Nitronic 60 sleeve and bushing

[haze10]

I am trying to select a sleeve and bushing material. Operation is a bottom auger for a large chemical vat processing paint pigment, 50% solids and water, somewhat abrasive. There is a large universal joint at the bottom that drives the auger. The cross of the joint has pressed on replaceable sleeves, the yokes of the joint have stationary bushings. 304SS is the auger and vessel material and it has held up well. The old sleeve and bushings were carbon steel 4140 Rc50. They have to run dry except for the water to avoid contamination. The 4140 would corrode badly. I am looking to go to Nitronic 60 for both the sleeve and bushing. Little concerned because the rockwell is only 20 on the Nitronic 60 and it can't be heat hardened. Loading is mild by the auger, the cross rocks about 15 degrees in the yoke, as the auger spins at 30 rpm.
Should I be concerned that both the sleeve and the bushing will be have low hardness.
PS, I am not a materials engineer but a mechanical engineer.

 

[alexit]

I would not replace hard steel bushing with any type of soft one in abrasive application. The soft (304) auger might survive because the abrasives are not working in a narrow space (like the sleeve does.)

I would suggest SS like 420 or 440B/C hardened near 50Rc.

 

[sreid]

How about using Stellite facings for wear and corrosion resistance.

http://www.stellite.com/

 

[haze10]

Here is the problem. The equipment requires annual maintenance. We are getting about 9 months service out of the 4140 Rc50 bushing and sleeves. They appear to be subject more to corrosion than to wear. My goal is to try to get 18 months service to guarantee it makes the one year service interval. Because they are replace frequently, I don't want to go for anything too exotic or expensive. Clearances are on the loose side. The ideal would be to select a material that can be turned on a CNC machine, to a profile of say 30um and not require grinding or heat treating. The sleeve is pressed on the cross with 0.001 interference. Sleeve and bushing have about .005 to .008" clearance. If I go with material that displaces with heat treatment, than the components have to be remachined. So then its rough machining, heat treating, and grinding. Driving the cost of each piece over $100. The Nitronic 60 brochure states that wear is not changed with a profile from 6 to 30 um, and that the self mating pieces will not gall with 50kspi pressure. Wear is also very low.

Researching all the stainlesses, they all have very low galling thresholds, even 17-4 when hardened to 42Rc. One person suggested I try 17-4 annealed and machined to fit at Rc28 on one piece, say the bushing, and 17-4 H950 at Rc42. He claimed that the 12 point difference in hardness would prevent galling. But I can't find any literature to support that.

The other option is to stay with steel, like 8620, carburized to Rc60, then ground. While this may be a better bearing surface, the corrosion would probably be the same.

Is there any ideas out there to help me. Coatings are just to expensive and time consuming. Often the bushings have to be in a couple day for emergency repairs, and while close they are not all identical. I need a material that can be handled and the local machine shop, preferably without heat treating and grinding. Carbon or SS is acceptable.

Here's the link

http://www.hpalloy.com/DataSheets/ni60.htm

just below the title click, see full brochure.

thanks again to all you experts.

 

[TVP]

My first thought is similar to alexit-- a martensitic stainless steel like Type 420 or 440C. Both can be quenched and tempered to 50+ HRC. You can obtain more information from the following links:

http://www.alleghenyludlum.com/ludlum/pages/products/xq/asp/P.81/qx/product.html

http://www.alleghenyludlum.com/ludlum/pages/products/xq/asp/P.84/qx/product.html

Click on Technical Data Bluesheet for both of these links

http://www.timken.com/products/spec..._info/PDF_Files/vac_arc/Bearing/440 N-DUR.pdf

http://www.cartech.com/

 

[haze10]

The 440 looks interesting. But what about galling. Would you make both the bushing and the sleeve from the same material, and would you harden them to the same rockwell, or to different rockwells. How much dimension change would you expect after heat treating. What finsh profile would you recommend.

 

[alexit]

440 is typically air-quenched, if you can live with 0.03mm distortion during heat treatment this would not require secondary operation.

Galling will always be problem if both are same material and operating poor lubrication. People who say to keep hardness very different to prevent galling are having different experience than I, we always get galling problem no matter what.

