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Technical Discussions
Vince
Vince
08:22 Sep-16-2004
Surface etching to detect hot cracks

Hello,
I have A206 castings that potentially contain hot cracks. Normally, I would apply die penetrant to inspect for cracks. However, the castings have been shot-blasted at the foundry and painted, so any cracks that might been there are masked.
Is there a way to remove a small amount of surface material along with pain to expose existing cracks so that they could be detected with die pen? Would chemical etching of the surface work, and if so, what is the right way to apply it? Any advice, comment will be greatly appreciated.


 
 Reply 
 
Ed T.
Ed T.
08:26 Sep-17-2004
Re: Surface etching to detect hot cracks
----------- Start Original Message -----------
: Hello,
: I have A206 castings that potentially contain hot cracks. Normally, I would apply die penetrant to inspect for cracks. However, the castings have been shot-blasted at the foundry and painted, so any cracks that might been there are masked.
: Is there a way to remove a small amount of surface material along with pain to expose existing cracks so that they could be detected with die pen? Would chemical etching of the surface work, and if so, what is the right way to apply it? Any advice, comment will be greatly appreciated.
------------ End Original Message ------------

They should not paint these castings prior to NDT.


 
 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
08:41 Sep-17-2004
Re: Surface etching to detect hot cracks
----------- Start Original Message -----------
: : Hello,
: : I have A206 castings that potentially contain hot cracks. Normally, I would apply die penetrant to inspect for cracks. However, the castings have been shot-blasted at the foundry and painted, so any cracks that might been there are masked.
: : Is there a way to remove a small amount of surface material along with pain to expose existing cracks so that they could be detected with die pen? Would chemical etching of the surface work, and if so, what is the right way to apply it? Any advice, comment will be greatly appreciated.
: They should not paint these castings prior to NDT.
------------ End Original Message ------------

Granted. But now that they have done it, yes, it should be possible to etch the surface after removing the paint, so that the cracks are exposed. But I would suggest an alternative. I am assuming that the casting is ferrous and is magentic. If so, a magnetic particle test will reveal the cracks without the need to remove the paint or to etch the material. Try that and cross check the same with Dye Penetrant. If the correspondence is good, you could adopt the process.



 
 Reply 
 
DJ Kallhof
DJ Kallhof
04:30 Sep-20-2004
Re: Surface etching to detect hot cracks
----------- Start Original Message -----------
: Hello,
: I have A206 castings that potentially contain hot cracks. Normally, I would apply die penetrant to inspect for cracks. However, the castings have been shot-blasted at the foundry and painted, so any cracks that might been there are masked.
: Is there a way to remove a small amount of surface material along with pain to expose existing cracks so that they could be detected with die pen? Would chemical etching of the surface work, and if so, what is the right way to apply it? Any advice, comment will be greatly appreciated.
------------ End Original Message ------------

The Hot Tearing you are concerned with may cost more to find than the castings are worth. If it is cost effective I would suggest the following steps. First, you need to find out the parameters of the grit blast ie. the grit size , the p.s.i., the working distance and finally the duration of the blast. I am assuming the A206 you are refering to is an Aluminum casting alloy comparable to UNS A02060. The material is soft as you know and peening of the defects will not take alot of effort. You will need an Alkaline solution to strip the paint. Aluminum alloys can be etched with a Sodium Hydroxide, distilled water mixture (200 grams/ per liter) without to much effort. To prove the process take similar samples known to contain the cracking you seek. Grit blast them to the same parameters as the parts in question. Do a dye penetrant inspection to verify the peening of the defects. Etch them and follow it with another dye penetrant inspection to find if they reveal the defects that were evident prior to the blast. Good luck.


 
 Reply 
 
Vince
Vince
02:35 Sep-20-2004
Re: Surface etching to detect hot cracks
----------- Start Original Message -----------
: : : Hello,
: : : I have A206 castings that potentially contain hot cracks. Normally, I would apply die penetrant to inspect for cracks. However, the castings have been shot-blasted at the foundry and painted, so any cracks that might been there are masked.
: : : Is there a way to remove a small amount of surface material along with pain to expose existing cracks so that they could be detected with die pen? Would chemical etching of the surface work, and if so, what is the right way to apply it? Any advice, comment will be greatly appreciated.
: : They should not paint these castings prior to NDT.
: Granted. But now that they have done it, yes, it should be possible to etch the surface after removing the paint, so that the cracks are exposed. But I would suggest an alternative. I am assuming that the casting is ferrous and is magentic. If so, a magnetic particle test will reveal the cracks without the need to remove the paint or to etch the material. Try that and cross check the same with Dye Penetrant. If the correspondence is good, you could adopt the process.
------------ End Original Message ------------

Thank you for your replies.

First of all I apologize for my spelling mistakes.
A206 is a cast aluminum alloy, non-ferrous, non-magnetic, so mag particle inspection does not apply. Also, the castings are painted and that's a fact we are trying to overcome now. Regardless of whether they are painted or not, the key to the solution is to find the right method of etching cast surfaces to remove a mil or two to expose potential hot cracks. Will Sodium Hydroxide be aggressive enough to remove that much material? Are there any corrosion related issues I should be concerned with after the etching?




