Preheat — What It Is and Why It Matters
Preheat is the application of heat to the base metal before welding to slow the cooling rate in the weld zone. By raising the base metal temperature before the arc strikes, the temperature differential between the hot weld zone and the surrounding metal is reduced, which slows cooling and prevents the formation of hard, brittle microstructures that are susceptible to cracking.
Preheat is required on high-carbon steels, alloy steels, thick sections of any carbon steel, restrained joints, and when welding in cold ambient conditions. The required preheat temperature depends on material composition, thickness, joint restraint, and the specific welding code. AWS D1.1, ASME Section IX, and other codes provide preheat tables based on these variables.
Preheat is applied using oxy-fuel torches, electric resistance heating pads, induction heating coils, or forced-air heaters. Temperature is verified using temperature-indicating crayons (Tempilstik), contact pyrometers, or infrared thermometers. Preheat must be verified at the specified distance from the joint edge and maintained until welding in that area is complete.
Frequently Asked Questions
When is preheat required?
Preheat is typically required when the base metal carbon equivalent exceeds a threshold value, when material thickness exceeds code-specific limits, when ambient temperature is below 32°F (0°C), for highly restrained joints, and when using non-low-hydrogen welding processes on carbon steel over 1" thick. Always check the applicable code or WPS for specific requirements.
How is preheat temperature measured?
Temperature-indicating crayons (like Tempilstik) melt at a specific temperature and are the simplest method. Digital contact pyrometers and infrared thermometers provide more precise readings. Measure at the specified distance from the joint edge (typically 3" or one plate thickness, whichever is greater) to ensure the preheat has soaked through the material, not just surface-heated it.