P91 Weldability Issues

[Mr168]

We are having issues with all of our P91 pipe spools.

The spools supposedly have their end preps machined by the supplier after furnace PWHT. Deoxaluminite primer is applied to the end preps prior to transport, supposedly after PWHT and prior to shipping.

In the field, we are having weldability issues. The filler material is not wetting out and tying into the sidewalls properly, we are having to run it hotter than usual to get adequate penetration and root fusion with GTAW, excessive "trash" works its way to the top of the weld pool and eventually either sparks off or curls up into a tiny "sheet".

Everything equates to the issues you have when welding over heavy scale/oxides. They are having to take a hard stone grinding wheel to the face of the material before they even hit sound metal that begins to throw off sparks. The issue is noticeably worse on spool pieces with the additional furnace bake after induction bending. On the few spools where they have field machined the preps to bring the face up to square and a significant portion of material was removed, the weldability improvement was described as "night and day".

Initially I thought possible decarburization issues from the furnace bake, or chrome carbides working their way to the surface. But if they supposedly aren't baking the spools after machining or priming the end preps on them, what the heck is going on here? There have been no issues with our P1 spools, only the P5B's.

 

[metengr]

Deoxaluminite primer is applied to the end preps prior to transport, supposedly after PWHT and prior to shipping.

Your welders need to take the time and completely remove the protective surface coating prior to welding. There is nothing else but this.

 

[Mr168]

They have been removing the coating. They have tried both chemical removal, wire brushing, and other mechanical methods. Even after taking .040+ of metal off the face of the joint after removing the coating, they were still having issues.

 

[metengr]

What about painted pipe spools? Are they getting good grounding contact for welding? I have also seen this before.

 

[stanweld]

From your description, it would appear that the pipe spool supplier has machinbed the bevels prior to heat treatment, then coated the bevels for shipment. You should be able to determine this visually. We always abrasively prepare the bevels in P91 pipe to bright metal prior to welding.

 

[Mr168]

Metengr: Good ground contact. Even with the unpainted spools, the scale is so heavy on the body of the pipe that they always grind fresh metal to ground to, particularly with our orbital equipment.

Stanweld: That was along the lines of what I was thinking, however, the fine grooves from the cutting equipment are still visible from what I've been told, making it very hard to identify visually, especially with the primer on the surface.

A large part of the issue is that we are holding a very tight tolerance on the end preps of these spools, so taking hard grinders and field machining to the face of these preps is both opening up our joint geometry and creating less than ideal conditions for our OW equipment. Not to mention the significant amount of time it's taking us to re-prep these surfaces that are supposed to be arriving in the "ready to weld" condition.

 

[metengr]

Mr. 168
After re-reading your OP, the statements below concern me;

The issue is noticeably worse on spool pieces with the additional furnace bake after induction bending.

After induction bending the pipe spool must be subjected to a normalize and temper heat treatment. Was this done? Did the vendor perform any weld build-up to the pipe spools?

Even after taking .040+ of metal off the face of the joint after removing the coating, they were still having issues.

Are you absolutely sure about this? If you are removing the coating visually and machining an additional 0.040" of metal from the weld face (there is something else going on, if you cannot draw an arc for welding).

Initially I thought possible decarburization issues from the furnace bake, or chrome carbides working their way to the surface.

No, the above would not result in the ability to strike an arc.

There must be some type of residual surface contaminant that affects the ability to sustain a hot enough arc for welding.

 

[Mr168]

-Yes, all the pipe coming to us has N&T being performed after the induction bends.

-They were removing the primer by wire brushing, and then taking off the surface material whether by grinding, or by machining. As for the exact amount of material being removed, the .040 was an estimate given to me by the guys doing the machining.

-Please note that it is not an arc stability issue. It is more an issue of the filler's inability to wet out and tie into the root and sidewalls, the arc itself is stable. The only time there would be an arc stability issue was when the tungsten would pick up a contaminant that decided to jump off the sidewall or out of the "fish eye" at the top of the weld pool, at which point they would stop.

I'm going to have them send over the heat/lot numbers of both the pipe spools and the filler material itself to try to rule out any anomalies there as well.

 

[DABwilldo]

If the problem is noticably worse after the induction heating-bending and furnace bake, could your issues be related to some kind of contamination in the furnace atmosphere at your heat treater? What is the inert atmosphere in the furnace? Or is it a vacuum furnace?

You're already on the trail of the other lead I would pursue; check out the filler metal being used.

Does the problem only occur in the field? Or is it reproducible in the shop, with different welding equipment? It's a remote possibility, but if there's only 1 or 2 sets of welding equipment in use, you could have an equipment problem? Or does the equipment perform OK when welding other materials?

 

[Mr168]

DAB: It's possible that it is a furnace-related issue. Unfortunately, there are heavy political ramifications with trying to get the desired information from the supplier. Can't go into detail online, but we are trying to eliminate all outside possibilities before tracing it back to the shops.

The problem is only occurring in the field for us, but anything done in the shops is always freshly machined. The welding equipment shouldn't be an issue, as we have quite a few AMI units, and the problems are the same between units, and are nonexistent when doing plain carbon spools.

I don't suspect the filler itself to be the issue either. They fought similar issues on the previous project, different manufacturer for the filler, and have already used multiple lots on the current project, though I'm awaiting CMTRs for verification.

I did just find out that all the welding end preps are sandblasted by the supplier prior to application of the paint, so that would eliminate just about any visual cues associated with scale on the end preps.

 

[DABwilldo]

Mr168,

I understand and sympathize with the 'political' issues you face in dealing with your heat treater. I've been in similar circumstances myself (I think).

So, sounds like power supplies and filler metal are not a part of the problem.

I'm curious now about this sand blasting you mentioned.

Is this a generic term, or is the abrasive media really SiO2? Do you have any feeling for whether or not this process is leaving any residue on the surface of your pipe joints? Did the supplier performing this operation recently make any changes to the media or procedure used? There are a wide varieties of abrasive media in use depending on the application; SiO2, Al2O3, ZrO2, and various other natural minerals, and even organic media like crushed walnut shells. Is it possible that the wrong or a different media is used on your pieces? Seems plausible to me that some abrasives might leave residue on the pipe, and that depending on the filler metals and base materials involved you might see a problem with some parts, but not all parts....

Just tossing out some ideas that occurred to me as I read your last posting...

 

[Mr168]

I will have to do some poking and prodding into the actual media being used.

While typically I'm not a fan of any media blasting on a weld end prep, I'm less concerned with the potential for material embedding itself into the surface (if they're taking .040 off and still have problems, I doubt that's the issue), and more concerned with it's potential use for eliminating visual cues of what could be a scale after PWHT.

 

[stanweld]

I would offer one other thought; have you checked for arc blow due to magnetic effects?

 

[Mr168]

Yes. I have had them change ground locations, wrap the leads around the pipe, etc, just to rule that out.

 

[Mr168]

After some further investigation, we have verified that some of the end preps were in fact being PWHT'd in the furnace not only after the ends had been machined, but after deoxiluminate primer was applied, effectively baking it into the surface. They would then touch up/recoat with primer prior to shipping.

 

[Mr168]

As a bit of a follow-up, we are looking at tentatively popping Brinells on the faces of these P91 bevels.


What I find interesting, is that when hitting the land on the J-prep with a file, there is virtually no initial bite, indicating increased hardness. If anything, I would have figured that surface decarburization after furnace bake would have created a superficially soft surface.

What is the likelihood that the thin extension of the J-prep is rapidly cooling, increasing surface hardness? Are there any other considerations that I'm overlooking in regards to PWHT of a P91 spool piece after the bevel has been machined?