Mechanical Finishing Clinic: Blasting Media That Reduce or Eliminate Ferrous Surface Contamination
Q. We are blasting our stainless parts in a blast cabinet with a finer steel shot. We are concerned about ferrous contamination but need the impact the steel media produces for our parts. What media do you recommend?
A. Ceramic blast shot is our best recommendation. Stainless blast media is a standard recommendation, but keep in mind that stainless is 50-percent iron.
Ceramic blast media are spherical media with high mechanical strength and wear rates. Ceramic is impact-resistant and creates little dust. Ceramic shot maintains its round consistency and are chemically inert. Ceramic blast media processes produce a smooth, bright satin finish and the ceramic shot’s density creates higher impact speed, making it a good choice for deburring and peening.
Blast pressure recommendations for ceramic media are between 40-65 psi with media cycle lives between 70-90 times through the blast system. Ceramic shot media is versatile and can be blasted with all delivery systems (air, wheel, and water). Ceramic shot is a standard peening material for titanium parts. Ceramic media also comes in grits that are excellent for etching parts with extended media life.
Stainless blast media is available in shot (cut and conditioned wire, or caste) and is available in 304, 316, and 400 alloys. Stainless is a softer but heavier media that is a good choice for reduced blast process times, deburring, and reduced surface contamination.
Stainless media produces a brighter finish with reduced blast machine wear. It obtains some of the top media recovery rates with up to 90-140 cycles through the blast system. Blasting pressure recommendations are from 40 to 90 psi.
Ferrous contamination on stainless surfaces in the welded affected areas is often caused by welding heat drawing the carbon to the surface. Blasting will not remove the carbon. Chemical passivation or electro-polishing is required to remove the carbon in the weld heat precipitation zones.
Originally published in the January 2017 issue.
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This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.
When choosing vibratory media, understand the size, shape, starting roughness condition and metallurgical structure of the part.