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Forge-welding,Blacksmith's Magic Process.SOLUTIONS with Effective, Practical Advice
Forge-welding is one of the most ancient joining technique performed for centuries by blacksmiths. It is a solid state process whereby the melting temperature is not reached. Mighty hammer blows cause permanent deformation and assure metallurgical contact between two elements to be welded together. I reported briefly of the vivid impression I still retain in my memory, having seen it as a child, forge welding applied in practice, under my eyes, many years ago. The note on this experience was published (2) in Issue 35 of Practical Welding Letter for July 2006. An Article on Field Assisted Sintering was published (7) in Issue 155 of Practical Welding Letter for July 2016.
The main requirement for Forge welding is that the materials be forgeable. That means malleable and deformable under pressure when hot, without cracking.
Cast iron is not forgeable and cannot be forge welded. To perform Forge welding, the ends of two mild steel bars are normally heated together to white color in a coke forge, or forge charcoal furnace, fed with compressed air to enhance the temperature of combustion. As soon as the right temperature is reached, the bars are quickly withdrawn by two blacksmiths who stand at opposite sides of the anvil. Experienced blacksmiths judge the correct temperature for Forge-welding and its uniformity throughout the material to be welded, by looking only at the color, without the help of any instrument. The glowing ends are superposed, forming a bulge, and welded together by hammer blows stricken in rapid succession alternatively by the two workers. In the intervals between blows the resulting welded bar is slowly rotated on its axis, to achieve uniform thickness. Any occasional contaminant or oxide present on the surfaces is forced outside of the joint by the hammer blows, so that joining and coalescence occur on a clean interface. In a few minutes the welding is accomplished, the bulge is brought down to the original size of the bar and coalescence is generally so good that no parting line can be detected. At too low a temperature, the metal is not sufficiently malleable and welding will not take place. Overheating is to be avoided, otherwise a brittle joint may occur. The metal can become severely oxidized with rough and spongy appearance. Modern automatic or semi automatic applications use power driven hammers, and include the production of tubing and of clad metal. Forge-welding may be used also for maintenance or repair, or in fabrication of sculptures and artistic creations. Besides carbon and low alloy steels, Forge-welding has been applied also to other metals, with or without the use of a flux. Nowadays though, it is largely superseded by other more controllable solid state processes. This process has nowadays mainly historic interest. It is important to understand the importance of the different factors influencing the outcome, for considering the main features of different processes derived from Forge-welding.
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