I'm having trouble getting the temperatures I need to pour iron. I'm using waste oil in a forced air burner with a preheater. In the last several attempts, the iron charge would melt, but only into a sort of slush. When pouring a small amount of liquid would come out, leaving a semi-solid mess in the crucible. Leaving it in the furnace longer doesn't help. It seems I'm getting close to temperature but just not hot enough for a clean melt. The weird thing is, I've used this furnace to melt iron consistently in the past. Some details: Furnace: 20 cm id, 23 cm inside height Crucible 1: 13 cm od, 16 cm height Crucible 2: 11 cm od, 14 cm height Lid is flat with 8 cm vent hole The tuyere enters at about the furnace floor. Plinth is 25mm. Things I've tried so far: - There seemed to be a cold spot on the furnace wall and one side of the crucible. I suspected it was taking the oil too long to vaporize, so I cut it with about 50% kerosene. The last burn was much cleaner, no evidence of a cold spot, and I could see flame starting right where the burner enters the tuyere. - I've tried two different size crucibles as listed above. - Increased the plinth from 25 mm to 50 mm - I'm mostly running a neutral to slightly rich flame. - To create the preheater, I split the burner tube into an array of 1/2" tubes. Flame exits the furnace impinging on these tubes, then they rejoin before the oil enters at the burner exit. I've tried supplementing this with a propane torch to heat longer and better sections of the tubes. Without airflow, the tubes get red hot, so there's considerable heat going into the air before it hits the burner. - Added a spacer to increase the furnace height by 25mm. - Leave the crucible in the furnace longer or reheat to solid mass after pouring of any liquid. Even 1/2 hour extra made no difference. So any ideas on why this is happening, and what to do about it?
I had similar issues with my oil burner furnace and had trouble getting the iron much above softening temperature. I did have one super fast dream run with an A25 crucible in a 28cm/11" bore and it turned out what I thought was kerosene/Jet A1 had some mystery additive that made the 20L plastic bucket go soft and sag a bit. I'm assuming some sort of solvent was tipped in and this burnt much faster. By monitoring the rate of temperature rise as the fuel air mix was varied I could get the furnace to at least 1350 deg C as that's the point that K type thermocouples melt. The A25 crucible was a snug fit to the furnace bore so I switched to a much smaller crucible like an A6 or similar to increase the volume available for combustion and this worked well. I'm now in the process of building a larger furnace that will have faster rate of burn due to the increased volume. So the upside down crucible on the right of the image is an A25 and iron only gets soft in it whereas the A6 gets plenty hot enough as there's more volume to support a higher rate.
Thank you for the reply. I did try another run using diesel instead of oil to get a more volatile fuel, and with a smaller crucible as you suggest. I ended up with a similar problem with a liquid/slush mixture, however letting it soak longer seemed to help. I did 3 batches and by the time I got to the third, it poured ok. Alas, it didn't quite hold enough iron to fill the mould, so I'll need to figure out something else. I did notice quite a bit of ash, which surprised me. In addition, there was a buildup of carbon on the side of the crucible opposite the tuyere, so possibly a cold spot in the furnace? Anyway, I seem to be getting closer; I just need some compromise of space in the furnace vs volume of the crucible, and stick with a lighter fuel.
