Pitting of SA106 GrB pipe both on OD and ID

[dharhay]

We have been using 2" lengths of 2" XS SA106 GrB pipe in pairs to hold a tapped bar. One difficult application cycled this loaded pair of pipes nearly 200000 times and failed. The pipes are welded on one tangent (top and bottom). The failure has been attributed to crack initiation and propigaton at multiple microscopic pits, along the loading line.

We received the cut pipe by the several hundreds, they look fine and are relatively rust free. This is after all, pipe. So, the 2" sections are cut from 20, 40 ft lengths that could be sitting out of doors.
Is it reasonable to expect the pitting? Is seamless process the culprit? Is there a process to ask for or to ensure is not done that would eliminate the pits?

Thanks for your input

Dave Harhay
Sypris Technologies
Tube Turns Products

 

[metengr]

Not enough information to base any recommendation with what look like typical surface artifacts in SA 106 pipe material. Your description of the failure sounds more like it has nothing to do with typical surface artifacts and more to do with the location of the weld. I would bet the failure was located along the toe of the weld at the tangent line.

Do you have a picture of the actual failure and location?

 

[dharhay]

Thanks for the response metengr. You are a valued contributor. I have attached a photo of the failed mount. Note that a third party evaluation shows cracks propagating from the root of pits on the ID and there are multiple sites that are along the load path that parallels the weld which is on the OD, but failure is from the ID.

Dave Harhay
Sypris Technologies
Tube Turns Products

 

[weldstan]

Your photo only proves metengr's diagnosis. Do you have a picture of the fracture surface?

 

[dharhay]

Thanks weldstan. My opinion is that you are jumping to the weld. My question was are the pits in the pipe 'normal' to see without the knowledge of the failure in our minds.
The weld is there making that location on the cutoff very stiff. The pipe cutoff will always break near that weld just like a diving board will always break near its mount and not near the tip. If there were no pits, I would say that there would be longer life.

I cant show many more photos, but I have attached the failure surface. The multiple fracture locations are on the ID and the weld is at the bottom of the photo.

Thanks again for adding your opinions.

Dave Harhay
Sypris Technologies
Tube Turns Products

 

[metengr]

dharhay:
Thanks for the follow-up. I see what look like ratchet marks along the ID surface of the pipe and beach markings, which confirms fatigue crack initiation and propagation. From the looks of it, it is probably low cycle fatigue because the pits would act as local stress concentration, resulting in fatigue crack propagation for most of the life.

Your source of the problem is not really the pipe ID or OD surface condition, as much as this is an inadequate design related to a low cycle fatigue application. I think you need to re-think the design to improve fatigue strength (either increase strength or modify the location of the weld. Another way to view it is the current design is inadequate to handle "One difficult application".

 

[Guest102023]

FWIW I agree entirely with metengr's assessment and conclusions.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[dharhay]

Thanks all for adding your input. I agree that in order to go forward, we would need a design change to allow this application and other challenging applications be trouble free as all the others through the decades. I have learned a lot from this failure.

Dave Harhay
Sypris Technologies
Tube Turns Products

 

[weldstan]

dhahhay,

Some of the ID pits shown on the photomicrograph are considered normal for the seamless pipe. The tight, crack-like discontinuities are not and I suspect they are the initiating sites for fatigue cracking. You could remove the ID surface layer by machining or grinding with a fine stone. You might also try a welded and drawn pipe.

Was all the pipe from the same heat by a single manufacturer?

 

[racookpe1978]

Both of the photo's are of too small an area to help. Good detail of the specific failure point. But that's not what caused the problem, that's a symptom of the final failure of the problem.

I can't tell without looking at several photo's of the whole assembly and how it was made, and how it failed, and how the good ones held up. Make sense?

 

[rconner]

Maybe just about anything that has ever failed or been broken, meeting normal quality expectations/requirements or not, in the history of the world has to some extent had the fractures propagated by microscopic discontinuities or flaws --

 

[mrfailure]

Just because pits are present does not mean they caused your failure. Cracks will initiate at the weakest point - in this case pits. Fatigue in this system would have initiated elsewhere if they were not present.