Author Topic: Casting iron using Thermite instead of a furnace  (Read 16452 times)

Offline vtsteam

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Re: Casting iron using Thermite instead of a furnace
« Reply #25 on: February 01, 2013, 10:40:02 AM »
I have the Lindsay Book reprint of "Thermit Welding Process" by Richard N Hart. I'd recommend reading it if you contemplate casting with thermite. I don't see any reason why it can't be done, but control of the quality of the casting might be a problem.

In answer to the original questions:

The aluminum oxide becomes part of the slag in the melt -- if given enough time it will rise to the surface. Thermite welding took advantage of this with a conical melting chamber which was tapped into the weld mold at the bottom after a brief delay. I would say it probably takes some nerve to tap, as the thermite reaction can produce heats up to 2700 C -- considerably higher then the melting point of steel or iron.

It may produce splatter even in a commercial application, or near explosive ejection in a home made experiment which wasn't properly calculated and controlled. Traditionally the reaction was cooled by additions of steel punchings in the mix, and by careful melt chamber and mold design. An experiment might not get these right by comparison with developed and tested equipment and know how used for rail repair at the turn of the century. While splatter might be okay if you aren't close to the test melt, as when people play with the reaction on a very small scale on Youtube, tapping for a pour would require up close and personal activity which isn't very appealing.

The volume and weight of thermite required would be many times that of the finished piece, so it could become a fairly serious sized reaction requiring a big melting vessel to produce a relatively small piece. Gating and risering is essential because of the heat generated -- that adds to the weight of materials, as does the aluminum, and the iron oxide has lower density than the finished iron, so is volumetrically large. Powdering also increases the volume -- it needs a big melt vessel lined with VERY refractory material. Remember this is potentially a 2300 C pour with ultimate heat available of of 2700 C. Typically the lining of the melt vessel was 2-3 inches thick for a rail repair, made of "magnesia" refractory and needed frequent replacement. The temperature of the thermite reaction will melt asbestos -- in fact asbestos sheet was used as an automatic tapping valve for casting repairs. It would melt in a few seconds and tap the steel. This isn't you usual hobby furnace pour.

The materials themselves also were fairly specific -- grain size and composition of both the aluminum and iron oxide were specified in order to produce quality castings. It wasn't just rust they were using, but a product of the foundry industry -- with specific composition and grain size relative to the aluminum.

All of this says to me that though it may well be technically possible to cast using thermite on a small scale, is it practical or desirable?

It is possible to cast iron with a fairly simple propane, oil, or charcoal or coke furnace, made with widely available materials, using well developed practices at a hobby level. And that furnace is re-usable, and predictable. I guess it depends on where the interest lies. If you are interested in process rather than product, maybe that is appealing.

One interesting thing mentioned in the book is that a thermite reaction WAS used to introduce nickel and titanium to a conventional (not thermite) steel melt in commercial practice. These were actually thermites with the oxides of those materials, not iron oxide. And they were plunged in a can into the steel melt. Interestingly, also, it didn't work for an iron melt, because the temperature wasn't high enough to complete the reaction well enough to produce a slag, and this DECREASED the quality of the iron rather than increasing it.

Well hope this helps -- not trying to discourage anyone -- just, it may be more complicated than scraping some rust into some aluminum powder from the bandsaw -- at least if trying to do any casting with it.

The above book is recommended reading if you are contemplating thermite as a melting process.
I love it when a Plan B comes together!
Steve
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Offline bhowden

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Re: Casting iron using Thermite instead of a furnace
« Reply #26 on: February 01, 2013, 04:05:03 PM »
Wow, great response!  Thanks for the input as that answers a lot of questions.

Brian