During the manufacturing process of welding, a large amount of heat is used to create a molten weld pool. As the weld pool cools, it fuses creating a strong joint between two parts using the weld metal. As the temperatures seen in welding are extremely high, how the weld pool cools is important to control. This is where weld preheating is used.

Below we will discuss what is weld preheating, how we preheat, how it is controlled and the advantages and disadvantages of the process.

What is Weld Preheating?

Weld preheating is when the surrounding material around a weld is heated to a specific temperature before starting the weld. This temperature should be maintained throughout the welding process. Parameters affecting the required temperature depend on the type of material, its thickness and environmental factors. For example, high carbon steel (>0.2%) will always need preheating.

Why is Weld Preheating Important?

Preheating is an essential part of the welding process and is important in ensuring good and repeatable weld quality.

• – Preheating the surrounding material slows the rate of cooling of the weld pool which affects the hardness of the weld. The result is a desirable grain structure of the weld and a reduction in martensitic formation.
• – Slowing the rate of cooling encourages hydrogen diffusion from the weld area by increasing the time spent in an elevated area. The elevated temperature provides a greater diffusion rate for hydrogen compared to room temperature, reducing the risk of hydrogen cracking.
• – Helps remove any moisture and dirt from the material that may potentially contaminate the weld and lead to weld defects.

When To Use Weld Preheating

Several factors determine weld preheating, including the type and thickness of the base material. Steels over 25 mm of thickness and more than 0.2% carbon tend to require a greater preheat temperature than other types of steel.

However, the WPS (welding procedure specification) should state the required preheating temperature if required. It is common for the WPS to state an acceptable temperature range, and sometimes it will say the amount of time before the welding the material must undergo preheating.

Different Methods of Weld Preheating

For preheating in welding, heat is applied to the surrounding area of the weld or the entire component. There are four common methods of weld preheating.

Ovens

Ovens are used for weld preheating to heat an entire part to a specific temperature and maintain it throughout the process. With the oven set to the correct temperature, there is good control of the process. The nature of the heating process results in uniform heating of the component.

However, if the components are large, a large oven is required, which means it will be expensive to run and does not offer flexibility in terms of mobility.

Induction Heating

Induction heating creates a magnetic field in the component, which results in eddy currents. This process results in the component being heated from the inside out. The welding preheat process can be achieved using blankets or cables, which will both give localised heating and will not uniformly heat the entire component, however, it will uniformly heat the desired area. Unlike an oven, induction heating for weld preheating is a lot more efficient and can be achieved in a short time.

A disadvantage of using induction heating for weld preheating is that it has a large initial cost, and the operator will require training to use it correctly.

Open Flame 

Open flame heating for weld preheating consists of the operator using a gas torch to use a flame to apply heat directly to the area. An advantage of this process is that it is straightforward to carry out and requires little to no training for the operator.

However, the process results in uneven heating of the area, and it is difficult to maintain the temperature of the area. Additionally, the process naturally introduces health and safety issues due to the introduction of flames in the workplace and harmful compounds such as soot.

Resistance Heating

Ceramic pads are used to implement resistance heating for weld preheating. The ceramic pads are heated to a specific temperature and transfer the heat to the material where they are in contact. The method allows for accurate control of the metal temperature and can be used on small- and large-scale projects.

Resistance heating is very expensive and proves to be time-consuming as you often have to contract the work out to a third party. There is a long set-up time for this method, with it taking potentially up to 1 hour to just set the pads up.