Charpy Impact testing W.T.F.

Date 07-08-2015 18:12

This is more of a sympathetic agreement than an answer; we have had to have charpy testing done for several of our customers as well.
Because there are so many variables that can affect the results, ASTM A673 states "an impact test shall consist of three specimens..." and then says to use the "average" of the three tests to come up with your actual values (it also states that none of the three can fall below the specified minimum). Hmmm...how could we apply this methodology to NDT...?

By js55

Date 07-08-2015 20:02

MRW,
It isn't intended to necessarily be accurate. Its intended as an inexpensive quick method to determine if a material is in the brittle range. Who cares if you numbers go from 100 to 150? Or even 50 to 150. All those numbers are in the ductile range.
There are simply too many variables to consider even with the same a material. Materials are not entirely homogeneous.
If you want mathematical accuracy you will go with CTOD's.
Be careful what you wish for.

By MRWeldSoCal (***)

Date 07-08-2015 20:33

JS,

When I spoke with the person who calibrates the machines they spoke of these factors. Now were also just using base metal, no welds, so that eliminates all those factors, then the anvil has to be perfect, the 1300$ damn tongs need to be perfect. Lastly the cooling of the sample is the only factor left. So were doing another test at room temp and it was a total disappointment.

How much value is an engineer or company putting into these numbers? for the example of the HY-100 we are using we have had the range i spoke of, but broken at room temp it broke lower than frozen at 0. So where does that go?

I like how you said a quick way to see if its ductile. haha maybe if you're the guy sending it out or have access to a full CNC lab. But over here we machine down the blocks, notch, and surface grind to sample out of just base metal it takes some time, out of a weld and to be precise takes longer. Its faster to do a tensile pull.

Can yo also explain CTOD's im not familiar I guess.

Jordan

By welderbrent (*****)

Date 07-08-2015 23:03

Jordan,

For some good information on Charpy's take a look at Linnert's 'Welding Metallurgy' chapter 3 in Vol.1. Available at the AWS bookstore and other places.

As JS says, it is an interesting study to find all the variations to how the test is run, what is being looked for, and how to 'read' the results. I don't understand it even after reading through chapter 3 but have a better understanding of the process and how it came into existence. In my understanding, probably flawed, it is a very undependable system but the cheapest thing they have come up with to get them some idea of certain metallurgical properties of the parts in question.

He Is In Control, Have a Great Day, Brent

By MRWeldSoCal (***)

Date 07-08-2015 23:21

Brent,

Yes I have that book, I refer to it as my welding bible. Ill read up tonight on it.

Thanks

Jordan

By welderbrent (*****)

Date 07-09-2015 00:02

Don't have it with me tonight as I am studying for MT next week but it is about 1/2 way through the chapter.

Brent

By js55

Date 07-09-2015 16:42

MRW,
You make me wonder why such desperate breathlessness. Are you getting numbers that are non-compliant?
Just how good of number do you need?

As for CTOD there are far better explanation on the internet than I could pull out of my head. I suppose I could brush up and appear smart, but nah.

In any case the idea is that with this test you get actual material property information and can tie this into equations. This cannot be done, and is not intended to be done with CVN's. Despite the criticality of the CVN notch (the radius especially), as you indicated, this is still a quick cheap proof test. And that's all.

By MRWeldSoCal (***)

Date 07-10-2015 18:46

Its not the need for the number its just a question of why. Why is there such a different in this one process. we can run tension and get very consistent, but with this its all over the board.

