Forge welding is a combination of heat, timing and surface preparation. The heat sources we will discuss in this article are gas (propane or natural gas) and coal/coke. Welding is a little more difficult in gas fires because of temperature limitations and the fact that gas fires tend to be oxidizing (excess oxygen).
When welding in a gas fire I preheat the material to a bright red heat and brush. Flux is then applied (if flux does not melt then heat is insufficient) and the piece is returned to the fire. I allow the piece to remain in the fire until a full welding heat with lemon/yellow color is reached. The piece is removed and immediately welded on the anvil with quick light blows. As the material cools force of the blows is increased. A second finishing heat may be necessary to blend weld seams and reduce material to desired dimensions.
Welding in a coal forge is similar, although easier because of the high, fast heat and the fact that a reducing fire (no oxygen) is obtainable and sustainable. The timing aspect of forge welding is to forge while you have a welding heat, that is, strike while the iron is hot. I may seem to be stating the obvious but insufficient heat, a cold anvil and disorganization of the smith are the leading causes for missed welds. To be sure you can recognize a welding heat take two long pieces of steel, ‘apply flux’ and hold one end of each piece in the fire. When the two pieces will stick together firmly, they are at a welding heat.
Our anvils are a heat sink. They will effectively pull the heat from the materiel being welded. The size of the anvil and the ambient temperature both affect the length of time an anvil must be preheated in order to achieve consistent forge welds.
Disorganization of the smith is the hardest obstacle to overcome. This will take time and practice. While your material is in the fire, locate your hammer and organize your thoughts on what you wish to accomplish in this heat. A moment’s hesitation between forge and anvil can be enough to rob you of your welding heat.
Surface preparation consists of scarfing, cleaning and fluxing. The scarf is the preparation of forging the material so that it fits tightly together and allows for stock reduction while forge welding. There are a number of different scarfs and variations of each. The chain link scarf has the ends of the material forged to fit together with overlapping tapers. The step scarf has a set down slightly less than half the thickness of the material. These steps are fit tightly together so that the joint before welding is one and a half times thicker than the parent material. Regardless of the type of scarf used the ends of the material should fit together tightly enough that scale does not form in the joint.
The final surface preparation is to apply flux. The job of the flux is to keep the material clean while it reaches a welding heat. Apply enough flux to cover the entire joint. Flux is like snuff; a pinch is all it takes and drips are a waste.
Forge welding has long been thought to be a mysterious and fickle technique with a high rate of failure. Forge welding however is simply a combination of heat, timing and surface preparation. When all three of these elements are present you can be assured of a high quality forge weld almost every time.