Dark residue on shot peened surface

Posted by: Babar

Dark residue on shot peened surface - 12/11/19 02:51 AM

Dear all,

I am facing a problem of presence of dark residue on shot peened surface. I dont have deep knowledge of this process so I apologize if the question is very basic.

Practiaclly we are shot peening some parts made of Fe ARMCO or Nickle, but post shot peening if we rub a cotton bud on the surface it becomes dark. We are using glass beads type AGB10. My assumption is that this is because the glass beads are getting broken and leaving their residue. I tried to change the air pressure but nothing changed. Is it normal? The parts that we are shot peening have to be in an X ray tube so surface cleaning is very critical to us.
Posted by: Socrates

Re: Dark residue on shot peened surface - 12/11/19 02:23 PM

Both nickel and Armco iron must have oxide coatings. Nickel oxide as Ni2O3 is black as is Fe3O4 on Armco iron. Peening produces nascent surfaces that readily convert to these oxides when exposed to air. Post-peening removal will reduce the thickness of the black coatings.
Posted by: Babar

Re: Dark residue on shot peened surface - 12/12/19 02:38 AM

Thank you for your response. I have proposed to add cleaning with Acetone or pure Alcohol in the process post-peening.

Is there any way to reduce or prevent this oxidation? Because addition of this cleaning increase lead time of product.

Currently we are using 5 bar pressure AGB10 beads, but the distance between nozzle exit and part is not controlled and operator can move and rotate the part freely.
Posted by: Socrates

Re: Dark residue on shot peened surface - 12/12/19 01:58 PM

I do not know of any way of preventing oxidation of non-noble metals - oxides must form when exposed to air. How about reverse thinking? Is the dark coating unacceptable to your customer? If it is being formed consistently then it is useful proof of coverage! That is the reverse of using lacquer coatings that have to be blasted off to prove coverage.
Posted by: Babar

Re: Dark residue on shot peened surface - 12/13/19 05:28 AM

I understand. Well; that means I must have to add a post peening cleaning procedure.

Just to add knowledge non-related to this topic: In X ray tubes presence of any kind of pollutant can cause pre mature failure. So high care is taken to clean manically each and every part which has to be inside of the tube. Here the part being shotpeened is a focalizer which is inside of the tube.