What are the basics of melting iron?

Discussion in 'General foundry chat' started by Mark's castings, Nov 21, 2018.

  1. So what's the basics for successfully melting iron?. Apart from the obvious heating energy requirements do you just throw some scrap iron in the crucible, heat it up until it melts, skim the slag and pour?. Will things run better with some flux, I've heard that limestone is used?. What about adding carbon during the melt?.

    I'd appreciate any descriptions of how iron foundry users go about their melts.
  2. PatJ

    PatJ Silver

    (This is how I melt iron. Other's methods may vary.)
    Edit: I have to credit ironsides for giving me quite a bit of this information. Melterskelter also contributed significantly to this information.

    Assuming the burner is running hot (close to optimal combustion) and producing the heat required to melt iron (for a 13" dia x 14" tall furnace, your fuel rate should be in the 3 gal/hr range), I do the following:

    1. Fill the crucible with iron up to the top before you start the burner, but not over the top. Smaller pieces of iron seem to melt faster and produce a pool of metal quicker than larger pieces.
    2. Once a pool of metal has formed, hold each additional piece of scrap over the exhaust stream with tongs, and heat the metal to drive off any moisture, and then drop the piece into the pool.
    3. To prevent oxidation, immediately push each added scrap piece completely under the surface of the molten pool. This also breaks up the slag and keep it from forming a hard cap on top the pool.
    4. Let each piece of scrap melt before adding the next piece. If you have small pieces of scrap you can add several at once. Feel around in the melt with a steel rod to make sure everything is completely melted before adding additional scrap.
    5. When the crucible contains the desired amount of molten metal, skim off the slag, sometimes first with the lid closed and the burner running, and then with the lid open and the burner off; but I keep the crucible in the furnace.
    I know of one guy who skims and adds ferrosilicon with the crucible in the furnace and the burner running, but that can get a bit hot to work with (your choice).
    6. Some people add dry powdered calcium carbonate (limestone) to help skim the slag. I tried it once and it made the slag a bit gummy.
    Limestone is a suppose to be a flux, but I have not tried adding it into the crucible prior to a melt, and I have not seen anyone else add it either for a flux or use it as a slag coagulant.
    7. Vermiculite is suppose to work better at slag coagulation, and supposedly you can insert a rod and roll the slag onto the rod, where it will adhere, similar to a glass blowing technique to get glass on the rod. Vermiculite is available in the garden section of hardware stores. I bought some and then misplaced it, so have not used it, but I heard that it works.
    8. Once the slag is removed, measure out approzimately 0.35 oz/lb of iron of ferrosilicon (75%) (this amount varies among people who cast iron). It must be kept dry. Dump the ferrosilicon on top the melt with the crucible still in the furnace, and stir it in with a graphite rod until it is dissolved. The melt will become noticeably more fluid after the ferrosilicon is added. Skim one final time, and then lift the crucible out of the furnace and pour.
    You must pour within about 4 minutes or less of adding the ferrosilicon, else the effects will be lost. You should pour as soon as possible anyway (within perhaps 60 seconds of removing the crucible from the furnace) because the melt temperature drops very quickly once the crucible is out of the furnace.

    I don't use a pyrometer with iron since those tend to be expensive.
    If the iron begins to roil/bubble after the slag is removed, the Navy Foundry Manual states that this is about 2,800 F.
    I generally like to err on the side of the pour being a little on the hot side, rather than it being too cool and not completely filling the mold.

    Everything that goes into the molten meltal (skimmers, rods, scrap, graphite rod, etc.) needs to be preheated in the flue stream to drive off moisture.

    I poured some hot iron into a steel ingot mold that I had preheated with propane, and apparently I did not get the molds hot enough, and the metal popped/exploded when it contacted the mold, and blew molten iron back on me, with some of it running down my gloves and causing 3rd degree burns on my hands.

    You need a heat shield on your pouring shank, else your glove will overheat.
    You need tools with long handles, else your glove will overheat.

    You need full face shield with hardhat, with a shaded lens to stop the the IR/UV from burning the eyes.
    I have found that a #5 shade is not enough, and so wear #5 glasses under a #5 face shield.
    The UV/IR and radiated heat is intense when the furnace is open.
    You need full leathers (or the silver type clothing), and leather boots with the pants worn over the boots to shield off falling/splattering iron.

    Iron pours are incredibly hot, so take the necessary precautions to do it safely.
    To achieve iron temperatures, your entire system must be in excellent shape, and must be designed to operate at iron-rated temperatures, ie: the furnace refractory, the crucible, the furnace insulation, etc.
    As a minimum, materials used for iron melting should be rated for 3,000 F or very close to that.

