Preheating in welding is used to help ensure weld quality and reduce the occurrence of cracking and other problems that can result in costly rework. Welding preheat is commonly used before welding steel or steel alloy pipes or plates that are 1 inch thick or more. Preheating is often required in shop and field welding for oil and gas, transmission pipelines, power plants, structural construction, mining, shipbuilding and heavy equipment applications.
The process of preheating involves heating the area around the weld joint or the entire part to a specified temperature before welding. This reduces the cooling rate of the weld and drives out moisture, which in turn helps prevent hydrogen buildup and the potential for cracking.
Several methods can be used for welding preheat, including induction, open flame, resistance heating and convection ovens. Each one has benefits and drawbacks depending on the application. The best preheating method for a specific application often depends on the material thickness, the size of the weldment, the project timeline and budget, and available personnel and expertise.
When to use welding preheat
Determining if a welding application requires preheat depends on several factors, including the type and thickness of the base material. It’s typically dictated by the welding code being used. To meet the requirements of the code, the welding procedure specification (WPS) for the job will outline the minimum and maximum preheat temperatures as well as the necessary duration of preheating. Often, a part must be held within a specific temperature range for a certain amount of time — such as between 250 degrees and 400 degrees Fahrenheit for 30 minutes — before welding can start.
Welders typically must monitor the base metal’s temperature between weld passes to ensure the material remains within the required range. Common temperature verification tools include crayons, thermocouples, infrared thermometers and thermal imaging cameras.