Is preheating required before welding? Does a welder (machine) need to warm up? Do you need to heat the materials before striking a welding arc on them? These are some questions we’ll look into below.
In many welding processes and applications, preheating is a crucial step. Preheating helps slow the cooling rate of the finished weld. It also reduces cracking risk in the joint by lowering the hydrogen amounts in the finished weld.
Preheating or externally heating the material before welding them can add an additional step to the whole process, but in the long term, it can prove to be beneficial.
It can definitely save you a lot of hassle, time, and money by reducing the risk of a potential improper weld that might require reworking in near future.
The importance of preheating a welder
Does a welder (machine) need to warm up or not is a question for later, but it is definitely true that preheating the materials can be beneficial at times. When you preheat the material, its temperature difference to the welding arc reduces, which benefits the weld in several ways.
- Firstly, it helps reduce the shrinkage stress that can lead to risk of cracking and distortion in the finished weld. Hot metals expand and contract on cooling down. Thus, if there is a large temperature difference between the molten pool and the base material, it can result in internal stresses because the weld area tries to normalize the temperature difference. This can lead to cracks and distortion in the final result. Preheating the base material can reduce this risk.
- Second important thing, preheating slows down the rate of cooling in the finished weld. This is especially important in some metals that are more susceptible to hardness at higher temperatures such as cast iron. If the weld joint cools down faster, it can sometimes result in brittle joint which is less ductile and usually weaker. Preheating helps prevent this from happening, giving you a less brittle, more ductile finished weld. A slowed cooling rate also allows hydrogen to escape the molten weld pool before it hardens completely, minimizing the risk of cracking.
- Another important aspect of preheating is that it allows to start with an elevated temperature in the base material. This ensures proper penetration, especially when using thicker materials. By preheating, you can get optimal penetration even if you use less heat in the welding arc.
When is preheating required?
Preheating is important and very useful in certain situations. For instance, if you are working with highly restrained well joints or thick materials. As we saw above, it can be very beneficial when welding thick pieces.
However, the thickness that determines the need of preheating varies depending on the material of the pieces. Preheating is also important when welding base materials such as cast iron, that tend to be more brittle.
Sometimes, preheating requirement is specified by the manufacturer of the base material. This can be found in a table that specifies temperatures for preheating for different materials and varying thickness.
Preheating is also important when welding materials that have a high carbon equivalency. These materials can produce very strong weld joints, but can also result in brittle and less ductile joints. Preheating them can prove to be helpful in such cases.
How are base materials preheated?
Preheating using an open flame is a common preheating method. It is commonly used because of its ease of setup and portability. The initial cost is also very low and process to preheat using open flame torches is something welders are usually familiar with.
However, much of the heat being produced from an open flame is lost in the surrounding and it can be an inefficient method as compared to others.
Maintaining consistent temperature levels is also very difficult. For these reasons, other preheating methods and tools are used. It also poses various safety hazards such as getting burns.
Using an oven or furnace to preheat parts is an option, especially for smaller parts. Large ovens or furnaces can be used for larger parts. Induction heating is another effective preheating method that can be provide consistent heat throughout the welding area of the base material. Induction heating is considered as a safe preheating method.
This method is the most efficient method out of three as it minimizes the possibility of heat loss. This is because induction heating is done through passing alternate current.
A coil is placed around the base material or the part that needs preheating. The eddy current passes through the coil to the part as a result of the alternating magnetic fields produced around the coil. The resistance of the material to this current is what causes it to heat.
What to consider for preheating?
The preheating guidelines for various materials given by their manufacturers should be followed during the process. Along with that, there some general practices that should be followed as well.
- When using open flame for preheating, the distance of the flame from the joint is an important aspect to achieve proper preheating. The correct distance varies from material to material and application procedures or welding codes.
- It is best to preheat a large area around the welding joint to ensure that proper temperature will be maintained throughout the material. It minimizes the risk of heat being sucked away by the colder areas. Therefore, preheat a wide area around the weld joint for better results.
- The preheating temperature should be directly verified before beginning the welding process. This is usually done using infrared thermometers or several other devices for heat-measuring.
Does a welder need to warm up?
As seen above, preheating is important for many situations of welding applications. But does a welder (machine) needs to warm up?
This solely depends on the welding machine that you are using. Some welders (machines) can take time to accumulate enough heat required for the weld arc and in those cases you may need to warm the machine up before beginning the welding process.
Usually most welding machines do not require preheating and can be used directly after powering them up.