What is Preheat in welding? Find out more here

A crucial step in many welding applications, preheating slows the rate of cooling in a finished weld, lowers the amount of hydrogen in it, and reduces the risk of cracking. Artificially 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 you both time and money by reducing the potential for a failed weld that requires rework.

With several types of equipment supplied for preheating the base material, all have benefits and drawbacks. A specific application depends on several factors, including any code requirements that may apply. Consider these tips and best practices that contribute to proper preheating and a high-quality weld.

Why Is Preheating Important?

Preheating minimizes the temperature difference between the welding arc and the base material. This benefits the weldment in several ways.

First, 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 molten weld pool and the relatively cool base material can result in internal stresses as the weldment tries to normalize those temperature differences. These internal stresses increase the risk of cracking and distortion.

Second, proper preheating helps to slow the cooling rate of the finished weld and reduce hardness in the heat-affected zone (HAZ), which creates a weld that is less brittle and more ductile. These characteristics are especially important for materials more susceptible to hardness at elevated temperatures, such as 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 to help minimize cracking.

Last, preheating 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, because the base material starts out at an elevated temperature.

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

When Should You Preheat?

Preheating is especially important when welding:

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, and when welding dissimilar materials.
When recommended by the base material manufacturer. This information often can be found in a table that specifies preheat temperature ranges for a given material thickness.
Preheating also can be good for materials with a high-carbon equivalency, such as AISI 4130 and 4140. High carbon levels and/or additional alloys can make the material stronger and harder, but also more brittle and less ductile, which can lead to potential cracking issues.

How Are Parts Preheated?

Once you have determined that the welding application requires preheating, consider the best method to use.

Methods of preheating

In welding preheat, the heat can be applied directly around the area of the weld joint, or the entire part can be heated. There are four common welding preheat methods:

Induction
How it works: Induction creates a magnetic field that generates eddy currents within the base metal, heating it internally from within. Induction 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. Induction produces a uniform heated area — making it easy to achieve and stay within even a strict temperature window. The process is also extremely efficient and does not require personnel to watch it. Induction provides safety benefits, since the output coils don’t get hot or create an uncomfortably hot environment for welders. The process also provides the flexibility to heat many different sizes and shapes of parts, and delivers localized heating where the part is.

Open flame
How it works: Operators use a fuel gas and compressed air torch (sometimes called rosebuds) to apply flame directly to the metal part.
Pros: If welding operations already have a torch, additional equipment doesn’t need to be purchased for preheating. Flame preheating also requires little operator training. 

Resistance heating
How it works: Resistance heating uses electrically heated ceramic pads placed on the base metal. The heated tiles transfer heat to the part through radiant heat and conductive heat where the pads are in contact with the part.
Pros: Preheating with resistance can deliver temperature consistency if the system is working accurately and no pads are broken. It’s also a method that can be used for larger projects and parts.

Ovens
How it works: Ovens used for welding preheat use convection heating. The entire part is placed inside the oven for preheating.
Pros: Uniform heating is possible since the entire part is inside the oven. It’s also a good option when an entire large part must be heated, or when the operation needs to do batch heating of many parts at once.

Industries where pre-heat treament equipment is often used

Oil & Gas
(Petro)chemical & Refineries
Power
Food & Pharmaceuticals
Metals & Mining
Infra & Rail
Renewables
Manufacturing
Pulp & Paper