Silicon-Carbide Crucible for Iron

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.



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.


More later...

Denis

Also the temperature rating is lower by about 200 deg C: silicon carbide vs 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.

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

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? .
https://www.refractory-ceramic.com/...h-temperature-resistance-silicon-carbide.html

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

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.


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


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.




From above.


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

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.


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.



Finger detail.


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


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

Denis

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.


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.

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

Denis

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

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

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

When it comes to choosing the right crucible for your melting needs, the choice between Silicon Carbide Graphite and clay graphite materials can be a game-changer. Both types of crucibles offer unique advantages, but they excel in different applications. Understanding these differences could be the key to optimizing your workflow, reducing downtime, and improving your melting efficiency.

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Graphite silicon carbide crucibles are engineered for extreme durability and excellent thermal conductivity. They thrive in high-temperature environments, making them ideal for melting non-ferrous metals like aluminum, copper, and brass. With a combination of graphite's lubricating properties and silicon carbide's strength, these crucibles offer superior resistance to thermal shock, abrasion, and chemical erosion.

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On the other hand, clay graphite crucibles are your go-to for lower-temperature applications, particularly for casting metals like gold and silver. Their composition includes natural clay, making them more affordable, but slightly less resistant to high temperatures compared to their silicon carbide counterparts. That being said, clay graphite crucibles are still highly reliable, especially for smaller scale operations or when cost-efficiency is a priority.