Preheat in Welding: What Is It and When Should You Use It?

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.
• Cons: Induction can have a higher initial cost and requires some operator training to ensure proper wrapping techniques.

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.
• Cons: Drawbacks of flame preheating include inconsistent temperature uniformity, a slower time to temperature since the process is inefficient, and comfort and safety issues for operators working near the soot, smoke and gases produced while using open flame. Flame preheating also involves the extra expense of the gas storage and distribution to the welding cells and requires personnel to be on fire watch* during the process. Flame can cost $40 to $60 in consumable fuel per hour.

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.
• Cons: The cost of using resistance can be higher, especially since the method is often contracted out to a third party who controls the schedule and timeline. It can also be time-consuming — sometimes taking an hour or more to arrange and secure the heating pads to the part — and the required cool-down time adds to the time for setup and removal. Resistance heating elements also need insulation to help retain heat. The insulation and ceramic heating pads are costly consumables that need to be replaced occasionally, and the work environment can be an issue for these fragile components. Broken pads can cause hot and cold spots in the part. In addition, resistance heating elements get extremely hot and can pose a safety hazard for operators as well as affect operator comfort by adding to the ambient temperature. With resistance heating, electricians and a costly electrical drop are also necessary. During the heating cycle, the power source becomes a “spark generator” and require continued maintenance supervision from the mechanical contactors.

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.
• Cons: Ovens can be quite large and require connection to an electrical outlet, so they are often permanently installed in one location and don’t offer mobility. As a result, parts must be brought to the oven for preheating, which can be difficult when very large parts are involved. These systems also typically require a substantial electrical connection and system, which adds expense. Also, the oven may have to be preheated for several hours before it’s used, and it can put out enough heat to warm up the area around the oven, negatively affecting worker comfort. Outsourcing oven preheating causes a loss of control over the schedule and timeline.

You can avoid the welding problems that threaten construction schedules and drive up costs.