Pre-heat and post-heat treatment machines for welding

What is heat treatment in welding?

The process involves heating the welded area to a specific temperature. Then holding it at that temperature for a set duration, followed by cooling the material at a controlled rate.

What role does heat treatment in welding play?

It's a series of processes carried out on welded materials to improve their properties. It relieves internal stresses in the material and improves the weld strength. Thus preventing cracking or distortion of the metal. Heat treating plays a critical role in ensuring that welded materials are strong, durable, and free from stress and distortion.

Advantages of pre-heat and post-heat weld treatment.

Introducing heat into the base metal by an external heat source does add another step to the welding process. But in the long term it can save both time and money. Reducing the potential for a failed weld that requires rework.

• It helps to lessen shrinkage stresses that can lead to cracking and distortion. Because hot materials expand and cool ones contract, a large temperature variance between the weld pool and base material occurs. This can result in internal stresses as the weldment tries to normalize those temperature differences. These internal stresses increase the risk of cracking and distortion.
• Preheating helps to slow the cooling rate of the finished weld, reducing hardness in the heat-affected zone. This creates a weld that is less brittle and more ductile. These characteristics are important for materials more susceptible to hardness at elevated temperatures. These include cast iron, medium and high-carbon steel, or high-carbon-equivalency steel. Slowing the cooling rate also allows hydrogen to escape the weld puddle as it hardens. Again helping to minimise cracking.
• It also introduces the necessary heat into the weld area to ensure proper penetration. This benefits thick materials and those that conduct heat quickly. By preheating, you can use less heat in the welding arc and still achieve optimal penetration. This is because the base material starts out at an elevated temperature.

When Should You Preheat Welded Joints?

Preheating is especially important when welding the following:

• Highly restrained weld joints.
• Thick materials (the rule of thumb on thickness and when to preheat varies by material type).
• Base materials that tend to be more brittle, such as cast iron.
• When welding dissimilar materials.
• When recommended by the base material manufacturer.

This information is often found in a table that specifies preheat temperature ranges for a given material thickness. Preheating can be beneficial for materials with a high-carbon equivalency. Such as AISI 4130 and 4140. High carbon levels and/or the addition of alloys can make the material stronger and harder. Yet also more brittle and less ductile, leading to potential cracking issues.

Industries where pre-heat treatment equipment is used.

• oil & gas
• (petro) chemical & refineries
• power
• food & pharmaceuticals
• metals & mining
• infra & rail
• renewables
• structural steel
• pulp & paper

Normally a preheat maintenance i.e. preheat throughout the welding is required for more exotic materials such as Cr-Mo steels, HSLA steels, etc.

Types of heat treatment for welding applications.

• Induction - creating a magnetic field that generates eddy currents within the base metal. Induction heating involves heating internally from within. Accessories, such as cables or blankets, are placed on the part to generate the magnetic field. Pros: Induction offers quick setup, often in less than five minutes, and a fast time to temperature. The process produces a uniform heated area. This makes it easy to achieve and stay within a strict temperature window. The process is also efficient and doesn't need personnel to watch it. Induction provides safety benefits, since the output coils don’t get hot. Avoiding an uncomfortably hot environment for welders. The process also provides the flexibility to heat many different part sizes and shapes. While delivering localised heating where the part is.
  

• Open flame - Operators use a fuel gas and compressed air torch (rosebuds) to apply a flame directly to the part. Pros: If welding operations already have a flame torch, extra equipment isn't needed for preheating. Flame preheating also requires little operator training. 
• Resistance heating - uses electrically heated ceramic pads placed on the base metal. The heated tiles use radiant and conductive heat, transferred to the part via heating pads. Pros: Preheating with resistance can deliver temperature consistency when all elements are working. It’s an ideal method for larger projects and parts, when using multiple heating elements.
• Ovens - used for welding preheat via convection heating. The entire part goes inside the oven for preheating. Pros: Uniform heating is possible since the whole piece is inside the oven. It’s also a good option when the operation needs to do batch heating of many parts at once.