 
 Reply 
 
DJ Kallhof
DJ Kallhof
03:27 Sep-21-2004
Re: Surface etching to detect hot cracks
----------- Start Original Message -----------
: : : : Hello,
: : : : I have A206 castings that potentially contain hot cracks. Normally, I would apply die penetrant to inspect for cracks. However, the castings have been shot-blasted at the foundry and painted, so any cracks that might been there are masked.
: : : : Is there a way to remove a small amount of surface material along with pain to expose existing cracks so that they could be detected with die pen? Would chemical etching of the surface work, and if so, what is the right way to apply it? Any advice, comment will be greatly appreciated.
: : : They should not paint these castings prior to NDT.
: : Granted. But now that they have done it, yes, it should be possible to etch the surface after removing the paint, so that the cracks are exposed. But I would suggest an alternative. I am assuming that the casting is ferrous and is magentic. If so, a magnetic particle test will reveal the cracks without the need to remove the paint or to etch the material. Try that and cross check the same with Dye Penetrant. If the correspondence is good, you could adopt the process.
: Thank you for your replies.
: First of all I apologize for my spelling mistakes.
: A206 is a cast aluminum alloy, non-ferrous, non-magnetic, so mag particle inspection does not apply. Also, the castings are painted and that's a fact we are trying to overcome now. Regardless of whether they are painted or not, the key to the solution is to find the right method of etching cast surfaces to remove a mil or two to expose potential hot cracks. Will Sodium Hydroxide be aggressive enough to remove that much material? Are there any corrosion related issues I should be concerned with after the etching?
------------ End Original Message ------------
By processing known crack samples through a blast operation similar to the parts in question and following it with an acid etch you will be able to determine the effectiveness of the acid etch. Typically, acid etching only removes one to two tenths as in .0001" to .0002". You could make up a stronger bath and do stock loss tests to determine the parameters required to remove whatever amount of material you desire. I know of no corrosion related issues following acid etching provided the acid is properly dilluted or neutralized and adequately flushed from the surface. This needs to be done to facilitate the subsequent fluorescent penetrant inspection as well.




 
 Reply 
 
David Forsyth
R & D
TRI/Austin, USA, Joined Nov 2001, 41

David Forsyth

R & D
TRI/Austin,
USA,
Joined Nov 2001
41
05:38 Sep-21-2004
Re: Surface etching to detect hot cracks
(cut)
>>Will Sodium Hydroxide be aggressive enough to remove that much material? Are there any corrosion related issues I should be concerned with after the etching?

>By processing known crack samples through a blast operation similar to the parts in question and following it with an acid etch you will be able to determine the effectiveness of the acid etch. Typically, acid etching only removes one to two tenths as in .0001" to .0002". You could make up a stronger bath and do stock loss tests to determine the parameters required to remove whatever amount of material you desire. I know of no corrosion related issues following acid etching provided the acid is properly dilluted or neutralized and adequately flushed from the surface. This needs to be done to facilitate the subsequent fluorescent penetrant inspection as well.
-----

In fatigue-critical applications, there are many, many documented failures due to fatigue cracks nucleating at pits left from chem-milling processes. This is not so much acontinuing corrosion issue as a surface finish issue. This is something which you should keep in mind.

In 2000 series Al for example, the intrinsic discontinuities in the material which are the predominant sites of crack nucleation are on the order of 0.001 - 0.003" in diameter. The unfortunate congruence of pits from chem milling and these alloying particles can greatly reduce fatigue life. See papers at http://www.apesolutions.com/frm_link.htm for more information.

Dave.


 
 Reply 
 
Vince
Vince
06:12 Sep-21-2004
Re: Surface etching to detect hot cracks
Will Sodium Hydroxide be aggressive enough to remove that much material? Are there any corrosion related issues I should be concerned with after the etching?
------------ End Original Message ------------

I ran a test on a block of A206 by submerging it into Sodium Hydroxide solution (200g/liter). The solution preferentially attacked sites of Al-Cu intermetallic compound at the surface. The next step is to test 100% Nitic acid etch. The other test will involve etching test blocks with a solution of 24% H2SO4+7%HF. I appreciate your warning about fatigue related issues due to formation of pits.


 
 Reply 
 
mahinda
mahinda
01:10 Nov-17-2004
Re: Surface etching to detect hot cracks
----------- Start Original Message -----------
: Will Sodium Hydroxide be aggressive enough to remove that much material? Are there any corrosion related issues I should be concerned with after the etching?
: I ran a test on a block of A206 by submerging it into Sodium Hydroxide solution (200g/liter). The solution preferentially attacked sites of Al-Cu intermetallic compound at the surface. The next step is to test 100% Nitic acid etch. The other test will involve etching test blocks with a solution of 24% H2SO4+7%HF. I appreciate your warning about fatigue related issues due to formation of pits.
------------ End Original Message ------------




 
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