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Technical Discussions
Amar Rajadnya
Amar Rajadnya
10:57 Mar-22-2010
Diffraction patterns in casting radiography..Is it casting defect?

Hi,

I produces investment casting of 316 grade (ASTM F 745-07) for orthopaedic implants. My recent casting bach shows mottling or columnar structure as per reference films, which idicates that it is a diffraction defect...! The radiography interpreter is confused with this pattern and supposes as type of shrinkage..
Let me know is diffraction patterns are really casting defects..? If not then what are the causes of such radiographic patterns..?

Thanks

Amar

arajadnya@yahhoo.com

    
 
 Reply 
 
S.V.Swamy
Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex , India, Joined Feb 2001, 787

S.V.Swamy

Engineering, - Material Testing Inspection & Quality Control
Retired from Nuclear Fuel Complex ,
India,
Joined Feb 2001
787
12:41 Mar-22-2010
Re: Diffraction patterns in casting radiography..Is it casting defect?
In Reply to Amar Rajadnya at 10:57 Mar-22-2010 (Opening).

The diffraction patterns per se may not be defects in casting, since they are sometimes produced by columnar grain growth. We had similar problems in some austenitic stainless castings produced through investment casting process. The Quality Manual of the customer and or their designer is the person competent to handle such problems. Whether any physical defects are there in the castings can easily be checked by cutting and polishing / etching of a few specimens.

Fitness for use is the guiding principle in such cases and I could help all the parties involved in one case where no clear standard existed.

Wish you good luck.

Regards

Swamy

    
 
 Reply 
 
Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
13:19 Mar-22-2010
Re: Diffraction patterns in casting radiography..Is it casting defect?
In Reply to S.V.Swamy at 12:41 Mar-22-2010 .

Amar

Diffraction mottling is a known phenomenon in the radiographic imaging of austenitic welds and castings. Increasing kV can reduce the effect. If this happens you will know definitely its not a discontinuity but as Mr Swamy says due to columnar grain structure.

    
 
 Reply 
 
Amar Rajadnya
Amar Rajadnya
13:34 Mar-22-2010
Re: Diffraction patterns in casting radiography..Is it casting defect?
In Reply to S.V.Swamy at 12:41 Mar-22-2010 .

Dear Mr. Swamy / Nigel,

Many thanks,

Is there any literature or papers on similar topic...?? Did mottling or columnar appearance hide actual defects ?? Is there any standard by which I can convince my radiography lab...? Should I take sample radiography from another lab...? Will the results same in any lab...?

Now my problem is how to prove this point authentically...!!

Thanks in advance.!

Amar

    
 
 Reply 
 
Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
14:06 Mar-22-2010
Re: Diffraction patterns in casting radiography..Is it casting defect?
In Reply to Amar Rajadnya at 13:34 Mar-22-2010 .

Amar

Search ndt.net and Google for "diffraction mottling". Some reference radiograph sets will show examples, occasinal articles have appeared in journals.

Diffraction patterns are NOT defects, they are patterns caused by the preferential directional scattering of the incident photons on the cloumnar grains.

Increase the kV from your original exposure parameters and this should disappear.

    
 
 Reply 
 
JB Goradia
JB Goradia
04:53 Mar-23-2010
Re: Diffraction patterns in casting radiography..Is it casting defect?
In Reply to Nigel Armstrong at 14:06 Mar-22-2010 .

Most of the time diffraction mottling is identifiable because in case of flaws of less density, radiation energy is not absorbed whereas in case of diffraction mottling radiation energy is absorbed equally only direction changes.

    
 
 Reply 
 
Amar Rajadnya
Amar Rajadnya
03:50 Mar-24-2010
Re: Diffraction patterns in casting radiography..Is it casting defect?
In Reply to JB Goradia at 04:53 Mar-23-2010 .

Thanks,

More replies awaiting....Let me know, is there ant use of cutting the specimen....Did micro structure pattern will identify the difference between diffraction pattern and shrinkage...? How?

What should be ideal KV (energy level) for radiography. The casting portion is of thickness of 9 mm.

Thanks...

Amar

    
 
 Reply 
 
Stan
NDT Inspector,
Canada, Joined Jan 2009, 31

Stan

NDT Inspector,
Canada,
Joined Jan 2009
31
15:09 Mar-25-2010
Re: Diffraction patterns in casting radiography..Is it casting defect?
In Reply to Amar Rajadnya at 03:50 Mar-24-2010 .

Hi Guys:
According to the ASTM NDT Handbook Volume 4, Radiographic Testing, the best way to determine if an indication on a radiographic image is mottleing caused by diffraction is to take a second exposure at approximately 5 degrees to the first. If the indication is caused by porosity or segregation, the image will be little changed from the first. If the image has a different pattern, then it is most likely due to diffraction.
The book goes on to mention that raising the KV and using lead screens may eliminate the diffraction patterns.
Stan

    
 
 Reply 
 
Amar Rajadnya
Amar Rajadnya
19:05 Mar-26-2010
Re: Diffraction patterns in casting radiography..Is it casting defect?
In Reply to Stan at 15:09 Mar-25-2010 .

I tried increased Kv which in turn minimized the diffraction pattern..I tried 2 separate radio-graphs for same casting, the pattern position changes by doing it..further I also tried cutting the specimen and etching but nothing was found under 50X microscope ...

My question now is specimen cutting, polishing and etching methods are meant for just visual inspection thro' microscope or grain structure can determine the shrinkage type....?

Any specific standard for judging the shrinkage by observing micro structure....

In my case, it seems the lab. 'needs' some reason to reject the casting though they are not getting 'anything'..? Is any other logical method left to find out the shrinkage inside the casting...??


Regards,

Amar

    
 
 Reply 
 
rishikesh kumar
rishikesh kumar
07:19 Apr-05-2014
Re: Diffraction patterns in casting radiography..Is it casting defect?
In Reply to Amar Rajadnya at 19:05 Mar-26-2010 .

Diffraction mottling is not a defect but is it allowed on the radiograph, especially when it can be removed or improved by increasing the radiation energy?
The supplier of austenitic steel investment casting used to do 5% RT using Iridium and mottling was never seen. But now they are using X-Ray and all the radiographs are showing mottling. Can we accept the radiograph as it is?

    
 
 Reply 
 
Gerald R. Reams
Engineering,
Industry, USA, Joined Aug 2012, 180

Gerald R. Reams

Engineering,
Industry,
USA,
Joined Aug 2012
180
15:57 Apr-05-2014
Re: Diffraction patterns in casting radiography..Is it casting defect?
In Reply to rishikesh kumar at 07:19 Apr-05-2014 .

In your case, the interpretation of diffraction or relevant indication cannot be determined with only the one radiographic view. Many alloys present this same problem. I always use 2 radiographs taken at different angles. If the pattern changes or disappears, I interpret it as diffraction pattern and not a relevant indication.

    
 
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