Silicon-Carbide Crucible for Iron

Discussion in 'Foundry tools and flasks' started by Melterskelter, Feb 13, 2022.

  1. Melterskelter

    Melterskelter Gold Banner Member

    At the suggestion of a mentor at a commercial foundry I decided to buy a Si-C crucible and give it a try. I have been using Super Salamander clay graphite to date and they have given generally goos service. But the mentor said the Si-C should do well and the sales guy at PMC said the Si-C should work---so, let's see. It might be more heat conductive and therefore give me a shorter melt time.

    Problems with Si-C for me are sizes available. So far, the largest I've found is about like an A20. I would like to have a larger one at some point if this one works out well from a durability and conductivity standpoint. The Si-C crucible I bout is about 1/2 the price of a comparable Super Salamander.

    The other difference in the Si-C crucible that impacts my use besides its smaller size is the is difference in shape. The super Salamanders taper, but the Si-C crucible has vertical sides which require a different strategy for shank or trolley design.

    Crucible1.JPG

    I've just gotten a good start on the trolley hoop that will lift the crucible. Because of the straight sides, I had to add "legs" to the hoop.
    Crucible2.JPG

    More later...

    Denis
     
    Last edited: Feb 13, 2022
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Interesting. I always thought SiC was not recommended for Ferrous duty, at least that's what Morgan's literature says about their SiC crucibles. I understood it to be because of SiC's solubility in iron, and while the same is true for carbon (ala clay graphite), it supposedly occurs much more rapidly with SiC. Should be interesting to see how they hold up. If they're economical, seems worth a try. Morgan's SiC crucibles were more expensive than their clay graphite crucibles last I checked.

    Best,
    Kelly
     
  3. Also the temperature rating is lower by about 200 deg C: 1400 silicon carbide vs 1600 for Salamander Super crucibles. I have personally seen a well used, thin A20 silicon carbide go soft in that the rim could be pushed inwards at molten bronze temperatures. I'd been told by a Morgan rep that clay graphite crucibles have silicon carbide in them and silicon carbide crucibles have clay and graphite in them, it's just the ratios are different between the types.

    That said I've heard of a guy using silicon carbide crucibles for iron use, I just don't know how long they lasted. For aluminium and bronze they are more durable than clay graphite and worth the extra cost.



    temp-overview-web.png
     
    Last edited: Feb 13, 2022
  4. Melterskelter

    Melterskelter Gold Banner Member

    I am aware of the fact that the SI-C is said by some not to be OK for iron. But then the commercial foundry guy in the business for 30 yrs scoffed at the idea that clay graphite was needed. And the PMC guy said the Si-C work ok. So, I just think I need to find out. From what I read there are several formulations of Si-C crucibles. Getting precise information on the formulation that various sellers carry seems to be nearly impossible. I will tell you more in a few days.

    Denis
     
  5. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    More durable or just more conductive and higher performing in that regard?

    Best,
    Kelly
     
  6. Zapins

    Zapins Gold

    Subbed. Test it! I need to cast iron soon for a project. I wanna make a new fire pit grate.
     

  7. Without directly testing it myself, I was under the impression that silicon carbide is more chemically resistant to fluxes. I'd theorized that if the SiC content is high enough then the other materials would get eroded and leave a layer of SiC in contact with the melt to resist fluxes and corrosive metal oxides.

    Edit: according to this website, pure solid silicon carbide crucibles have excellent chemical resistance at high temperatures and resistance to thermal shock.....Anyone want to guess what an A25 in solid silicon carbide would cost?:eek:.
    https://www.refractory-ceramic.com/...h-temperature-resistance-silicon-carbide.html
     
    Last edited: Feb 13, 2022
  8. Tobho Mott

    Tobho Mott Gold Banner Member

    Tim Smelko told me several years ago that the Silicon carbide crucibles they carried at his foundry supply business were good for iron service. Always sat a little funny with me due to the aforementioned common hobbyist wisdom about needing clay graphite for iron, and I haven't tried it to see for myself... Maybe there is a wide range of quality in Si-C crucibles and not all are good for iron? I vaguely recall an old alloyavenue thread where someone said they ordered a silicon carbide crucible that appeared to be just a cheap clay graphite that only had the SiC in a thin glaze layer which began chipping off early on. Quite sure mine is the real deal though. It is a lot lumpier looking than the blue one pictured above. The 2 #70 SiC's I got from Bill J looks very similar to my #12 only bigger, and quite different than the smooth sided graphite crucibles I've seen. But that makes sense as those 2 almost certainly came from Smelko's too. They cost more than slamander supers. Looking forward to seeing how yours holds up, Denis!

    Jeff
     
  9. Melterskelter

    Melterskelter Gold Banner Member

    Here is a bit more progress and these photos may make it clearer as to how the legs on the hoop which attaches to my trolley will work.

    The hoop connects to the trolley with a roll pin so I can quickly change out the hoop for the Si-C crucible and insert the Salamander hoop.
    Crucible Hoop2.JPG

    Here is a detail of the necked-down 1" stem welded to the hoop and showing the pin/hole.
    Crucible hoop1.JPG

    I have to have a support for the crucible that allows the legs to clear the base. That positioning of the hoop accomodates my lifting tongs. I will make a base for the support that will be screwed to it to provide a solid weighted base.


    Crucible Hoop3.JPG

    From above.
    Crucible Hoop4.JPG

    It is unfortunate that it takes all this futzing around to answer the simple suitability-for-use question. But, I could not come up with a simpler solution for my setup.

    Denis
     
    Last edited: Feb 15, 2022
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  10. Billy Elmore

    Billy Elmore Silver

    I had the same issue with my new crucible...barely tapered on the side. Keep us posted on your results!
     
