Mark IV

An Attempt at a multi-function furnace

The problems that presented themselves in the previous few furnaces were becoming issues that I thought I could address with a new furnace. The Mark IV was the most advanced furnace to date and it was going to be my best! I scrapped the transformer design as it was unwieldy and concentrated on creating a compact furnace with enough power to get the job done, but not so much that it cooked itself in the process. A unique phenomenon became apparent in the Mark III furnace design which I had read about in researching large industrial ac furnaces. Namely "Boring Through"...

Solving the Boring Through issue

In a large industrial arc furnace for melting ferrous metals, the initial stages of the burn are known as the "boring through" phase or the "boring phase", which I am sure is anything but uninteresting. This occurs in arc furnaces melting material which is not particularly thermally conductive (doesn't transmit heat well) and simultaneously is much more refractory than other metals (higher melting point). Steel is one of these materials. While it is a metal, and does transmit heat better than say... wood, it has no where near the conductivity of metals such as copper and aluminum. So the issue lies in the incredible discrepancy between the heat applied (by the arc), which may be may tens of thousands of degrees Fahrenheit, and the tendency of steel to NOT transmit that heat to the material around it. So what happens is that the metal beneath the electrode is instantly liquified, but a few centimeters beyond that, the material is still well below its melting temperature, which means the electrode continues "boring" down through the metal until the entire quantity of material in the crucible is above the melting point. This is compounded by the fact that steel melts at between 2,500°F and 2,700°F. It doesn't transmit heat well, and it requires a lot of heat to be transmitted to melt. This causes the furnace to bore a hole down the center of the solid metal and begin pooling material on the bottom of the crucible. In a large arc furnace, enough molten metal accumulates here that the electrode can arc against the puddle and eventually bring the entire pot up to melting point. In my furnace, due to its small size, what was happening instead, is that a few drips would form at the bottom. When the arc finally got there, the rest of the crucible wasn't sufficiently heated to begin melting other steel and adding it to the puddle. Instead, it would arc against that small puddle until it had burned it away, then arc directly against the electrode and bottom of the crucible. Destroying both.

The solution here, was some sort of preheat mechanism. I needed a way to warm all the metal in the crucible up to about 1,000°F so that as the arc started, it would melt the metal faster and create a pool at the bottom that it could safely arc against while the whole crucible came up to the required temperature