J

By js55

Date 07-10-2015 20:33 Edited 07-10-2015 20:35

MRW,
Keeping in mind I am not a mechanical engineer, I think I can comfortably say that the tension test essentially measures, without getting too technical, simply the resistance of the atomic structure to slow deformation in a single direction.
For CVN's you are measuring not only strength to a certain extent, but ductility as well, and the ability of the material to absorb fracture at a high rate of speed. Also, metallurgical discontinuities such as laminations, sulfides, carbides, grain size, manufacturing methods (i.e. hot rolling, normalizing, normalizing and tempering, etc.)as well as rolling direction. None of which would effect tensile testing to any great degree (with a few notable exceptions).
However, having said this I think that if you are testing the EXACT same material under the EXACT same conditions you will find a very respectable consistency. Enough that you could possibly judge the testing ability of your lab. Especially considering that the machining of the CVN specimen is much more critical than a tensile test.
Oh, and one more thing, colder temp testing makes it even worse. Especially cryo. If the tester stops to wipe his runny nose in moving the specimen from the bath to the anvil you can vary the temperature by 30deg to 40deg or more.

By MRWeldSoCal (***)

Date 07-10-2015 21:01

Cryp is more than we are doing yes. I believe our system is working good. But we have used the same plate the entire time. Next were going to send them out as room temp to finally eliminate any other factors.

So what does it mean if 3 different labs send back far different results and there isnt any more factors?

By CWI555

Date 07-13-2015 12:15

It means one of two things. Either your plate is inconsistent or the labs are. Ocums razor would say the plate is inconsistent. However, that does not preclude the labs being inconsistent. The CVN is one of the more consistent cheaper test, but that doesn't mean the low dollar tech task with it is doing it right.
If your certain it's not the plate, machine your own samples replete with a second control sample for each location. Have the test run on the first sample. If If the results from one lab are extremely different, have the control sample from from the suspect lab ran at one you believe is accurate or at least in the ball park. That should Identify a sketchy lab.

My .02 worth

By MRWeldSoCal (***)

Date 07-13-2015 15:54

That's already what we've been doing. We've tested 5 labs already. This next round is at room temperature to eliminate the temperature being dealt with improperly. Ill be sure to post the results of those.

By CWI555

Date 07-13-2015 16:44

"But we have used the same plate the entire time"
Then there is only one other logical variable left.

By MRWeldSoCal (***)

Date 07-13-2015 17:26

Yea, exactly. Thats the last factor. Well have to see what the temp results come to be.

Jordan

By aevald

Date 07-09-2015 20:32 Edited 07-09-2015 20:34

Hello Jordan, I have briefly taken a look at this conversation and would add this little tidbit. Much of this sort of topic has come about as of late and probably relatively new if it was put onto a welding history timeline. Changes in material types, their uses, filler metal availability and types, and a whole lot of other things have driven a number of different manners in which the industry tries to justify and verify applications, use, and suitability of purpose.

This was driven home to me the most when I was involved in a bit of wind tower work. They were requiring E7018-1 electrodes, initially I had no idea what the "-1" was representative of. Upon checking it out I found that it referred to Charpy impact values at reduced temperatures and was a requirement due to the types of materials that these towers are constructed of and also to take into account where they might be located. They were essentially building them to be suitable for service and to withstand environmental issues regardless of where they were to be deployed.

Sometimes particular testing requirements are part of a bigger picture. In particular, testing of one form or another might be verified by additional testing by other methods. Giving particular ranges of acceptability may very well take into account an absolute no-problem answer to what is needed, whereas if the "complete" range was taken into account this might leave a gray-area that could lead to problems if there were any unaccounted for exceptions to equipment accuracy or other variables that could lead to failure.

I was involved in a project where assessment for fitness of service and quality of welding was being analyzed and verified with UT. In addition to this, complete destructive testing was being done to verify the UT results. The UT in this case was being used in a manner that wasn't consistent with it's intended use and design so they wanted to be sure that it would be sufficiently accurate and reliable for what they planned to use it for. Parts were UT'd, and the results were mapped. The sample was physically removed and impact testing was done, X-ray was also done, and likely a few other things that I was not made aware were included. Final results of this process verified that using UT in this instance "would" result in information that could be considered accurate enough and be fit for this application/use.

