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Welding FAQ

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Also see: Welding | CWR & Thermite Welding | Holland Welding

From: "Gordon Davids Date: Fri Jan 9, 2004 4:55 pm

There are two basic methods used to weld rails together.

In a welding plant the common method now is the electric flash-butt process. The rail ends to be welded are brought together and a large electric current is passed through them via electrodes clamped to the rail bases. When the current has heated them to a predetermined temperature (yellow, at least) and a plastic state, they are drawn apart and then pushed together with a very high force. That way, the steel in the two rails is fused all the way through the cross section.

The upset metal is then sheared off, and the rail head is contoured by grinding. The resulting strings are nominally 1/4 mile long, but the actual length is only limited by the capacity of the storage tracks at the plant, and the length of the rail trains used to transport the rail strings. The strings are usually pushed directly into the rail train as they are assembled.

Field welding is entirely different, and normally uses a thermite process. The rail ends to be welded are squared and ground clean, and separated by about one inch. Depending on the location of ties, one tie might be pulled out, or two ties moved back to make room for the mould.

A sand mould, sometimes in a sheet steel housing, sometimes not, is clamped in two halves over both rail ends and sealed with "luting" material, formerly asbestos, now something else. A pot is placed on top of the mould. The bottom of the pot is closed, sometimes with a self-destroying plug but usually the tap is controlled by the welder. The pot is filled with a measured amount of thermite - a mixture of aluminum chips, iron oxide, steel pellets and alloying material. A small amount of magnesium powder is set on top of the thermite mix as an igniter, and then the magnesium is touched off with a spark igniter.

When the thermite is ignited, the aluminum reduces the iron oxide and alloying material to molten steel, melts the steel pellets, and produces aluminum oxide and other material as slag. The slag floats to the top of the pot (we hope) and the molten steel is tapped into the mould by the welder. After it cools, the upset metal is chipped, sheared, or burnt away, and the rail head sides and surface are restored by grinding.

At least, that's how it was done 32 years ago on the D&H, and I don't think the principles have changed very much. Only the quality control has been improved, to protect the innocent. Any questions, class? Gordon
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