Forge welding can be used for a range of different metals including high and low-carbon steels, wrought iron and some cast irons. It is also possible to forge weld aluminium alloys, however, it is difficult to forge weld bronze, brass and copper.
Copper-based alloys can be welded although it is difficult due to the tendency of copper to absorb oxygen while being heated. Titanium also has a tendency to absorb oxygen while molten, but in this instance the weld created with a forge weld is often stronger than a fusion weld or other pressure-based techniques.
With dissimilar metals, the lower melting temperature eutectic between the materials often means that the weld is stronger than the individual metals.
Techniques
The oldest forge welding process is manual hammering. This ancient method involves heating the metal to the required temperature before coating it in flux, overlapping the weld surfaces and then using a hand-held hammer to repeatedly strike the joint. The hammer blows, coupled with the formation of the joint itself through bevelling or rounding the surfaces, allows the flux to flow out. The hammer blows are only moderately hard so as not to cause the flux to blast from the joint as soon as the first blow is struck. The invention of mechanical hammers, originally powered by waterwheels but since also using compressed air, electricity, gas engines, or steam for power, revolutionised the basic forging method further.
Forge welding can also be achieved using a die, where the pieces of metal are heated and then put into a die where the pressure is applied, or with roll welding, where the heated metals are overlapped before being passed through high pressure rollers to force them together.
Modern forge welding is often automated, while other techniques include resistance forge welding where the press or die is electrified as a means of producing heat for the weld. Hydrogen gas and induction heating can also be used to burn out oxides in an oxygen reactive environment in a process known as shielded active-gas forge welding.
Temperature
The temperatures used in forge welding typically range from 50-90% of the metal melting temperature. Both welding surfaces need to be heated to the same temperature and welded before cooling down.
With steel, the carbon will mix with gamma iron to form a solid solution called austenite at a certain temperature. Beyond this, the material changes to delta iron and becomes magnetic once more, so a blacksmith can check the correct heat has been reached by trying to attach a magnet. Steel may take on a wet or gloss appearance when it reaches the welding temperature, although care must be taken not to overheat the metal so that it gives off sparks as a result of rapid oxidation as this will lead to a brittle weld.
Decarburisation
When steel is heated to an austenising temperature, carbon begins to diffuse through the iron. The rate of diffusion increases as the temperature increases and the carbon combines with oxygen to form carbon dioxide, allowing the carbon to diffuse out of the steel into the surrounding air. However, this means that the carbon content is reduced in the metal, making it softer. As a result, blacksmithing is completed quickly to reduce decarburisation and prevent the steel from becoming too soft.
Decarburisation can be mitigated by using a steel with a higher carbon content than required. Using a high carbon content metal means that the correct carbon levels can be attained following welding and the associated decarburisation.
Flux
For forge welding to work, the weld surfaces need to be clean. Oxides can form on the surface of the metals and impurities like sulphur and phosphorous migrate to the surface during welding. To prevent the welding surfaces from oxidising and creating a poor quality weld, a flux is used. The flux, which can be made up from a mixture of substances or simply a fine silica sand or borax mixed with iron filings, combines with the oxides as they form, lowering their melting temperature and viscosity. This allows the oxides to flow out of the joint as the two workpieces are beaten or pressed together to create a strong weld.
Summary
To summarise the forge welding process in four steps:
1. Preparation
Make sure you are using the correct forging temperature to match with the metals you are joining. They should turn yellow when heated in the forge, but heating them beyond this can cause oxidation. It is also important to make sure the weld surfaces are clean in order to create the best possible weld.
2. Flux
Once heated, remove the metals from the forge and sprinkle them with flux (see above). This will shield the metal and prevent oxidation that could lead to slag or scales forming on the surface and ruining the weld.
3. Heat
Once you have applied the flux, place the metal back into the forge and bring it back up to a bright yellow heat. If you leave the metal in too long it will spark, meaning that the metal is contaminated. The required temperature will differ according to the metal you are using.
4. Making the Join
When the metal glows yellow you need to quickly remove it from the forge to prevent it becoming soft. At this point you can either place the workpieces in a press or use a manual or powered hammer to form the weld.The hammering needs to continue until a clean and finished weld with no weld lines is created.
The primary advantage of forge welding is that it can produce high strength, high quality welds quickly and easily without the use of filler materials.
However, producing high quality welds and preventing contamination requires skill and expertise. When done manually, forge welding is time consuming, while hammering too hard or getting the temperature wrong can ruin the weld.
Forge welding larger items can also be difficult due to the need for a bigger heat source, while manual welding of large items was difficult due to the materials cooling down before the weld is completed.
Forge welding has been used for centuries to create items ranging from armour and weapons to tools, farming implements, cookware, gates, fences, and even pressure vessels.
Due to its versatility and ability to join different metals, forge welding has found a huge range of different applications over the years.
Is forge welding strong?
Forge welding creates a strong bond, often stronger than that which can be achieved with fusion welding, where the metal is liquefied. The combination of pressure and high heat increases the strength of the material along with the deformed particle structure.
Can you forge weld without flux?
You should always use flux when forge welding as it removes oxidation that could otherwise cause an imperfect weld.
Can you forge weld stainless steel?
You can forge weld stainless steel but it must be passivated to improve resistance to corrosion. Some grades of steel, such as 300 grade, may require more hammering to complete the forging. Forging stainless steel enhances its strength and corrosion resistance as well as creating a continuous grain flow through the part.
Can you forge weld copper to steel?
It is possible to forge copper to steel, although copper tended to be hammered while cold and only heated to harden the metal. Copper (and bronze or brass) do not forge weld easily as copper-based alloys tend to absorb oxygen during heating.
Can you forge weld aluminium?
Aluminium can be forged but it needs to be done at a low heat due to the metal’s low melting temperature. This low capacity to retain heat is due to the low density of aluminium, meaning that hot tooling is recommended to prevent rapid cooling of the workpiece during forging. Cold forging is often the preferred method for softer metals like aluminium, being a less expensive technique that requires little or no finishing.
Forge welding is a process that involves heating two pieces of metal before hammering, pressing or rolling them together to create a joint. Originally developed in the Middle Ages, it is one of the oldest known methods for joining metals and is an essential skill for blacksmiths.
It is important to understand the ideal temperatures for forging different metals to prevent them from getting too hot during the process. This solid-state, diffusion welding process uses temperatures lower than the melting point of the materials and doesn’t require any filler material.
Being an ancient and versatile welding method, forge welding has been used for a wide range of applications over the centuries.