Residual magnetism vs relevant indications 2

[Tmoose]

Some MT testing was recently performed on some Abrasion Resistant iron castings.
We finish machine the two "sides" of the castings in a surface grinder with coolant, holding the part with a magnetic chuck.
After one side is machined, the part is flipped, and the opposite surface is machined (ground).

Non fluorescent LPT testing by our level II inspector showed absolutely NO indications.

A third party Level II inspector came in and did wet fluorescent MT inspection, and found what he called "chatter" indications on both sides of nearly every part, caused by improper grinding practices.
He decreed the indications relevant, and the parts no good. ( The acceptance criteria are our call )

Attached is a picture showing the "chatter" pattern, and the pole spacing on the mag chuck in the surface grinder.

http://files.engineering.com/getfil...0130&file=CHATTER_MEDLEY_ANNOT_anonymous_.jpg

We are about to begin a few tests for residual magnetism with a tiger stripe pattern like the "chatter", and if any is found, a thorough demagnetization and repeat MT inspection of a few blades by the 3rd party. Also a chemical etch of some kind to see if there hardness etc variations that correspond to the chattter.

** What are your opinions of the similarity of the "chatter" indications and the mag chuck pole spacing ?

thanks,

Dan T

 

[metengr]

I have seen what are called ghost or phantom indications (light streaks that were fuzzy in appearance) in material that was subjected to wet fluorescent (WF)MT. These were not relevant indications - an actual break in the applied magnetic field caused by a linear indication. Going through a demag and re-inspection cycle using WFMT will be beneficial to resolve the dispute.

 

[JR97]

Tmoose,

Most certainly non-relevant indications caused by localized residual magnetism (magnetic writing). If the indications were a result of "grinding chatter" they would be much sharper and more distinct. I wouldn't even bother doing anything other than demag and retest. Actually a bit disappointed your 3rd party MT inspector would even consider them to be related to the grinding.

JR97

 

[brimmer]

Get a new MT person. If he can only describe these as "chatter" that's your first clue, if there are relevant indications he should be able to describe what they might be or characterize better. They do not look like grinding indications to me. Try to deamg, can also use black on white MPI to get a better visual clue as to if there is anything there.

 

[Tmoose]

thanks all.

The retest is scheduled for Monday.
I'll let you know how it goes.

thanks,

Dan T

 

[arunmrao]

Tmoose

Interesting discussions. Can you please describe the casting a bit more and its application. In my limited experience I have had no opportunity for such issues in high chrome irons.
Thanks

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

Mahatma Gandhi.

 

[Tmoose]

Hi Arunmrao,

These particular parts (Induced draft fan blades handling pulverized coal in high speed coal pulverizers , in this case) have been the subject matter of most of my posts lately.

https://www.eng-tips.com/viewthread.cfm?qid=437162

We've been making these parts, and many others, from these type materials successfully for DECADES.
Some parts are stationary, but some like these fan blades are subject to significant "centrifugal" forces so get more scrutiny.
The recent issues stem from seemingly innocent cost saving changes that as is often the case snowballed right to H*ll.
Some examples are bringing procurement in-house, changing suppliers, and perhaps most unfortunately from our perspective, customers' experienced maintenance crews retiring or being whacked by RIFs with little effort to have the new guys work with those on their way out.

The strong tendency for grinding "checks" ( CRACKS!!) even with quite modest levels of casting clean up grinding, was well know for all that time, so various NDT inspections have been part of life as well.

You said " no opportunity for such issues in high chrome irons ." My hunch is that may simply mean nobody ever looked .

 

[arunmrao]

Thanks Dan for your excellent response. A star comes your way. Another rather juvenile question regarding grinding cracks. Did you find any variation in Retained Austenite levels and what is the upper limit of R.A acceptable.

Thanks once again.

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

Mahatma Gandhi.

 

[Tmoose]

RE: Retained austenite - quick answer - we don't find any variation because we don't look.

We've been buying these parts to basic ASTM A 532 specs for DECADES.
We specify Chemistry, and minimum hardness, as does ASTM S 532 ( except for "softened" castings, that specify a maximum hardness) . We actually tighten the chemisty limits a little bit.

This is what A 532 has to say about RA and microstsructure .

​

"7.1 The alloys covered by this specification are expected to have microstructures that consist essentially of carbides, martensite,
bainite, austenite; and in exceptional cases minor amounts of graphite or pearlite.

​

7.2 The microstructure will not be routinely determined nor reported except in accordance with special agreement between
the manufacturer and the purchaser, or in cases of dispute."

 

[Tmoose]

Well, the 3rd party demagged four of the "bad" parts, re-magged as part of a new MT test, and the "chatter" is GONE.

But some new questions have reared their ugly heads.

The report is being revised thusly.

https://files.engineering.com/getfi...08-81a3-dba03246a105&file=chatter_re-test.PNG

Now the call from the 3rd party tester is unclear if all the parts need to be de-magged and re-tested, for fear the residual chuck magnetism (obviously) could not be overpowered by the MT yoke magnetism.

​

" I also confirmed with our level 3 about us damaging (sic - demagnetizing intended ) and the results to change.

​

As this is something we don’t normally do. Since we are remagnetizing the parts in 2 different directions.

​

All we can think of as now, is that these parts must be holding magnetism much higher than the yoke can produce.

​

So really I’m not sure of the results on the ones that we tested due to the magnetism and the fact that they were visible die penetrant tested before florescent MT "

RE: residual chuck striped magnetism overpowering the MT yoke magnetism. A QQI was used the first part, and the 3rd party Level 2 and customer's Level 3 inspectors were satisfied.

I'd appreciate comments as to the necessity of re-testing these parts.

thanks,
Dan T

 

[metengr]

So really I’m not sure of the results on the ones that we tested due to the magnetism and the fact that they were visible die penetrant tested before florescent MT "

I would stick with one NDT method for acceptance. If you performed fluorescent dye penetrant inspection, this is a very sensitive surface examination method and should be acceptable.

 

[Tmoose]

Hi metengr,

We normally due non-fluorescent LPT.
The customer does, and insisted we have done, wet fluorescent MT.

The quote "So really I’m not sure of the results on the ones that we tested due to the magnetism and the fact that they were visible die penetrant tested before florescent MT " was a recent one from the 3rd party inspector (we used on April 5 in front of the Customer's level III inspector).
It was made a few days ago, after the relevant "chatter" indications were pretty well proven to be non-relevant and due to residual magnetism.

My question is "Do very minimal visible die penetrant residuals interfere with MT?".
My guess is NOT.

thanks,

Dan T

 

[metengr]

Hi Dan
Your correct. I would make sure the components are cleaned and rinsed of all developer before wet fluorescent MT for final acceptance.