strange tensile results

[PriamEngineering]

Hello, can anyone advise me on the following problem?

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We have bought some components from a machine shop. Tubular forgings. It's offshore equipment. The material spec is a F22 (90ksi Yield) forging. The material certs from the foundry state the material meets all yeild, UTS, NACE (hardness) and charpy requirements. We perform a lot of FEA here and as part of the input to that, we decided to get the true stress - true strain curve measured to ASTM A370. The results that came back showed that at midwall (t/2) the material had a low yield strength and inconsistency between results. The tech who performed the test said that the tensile test specimend failed in a strange way. That is instead of following the usual pattern of the crack in the specimen growing orthogonal to the direction of loading, significant cracking was observed in the direction of load. with the final crack being in the expected direction. also the final fracture faces had a strange surface crack emminating in three places ( a bit like a mercedes-benz symbol). Anyway being a Mechanical I am looking for any opinions before I go back to the Foundry. Please let me know if you can't see the pictures. We got 9 samples tested. all exhibited similar failure modes (tri-laminar tearing with no classic cup and cone failures). Anyone seen this before?

during the test
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showing the fracture face
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showing the two halfs after fracture
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Thanks in advance

Derek

http://www.priamengineering.co.uk

 

[metengr]

Very interesting tensile failures. It could be that significant banding or segregation occurred from resulting in a longitudinal plane of weakness, especially when necking occurs during tensile testing.

Have you had any samples sent to a metallurgical lab to confirm the chemistry and most important hardness and microstructure?

 

[metengr]

Sorry, I hit the send button before final editing;

...banding or segregation occurred from either the original heat of material or forming...

 

[arunmrao]

Agree with metengr's comments.It will be interesting to see the microstructure at the fractured surfaceThere is an inconsistency in quality caused by segregation.


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

Mahatma Gandhi.

 

[weldstan]

Also agree with metengr and arunmrao. I have seen this type of fracture before and it was due to segregation and very large grain size.

 

[PriamEngineering]

Thanks for the feedback so far. Can you explain to me what you mean by segregation? I will ask for the chemistry, hardness and microstructure to be reported on.

thanks

Derek

http://www.priamengineering.co.uk

 

[metengr]

Chemical segregation in steel castings or ingots refers to regions in the casting where the chemical composition may be rich or lean in certain alloying elements. Banding refers to a preferred texture of the metal grains and is confirmed by metallographic examination (microstructure).

 

[MagBen]

That looks to me inclusions (dirty), instead of segregation. Segregation is less likely for this low alloy steel.

 

[DeleteriousPhases]

having a look at the microstructure would be great. I am in agreement with metengr. are all test pieces taken from the same heat?

 

[PriamEngineering]

I have asked for this to be done. I am not sure of the heat treatment process involved in this forging. Normally I would go for a harden and temper but as this hs to be NACE compliant the harness has an upper limit. So maybe annealing would be involved? ? Would the presence of pearlite or martensite indicate a poor heat treatment?

http://www.priamengineering.co.uk

 

[PriamEngineering]

Update. I have recieved the micro etching report and it states that the grain structure is:
"Microstructure consisting of tempered martensite with some grains indicative of low carbon martensite and probably some bainite"
is this what you would expect from a correctly normalized, hardened and tempered carbon steel (0.1% -0.2% by weight).

Thanks in advance

http://www.priamengineering.co.uk

 

[metengr]

The description is too vague regarding the microstructure. Did the microstructure exhibit banding or inclusions along a preferred plane? Did you explain to the lab the nature of the failures for the tensile specimens? I would suggest microhardness testing to corroborate the metallographic examination results. Additional work needs to be done to explain the observed tensile failures.