Stainless Particulates 4

[joeatuml]

I'm trying to reduce the amount of particulates generated by stainless on stainless contact. The steel sees temps of 245C so most plastics are out of the question to reduce friction. What is the best type of finish on steel to lessen the amount of friction and particulate generation with SS on SS?

 

[CoryPad]

Coatings containing polytetrafluoroethylene, molybdenum disulfide, and/or graphite can be used around that temperature for friction reduction.

 

[mgh70]

I've had good results using Colmony or Wallex coatings if you are looking for hard-facing material. Though, I don't know what your application or budget is.

http://www.wallcolmonoy.com/products-capabilities/surfacing-alloys/

-M

 

[EdStainless]

You need to see if you can make them two different types of stainless steel.
One should be hard, either a martensitic like 440C or a PH (17-4, 17-7, 15-5, 13-8) and this part should be very smooth.
The other could then be a high Mn + N austenitic, these alloys are very resistant to galling (the source of your particles).
Either moly disulfide or tungsten disulfide would be good coating.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

What are the contact stress levels involved? What is the amount of relative motion existing at the interface?

The metal-metal contacts between stainless steel surfaces don't really produce loose particulates. Instead, there is usually mechanical adhesion between the contacting surface asperity points that is immediately followed by shearing of the local bond. This repeated bonding/shearing process results in a transfer of material from one surface to the other, and produces the classic surface damage seen with galling.

As EdStainless notes, one approach to minimizing galling with stainless steels is to use materials that have a difference in hardness. Another approach is to use an inert surface coating that inhibits the mechanical adhesion process between the contacting stainless steel surfaces. MoS2 dry film coatings work well for this application. It is good for the temps you need, it is easy to apply, it has low friction properties, and it can handle fairly high contact pressures.

 

[joeatuml]

Thanks for the suggestions.

Basically we have a bakers cart, and a tray which weighs approximately 20-30lbs and then we slide the tray onto the bakers cart.

http://ecx.images-amazon.com/images/I/41a6DgZrFBL._SY300_.jpg

Kind of similar to the link. We see tiny particulates 50 to 200 microns in our products and when you wipe the rails of the carts you can get these particulates as well.

I would like to avoid any coatings since this could then chip up. We are going to reduce the surface area of the sliding (make tabs that the trays can be placed on) and then do a different steel finish for the trays. I was thinking a tumbled finish would be best to basically rough it up to begin with and looking for any other suggestions.

 

[EdStainless]

You want the sliding surfaces hard and smooth.
You could come up with a system to roll them with a steel roller to harden them.
This would make them less prone to wear.
But in reality until one of the surfaces is non-metallic you will have this issue.

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P.E. Metallurgy, Plymouth Tube

 

[ornerynorsk]

Does your solution need to conform to NSF, or is this a non-food application?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[ornerynorsk]

McMaster Carr has slippery tape with service temps up to 500 F.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[tbuelna]

joeatumi- Thanks for the additional details. They help a lot.

I'd suggest adding a couple small raised pads on each rail surface of the cart that the bottom of the tray can slide on. The pads can be made from something like PEEK which is good to 260degC, has excellent wear properties, and is suitable for food service. Make the pad about 3/4" dia and use three or four equally spaced on each rail. Put a slight crown profile on the pad surface so there is no edge loading and the contact is always uniform as the tray slides over the pads.

Design the pad so that it can be easily installed and replaced. Maybe use something like a barb on the back side of the pad that snaps into a hole in the rail. PEEK rod stock is fairly expensive (~$30/ft for 3/4" dia) but each pad will only require a small amount of material. The pads can be produced in large quantity using a CNC screw machine for minimal cost. Also, burnish the wear surface of new pads before using them for best results.

Looking at the picture of the cart you provided, I think the reason you are generating metallic debris is due to the (relatively) sharp leading edges of the cart rails scraping material from the tray surfaces when the trays are initially slid onto the rails tilted/skewed. So besides the pads on the horizontal rail surfaces, you might also want to add some plastic bumpers on the leading edges of the vertical rail flanges.

Hope that helps.
Terry