    I use Morgan "Salamander-Super" clay graphite crucibles exclusively, and they are rated in 2,912 F, and also ferrous-metal rated. Not all crucibles are ferrous-metal rated, and not all crucibles are rated for 2,912 F.
    Morgan recommends a preheating schedule for heating the crucible, to temper it before first use.
    Morgan also recommends gripping the crucible on the lower half only when lifting with crucible tongs.

    I have never added carbon to a melt, and although that is mentioned in one of the commercial furnace instruction manuals, I have never seen anyone anywhere use carbon in a melt.

    The melting point of gray iron is somewhere in the range of about 2,100 -2,200 F, depending on the Class of gray iron. I have melted Class 40 gray iron without problems, and it has a higher melting point than the lower Classes of iron.

    For thin intricate parts, the Navy Foundry Manual recommends pour temperatures up to 2,700 F.
    For thicker castings with perhaps a 2" thickness, pour temperatures can be as low as 2,350 F.
    Note that if your iron alloy has a melting point in the lower end of the range, then your pour temperatures may shift down accordingly.

    If the iron begins to roil/bubble after the slag is removed, the Navy Foundry Manual states that this is about 2,800 F.
    Since I don't use a pyrometer, then I generally like to err on the side of the melt being a little hot, rather than it being too cool and not completely filling the mold.

    For sand molds and cores, I use either a resin-based binder, or a water-based binder.
    The resin binder works exceptionally well with iron, and can be used to make cores also.

    For green sand molds, good luck, you are on your own, but as I understand it, you need a certain amount of coal dust mixed into the sand to give a good surface finish.

    I have been told that a good mold wash will help with the surface finish of iron, and I am going to try that. Dry out the mold wash (either by air drying, or light flaming with a propane torch) before pouring iron into the mold, else the iron will be ejected suddenly.

    I have been told that iron castings should remain in the mold overnight and allowed to cool as slowly as possible so that you get good grain formation and easy machinability.

    Edit 04:
    I have omitted the ferrosilicon from castings that were perhaps 1.25" thick or thicker, and they could easily be machined.
    Thin castings (perhaps 1/4" thick) without ferrosilicon had the hardness of tool steel and could not be machined other than by grinding.
    Last edited: Nov 22, 2018
    Jimmy Cogg likes this.
  3. Pat, that's a fairly comprehensive guide to help a beginner, I'll base my next try on it. For this last attempt: I cut up several 1.25Kg iron weights, they are pretty small and when grooved with a grinder and broken show a fine grey grain. I cut up five of them for 6.25Kg total and sat them around the furnace opening to preheat to red. By the time I was ready with the protective gear, they were glowing red and some were fragile and easily broke with the tongs. At this point I loaded them into the crucible and threw in a handful of peanut coking coal and shut the furnace lid. The resulting mass is shown below, it was stuck hard in the crucible but shrank enough to remove after cooling. There were several unmelted weights at the bottom of the crucible so the furnace wasn't hot enough after all my fiddling around with burn rates.

    Edit: My protective plastic full face mask saved me three or four times as random lumps of burning coal that had been spilled into the furnace were blown back out at random intervals and were deflected by the face mask without even causing any damage to the clear plastic. One lump landed on my woollen long sleeved shirt and was brushed off before it caused even serious burns to the shirt fabric.

    I think next time I'll follow your advice and start with a small pool of molten iron to add to. First thing is to get the furnace fuel system rate sorted out, I think there's liquid oil in the tuyere getting behind the refractory.

    This was a hard solid mass (when hot) sitting over a over a void containing a few unmelted weights
    iron melt fail 1.jpg

    One of the weights in the bottom of the crucible showing partial melting.
    iron melt fail 2.jpg
    Last edited: Nov 21, 2018
  4. PatJ

    PatJ Silver

    Your melt is too cold, which means the burner is not operating hot enough.
    That was the story of my life for six years, but you don't need to repeat my mistakes, and thus the reason I am posting this stuff about iron melting.
  5. That's what I suspected, I'll have to sort out the furnace fuel leak issue, I think unburnt liquid oil is getting behind the refractory and igniting at the bottom hatch area: If I make a stainless tongue to run under the nozzle and guide any liquid down hill into the chamber I think it'll fix it.
  6. Melterskelter

    Melterskelter Gold Banner Member

    18 Prism (1).JPG 18 Prism (2).JPG 18 Prism (4).JPG 18 Prism (5).JPG 18SEPrism (1).JPG View attachment 4602 View attachment 4603 A few notes based on my experience (still on the fairly steep part of the learning curve) over the last year and 75 iron melts averaging 50 pounds:

    Adding carbon to your melt might seem like a good idea and is done in industrial settings when using pig iron, steel and carbon plus silicon to make grey iron. I recently discussed this with a medium-sized commercial foundry owner who has mentored me. He says you have to get your iron to 3000 degrees to dissolve the carbon. He is running a couple of 3000 pound capacity induction furnaces and can get there with the twist of a knob on the control panel. I have never come close to 3000 with my diesel furnace as 2650 takes some time and 3000 would be a challenge.

