The Temperature Of Forge Welding

A primary requirement for forge welding is that both weld surfaces need to be heated to the same temperature and welded before they cool too much. When steel reaches the proper temperature, it begins to weld very readily, so a thin rod or nail heated to the same temperature will tend to stick at first contact, requiring it to be bent or twisted loose. One of the simplest ways to tell if iron or steel is hot enough is to stick a magnet to it. When iron or steel cross the A2 critical temperature, it begins to change into the allotrope called gamma iron. When this happens, the steel or iron becomes non-magnetic. In steel, the carbon begins to mix with gamma iron at the A3temperature, forming a solid solution called austenite. When it crosses the A4 critical temperature, it changes into delta iron, which is magnetic. Therefore, a blacksmith can tell when the welding temperature is reached by placing a magnet in contact with the metal. When red or orange-hot, a magnet will not stick to the metal, but when the welding temperature is crossed, the magnet will again stick to it. The steel may take on a glossy or wet appearance at the welding temperature. Care must be taken to avoid overheating the metal to the point that it gives off sparks from rapid oxidation (burning), or else the weld will be poor and brittle.