 amps with 1/8" for the 80B8 Root and around 120 amps with 3/32" Inconel for the fill and cap.

We found that a preheat of 225 F with an interpass temp of 375-400 F worked best, allowing it to cool back down around that 225 range between passes. Allowing the base metal to get up to that 500-550F range that is allowed yielded poor results. The base material itself becomes pretty difficult to work with when going over the 400ish range. While I'm sure it's possible and that there are welders far more skilled, when we got that much heat into the base material it did not fuse as well as you'd think. It may have looked like it at the time but, bending it showed otherwise. Consistent filling to help keep the puddle at a more stable temperature also helped. Make sure you watch the bevels break down fully or that root will suck back hard.

I hope this helps a bit, and if you have any additional questions feel free to ask.

 

Thanks for sharing all that information, especially the welding characteristics.

My only comment is about the high interpass (500F) you tried. P91 has a low ceiling for reversion to austenite (helped mainly by Ni), so I would probably limit it to around 400F, strictly enforced, which you did.
You want substantial interpass cooling to permit fuller martensite transformation.
Not surprised about the cap characteristics; it is typically the biggest and widest pass, and it is not subsequently tempered, which is important and even necessary.
Did you do Vickers hardness testing as part of the qualification?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."