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What is the best material for cryo ball mill cup assembly?

  1. May 23, 2015 #1
    I am working with a set of alloys that must first be ground up in a ball mill at cryogenic temperatures. It is important that no contamination makes it into the material and careful steps are taken to ensure this end.

    Unfortunately when we ran a set of samples through our RBS lab and then again through PIXE we saw that we had Fe and Cr contamination. At first it was thought that it came from the cutting tools used to prepare the samples so we ran another sample that was cut using EDM. The results were the same. The only other contact the material has with a stainless steel is from the ball milling step.

    We have been using a 440C for the cup and balls but it would seem a 316L may be a better fit for this application. I know 440C does not typically do well at cryo-temps but that is what other labs have been using so we didn't expect for there to be problems of this type.

    Some things to consider for the new cup and ball material are machinability, cost, availability, cryogenic properties, vibration and fatigue resistance, and ability to be sealed (cups are filled in an inert Ar atmosphere). Another possibility is a heat treatment of the current 440C materials, however I am unsure what the best approach would be in that regard.
     
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  3. May 23, 2015 #2

    Q_Goest

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    Hi mesa. Where do you think the iron and chromium are coming from, the balls or the barrel? Do you know how much is coming off and getting into your material? Is there an acceptable limit for the amount of contamination?
     
  4. May 25, 2015 #3
    Both the balls and the cup are made of 440C stainless although it would seem likely that the primary source of contamination is from the balls? The exact level of contamination is still to be determined, I only had the lab for a couple hours and our proton beam was misbehaving and I had data to collect on samples for other projects as well.

    Talking with Barry (the tech who runs the lab) he suspects we are on the order of about a 1% contamination when looking at the rough data, a serious problem. I am going back in tomorrow to get more exact values.
     
  5. May 25, 2015 #4

    Q_Goest

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    By "cup" do you mean the thing the balls roll around in? Sorry, I'm not very familiar with this type of grinding machine. If the balls are banging against each other, it makes sense that they would wear most I guess. I would think you could measure the weight of the balls before and after to determine metal loss.

    I'm more familiar with the cryogenic milling machines that have a stator and a rotor and the two parts never come into contact. There's a very fine gap between the two and whatever is being milled is broken up and then the small bits are being thrown back and forth like ping pong balls inside the mill. They can impact each other or they'll impact the stator or rotor. The powder produced is very fine and of uniform consistency. Materials for those parts are typically Stellite, CREUSABRO or wear resistent white iron such as NiHard. I've also seen some precipitation hardened materials used but they are vastly inferior and tend to wear quickly. The amount of contamination is many orders of magnitude less than the 1% you've suggested. It's so small, I don't know it could even be measured, but I suspect that's because they don't have metal parts (balls) impacting each other.
     
  6. May 26, 2015 #5
    That is correct, the cup holds the balls and the assembly is located inside of the cryo-chamber. The weight of the balls could certainly be measured before and after milling operations and is an excellent suggestion.



    It is this material that we start with, 325 mesh and then we pulverize it from there.

    Precipitation hardened materials can give neat properties like stress corrosion resistance but usually come with sacrifices in other characteristics, not much surprise they wear quickly.

    I believe the issue we are having is from the high carbon content at cryo-temps. This type of interstitial impurity is great for room temperature applications when it comes to wear but I believe is stressing the lattice to too high a degree at these extreme temperatures causing micro spallation on impact.

    This is why I believe a 316L would be better suited for this application, it has a low carbon content and works well at room temperature with a concurrent increase in strength at cryo-temps. Any thoughts on this?
     
  7. May 27, 2015 #6

    Q_Goest

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    The increase in strength of austenetic stainless at cryogenic temperature is excellent. Of course, you could also improve the strength through cold working. That said, when it's used on the types of mills I'm familiar with (the ones without balls that don't have metal/metal impact) the 300 series material does not do well even in cryogenic conditions. It's no better in fact than the precipitation hardened stainless. At -200 F, the other materials I've suggested above work very well, but again, there's no metal/metal impact.

    Although I can appreciate your concern about carbon content and micro spallation on impact, the austenetic stainless steels have a tendency to gall or transfer material between them when they rub. That's true at cryogenic temperature as well. I"ve had lots of experience destroying stainless steel parts due to SS/SS rubbing at very low temperature. It galls just as bad at low temperature as it does at ambient. For metal/metal contact, I might try one of the Nitronic grades of stainless.

    I wonder if reducing the amount of metal/metal impact would help reduce contamination. Reducing the number of balls might help.

    What strikes me is simply that to avoid contamination, a ball mill is probably not a good choice of mill.
     
  8. May 27, 2015 #7
    That would certainly be a strike against this 'replacement' material. I know the manufacturers make 316 cup assemblies, I am surprised issues with galling do not improve much at cryogenic temperatures.

    It certainly could. Trying to find a balance between the amount of contact time in the mill to produce the desired result is certainly worth more exploration. Another excellent suggestion.

    The idea is to maximize the Hall-Petch effect by artificially creating nano crystalline grains that are then subjected to ECAE (equal channel angular extrusion). Other labs are also using this process and the results (excluding this contamination) are astounding. It seems worth trying to fix this problem.

    I am still looking into Stellite, CREUSABRO and NiHard. Another option could be to switch to ceramic balls, these are also used extensively with these mills although there is essentially no data on their use at cryogenics temperatures.
     
  9. May 27, 2015 #8

    Q_Goest

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    I don't have any experience with ceramics, so I'd be interested in what you find out.

    I see Nitronic balls are readily available:
    https://www.google.com/?gws_rd=ssl#q=nitronic+balls

    Nitronic 60 in particular has terrific galling and wear resistance which I think is essentially what you're looking for; not to mention excellent properties at cryogenic temperature (ie: remains ductile).
    http://www.nickelinstitute.org/~/Me...lingCharacteristicsofStainlessSteel_9006_.pdf
    https://www.hpalloy.com/alloys/descriptions/NITRONIC60.aspx
    https://www.google.com/?gws_rd=ssl#q=nitronic+galling+wear+
     
  10. May 28, 2015 #9
    It seems from this information a 440C cup with your suggestion of Nitronic 60 balls is the way to go. Many suppliers stock the Nitronic 50 SS balls but so far I have turned up none who have them available in N60. There is one company (US Ball) found so far who can manufacture these in the N60 grade, although I am still waiting on a quote. If too high they can be machined on campus from round stock (substantially better availability than spheres).

    Ceramics are worth exploring further although the information found so far does not bode well for this application. Good thing there are so many to choose from.

    Thank you for your insights on this project Q_Goest, if I come across something interesting I'll send it your way.
     
  11. May 28, 2015 #10

    Q_Goest

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    I'd appreciate that, thanks. Feel free to shoot me a PM on what you come up with.
     
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