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Joined: Jan 2005
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We are examining 4340 steel parts (46 HRC) that have been shot peened and chromium plated. There appears to be particles imbedded into the steel under the chromium plating. This seems to be a surface condition. Is it possible to "push" some residual heat treating scale that did not clean up or some broken shot into surface of this material? Some of these particles are 0.0005" into the steel with chromium plating behind it or around it. The shot used was S-230 regular hardness. Your opinion please. Thanks.


Jack Champaigne (Dr. Peener)
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Yes, it may be possible. I haven't heard of this specific problem before. If you don't maintain the media properly, i.e. allow broken shot, then this might get pushed into the surface. I suppose it's possible that heat treat scale could also get into the surface. Do you use any other de-contamination (chemical)?


Jack Champaigne (Dr. Peener)
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Normally before plating there will be a called out for grit blasting at a required pressure and than you need to use chemical to clean the surface e.g. sulphuric acid (dipping method)for not more than 30 seconds, rinse and than plate.
Important is that your sieve must be able to remove the broken shots.
If that is a heat treated part my advise is to grit blast the part before shot peen.This will reduce the scales from embarking into the parent metal.
You may want to try this solution.

Regards: Tessensohn Albert


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