cracks in malleable iron threaded 90° elbow 2

[Eduardo1982]

Hey guys. In a construction project we were facing some problems with some malleable iron threaded 90° elbows. During some pneumatic tests two elbows failed (I attach some picturees). We think that the problem is bad practise during construction by the contractor (due to excessive wrenchs marks). However the client is asking for a deeply root cause analysis with emphasys in the material properties. We got some quality certificates of these material but are too basic (most of them just say that mechanical properties met the requirements of ASTM A197. 90° elbows are class 150 and correspond to ASME B16.3 fittings). I was thinking to send the failed fittings to a laboratory for metalography test (probably to check microestructure if everything is OK) but I am not sure if it could be enough.. I would like to know if with this failed fitting can I get some tensile test.. I need some advice of you guys that maybe have much wide experience in these fields!!
Any help will be helpfull.

 

[btrueblood]

The wrench marks on the fittings are the big clue. There would be no need for aggressive tightening on pneumatic service lines in 1/2" NPT, unless something was amiss. As Ornery and Cpro (and I) say, the cracking shown (longitudinal) exactly matches the failures we have seen from over-tightening tests we have done both for outside fittings purchased as well as our internally produced threads, both internal and external. It's the most common failure mode for tapered pipe threads. I've never seen a circumferential crack in a threaded (taper) pipe fitting. I'd not waste a dime on external consultants, but then it's not my project nor my customer.

 

[MintJulep]

Can you get some larger fittings from the same supply? Large enough to cut a decent tensile specimen from?

Do basic metallurgy and hardness on both your failed and the larger fittings - hopefully this will demonstrate that they are "the same".

Then if the tensile tests of the specimens from the larger fittings meet the requirements you can infer that the small fittings also meet.

It's a long way to go, and expensive for testing that will in all likelihood tell you that the fittings are exactly what they are advertised to be.

But that seems to be where you are. Did you purchase the fittings?

 

[Eduardo1982]

To answer MintJulep questions. It is not possible to have large fittings so I can not get this specimen.
I wasthinking to do hardness test and microscopy to check microstructure.
The fittings were purchased by the company I was worked for. These came from China.
I wanna thank to all you guys who kindly post your experience and knowledge, really thanks!.

 

[mrfailure]

You might be able to get a sub-size tensile out of your T-section to allow direct comparison with ASTM A197.

 

[desertfox]

Hi Eduardo
I would send the failed items to a lab, it would probably cost more to do your own testing unless you have all the right equipment to hand like a tensile testing machine etc.

Looking at the crack it appears to be caused by a hoop stress component which would be present normally under internal pressure may also be caused by other reasons mentioned by others. Was the installer given any instruction relating to the fittings being tightened? Or are they deemed experienced contractors who do these installations regularly? The reason I ask if it was poor installation and these installers were rough, I would of expected more to fail from being over tightened in the quantity of around 500 you state if the installers were doing a bad job.

Either way if you are intending claiming from the contractors then you need to get independent proof as to what did or didn't cause the failure and it also helps if you can show that installation instructions were given to the contractor and they weren't followed.

I found this link to a supplier of malleable fittings which gives torque and number of turns for tightening the fittings which might help,

https://www.gfps.com/content/dam/gf...talogue-malleable-cast-iron-fittings-2013.pdf

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Larry-MTS]

The malleable iron fittingss are extremly CHEAP If the quantity is small, I suggest you just discard the failed and change your supplier. Find some new manufacturers/stockists, there are so many. As you know, you can not get the proper specimen unless your supplier provide it with the same casting heat number. The mechanical test is too costly comparing to the product itself and it is unrealistic. These fittings are mainly used for low pressure and non-crucial applications. You sure you want to make your business/project more expensive?

 

[Eduardo1982]

Hey guys.. well I wanna tell you that I made some tests over these two samples and I am going to summary the results:
- Hardness test in both samples: (average value is 75 HRB.. 133 HB.. estimated tensile strenght 45.6 Kgs/mm2)
- Metalography:
Pictures shown in 200x microscopy.

 

[Eduardo1982]

Hey guys..
Following the issue comment several months ago, I got some pics from fracture zone of Tee fitting.
In this section clearly appears a "White" zone and a "black" one. I guess these could be some inclussions.. what do you think about that?

 

You will not see inclusions on these fracture surfaces unless they are grotesquely large. Generally there is little information to be gleaned from a cast iron fracture surface.

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

 

[arunmrao]

If you are seeing a mottled fracture,,, it indicates a poor malleabilization. The original white iron has not transformed into malleable iron.

But again,, I wonder if this can e true for such a small castings,unless the foundry has very poor controls.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[Eduardo1982]

and do you know if there are any book, recommended literature or any other that can gives typical photos of inclusions on malleable cast iron?

 

[Compositepro]

It seems like an "incident" is becoming a career.

 

[3DDave]

I think the answer is on-site QA. If samples fracture with little deformation the box with the rest goes back to the supplier. If they are breaking because they are junk, there's no point paying for them. If they are breaking from overenthusiastic pipe installers, there's no point in ruining good product.

Look at the history of the successful Crystal Palace at the 1851 Great Exhibition as described in the book "To Engineer is Human" by Henry Petroski. It is followed by a history of a similarly quick-design/fab disaster by a company that did not do on-site verifications.