The carbon deposits indicate a fuel/air mixture imbalance. After fitting a K type thermocouple I was able to get real time feedback on the best fuel/air mixture that gave the fastest rising temperature. I noticed that the ratio had to be adjusted several times during the run as temperatures increased and the furnace got hotter. Iron runs are typically longer than brass or aluminium by close to a factor of two. A thermocouple has to be removed at much above 1300 degrees C to prevent it melting but by that point the tuning is right and the furnace needs more run time to melt the iron. Fitted a K type thermocouple to my furnace to help fuel/air mix
I've finally had some success casting iron. Mark, I did take your advice to use a k-type thermocouple with one small modification: I forgot about the thermocouple and left it in the garage until it was too late and immediately burned the tip off. I feel that was a critical factor. On my next attempt, I was barely getting any flame at all. I shut down and cleaned wood chips, pine needles and some unidentified solids out the the burner needle valve. After that the fuel flowed much better! In the meantime, I had purchased a 1600C infrared temperature sensor, so could monitor what was happening at the top end. Although I was able to melt iron, and got some great castings out of it, I'm not completely happy with the furnace yet. My measurements consistently hovered about 1540C despite adjusting the air and fuel. I recharged the crucible 6 times and each pour was cold on top, with the iron looking slushy. I could stir it into the very liquid material in the bottom, but soon found it freezing on top again. In the end I scrapped that off and poured the molten iron underneath. So, I'm still not hot enough, and it seems in the top of the furnace in particular. I got to thinking about why that would be. We've had discussion about air/fuel mixture, but I think that's only part of the equation. I'm starting to think there's 4 main components to a uniform and sufficiently hot furnace: - The air/fuel mixture of course. This link https://www.engineeringtoolbox.com/adiabatic-flame-temperature-d_996.html shows max flame temperatures for most hydrocarbons will be in the ~2000C range, so there's room to move on a/f and we don't have to hit the perfect mix. - Start soon. The air blows the fuel into the furnace where it vaporizes and ignites, but it takes time and creates a cold spot at the exit burner. I've seen this be longer or shorter from time to time. This has to be due to the air velocity and therefore indirectly by the air/fuel mix. - Fill the furnace. The flame needs to be big enough to fill the furnace volume. Thus lots of air and fuel, but that also affects the points above. - Don't lose it out the top. This is perhaps more complicated. There's a vent hole in the lid to exhaust the flame. If you look in the hole, it's really really hot. Therefore the contents of the furnace are experiencing a really really cold external, as viewed through the vent. Heat loss goes up as the 4th power of the temperature, so that 1800K crucible is loosing a lot to the 300K exterior. I think this is why a reducing flame shooting half a meter out the top gets good results. If the flame is oxidizing or neutral, it's transparent. It may be the hottest flame due to air/fuel mix, but the furnace is losing more out the vent. That long yellow exhaust flame may look wasteful, but it's shielding the crucible from the cold cruel world outside. I don't know how to balance all of these, and I suspect it also depends on the size of the furnace, the burner design, crucible size and perhaps some other things. The fact that the bottom of my crucible is hotter than the top leads me to speculate that the last component is important and excess fuel has a benefit. I'm just not sure how to quantify this or to find the right balance. I have had more success this time though, so I'll keep trying and post more if I gain any more insight.
Those pesky "Unidentified solids"! They ruin the day every time! Uninformed idea: Is this a worthwhile and testable property? i.e. could you LOWER the plinth and see if the heat moves up the crucible (or melt)?
I'm wondering if your blower is supplying more pressure and velocity than needed, in addition to the preheated air containing lower oxygen density. Pete
Also you will end up with a thick crust of molten iron oxides on top of the usable iron that will need skimming and removing. Sometime you can skim the melt and add some perlite which will coagulate a solid lump on the surface aiding skimming.
Weird. I have a propane furnace made from a 10 liter (2.6 gal) paint can lined with a 2" layer of Kaowool, coated with 'rigidizer' (painted with a porridge of Fermit refractory cement). This works awesome. Cast iron can be melted within 20 minutes starting from a cold furnace and its surface is clean like a mirror. I can only view the inside when using dark goggles, too bright for the naked (face shield protected) eye. After 20 minutes my thermocouple (put into the top of the furnace horizontally in a hole in the wall on the level just under the crucible top) shows 1400 C and sometimes higher. The trick is the very good insulation of Kaowool compared to heat soaking concrete like Mizzou (which I used in the past and did not get that hot). And I use forced air with a blower which increses the temperature compared to natual draft. Jet A-1 can burn very hot: the temperature inside aircraft engines is usually 1600 C, but they are designed to propel an aircraft and not heating a crucible with metal. Propane is easier in furnaces as no hassle with pumps or atomizers is needed for liquid fuel with the increased fire risk. For iron temperatures (and copper as well) I use an S type thermocouple because it is much more durable and resists long time exposure up till 1600 C while type K can only keep 1100 C continuously and 1300 C for short time only.
