Charpy impact on failed piping piece 1

[mat211743]

Hello Friends, We had a failure (fish mouth) in our piping (106 Gr.B) due to freezing . There are theories that it might have failed due to local overpressure because of water trapped between two ice plugs or the pipe froze 100% and burst. One of the suggestions was to do Charpy Impact on the failed piece to determine if the failure was actually due to brittle fracture.

(1)A 106 Gr.B is typically rated upto -29C. Conducting charpy on A106 Gr.B above -29C is not going to fail the material by brittle fracture though it had failed by brittle fracture during service. is this true?
(2)The brittle fracture is best assessed by fracture surface rather than Charpy impact. is this right?
(3)Is there a value in doing post charpy impact testing on a failed piece above -29C?
(4)I believe, if we know the failure mode is brittle, we can do a post testing at temperatures lower than
-29C to determine the possible ductile - brittle transition temperature for that particular heat of material. We can only estimate that at the time of failure what might have been the possible temperature.Is this right?

Thanks.

 

[metengr]

Why not examine the fracture surfaces using a metallurgical lab to confirm brittle or ductile fracture? Why conduct CVN testing with machining costs and testing?

 

[EdStainless]

I am with Metengr on this one (as usual).
An examination will tell you how ductile the fracture is.
I have seen pipe a lot stronger and tougher than this split by freezing water.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[desertfox]

I agee with the other posters about determining type of fracture, one other thing you could do is a hardness test on the material to ensure it was in specification when first built.
I had an experience where a hardness test indicated the strength of the pipe was outside the specification.

 

[weldstan]

you indicated "fish mouth" appearance, which is indicative of ductility. Fracture surface analysis per metengr will further define failure mode.

 

[mat211743]

Yes we are sending the samples to the metallurgical lab for further analysis. My point is fracture surface analysis is the best way to go rather than conducting charpy testing. I am trying to identify the best method to analyze the fracture mechanism. I believe post failure charpy testing is not a suitable method here.

Weldstan, my initial impression with fishmouth appearance is that the failure ductile in nature. That is why i specifically mentioned in my thread.

 

[racookpe1978]

How long was it in service?

Granted, the conditions were "right at" the transition temperatures, but a single casting (?) properties can vary between one end and the other (cool rate, crystal structure, surface flaws, inclusions, holes, stress risers, etc) so the results of a impact test on one end might not show what failed at the other end under the higher stresses there. So, the break happened at one location at one temperature, and another part that looks identical "may not" break at exactly the same temperature. A little higher or a little lower.

And you may get a brittle failure "Start" at a flaw, then that failure reduces the area until ductile failure splits the rest of the part. Or vice versa.

How accurate do you know actual temperatures at the location of the break? They can vary also across the single piece.

If the Charpy test is done on a sample piece after the examination, remember to do the test at the lowest temperature that was possible at the day in question. Again, small temperature changes will matter there.

 

[1gibson]

"One of the suggestions was to do Charpy Impact on the failed piece to determine if the failure was actually due to brittle fracture."

At best, that would determine that it "could have been" due to brittle fracture, but you won't even get that far.
What is the minimum value for absorbed energy would mean it could have been a brittle fracture? There is no real answer, it's a waste of time and effort. Even if you had a value that was determined reasonably, you still can't quantify the force from the ice well enough to draw any conclusions.

Did the charpy suggestion come from someone who "knows just enough to be dangerous" and is probably proud to make that claim? It's not just a catch phrase, there is some truth to it.

 

[mrfailure]

I think discussion of whether the fracture is brittle or ductile misses the point. The first thing you should realize is the fishmouth rupture indicates hoop stress of the material has been exceeded. While determining whether fracture is brittle or ductile can give some helpful information, the fracture mode is an effect, not a cause. For the suspected cause of internal ice formation (which commonly does cause carbon steel piping to burst), you may well be able to get a ductile "fish mouth" rupture exhibiting plastic deformation but still be above DBTT. Also note that pipe rating does not account for freezing within the pipe as water expands to ice.