Letter 9017

Hard wear surface for 17-4 PH stainless. 

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We are building small harmonic drive units for some miniature research robots (5 cm scale). The harmonic drives function with a cup-shaped "flexspline" with external spline teeth mating to a fixed spline ring with a slightly larger number of internal teeth. Rollers cause the spline contact point to walk around, producing output rotation of the flexspline due to the difference in numbers of teeth. Grease is used on the spline surfaces.

We need a material with good strength, wear resistance and machinability. We have tested 12L14 free-machining steel, but its strength and wear-resistance seem poor. We have found that 17-4 PH stainless machines nicely and can be hardened with minimum distortion, but I understand that galling may be a problem when it is run against itself. So we're considering surface treatments to minimize wear and galling. One recommended treatment is a "thin-dense" or "nodular" chrome plating that is reportedly very thin and uniform, with buildup of .0001-.0002" and surface hardness of about 72 Rc. Part tolerances are on the order or a ten thousandth or an inch, so a thin uniform buildup is needed.

I'm seeking comments on this treatment, shops that can do it,
and recommendations for other treatments that would be good.

Thanks,

H. Ben Brown, Jr.
Carnegie Mellon University - Pittsburgh, PA, USA

Galling is most common when you try to run the same metal aginst the same metal.

Your original choice was a free cutting steel and would not have a good wear resistance. I think that you can easily get by with 4140 or 4340 or similar hardenable steel. Depending on shape, it may distort in heat treatment, so it is common to leave some grind stock for touch up. This material needs to be hardened in a vacuume furnace or one with a controled atmosphere or wraped up in a special SS foil bag to keep the surface from having a lot of heat scale. Sometimes they are copper plated to prevent decarburization on the surface. You might consider having it nitrided or case hardened. S-2 or D-2 should work well also, but are a lot higher priced and tougher to machine.

I really do not think that you need any coating. Chrome, EN and vapor depositions have their place, but I do not think that you need it for here, if I picture your part correctly.

James Watts
- FL

Hello Ben!

I think using 17-4 makes a lot of sense, for ease of machining and hardening reasons. As you say, little distortion in hardening, combined with pretty good mechanical properties, and corrosion resistance to boot. Are you using the H900 condition? The HRC 40-47 is attractive, but beware low ductility- that's one of the reasons that aerospace use of the material is commonly limited to H1025 & H1075.

My gut feeling is that you shouldn't have much of a problem, if your loads are sufficiently light and you keep the teeth greased. Galling tests that I've run here show 15-5 galls easily (280 psi) if clean, yet is very resistant (over 10 ksi load to gall) if oily. Those load paths are also the type that are forced to rub against each other- yours sounds like it can roll, giving you less of a problem. So, you may want to try out your device to see if your premonition is warranted.

When I have galling concerns, I too strongly consider chromium plating- we use it all the time on our 17-4 and 15-5 actuators. If your contact loads are low, then thin dense chrome plating per AMS2438 [link is to spec at TechStreet] will (not "should", but "will") work well. Further, you don't have to grind after coating. My contact loads are often higher than the thin dense chromium can tolerate on a 17-4 substrate, so I go with a thicker layer; say, 2 mils (50 microns) or so, after grinding. We electroplate on more, and grind off the extra, to ensure getting uniform coverage. To find a place that will do the work, look for "thin dense chrome" or "AMS 2438" on the supplier advertising. I'd guess there are firms that advertise here at finishing dot com which offer this service, so I'll not step on their toes and recommend someone else. Be sure to let the plater know that they're trying to plate stainless steel, so they can activate the surface properly.

Among the alternatives I'd consider would be nitriding the 17-4, to about 5 mils depth. Here you'd likely need to grind afterwards, because the surface gets ugly: you'd also likely get some microcracking on the surface, especially in the crown of the teeth. Another disadvantage would be that you lose most of your corrosion resistance of your stainless steels. Along a different tack, you might consider a galling resistant stainless steel, like nitronic 60 or Gall-tough, which have lower strengths (about 60 ksi yield, 100 ksi tensile).

I enjoyed my years at Carnegie (76-78), so I'd be happy to help if you've more questions or need something heat treated. Contact me at lgearhart@moog.com. And if you see Rick or Kathleen Carley, give them my regards!

Lee Gearhart
- East Aurora, NY

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