Enjoying Some Exothermic Welding, With Thermite!

There probably aren’t many people out there who aren’t aware of what thermite is and how it demonstrates the power of runaway exothermic reactions. Practical applications that don’t involve destroying something are maybe less known. This is where the use of thermite for creating welds is rather interesting, as shown in this video by [Finn] that is also embedded after the break.

In the video, one can see how [Finn] uses thermite charges to weld massive copper conductors together in a matter of seconds inside a graphite mold. Straight joints, T-joints, and others are a matter of putting the conductors into the mold, pushing a button and watching the fireworks. After a bit of cleaning the slag off, a solid, durable weld is left behind.

The official name for this process is ‘exothermic welding‘, and it has been in use since the 19th century. Back then it was used primarily for rail welding. These days it sees a lot of use in high-voltage wiring and other applications, as in the linked video. The obvious advantage of exothermic welding is that the resulting joint is strong and durable, on account of the two surfaces having been permanently joined.

21 thoughts on “Enjoying Some Exothermic Welding, With Thermite!

  1. What are the properties of copper cast in this way? I’ve noticed that low quality copper can get brittle. I can also imagine that such process can introduce thermal stress. (maybe not if you let it cool down very slowly, i am not really metalurgy expert)

    1. It should be pretty pure copper, so material property wise copper like (as long as it doesn’t go wrong and mix with the slag etc), not an expert on thermite cast copper though – heck on thermite cast anything.

      Copper and every one of its common alloys I’m aware of doesn’t give a monkeys about cooling rate. Heat it up enough that it goes soft and it stays soft no matter if you immediately cool it.

      Copper does however work harden and get brittle fairly easily. I’d suspect if you are noticing low quality copper being brittle after heat you haven’t got it up hot enough to anneal it so its just work hardening its way to a failure. Or its really not really copper at all.

    2. Copper (pure) is very ductile after cooling from casting temperature, somewhat more so if the cooling is rapid (IIRC, more trapped dislocations is the reason). Having welded copper using several processes (OA torch and GTAW most often, both autogeneneous and with fillers to control the weld properties), I can say that the primary problem is that the weld area tends to be TOO ductile and malleable in many cases (handy when making high pressure wire gaskets, not so handy for, say electrical wire where the material is strengthened by the drawing process, and there are several draws done after the final anneal and pickel), and the filler is often not pure copper so that the weld has better properties.

      The fillers here may include tin (as well as other elements that will alloy in the final product), and I would guess that the tin-bearing are for the joints to steel (rebar and the column). Pure copper may induce cracking with steel (the interface is really cool. This is a braze, technically, and the joint will include intermetallics and diffusion-formed alloys, like when soldering a PCB, and the properties may be very different than the base materials)

      1. “A herd of buffalo can only move as fast as the slowest buffalo. And when the herd is hunted, it is the slowest and weakest ones at the back that are killed first. This natural selection is good for the herd as a whole, because the general speed and health of the whole group keeps improving by the regular killing of the weakest members.In much the same way, the human brain can only operate as fast as the slowest brain cells. Excessive intake of alcohol, as we know, kills brain cells. But naturally it attacks the slowest and weakest brain cells first. In this way, regular consumption of beer eliminates the weaker brain cells, making the brain a faster and more efficient machine. That’s why you always feel smarter after a few beers.” – Cliff Clavin

    1. When rails are laid down in large sections, the more efficient method is “flash butt” (really) welding. Essentially the two ends of the rail to be joined are the two terminals of a welding generator – when they’re pressed together by vehicle-mounted equipment with rail-handling grips the rails fuse and the whole assembly moves on.

      Remote rail laying where the equipment isn’t available as well as smaller and more specialized things like switch frogs and repairs are often done with thermite – which is a little startling the first time you see it. A couple of guys lighting off a small volcano while they stand around arguing over soccer scores or how long until lunch.

    1. You probably have but didn’t realize what it was. Next time you are in a building with exposed I beam posts look at where they meet the floor and you might see a grounding conductor.

  2. In most cases a crimped or bolted connection is the way to go. Using thermite for the cases shown in the video, makes no sense. I’m used to inspecting earthing on 132 to 400 kV equipment, and there is a bolted connection ALWAYS used. Properly clean the area and bolt it together with a known torque – done. No mess.

    Even solid high voltage conductors are joined using shear joints.

    1. As mentioned by Moberis, this is for grounding only — either entombed inside building structure, or below grade in grounding grids under substations or main electrical rooms. Above ground? Not buried forever inside fireproofing on a steel structure? Yeah, bolts.

      1. A friend of mine works for a lighting rod company. And this is how they weld all the lengths of the copper conductors at each ground rod and for all of the lengths that are buried. And depending on the stage of construction they ether thermite weld or use bolted u-bolt connections above ground or inside the building.

  3. I used to work for an underground construction company. Lots of the Telcos we worked for required the ground wires to be bonded to the ground rods by use of a Cadweld Thermite weld. extremely tough if done correctly and the best connection you can get.

  4. As a telco tech who Cadwelded ground connections, I will agree with Taylorian73 above, our specifications called for Cadweld in almost all cases. After I was promoted to Tech Support Engineer I finally found out that that requirement had been implemented after the failure of both bolted and crimped joints in high voltage circumstances.

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