There are many reasons to use preheating when welding. In some applications, manufacturers preheat materials prior to welding. Sometimes preheating is required by specifications, sometimes it is an elective technique that conforms to best practices. Either way, it is critical to work with a manufacturer that understands this process and knows how to incorporate it into planning.
The 3 Reasons for Preheating
Manufacturers generally preheat for one of three reasons:
- 1. It is required: Welding codes or standards sometimes require preheating. Certain materials — such as high carbon or alloy steels — are prone to cracking during welding. Cracking, which is the worst kind of welding flaw, can occur when metal cools too fast. Preheating slows cooling of both liquid weld metal and base metals.
- 2. It increases productivity: Manufacturers sometimes choose to preheat steel because it increases productivity in three ways. First, it can reduce weld distortion. Second, it encourages weld metals to blend more fluidly into the surrounding areas, which can reduce the amount of required grinding. Finally, it reduces porosity by slowing the cooling enough to prevent gas bubbles from getting trapped as the metal solidifies.
- 3. It reduces stress: Preheating can work in conjunction with post-weld heating to bolster stress relief efforts. Some materials are especially prone to cracking, and the best results are achieved when post-weld stress-relief heating is preceded by major preheats of 500-600 degrees Fahrenheit.
Preheating and the Planning Process
Since it can take considerable time to heat a massive amount of steel, preheat requirements must be accounted for during planning. Since preheating involves several variables and complicated considerations, it is better for planners to flag this requirement early.
Planners must consider if the steel needs to cool overnight, or if teams will have to maintain the temperature so that less preheating is required the next morning. In some cases they will have to maintain a high temperature, finish welding and then, if necessary, move straight into post-weld stress relief without first letting it cool.
Often, manufacturers preheat because it is mandated by a welding code. Sometimes preheating is conducted by choice to achieve a benefit in productivity or stress relief. Either way, the need to preheat must be identified early in the planning process to enjoy optimal results.