UT (flaw detection) on a 6" round 18" long bar.

[noviceEng]

God day All,

I had a 6" round Bar UT inspected for cracks and the results came back as "flawless" and acceptable for use. Yet after machining off 1/2" from the OD using a manual lathe, we found numerous hair line cracks. When we confronted the inspection company they implied that the test was done accurately and that the machining process could have caused the cracks..... (rubbish... i know) Anyway what I don't understand is how come the UT inspection didn't pick up the cracks that were obviously there all along. Is it that the UT wasn't done properly or am I missing something?

thanks in advance.

 

[mfgenggear]

what type of material, hardness & machining process?
I seen grinding induce cracks from grinding.

I have also seen UT not detecting small sub surface hair line cracks.
In addition to UT if it's magnetic material I would recommend Magnuflux if it's not magnetic~ Flo. penetrant Inspection. Inprocess & final after parts are completed.

if there are any cracks at the surface it will be detected.

 

[noviceEng]

The mat. is EN19 Chrome with a BHN of 252. We turned it on a manual lathe and took off 7/16" using a carbide insert.

I was convinced that the UT test was questionable, so I had it done again using a different inspection company and guess what... they got the same results stating that the bar was O.K.
This issue can really make someone question the validity of UT inspections. I'll definitely being performing LP inspections before & after from now on.

 

[looslib]

The machining process itself can cause stress changes in the material. Therefore the inspection at one diameter may very well change from another diameter after machining.


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