    FeSi added to the melt is something I do routinely. Timing is crucial as the desired effects of added FeSi persist for only 5 to ten minutes after it is added. The amount to add is more difficult to determine. I found that adding 2 ounces to a fifty pound melt resulted in mildly hard iron---quite machinable but the items I pour have to be dead soft---Rc 0 to 5 (and that is the hardness I have measured in many commercial castings). With the smaller additions I was getting Rc 10 to 15. Then I went to 14 ounces per fifty and that did cure the hardness. I have been working down from there and am currently adding 6 ounces per fifty pounds of 75 % (there are various percentage FeSi inocculants sold) for a ..75% addition. I am starting to believe that the timing of the addition is the most important factor. Bear in mind that excess Si can cause imbalance in the iron resulting in excessive shrinkage and machining issues. It is unfortunate that small-scale casters usually do not have access to spectrometers as such devices could guide inocculant percentages and take out some of the voodoo.

    I have been able to use an accurate pyrometer and have found and also have been advised by two commercial casters that a good guideline for pouring temperature is to observe the pool of metal after skimming. When it no longer forms a dull thin "scum" layer and stays mirror-shiny you are at about 2650F. I have found this to be reliable in the 75 pours I have done over the past year. As you approach this temp you will see that the metal scums slowly over 10 seconds or so but does scum over. Once you are at or above 2650 the skimmed metal will be a glimmering pool and remain so.

    If you try to use a steel rod to mix your FeSi, it will quickly melt away---that is if you are up to good heat. The inexpensive graphite mixing rods sold on eBay are fairly robust and do the job well. I have used mine multiple times with no evidence of deterioration. I use a shop-mounted 1/2" diameter by
    12" long rod. Works great. I figured it would be brittle and break easily but that has not been the case.

    Adding carbon to green sand is essential to prevent burn-on and burn-in of sand on the casting. About 4% sea coal seems to work well for me with the casting being almost shiny and sand falling off easily. If you do not add carbon, you are going to have fine time removing sand from the casting surface.

    Vent you molds with a wire poker---see the video just posted by Ironsides where the foundrymen use a coat hanger wire to poke the mold many many times.

    Last edited: Nov 22, 2018
    Mark's castings and Tobho Mott like this.
  7. Ironsides

    Ironsides Silver

    Pat, you said that if the iron starts to boil/bubble it is 1540C ( 2800F ). I have my doubts about that.

    Here is an experiment you could try, put a re rod in the exhaust vent and see if it will melt. If the iron temperature is 1540C ( 2800F ) the furnace temperature will be a lot higher because the crucible is not a perfect conductor of heat so the inside furnace temperature should be 1650C ( 3000F) or more. Also re rod is a high carbon steel and will melt about 1490C ( 2710F ). So you should have 150C ( 300F ) above the melting point. I have tried to melt re rod in my exhaust vent and I have not succeeded yet. I would be very interested to see if you can do it with your new furnace.
  8. Thanks Denis for the practical advice, do you have rough fuel consumption and time taken to melt 50 pounds of iron?. I should get in touch with a nearby iron foundry that's an hour's drive away and see what they are doing too. As far as I know they use resin bound silica sand and use scrap steel as their feed stock for induction furnaces. I may be able to score some ferrosilicon but for early experiments I hope to substitute silicon carbide which gives a different iron crystal structure.

    I also have access to sand resin intended for iron foundry use that chars to a carbon rich layer: I think it allows the use of silica sand without a coating, but it couldn't hurt to spray a layer of graphite and alcohol on the mould before use.
    Last edited: Nov 22, 2018
  9. Melterskelter

    Melterskelter Gold Banner Member

    My present furnace is fairly high mass and soon to be replaced (I've gotten started on a new one) by a very very low mass furnace. I burn about 6-7 gallons to melt 50+ pounds. I know it can be done more efficiently. I burn just shy of 3 gallons per hour. So, if I run my furnace right, it takes just over 2 hours to pour 55 pounds of iron. A second 50 pound melt takes only an hour as I've already poured the heat into the furnace to bring it up to heat.

