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Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

06/13/2017 1:14 PM

Sir,

We have a Y-piece of P91 material which has three welded branches, each of 125 mm thickness. In one of the weld-HAZ, there was a surface crack ~ 12 mm in length perpendicular to the circumferential weld axis. When we tried to grind off this crack, multiple cracks appeared in vicinity of the original crack. We are puzzled as to how to remove the crack. The problem has come after the post weld heat treatment (PWHT). We do not know how to eliminate the crack & how to carry out re-post weld heat treatment. Please help!!!

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#1

Re: Crack in WELD-HAZ of 125 mm thick SA 335 P91 Y-Piece

06/13/2017 1:38 PM

I think that, as long as these cracks are confined to the welded area, you can grind out and start over. How dirty were the parts before welding started?

What welding protocol are you using (materials, technique, settings, etc.)?

Sounds like an incompatible welding stock was utilized, or maybe a false ID of the metal base material?

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#2

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

06/13/2017 10:34 PM

James's advice is spot on.

Still, how is it that grinding back to remove all cracked material is something that does not come first to mind? Are you qualified to be overseeing this project, ddn.verma?

What welding process was used?

Have you rechecked to procedure was what is required and that it was followed?

Have you rechecked the filler used was correct? How was the filler stored?

How was the joint prepped/cleaned prior to welding?

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#3

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

06/14/2017 9:33 AM

It's strange. Normally the crack should be long. Transverse crack indicates metallurgical failure. In addition to the above posts, preheating for this material (By the way, its classification changes. It is no longer P91, it is now P5b) is mandatory (180 * C) as is the PWHT at 750 * C minimun 2 Hours .Once finished the PWHT you should leave covered with insulation wool to avoid abrupt cooling

The ends of the Y were sealed to prevent drafts in the interior during the heat treatment?

It seems that you have to weld a new onY with new materiale and following all the indications:

1.- Material according to WPS/PQR

2,- Filler metal according to WPS/PQR

3.- Fit Up

4.- Preheat

5.- Post weld heat treament

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#4
In reply to #3

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

06/14/2017 9:39 AM

If the crack is longuitudinal, drill to 1,6 mm holes at both ends of the crack before start girding to remove the crack (Avoid propagation)

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#6
In reply to #3

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

06/14/2017 11:31 AM

Good point about plugging off the ends of the fitting/pipe with insulating materials to slow the cooling of the welded parts. I think they may have cooled faster than intended on the first run. Spot on advice!

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#7
In reply to #6

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

06/14/2017 11:57 AM

James

P 91 (current P5b) has always been delicate to weld, with many parameters to take into account. It is a very noble material, especially for steam pipes and steam equipment, MP and HP..........but welding it is a headache

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#8
In reply to #7

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

06/14/2017 12:05 PM

I guess it is for high temperature (and highest pressure) steam?

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#9
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Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

06/14/2017 12:11 PM

Yes is very good, also P22 is good, but more for LP and MP steam

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#5

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

06/14/2017 10:37 AM

It could be residual stress. How many times have you tried to repair in the same location?

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#10

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece t

06/19/2017 9:43 PM

Can you post pictures ?? What kind of piping system is this piece taken from ?

What is the maximum operating temperature for this steam system and was this temperature exceeded by the operators ?

High temperature alloy P91 materials have a 40 year old experience base in the power industry must be very carefully post-weld heat treated. A common type of failure unique to this material but due to poor heat treatment is called Type IV cracking.

http://www.twi-global.com/technical-knowledge/faqs/material-faqs/faq-what-is-type-iv-cracking-and-how-is-it-detected/

https://www.phase-trans.msm.cam.ac.uk/2006/MST7520.pdf

There is much information on the internet and on other forums about this problem

Many inexperienced piping fabrication teams fail when dealing with this challenging material.

You have some real trouble on your hands with this type of cracking on a high energy system....

Please tell us more about the failure and attempted repair

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#11
In reply to #10

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece t

06/20/2017 8:51 AM

An excellent introduction to P91 piping fabrication for the novice. The article is well written and outlines P91's advantages and disadvantages.

http://www.thefabricator.com/article/tubepipefabrication/welding-chromium-molybdenum-steel-pipe-for-power-plants

It is important to note that "wye" shaped piping weldments and fittings present the most difficult challenge for proper post weld heat treatment.

When can we learn more details about your specific piping problems and some pictures ? This is costing you a lot of money !!

Or, are you yet another poster that comes to this forum, demands immediate answers and then never is seen again ?

Can you tell us more so that we can help you ?

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#12

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

06/25/2017 4:23 AM

//We are puzzled as to how to remove the crack//

Use suitable grinding wheel to remove the crack. Of course, it is better to follow crack propagation arrest methods.

//We do not know how to eliminate the crack &

how to carry out re-post weld heat treatment//

Follow the same PWHT procedure to temper the repaired weld. If your PQR/WPS has no provision for second PWHT provision, Then qualify the welding procedure with two or three or four PWHTs as per the client requirement.

Reheat cracks are not quite common in SA 335 P91 welds as compared to SA 335 P22. There are plenty of documents have been published by EPRI regarding SA 335 P91 welding.

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#13

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

06/29/2017 9:41 AM

Verma,

You came to us with a complex severe problem, significant cracking in an expensive piece of power plant equipment.

You ended your posting with Please help !!!

Several people took their time and skills to try to help you.

They politely asked for more information in order to provide better free answers.

Like many others in the past, you have ignored all of them ....

Thank you for keeping a proud tradition alive on CR4.

BEST REGARDS ...

MJC

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#14
In reply to #13

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

07/03/2017 5:55 AM

Dear MJCronin Sir,

I greatly regret for the lapses. In fact, due to some health related issues, I had to be on leave from my company. I have re-joined my activities. I must obtain & send the requisite details at the earliest please.

Thanks & Regards,

DDN VERMA

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#15
In reply to #14

Re: Crack in WELD-HAZ of 125 mm Thick SA 335 P91 Y-Piece

07/03/2017 3:34 PM

There's a good old chap!

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