  11. Melterskelter

    Melterskelter Gold Banner Member

    Good to see you, Billy.

    More futzing---this time on the hoist tongs. They did grip the crucible as they were. But, I just did not like the the fact that they seemed to be at their limit of grip and really did not want to find out what happens if a full crucible of metal slips out of the tongs. So, the simplest thing I could think of was to make and add "fingers" to the hoist grips. I really like how they lift the crucible by putting pressure right on the turn of the "bilge" rather than gripping the vertical sides of the crucible. This feels very solid and positive.

    The 3/16" x 1" fingers are secured by 10-24 SHCS's to the tong grips.
    Crucible Hoist Mod2.JPG

    Close up of secured finger. And, yes, I number stamped them so they should be easily sorted back into place after they have been removed. The existing guide bars are the triangular shapes on the lower outside of the tong. They help guide the tong on when used with my current Salamander A20 and A25 crucibles.
    Crucible Hoist Mod1.JPG


    Finger detail.
    Crucible Hoist Mod3.JPG

    Side view of finger. The finger ends were generously ground with a radius both directions to ease any impacts with the crucible.
    Crucible Hoist Mod4.JPG

    Should be able to melt metal soon. Time and weather may allow tomorrow but it will be tight.

    Denis
     
    Last edited: Feb 15, 2022
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  12. Zapins

    Zapins Gold

    Lot of nice steel work there. I always inwardly groan when I have to make a new set of tongs/crucibles pourers.
     
  13. Melterskelter

    Melterskelter Gold Banner Member

    Thank you!

    I groan too. Seems like it takes sooo much time to get just a little done.

    I did check to make sure that everything plays nicely together and it does. I did finish the platform for the crucible by adding little ears to cednter the crucible and by adding a tab that will allow screwing the platform to its support so that it is very solid and not at risk of tipping or sliding around.

    Not my prettuiest welding on this thin guage steel, but it is done.
    Crucible Hoist Mod6.JPG

    One other bit of information is that I checked to compare the volume of my SUpewr Salamander A20 vs the no-brand A14 Si-C crucible. THe Super Salamander held 14 pounds 7 oz of dry sand and the Si-C held 12 pounds 7oz. Here I poured the sand from the full SI-C into the A20. Presently the A14 is the largest Si-C crucible carried by PMC Jewelry Supplies. Crucible Hoist Mod5.JPG

    I got one mold rammed up this morning and probably will not put fire to the crucible until tomorrow at thsi rate.

    Denis
     
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  14. Melterskelter

    Melterskelter Gold Banner Member

    Well, I melted 45 pounds of iron in it today. The melt seemed to go fairly fast but not dramatically different than the Super Salamander melts of a similar size. The crucible seemed to suffer no ill effects. This type of crucible seems to ooze little or no glaze-like material as opposed to the Salamander which definitely produces some glaze each melt.

    The crucible seemed perfectly solid and hard as I pulled it from the furnace. It did have a more uniform glowing appearance with no geographic lines showing as it glowed. The Salamanders seem to look like they are made up of a mosaic of pieces---I do not think this is truly representative of their actual structure, but that is how they look when glowing bright red.

    In summary, the crucible performed just fine. Time will tell if it is obviously more or less durable than the Morgan SS crucibles. If they are comparable in durability, they are much less expensive per melt as they are about 1/2 the price of comparable SS.

    Denis
     
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  15. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    That's significant cost delta.

    Did the X-shaped sheet metal crucible stand hold up OK? I was trying to decide if it would act like a fin and stay cool enough to stay strong for the brief time it served, or heat up fast and be at risk of fail under the heat and weight of the full crucible.

    Best,
    Kelly
     
  16. Melterskelter

    Melterskelter Gold Banner Member

    It did fine. I suppose there is a possibility of enough heat conducted (though actual contact points are small) or, more likely, radiated heat to cause trouble. But, I saw no evidence through post-pour discoloration or as I lifted the crucible of any glow to suggest that it heated remarkably. In its favor, the fins themselves with conduct heat to the air and radiate heat too. So, I think it is OK. I'll keep an eye on it. It would be easy enough to make a heavier one. I'll think about it.

    Denis
     
  17. Zapins

    Zapins Gold

    Where'd you buy the crucible from?

    Have you tried using ceramic shell for iron pours? Do you think ceramic shell can be used for iron?
     
  18. Tobho Mott

    Tobho Mott Gold Banner Member

  19. Zapins

    Zapins Gold

    Thanks! I did miss it.

    There is an iron foundry close by that I can probably bring shells to after baking and prepping them.

    I wonder if I can have them pour into a non preheated shell or if I'd need them to heat it up prior to casting?

    I've always preheated my shells for bronze but I'm not sure if thats even really necessary
     
  20. Melterskelter

    Melterskelter Gold Banner Member

    OK, Kelly, your question started to make me wonder about the lightweight stand I had made. In addition, I noted when I used it that I wished it was about 2 inches taller. So, I found some 1/8 x 3" material and tacked up a taller and stouter metal stand. It then screws to a wood base that is weighted when in use. I may later convert the wood portion to steel if needed.

    The metal portion is 8" tall and the wood base is 9. So, the crucible will be 17" off the ground. That makes for an easy transfer from my hoisting gear and allows a nice level lift for my trolley. I'll give it another whirl soon. The aluminum foil is an effective radiant heat reflector that prevents charring of the wood.

    Cucible Hoist Stand.JPG

    If I put 30 pounds or so of returns into the base, it feels nice and solid.

    As an aside, I think most folks would agree that welding 1/8" bar is a lot more pleasant than 22 gage.

    Denis
     
    Last edited: Feb 17, 2022

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