Letter 13050

Surface finishing... galling of 410 against 440C stainless steel 

I'm re-designing an external gear pump that handles diesel fuel, kerosene, heated crude oil, etc. Water in the fuel has caused the previous cast iron housing to rust. The pumping gears are made of 440C SSt (58Rc). I am making the new case from 410 SSt (annealed) to eliminate the corrosion problem and because it must closely match the coeff of thermal expansion of the 440C gears. I am concerned about galling of the 410. Can you recommend any proven surface treatments that resist galling, wear, and corrosion? The treated surfaces must also keep a flatness of .0002" to maintain the tight lateral clearance that exists between the gears and the casing end plates.

Bill Flavelle
- Atlanta, GA, USA

Silver plating and chrome plating are both used. Considering the hardness of the gears, though, chrome sounds more like it to me.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Standard PVD processes probably won't solve your problem -- the coating temperatures are too high. You should look at either ion implantation or ion beam assisted deposition. In ion implantation, the parts are bombarded with metal or gas ions (or a combination of both), forming a new alloy on the surface and a deep dislocation structure that improves the hardness and toughness of the surface. Ion implantation won't change the dimensions of your parts, so you can keep your tolerances. I've used the process extensively on cutting tools, where the life increase is typically around 200%. However, the data on galling is less clear. Theoretically, it should work, but it needs to be tested.

The alternative is to combine the ion implantation with deposition, putting down a coating such as chromium nitride or titanium nitride. The addition of the ion beam allows a good coating to be deposited at under 150 C, low enough to prevent degrading of the 400 stainless steel. Here, though, you face the "other side" problem -- the coated part may be too hard for the uncoated. There is also the problem of delamination, though coatings deposited by ion beam assistance have much higher adhesion than traditional PVD coatings.

Jim Treglio
- San Diego, California

Electroless nickel plating will maintain the flatness and provide excellent lubricity to these parts.

Todd Osmolski Charlotte Plating, Inc. Charlotte, NC USA

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