8. I like to keep the exterior of the furnace (I use a stainless sheet metal shell) cool to the touch.
It is as much about preventing contact burns as it is about efficiency.
A furnace that is cool to the touch on the exterior is not wasting energy.
The idea is to keep the heat inside the furnace, not radiate it off the exterior wall.
9. There are two schools of thought on preheating the scrap iron in the exhaust of a furnace.
Initially I preheated my scrap by setting the iron on the lid and letting it protrude into the exhaust opening.
A few years ago, I ran across the MIFCO furnace manual, and it mentioned never preheating iron scrap in the exhaust stream, due to the excessive oxidation it causes.
So I no longer put scrap iron on the lid of the furnace to preheat it.
I hold each piece of scrap iron (using tongs) in the exhaust stream for perhaps 30 seconds (depending on the size of the scrap piece), and then drop it into the crucible.
The added scrap must be pushed under the surface of the molten pool of iron in the crucible, to break up the slag, else the slag will get very dense and hard, and will become unmanageable.
MIFCO also mentions that scrap with any moisture on it will cause an explosion when it drops into the crucible.
All scrap iron and steel has moisture on its surface, but you cannot see it.
A dry steel ingot mold is not dry, and if you pour hot iron into an ingot mold without first heating it to perhaps 500 F, it will eject the iron right back into your face (I have 3rd degree burn scars on my hands from doing this, where molten iron splashed back onto my leather jacket, and then rolled down inside my gloves. You can't get gloves off fast enough when you have molten iron in them.
https://mifco.com/wp-content/uploads/2019/12/B-C-furnace-manual-revised-12-19.pdf
There is a section in the MIFCO manual listed above titled "Melting Gray Cast Iron", and several things mentioned in there I do not agree with, and I have proven this information to be false for me, perhaps because I am using an oil burner, not the MIFCO natural gas burner.
MIFCO says cast iron engine blocks and other iron machinery should not be used, but I have used many types of iron machinery scrap including machinery, and it makes excellent defect-free castings.
I have also used pure Class 40 gray iron, and it worked very well too, but no better than scrap machinery iron.
What I have heard is to not use any iron that has phosphorus in it, such as thin castings like bathtubs, radiators, etc. I think due to mechanical weakness of the castings.
Window sash weights in the US are junk metal and should not be used.
If I can break a piece of gray cast iron with a sledge hammer and get a nice clean consistent rough gray surface, I use it for scrap.
MIFCO also says that either clay graphite or silicon carbide crucibles can be used with iron, but the Morgan information sheets only list the clay-graphite "Salamander-Super" for ferrous metal, rated for iron temperatures.
The Morgan silicon carbide crucibles are not iron rated, and have a much lower operating temperature that would not necessarily be usable at iron temperatures.
And I don't add carbon to my melt, and have never had a problem with not adding that.
Don't confuse the MIFCO burner diagrams for an oil burner.
Natural gas and propane burners are not the same as oil burners (similar, but not the same).
I don't use drains in the bottom of my furnace.
I have never broken a crucible inside my furnace, but if I did, I would just turn the furnace on its side and operate it with the burner until the metal melted and ran out.
If you use good quality crucibles and don't use them beyond their life, then you should not see a broken crucible.
People break crucibles when they jam metal into them cold, and the metal expands and cracks the crucible, or they force too much scrap into a crucible, and crack it.
10. If your furnace and burner are operating correctly, then a 5/16" diameter steel rod used to stir iron when it is at pour temperature (perhaps 2,500-2,600 F, or maybe a little higher; I can't measure it since it melts the end off of a submersion pyrometer) will melt its submersed end off after about 30 seconds of stiring.
Anything you insert into the molten metal must be preheated to at least 500 F or more, else you could eject the metal from the crucible.
The only way I am aware of to handle the slag is to skim it off with a skimmer that has a heat shield on it.
Some have mentioned using vermiculite to get the slag to gather into a ball, but I don't use that, since if you did not get all the vermiculite out of the crucible, then that can cause inclusion defects in the casting.
Everyone I know who does iron just skims the slag at the last moment before the pour.
I skim slag, add ferrosilicon, stir, quickly skim any remaining slag, and then immediately pour.
I leave the crucible in the furnace during the slagging and adding ferrosilicon, since the metal will drop below pour temperature extremely rapidly after the crucible is removed from the furnace.
I am told that sooting the end of the skimmer will help prevent slag buildup on the end of the skimmer.
I just strike my skimmer on a piece of metal to clear it after each scoop of slag, and between melts I break the slag off the skimmer with a hammer.
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