In any job, there are the little tricks of the trade that save time, make your work easier, and allow you to produce the best final product that you can. Some have been passed down through the generations, some have come into existence as new materials, tools and techniques have emerged. But, are they all useful? Are they all they're cracked up to be?
In welding, one of these age old tips is preheating metal. Seems to make sense, right? But it is an undertaking that has been called into question lately. Many people's standpoint is that preheating is not a requirement, but merely a recommendation. Let's take a look at the possible reasons why you might want to preheat...
Firstly, it is said that preheating removes contaminants from the materials. This is true for contaminants that can be easily seen, such as oil and paint. Sure, heating with a flame will burn this away, but it will leave behind a residue of hydrocarbons, which are also a contaminant. Using an induction method will obviously not remove these paints and other visible surface contaminants. So, sorry, but the best way to clean up your surfaces is with a little elbow grease (no pun intended). Get in there with a grinder, wire brush and degreaser - these are an integral part of your welding tools.
Secondly, you hear people say that preheating removes moisture from the metal. This is scientifically false. Heating with a flame actually creates water through a simple chemical reaction. What you may see on the surface is this water accumulating. The reason this water stops building up after a while is that the material is so hot that the water turns to vapour instantly.
Thirdly, it is common to hear that preheating is important for increasing the ductility of the materials to decrease the risk of cracking. This is true, the material will become more ductile, therefore more resilient and forgiving; but this is still not the number one reason for preheating.
The reason that preheating is a requirement is to reduce the damaging effects of rapid cooling. Cold metals will quickly draw heat away from welds, causing a rapid cooling effect. This can lead to the formation of brittle micro-structures, which can lead to weakened joins. This is especially important when using high carbon steels.