Forge welding is a welding process of heating two or more pieces of wrought iron or steel until their surfaces are malleable and then hammering them together. Its use is ancient, doubtlessly being the first method devised for the joining of metals. Until the invention of electical and gas welding methods during the Industrial Revolution, it was the only available method.
Often a flux is used to keep the welding surfaces from oxidizing and producing a poor quality weld. The flux also mixes with the oxides that do form and lowers the melting temperature and the viscosity of the oxides. This enables the oxides to flow out of the joint when the two pieces are beaten together. A simple flux can be made from borax, sometimes with the addition of iron filings.
The welding temperature is above the forging temperature , and not so very far away from the melting point of the metal. Steel welds at a lower temperature than iron. The metal may take on a glossy, or wet, appearance at the welding temperature. Care must be taken to avoid "burning" the metal, which is overheating to the point that it gives off sparks from rapid oxidation.
Multiple layers of different kinds of iron and steel can be forge welded together to produce pattern welded or Damascus steel. A similar process is used to produce the blades of katana.