Other idea is what you say to 8620, carbonitride or such and secondary finish. We found for corrosion: get good core hardness (~Rc30), finish operation (we like hard turn in this range), then gas nitride (do not machine white layer after this), corrosion resistance and wear is very good, even better than 440. Problem for us was the distortion from last nitride. Maybe this work for you?

Is sleeve or bushing more problem? Do one in stainless and other in 8620 like written, solve galling and give trial for both materials...

 

[EdStainless]

I like the thought of using Nitronic (or any high Mn, high N stainless, like 201LN) for one member and 440C for the other.

With the austenitic stainless grades that show some wear and gaulling resistance, the reson is that they surface work harden a lot in service. When we cold draw tubing in those grades we can get over 150ksi UTS and mid 30's Rc without any trouble.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[haze10]

What about mixing the SS. Say use 440C for the bushing and 420HC or 416 for the sleeve to reduce galling. The bigger problem has been the stationary bushings. But both the sleeve and bushing show signs of wear and corrosion, just a bit more on the bushing. Then is when using the 4140 Rc50.
The stainless idea would definately solve the corrision problem. Using the 8620 and carburizing, finish cutting, and then nitriding it getting to expensive. Plus trying both doesn't prove that when self mated it would work (for the 440C). This is not a high load condition, I may get away with the SS not galling if I pick the right combination. 440C and Nitronic 60 show no galling to 50 ksi, but I am skeptical of the softness of the nitronic 60.

 

[haze10]

The other idea is to go with carbon steels, 4140 or 8620, carburize or heat treat, finish grind, and then electroplate hard chrome to 2/10" of .001" I could also do this to the 440C if that would stop the galling.

 

[haze10]

Here is a site that compares galling resistance of SS.

http://www.gocarlson.com/AlloyPDF/Nitronic60.pdf

 

[NickE]

Maybe one of the really high chrome tool steels?
(just a brainstorm thought, D7 chemistry says 11.5%-13.5%Wt-Cr -- almost stainless)

 

[TVP]

NickE,

The Cr is all tied up in carbide form. Not stainless at all.

 

[haze10]

Guys, I'm still lost.
440C, 416, Nitronic 60.
440 and 416 have a higher resistance to gall than 440c self mated. 416 and 316 is even higher. But the 316 may be too soft. If I went with 316 I could as easily go with the N60.

Slow rpms, moderate load, abrasive, run dry, simple and reasonably priced.

If I can't feel reasonably comfortable on a decision I have to revert back to 4140, and that was not really the best choice.

 

[rnd2]

haze10 Why not consider attaching a zinc anode so that it is in electrical contact with the universal joint?
That should alleviate galvanic corrosion which is probably occurring between the submerged part of the 304 auger and the 4140 universal joint, and at least let you select mechanically durable bushings and pins.

 

[NickE]

TVP- thats what I always thought, I had however heard some admittedly anecdotal reference to using the D-type tool steels in applications requiring light corrosion resistance. (Though that may have been in relation to A-type.)

 

[haze10]

Tool steel isn't the answer either as its too difficult to machine conventionally.

The easiest is N60 versus N60, straight machining, no grinding, no plating.

Next is N60 bushings versus 440C Rc60 sleeves. I have to heat treat and grind the sleeve, but the bushing is still just lathe turned, no plating.

Next is 440C Rc62 sleeve, versus 416 Rc53 bushing with 2/10" flash chrome. both pieces have to be machined, heat treated, ground, and the bushings chromed.

Next is 4140 or 8620 through hardened to Rc55, machined, heat treated, ground, flash chromed.

As you can see the N60 v N60 would be the simplest and cheapest way to go, but there is no telling how the relatively soft N60 would hold up to the grit, and to the physical loading. But I do believe the loading is relatively light as there is not great resistance in turning the auger in the slurry.

Help!

 

[alexit]

Ha, my last post never made it...

For to address galling issue, try 8620 turned bushing and gas nitride (do not machine white layer), corrosion resistance can be better than 440 and wear is very good as surface hardness Rc70. Do sleeve as 440C turned and then heat treated to Rc55. Geometric distortion of both 8620 and 440C should be less than 0.03mm from heat treat is this problem?

 

[haze10]

I would have to grind after heat treating. So the nitride coating would be removed.

 

[NickE]

how about Kohlsterizing some 300 series? would that prevent galling?