Preventing Accidents at Work

Exothermic Welding: How To Prepare For Successful Welding

21 January 2015

Henry Ellis

Permanently joining large pieces of metal in the field can be challenging, particularly if the objects are subject to large external stresses. Ordinary welding techniques are often inadequate to create a sufficiently-strong weld. To overcome this problem, exothermic metal fabrication welding was developed. When done correctly, exothermic welding creates sturdy joints resistant to lateral and longitudinal forces. However, exothermic welding does require attention to detail and careful preparation in order to be successful. Below are a few guidelines to keep in mind when using exothermic welding:

How exothermic welding works

Exothermic welding is simple in principle, and this makes it an ideal technique to use in difficult environments where conventional welding would not be possible. One industry that is dependent upon exothermic welding is railroading; joining rail ends in the field is a perfect application for this technology.

To perform exothermic welding, the two ends of metal to be joined are aligned and separated by no more than a few inches. A mold is clamped around the metal to form a secure "pocket" that will contain the weld. After that, a crucible filled with a metallic alloy and reactive metal is mounted atop the mold; the contents of the crucible are ignited, and the ensuing reaction creates a flow of molten metal that fills the mold. Both pieces are engulfed by the metal and are permanently joined at that moment.

What exothermic welding needs to be successful

Exothermic welding is not difficult to perform, which also lends itself to widespread use in field applications. For it to be of maximum benefit, though, there are certain things that must be done to ensure a strong weld:

• Clean, flat ends – both ends of the metal to be joined should be clean and flat. Each end should be parallel to the other, as well. Angle grinders, cutters and other power equipment can be used to fabricate the ends and remove rust, dirt and imperfections that might prevent a strong joint from being formed.

• Appropriate distance between ends – in the railroad industry, the ends of the rails are typically manipulated to be between one and three inches apart in order to be suitable for exothermic welding. If the gap is too narrow, the technique will not allow for enough material to flow into the mold. If it is too wide, the strength of the joint will compromised. Other applications outside the railroad industry require a similar distance alignment for maximum effectiveness.

• Sufficiently heated ends – exothermic welding will not form a resilient, strong bond unless the ends to be joined are able to adhere to the molten metal. One important task that must be done before applying an exothermic weld is heating the ends of the metal to a sufficiently high temperature. If the temperature is kept too low, the molten metal will cool too quickly and not bind. Heating the ends involves using a torch directly applied to the metal. However, if the temperature is raised too high, the pieces may sag or warp. It's important to know the recommended temperature range for each type of metal and its specific application.

• Proper composition of welding materials – in most cases, exothermic welding is accomplished by using iron oxide powder and aluminum powder for the reaction. The reaction creates molten iron, which fills the gaps and joins the ends of the metal; aluminum slag is produced as waste and is discarded. For some applications, the exact composition of the contents requires adjustment ahead of time. For example, mixing steel with the iron oxide can increase the strength of the joint. An incorrectly measured formulation can be disastrous if it leads to a weld failure in certain situations. That's why it's important to consult a metallurgist or welding engineer if there is doubt about what to use in a given situation.

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