Flame welding: an overview of the oldest welding method

Flame welding is the oldest known method of welding and is one of the pressure welding processes. Also known as forge welding, it uses less heat. However, the temperatures are also sufficient to soften the material. The workpieces are then joined using pressure. This is done with hammers. Hence the name forge welding. Similar to submerged arc welding, a flux is used. In the beginning this was fine flux sand, but today it is mainly borax that is used.

What is hot-dip welding?

Instead of a molten pool, the materials are made plastically deformable by the heat. The pressure brings the materials so close together that a new bond is created between the atoms in the metals of the materials.

While this process used to be very versatile, it has now been superseded by electric and gas welding. However, it is still used to join both similar and dissimilar metals. Nowadays, the processes are mostly automatic.

The exact amount of pressure applied can be adjusted and depends on the material used, its hardness and strength, as well as the temperature used. This is below the melting point of the materials. No filling material is required.

Due to the lower temperatures during welding, the heat is distributed evenly. This reduces the thermal stress that occurs at the weld seam. Compared to cold diffusion processes, more materials can be joined here because the heat makes them softer.

Which are weldable materials?

Forge welding is suitable for a whole range of different metals. These include steels with a high or low carbon content, cast and wrought iron, as well as aluminum alloys. Copper, bronze and brass can also be joined, albeit not quite as easily.

Alloys with copper can also be welded, but copper makes it a little difficult here because it readily absorbs oxygen as soon as it is heated. Titanium can also be joined in this way. In this case, however, titanium readily absorbs oxygen in its molten state, but a strong weld seam can be produced by simply heating it.

Joints of dissimilar metals often result in very strong joints due to the different melting temperatures, which can even be stronger than the joint between two workpieces of the same material.

Which are the application for hot-dip welding?

In the original method, the metal is first heated and then coated with a flux. The surfaces to be welded are placed on top of each other and then a joint is created with hammer blows. With the hammer blows and the formation of the joint, as well as with beveled or rounded surfaces, the flux is brought out of the forged areas.

The hammer blows must be hard enough to effect the connection. At the same time, however, they must not be too hard in order to prevent the flux from bouncing or being blown out of the joint immediately. In the past, mechanical hammers were often used, which were moved by water wheels. Today, compressed air, motors, steam or electricity are used.

Another form of forge welding is the use of a die. The workpieces are heated and placed on top of each other in the die. The necessary pressure is then built up there to create the joint.

Another method is roll welding. In this process, the metals are heated and placed on top of each other. They are then pressed together with high-pressure rollers, creating a joint. Today, forge welding is mainly automated and is often combined with other processes, such as resistance welding.

Which temperatures du you use during hot plate welding?

Temperatures generally between 950 °C and 1,500 °C are used for hot-dip welding or modern forge welding. The surfaces to be welded must be brought to the same temperature.

When steel is forged, the carbon mixes with gamma iron once the correct temperature is reached. This leads to the formation of a solid solution called austenite. Further processes turn the material into delta iron, which is magnetic. This allows a blacksmith to check whether the correct temperature has been used for the process by using a magnet.

When the steel reaches the right temperature, this can also be recognized by the fact that it takes on a wet or shiny appearance. It is important not to increase the temperature too much. This is because the rapid oxidation processes cause sparks and the weld seam becomes brittle.

What is decarburization

When steel reaches its austenitizing temperature, the carbon begins to escape. It separates from the iron and combines with the oxygen in the air to form carbon dioxide. This naturally means that the carbon content in the metal being processed is reduced. This makes the metal itself softer. This can be counteracted in two ways.

The first way is speed. If forge welding is carried out quickly enough, the amount of carbon that can escape is only small. This means that the metal retains sufficient hardness to be suitable for the intended use.

The second option is to use a steel whose carbon content is higher than the required proportion. The escape of the carbon then brings the metal to the carbon content that is ultimately required for the intended application.

How do materials flow?

Forge welding can only be successful if the welding surfaces are clean. The problem here is that oxides can form on the surface of the metals used during the process. They can also contain impurities such as sulphur and phosphorus. These migrate to the surface during the heating process and then affect the quality of the weld seam. This oxidizes or is generally not very strong or durable.

The use of a flux prevents the welding surfaces from oxidizing. It also helps to create a strong bond. A typical flux is fine quartz sand or borax mixed with iron filings. This flux binds the oxides and lowers the melting temperature and viscosity. Oxides can flow out of the joint as soon as the pressure is built up, so that the weld seam is strong and durable.

How does the execution of hot-dip welding work?

The process of hot-dip welding can be divided into a few steps. Preparation is the first step. The correct forging temperature must be determined and it must be ensured that the materials used can be joined.

During heating, the metals used should turn yellow, but no more. If this temperature is exceeded, oxidation processes can start. In addition, the welding surfaces must be cleaned to ensure the best quality of the weld.

Next, the flux must be applied. This is sprinkled over the metal to shield it and prevent oxidation from occurring. The metal is removed from the fire for this and then returned to the forge once the flux has been applied.

The metal is now heated again to a light yellow color. If the metal remains in the heat for too long, sparks will form, which is a sure sign of contamination. The exact temperatures that need to be reached depend on the metal being used.

As soon as the metal glows yellow, it must be removed from the fire and joined to the other metal. A press or hammer is used for this, which then creates the weld seam. The pressure must be applied with the hammer or press until the finished weld has been created.

What are the advantages and disadvantages, as well as applications of hot-dip welding?

The advantages of fire welding are that it is relatively easy to use and can be used to create very good weld seams. Also, no filler material is required.

The disadvantage, however, is that it requires a lot of specialist knowledge, experience and skill on the part of the user to ensure that the weld seams are of a really high quality and free of impurities. If forge welding is carried out manually, it is also very time-consuming. If it is hammered incorrectly or the wrong temperature is selected, the weld seam is quickly ruined.

If larger parts are to be joined using hot plate welding, a correspondingly larger heat source is required. In manual applications, it is very difficult to produce a complete joint before the temperatures drop too much.

Historically, forge welding has been used to make tools, equipment, armor and weapons for thousands of years. Nowadays, the process is also used to make other things, such as cookware, gates, pressure vessels and fences.