Chrome/Nickel coating to increase hardness 5

[luisd]

We are running a part on two rails (.75 diam) using steel linear motion ball bearings (open). They have to be steel because the area where they work is subjected to high temperature. The thing is the bearings are ruining my rails because the bearing balls (60/70 Rockwell) are harder than the rail (40 Rockwell). We really don’t want to change the rail material, and the .75 ball bearings only come with 4 tracks of balls (…from what I found). We are trying plating the rails with electroless nickel and hard chrome to increase the hardness. Does anyone have any experience on this and can give me any comments? Any alternative process to increase hardness without changing the material?
The tube is 17-4PH heat treated to H900.
Thanks

 

[NickE]

If your rails are 17-4ph H900, you might try a CH900 (I can get HRc of ~51-52 with 17-7PH @CH900 ) The C refers to maximally cold worked material. This causes a change form austenite to martinsite and allows a greater hardness after the H900 heat treatment. You also might see a higher hardness from using RH950 (the R is a refrigeration step)

GO here for info on 17-4(7) heat/mechanical treatments:

http://www.aksteel.com/pdf/markets_products/stainless/precipitation/17-4_PH_Data_Bulletin.pdf

Also there has been lots of discussion in the last few months about a process called Kolsterizing. (IIRC thats the name) I think that its a Salt bath process that uses nitrogen to produce a very hard case on stainless steels. It allows the stainless to keep its corrosion resistance.

go here:
thread330-49696

good luck!
nick

 

[Carburize]

High phosphorus electroless nickel heat treated at 500 F for 8 hours will develop a hardness of around 64 Rc. If your damage mechanism is only due to wear the plating may solve the problem, if however you have a heavy load issue you may get an interface separation problem due to the deformation of the lower strength substrate.

 

[mcguire]

Increasing hardness by cold work or precipitation hardening doesn't improve wear resistance. Hard chromium plating should give you the improvement inexpensively.

 

[unclesyd]

Hard Chrome or Electroless Nickel will have to have extra thickness to overcome the problem of deforming the base metal mentioned by Carburize.
We used .011" for Cr and .007" for the NI for any type load.

We had good luck with 17/4 liquid nitrided with the Kolene QPQ process.

What temperature do the rails and bearings see?

"Case 60" will take a pretty good temperature, much higher than the rollers

 

[unclesyd]

Forgot to add the websites.

QPQ process

http://www.kolene.com

Shafting
Both make a 440C linear support shaft.

http://www.nookindustries.com/linear/LinearShafting.cfm

http://www.thomsonind.com/Sections/Products/products_main.asp?PC=2000000&GI=1

 

[luisd]

Thanks to everyone, the information has been very useful.
We are plating the rails with hard chrome this week and probably do some wear tests. I’ll keep you posted with the results.
Discussing with the plating company a new question came up: If we use a hard dense chrome, what is the effect of welding an attachment direct to the chrome, or should we locally remove the chrome from the area to be welded. We have some advice that you can weld direct to/through the chrome, but if we are joining 17-4PH to a 316 Cres support structure should we remove the chrome?
Again, thank you very much.

 

[unclesyd]

The Cr has to be removed in either case. Do all the welding and machining prior to Cr plating. Everything that will happean is bad if you don't remove the Cr.

 

[SMF1964]

Welding to the 17-4PH will totally destroy any heat treatment fo the 17-4PH in the vicinity of the weld. Make sure you are not relying on the higher strength of the 17-4PH H900 that can't be sustained by 17-4PH in the fully annealed state, as that is what your heat affected zone will contain.

 

[EdStainless]

Look at microstructure of your shafts. Do you have full transformation? The RH950 heat treatment will help. Not just the hardness but hte microstructural uniformity also.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be slowed down.

 

[EdDanzer]

I doubt chrome plating will do much. The fractures in the chrome plated surface will only act as pebbles pushing into the soft under surface. An induction hardened surface will be required. Are the balls stainless?

 

[luisd]

Ok, we are trying the chrome plating. I'll keep you posted on the results.

Thanks to everyone for your help.
Best regards.

 

[Metalguy]

EdDanzer,

While waiting for the results, I suspect you are correct. One of the first failure anaylses I was involved with was because of a thick hard Cr plate instead of the required thin (0.001") plate. Sorta like loading a piece of glass that's sitting on a block of clay.

 

[najeebuddin]

The balls in the linear bearing are about 52 - 55 Rc, these are usually made of SAE 52100 bearing steel or equivalent. There are couple of things that you may want to consider. Easiet of all is to change the rails to 440 C stainless rather than PH 17-4. This is the most commonly used stainless rails with the linear bearings. Also the shafts are hardened to 50 - 55 Rc in this case and should give you very satisfactory results. By far this would be the cheapest solution. Since you do not want to change the material, the next thing is to take a look at the tolerance on the shafts used for the rails. Again the industry standard is dia. 0.7495/0.7490 for the rails to be used in association with 0.75" bearings.Since you are using on a softer shaft, no pre-load can be allowed. If the shaft dia. is any higher than stipulated above the pre-load could be a cause for brinnelling the shaft. Hard Chrome is too expensive a proposition and may not be a total solution because of flaking problems associated with it. Also the shaft needs to be ground and polished after plating to the required diameter.

 

[dcampbellpc]

You can also try a Ni/PTFE plating. Platers can generally get higher hardnesses by adding the teflon.