It sounds to me as if you have not been given all of the information pertaining to why you are being required to do this sort of thing. Patience on your part, as well as some strategically placed and composed questions might yield some answers that will explain your situation and give you some piece of mind in this case. Good luck and best regards, Allan

By MRWeldSoCal (***)

Date 07-10-2015 18:44

I totally get the application part of it. We did some work for a company that does oil wells and they had one going to the Baltic sea and therefore we had to do with a lot of impact testing at cold temperatures. I think my concern is more the inconsistency of other labs. They can all get close and relevant numbers in their own equipment, but none of them match each other. sample might all be within 10-15 pounds but they are 30 pounds from another labs results. I really enjoy the science of welding, but its frustrating trying to get whats real, it seems like value not so much an "accurate" number of the pounds it might actually take to break.

Jordan

By jon20013 (*****)

Date 07-10-2015 22:29

Jordan, if you really want to change the world try joining some Technical Committee's. All of the Technical Committee's I'm aware of welcome volunteers. Being a Member if Technical Committee's since 1986 has literally changed my life for the better.

As for the "why" with regard to CVN's, listen to what others are saying; CVN's are to Drop Weight Testing as Bend Tests are to Tensile Tests.

Also, think of The Titantic or perhaps more accurately, those WW1 ships that were sunk or otherwise damaged due to exposure to the icy cold waters of the North Atlantic...

Keep asking questions, that's one way our trade continues to flourish.

By MRWeldSoCal (***)

Date 07-10-2015 22:56

Thank you jon, Ill continue asking.

Funny thing i heard the other day regarding the titanic. A person said it was a conspiracy, they said there is no way ice can break steel. They said if you don't believe me go smash an ice cube with a hammer and see which breaks. I have never lost much faith in humanity till i heard that. I went on a chemistry rampage shortly after explaining impact strength and wrought iron riveted ships.

SHEESH

Jordan

By jon20013 (*****)

Date 07-11-2015 01:04

Haha! Not sure it's a good simile using an ice cube vs an iceberg, but it may be a pointless discussion anyway... try chilling "old world" steels down to very low temperatures and then strike them with a hammer and see what happens... steel can (and does) break when exposed to very low temperatures and then impacted; depending on how welding was accomplished (dozens and dozens of factors to consider) can definitely affect the ductility of the steel.

By welderbrent (*****)

Date 07-11-2015 02:56

So, Google 'Titanic Construction' and start reading.

At the last minute the crew tried to turn and avoid the berg. Hitting it more on the side instead of head on may have been their biggest mistake as the diagonal blow sheared most of the millions of rivets used to hold it together allowing it to gape open and no way they could close the sealable areas off fast enough nor the pumps being able to keep up with it. 3 years to build and 3 hours to sink.

Once past the hull the sides of the ship are thinner. It was going top speed (showing off) and didn't slow down though warned of danger in the area. The steel and iron used may have not been of the quality they were supposed to be but notice, iron. Brittle compared to materials used today. Take that with the angle of impact and most definitely you could 'break' it apart upon impact at the speeds it was going and the temperature that would have been involved with that much ice in the area.

And, for a different example than your hammer and ice cube: take and seal one end of a piece of tube steel. Then, fill the tube with water. Now, weld a cap on the other end and place it in a freezer. For quick results use a piece of 1" sq thin wall about 18" long. What happens. The tube with expand as the ice freezes and it will round it out and then break it, usually down the seam but not always. Look at fences, handrails, truck racks and other tubular steel items left out in the weather with spots where water can get in or condensation fills them up even just part way. Along comes winter and with the first cold night a guy goes out to his truck and the rack is all broken up.

You want to stand by that statement? (whoever made it that is) "no way ice can break steel" ! :eek:

Let me know how that works out for you.

He Is In Control, Have a Great Day, Brent

By MRWeldSoCal (***)

Date 07-13-2015 15:41

Brent,

Yea it wasnt my statement haha its was someone else!