The Shop > Metal Stuff
Oil fired crucible furnace
MetalCaster:
Ironman has 20 years experience with pouring iron using both crucibles and cupolets (if I understood him correctly), and more importantly a lot of experience with making green sand.
Both are really critical since no matter how good you are at melting and pouring iron, if your mould fails, then you have nothing.
Edit:
Someone corrected me on the word "cupola", and said that the small units that are top-fed are "cupolets", not "cupolas".
vtsteam:
I made several changes and cast iron today.
First change was the enlarged exhaust hole in the furnace lid.
Second change was reducing the size of the sprue from 1.25" to 1" in order to keep it choked with metal.
Third change was to add some of my 140 mesh sand to the 60 mesh greensand I have used recently. About 15% was added.
Fourth change was to ram somewhat lighter
Fifth change was to use no sodium carbonate as flux
Sixth change was to preheat the new crucible to red heat at moderate burner throttle before adding metal.
Seventh change was to alter the pattern to include a semicircular groove to form a greensand core. This piece willl later be bored out, and the cored channel will make this easier. It also reduces the the mass of metal poured..
First Try:
I'd like to say everything worked out, but unfortunately I poured short. The much larger amount of slag of a very pasty consistency fooled me into thinking there was more metal in the crucible than there was -- and that it was hotter than it was when i dipped in a poker and it came out clean. Also the gates were a little shallow, so basically I ended up with a nice bar ingot of about 3/4" thick, but not a part.
Second Try:
I could tell the pour was short, so I didn't wait very long to unmold. I decided to use the residual heat in the furnace to attempt another melt. So I quickly broke up more scrap, and fired up the furnace. While it was heating I rammed up a new mold. I decided that I'd try a riser in this one as an additional change, but make sure the crucible was full of metal, not just 1/3 slag. I figured my smaller sprue, and the new core groove would give me enough additional metal in a full crucible to afford an 1.25" riser. Also I'd put in slightly deeper gates, and be absolutely sure the metal under the slag was fluid.
I pulled the crucible out when I thought it was ready, and scraped the very thick sluggish slag out of the way where I would pour -- I couldn't scoop it all off easily, as I had when I used Sodium Carbonate -- that stuff really liquifies the slag.
I poured and watched the riser fill to the brim, and had enough metal left over to pour an ingot as well. Good!
In a couple minutes the riser started to shrink down -- which I took as a good sign! Both the sprue and riser dropped quite a bit. Here's a pic:
vtsteam:
The part came out well! The surface wasn't smooth I still don't have seacoal for a facing sand, but the added 140 mesh sand seems to have made for a finer surface grain than before. The lighter ramming doesn't seem to have caused any irregularity or coarse patterns, and the corners look good.
Bottom of the part:
vtsteam:
The top surface shows a big improvement over the last pour of this part. There are only 3 small shallow blow holes -- these will be easily removed with the machining allowance, and when the part is bored -- unless there are some hidden holes. I have a feeling that's all there are.
I believe the reduction in ramming pressure and smaller sprue (which I kept choked while pouring) helped trap less gas, and relieve it where present, creating fewer and smaller pockets. If I go to a shallower cope, it's possible venting and permeability will improve even further.
The greatest improvement was the elimination of the shrink depression. This was probably due to the addition of the riser. The greensand core that formed the channel might have helped as well by reducing the thickness in the center of the casting. Less thickess, less retained heat at the center. Since the channel also makes machining easier, it was definitely worthwhile adding it to the pattern.
I'm very happy with the results and feel I'm on the right track. Plus I have a good part!
Here's the top (dark areas are dampness from a rinse):
vtsteam:
And as a follow-up for furnace operation, the furnace consumed 2.5 gallons of diesel fuel total over the two melting sessions during a 4 hour period.
Actual time to melt for the first pour was 1 hour 5 minutes from lighting, and after a 45 minute cool down, the furnace ran for an additional hour to achieve the second melt -- which had more metal.
The crucible looked in good condition, though I couldn't remove a fair amount of pasty slag this time because without flux it was tough stuff to try to scrape out. I put the crucible back in the hot furnace to cool slowly, so I'll get a better look at it tomorrow. I think it is retaining wall thickness better than the first one had. It felt solid, not leathery during the pours.
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