Posted By: Dr Peener media contamination - 01/27/05 06:34 PM
Dear Dr. Peener,
I am a service engineer and we are a repair and overhaul facility for gas turbine engines and propeller assemblies.

We currently use Electronics Inc. "Y" Test Strips, P/N 970060.3, when qualifying some of our shot peen applications throughout our company and are very pleased with your product.

I am currently working on an R&D project and I was hoping you could answer some technical questions with regards to the use of shot peen media. Our overhaul data directs us to use either 550 cast steel shot or LD62 conditioned cut wire steel shot for peening applications. Note that I have not been able to find any reference anywhere referring to LD62 shot and I do not know if such a product exists. The material that we are peening is 7076-T61 Aluminum alloy.

Once the shot peening is complete, an iron decontamination process must be completed and an iron decontamination check follows to ensure there are no traces of iron remaining.

What I am looking at is to see if stainless steel shot can be used in place of the shot types listed above. In particular using SCW 62 Conditioned Stainless Steel (Type 302) Cut Wire Shot.

1) Would you know if this is a recommended product for shot peening aluminum alloys?

2) Is it known if this stainless media can contaminate an aluminum alloy surface much like cast iron / steel shot will from the shot peen process?

3) If there is a chance for surface contamination due to using stainless steel shot on aluminum alloys is there any known product that will remove the stainless steel contamination - much like using a Deoxidine / MetalPrep solution or a Sodium Hydroxide solution to remove iron contamination from aluminum alloys after shot peening.

4) Are there any process that can be performed to check for stainless steel contamination on aluminum alloys - much like current iron decontamination checks.

The reason I ask these questions is that if one looks at the galvanic difference between aluminum and type 302 stainless steel one will see that this difference is quite large. When this difference is large the galvanic corrosion rate is highly elevated. Thus we are unsure if the stainless steel contaminants (if any) on the aluminum surface could be detrimental.

Your help in this matter would be greatly appreciated.
Posted By: Dr Peener Re: media contamination - 01/27/05 06:39 PM
I have not heard of LD62 media. I sounds like it is an equivilent to SCW 62 condtioned stainless steel shot. I suggest you contact one of the cut-wire manufactures for a cross-reference.
Bob Gillespie
Premier Shot Company
1203 West 65th Street Suite 1600
Cleveland, OH 44102 USA

Bob has presented several papers at conferences detailing the contamination from cast steel, cut-wire and stainless cut-wire.
Posted By: JGirman Re: media contamination - 01/27/05 08:06 PM
I believe a better solution is to use ceramic shot (Zirshot) in this application. Zirshot does not create any iron contamination on the surface so there is no need for chemical treatment. It has the capabilities of peening at relatively high intensities, (10-15A)is very possible. In addition, Zirshot provides a very consistent surface finish especially on aluminum. I encourage you to explore this product.


Jeff Girman
Saint-Gobain Zirpro
Phone: (919) 593-4813
Posted By: Dr Peener Re: media contamination - 01/28/05 02:02 AM
From: Heribert Gray

Dear Dr. Peener

Spherical Conditioned Steel - SCCW 62 , hardness 60-65 HRC ( 640 HV1 ) cut wire is being used for such kind of Shot Peening - after thermal spraying for instance - to achieve a high penetration of the compressive residual stress affected zone below the surface area due to reason that you are familiar with.

Stainless steel Cut Wire ( even less Stainless Shot) is not recommended due to two reason:
1.) Stainless Steel is more precious than Aluminum, particular to the 7000 series Al- Alloys with regard to galvanic reaction.
Consequently, the galvanic corrosion occurs preferably and intense at the Al- Alloy!
Steel causes some discoloration on the parts in contrary to STEEL CUT WIRE but faces no major damage of the component made of Al-Alloy. The local galvanic corrosion stops as soon as the whitish Al-Oxide does insulate the flow of galvanic current.
2.) Do to the higher resistance of stainless steel alloys to any acids compared to steel, the removal of the contamination causes longer or more intensive chemical treatments.
This fact rises the risk of a hydrogen embrittlement in the Al-Alloy, particular in the case of 7000 series - Al-Alloys when being peak or overaged.
The longer the parts are exposed to acid treatment, the more Al-metal is dissolved as salt into the acid bath.

An approved alternative is the use of Zirconium Beads that are partially stabilized with Cerium Oxide but having no glassy Phase like Si-Oxide : 80% Zr-Dioxide with 20 % Ce-Dioxide, hardness 1100 HV
( 9 Mohs ) at crushing strength of 125 kg on 2 mm diameter beads.
Size: 1.2 - 1.7 mm
Shape : spherical
Appearance: smooth glossy satin finish
Specific weight: 6-6.1 g/cm³ - Apparent Density ~3.7 g/cm³

The beads are inert and leaving no chemically reacting residues on the surface to be exposed to Shot Peening !
These kind of professional peening is applied in the F-1 Racing Sports of TA-Plasma Sprayed High Temperature Components and Titanium Springs, high strength components and jet engine parts in the Aeronautical Industries, for Off-shore Oil Drilling Tools and last but not least for biomedical application ( implants ) .

The higher cost for such kind of media are paying off due to less or no cost for the post treatments like decontamination, at the bottom line!

Additionally, the requested intensity is reached faster due to the high hardness and the coverage can be adjusted to <= 100 % instead of >> 100 %.
This way the Peening Process in general is becoming less time consuming and less costly itself already.
The flip side of the coin is : this kind of media is limited to pressure pot peening or to suction peening with a gravimetric feeding system of the media.

so long

hope we could help


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