    I'd suggest the simplest approach first. That is use the resin sand that is designed for iron alone. You likely will gain little from a graphite coating as the manufacturer has spent years or decades tweaking it. A coating to improve smoothness may help and is commonly used in commercial settings. I use no coating. But, just use the basic stuff and add only if dissatisfied. There are already more than enough variables in play ;-)

    With respect to exhaust gas temp I have had a SS custom grill 1-2"above my exhaust port and it gets a cherry red but no hotter.

  10. PatJ

    PatJ Silver

    I bought some Velvacoat ZA 9078 mold wash, and a sprayer made for heavy slurries (Paasche Airbrush L Sprayer 1.32 mm with quart cup, size #2).
    I have not tried it yet, but hope that it works well with iron (it supposedly does).


    The sprayer is this type. Note that if you spray alcohol-based coatings, the spray is flamable, so do that outside away from ignition sources.

  11. PatJ

    PatJ Silver

    I have stirred in the past with a steel rod, but I don't think it was as large in diameter as a rebar.
    It was perhaps 3/8", and much to my surprise, it did melt off after 30 seconds of stirring the iron.
    I remember taking a photo of the missing end of the rod, but I am not sure where that photo is.

    When I use steel rod tools to skim and charge the furnace, the rods start to overheat and bend if I use them for very long in the exhaust stream.
    I suspect that my furnace would melt steel, but I am not sure I would actually achieve a good pour temperature.
    If I can melt the end off a steel rod off cleanly in about 30 seconds in a pool of iron, then it stands to reason that rebar would also melt easily, unless the alloy of the rebar has a much higher melting temperature than a mild steel rod.

    I will try to melt some rebar this weekend.
    The question is, if I melt rebar and pour it, what will I get?
    Will it cast in a usable form?
    Could I use it for a crankshaft?

    Here is some info about the strength of rebar at elevated temperatures:

    How rebar is made:

    This site gives a melting point of carbon steel at 2600-2800 F.

    This site says that steel melts at about 2,500 F.

    Last edited: Nov 22, 2018
  12. PatJ

    PatJ Silver

    Looking around the net, I don't think rebar melts at that high of a temperature.
    2710 F is the temperature I am finding for the melting point of pure iron.

    I am guessing that rebar melts at about 2,500 F, but check me on that.

    I believe I am getting close to an optimum burner temperature for light oil with forced air of 3,820 F, and also I use ITC100 coating on the interior of the furnace and lid, and so get more radiant heat reflected into the crucible.

    I definitely think rebar would melt in my exhaust stream.
    I will try it shortly and let you know.

    Last edited: Nov 22, 2018
  13. Peedee

    Peedee Silver

    I'm led to believe re-bar is inconsistent in its alloy content so that may not be a great measure of temp.
  14. PatJ

    PatJ Silver

    The MIFCO furnace manual mentions that Class 25 and 30 gray iron has a lower melting temperature and better fluidity than the higher classes of iron.
    In the past, I have melted and cast Class 40 iron without a problem, and it machined well even without ferrosilicon in the thicker pieces.

    The MIFCO furnace manual also mentions that steel or malleable iron scrap should not be added to the melt because they raise the melting point of the iron (which MIFCO says is hard on the crucible and furnace refractory), and have less desirable casting properties, are harder to pour, and the finished castings may be hard and difficult to machine.

    I recall ironsides video of mixing some sort of steel into the iron, and as I recall it was successful.
    I will post the video here.

  15. I don't know about melting rebar in the furnace but I was deceived by some iron in my crucible this last time: it looked like it was boiling at first, but then I realized it was oxidizing with the surface developing a blistered appearance as sparks came off the iron.

    I have a loan blower from a friend, it was made by Dawn, the Australian makers of offset vises, is iron and weighs about 30 to 40 Kg. I'll swap out the 1/2Hp three phase motor for an identical 1Hp single phase unit by the same manufacturer: GMF. It's a vintage unit and was part of the local town gas system, they were distributed through the pipe network keeping the coal gas moving through the pipes.

    http://www.dawntools.com.au/pdf/HISTORY DOCUMENTS/Dawn_Cast_ Iron_ Pressure_ Blowers_ Imperial.pdf

    Dawn 1F blower 1.jpg

    Dawn blower 1.jpg
    Last edited: Nov 23, 2018
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  16. PatJ

    PatJ Silver

    That is a nice blower.
    At one time I was using a Grizzly dust collector blower for my oil burner/furnace, but as it turns out, it provided too much air and apparently was cooling the furnace with fuel flow rates between 5 and 6 gallons per hour.
    Less is more as far as fuel and blowers when melting iron, and 3 gal/hr seems to be the sweet spot for most backyard furnaces for iron melts.
    I use a Toro leave blower running on the lowest speed, and it produces far less air on the low setting than what one may generally consider necessary for melting iron.

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