Varaiation in material properties
Varaiation in material properties
(OP)
We are running independant testing on materials that we recieve to check the accuracy of our vendors' MTRs. Obviously we don't expect every number on the original MTR to match the results of the independant testing 100%, but how much variation should I expect?
For example, we sent a 4130 bar specimen for testing, the yield strength came back at 72500psi, while the mill's MTR shows 83000psi. The UTS, elongation, and reduction of area all came back lower than reported on the MTR as well. FYI the bar diameter is 6".
For example, we sent a 4130 bar specimen for testing, the yield strength came back at 72500psi, while the mill's MTR shows 83000psi. The UTS, elongation, and reduction of area all came back lower than reported on the MTR as well. FYI the bar diameter is 6".





RE: Varaiation in material properties
Where was your sample cut from, near the edge or the center?
RE: Varaiation in material properties
RE: Varaiation in material properties
RE: Varaiation in material properties
RE: Varaiation in material properties
Did your lab pull multiple bars? single tests don't say much.
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Plymouth Tube
RE: Varaiation in material properties
We are using drops from our machine shop to send to our lab, so unfortunately we don't have alot of extra material to send off for multiple tests.
RE: Varaiation in material properties
RE: Varaiation in material properties
Agree with your comment metengr. Just for reemphasizing and reminding us this basic tenet a star comes your way. Thanks
RE: Varaiation in material properties
RE: Varaiation in material properties
I believe that Hart said that these were all longitudinal mid-radius samples. I am not concerned about that aspect.
I work with tubing and the specs there don't require many samples. We usually pull double the required number so that we can see what variation within a lot there actually may be.
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Plymouth Tube
RE: Varaiation in material properties
No, he said the mill did longitudinal samples, but the other lab did transverse samples. Metengr is right, the pull tests won't necessarily match on rolled bar stock in the two opposed directions.
RE: Varaiation in material properties
RE: Varaiation in material properties
The mechanical properties on the MTR do not represent the minimum, maximum, or even average properties of the material. Rather, they merely demonstrate that the material meets the specification requirements. You should expect your re-test results to meet the original specification requirements, but that is all you can expect.
For example, if the specificaiton the material was produced to required a minimum yield of 70,000 PSI, then both the MTR and your tests are in agreement. If, on the other hand, the material specification was a minimum yield of 80,000 PSI, then your test would indicate that the material is not in conformance.
Consider the producer. He heat treats the bar and takes a test from the end (since that is where he can get a sample without cutting the bar in two). He knows that the properties from the end will be higher than from mid-length, but his experience tells him that if he gets better than, say, 80,000 PSI, then any test from mid-length will be above 70,000 PSI. Therefore, he certies the material as meeting the specification and reports the actual result.
Now, consider the same producer, but his requirements are now 80,000 PSI minimum yield and he pulls the same test and gets the same result. If he uses that test to certify the material meets a minimum 80,000 PSI, then he has a problem with process control and he is producting material that does not meet the specification requirements.
Now, a lot of warehouse suppliers will take the MTR and make the assumption that it is a certification of the minimum properties, but is is not. Most producers are very careful to make sure the sepcificaiton they are certifying to is on their MTR. This is why.
Now, if you are asking if it is suprising to have 6" solid 4130 bar that has been normalized, quenched and tempered to have a yield strength of 83,000 when tested at mid-radius on the end of the bar and have the yield, UTS, and % El all show less when tested mid-lenght, then I would say, no, not suprising at all. In fact, that is what I would expect. 4130 is not very deep hardening and in a 6" bar, so I would expect the "end effect" from quenching to be rather pronounced.
rp
RE: Varaiation in material properties
The statement above about the purity and morphology holds very true as the size, orientation and elongation of the MnS inclusions plays a big roll in the physical properties of 4130.
Also mass effect plays a big roll in physical properties of 4130. As stated above everyone has to be on the same page.
RE: Varaiation in material properties
Where is this mill located? In the United States fraudulent mill certs is a ticket to a 8' x 8' jail cell. I would be shocked to find out a mill knowingly misrepresented their certs.
RE: Varaiation in material properties
rp
RE: Varaiation in material properties
WE had an incident where a company was building or repairing rail tank cars for us and was supplied supplied 304SS instead of required 316L SS. The leaking of one of the tank cars precipitated a full scale investigation by the government. I don't recall anyone going to jail but some hefty fines were paid as well as a considerable sum to our company.
http://www.phmsa.dot.gov/inspect-enforce
RE: Varaiation in material properties
Who within the mill would go to jail? Start with the cert auditors and the quality control personnel. If it is seen to be systematic through the organization - then company presidents would be charged.
Has it happened recently - from a mill? Not to my knowledge, since the penalties and risks are too steep. However, there are a couple of (and one very high) profile ceses of distributors getting caught on the wrong side of the law as they attempted to *cert-up* material. Federal indictments flowed and all involved are awaiting trial.