What is welding? Welding is distinct from other processes that we use to join metal, such as soldering and braising, in which a third material is added as a kind of glue. With welding, we take two distinct materials, two distinct objects. Through the addition of heat we create a molten pool of metal into which we add a filler rod. The resulting product is no longer two distinct pieces, but one homogenous unit. This piece of material can now then be cut, bent, or manipulated in any way that one would wish. It's not like you've added a third material that needs to be treated differently.
There are different kinds of welding. We use gas, acetylene and oxygen, as a heat source. Mig welding, arc welding, and tig welding, which all use an electrical source to create heat. But even though each of these are different and have their own distinct qualities, they're all essentially doing the same thing, that is taking two distinct pieces of material and joining them into one.
I want to demonstrate the welding process using an oxyacetylene torch and a filler rod. This is the easiest way to see how the physical process of welding occurs. It's always important when you're welding to wear the right safety equipment. Before I start the torch, I'm going to put my goggles down. To start the torch, I add a little bit of acetylene. I increase the size of the acetylene flame until it's burning nice and clear and hot. I add oxygen slowly until I get a nice sharp, defined cone of flame. The hottest part of the flame is at the tip of the blue, the very tip of the bright blue zinc. As you can see, as we start to heat the steel, it goes from a red to orange and starts to get shiny. And we can start to add our filler material. And you can actually see that molten pool of metal forming as we move along with the filler rod completing the operation.
Now that the oxygen-acetylene welding process is complete, we can examine the results. We had two separate pieces of material. We had a common molten pool of metal, into which we added our filler rod. And now we have a complete, single, homogenous piece of material. Two pieces of steel joined into one.