Bringing the joint area to a minimum temperature before the first arc, typically 50 to 150 °C for carbon and low-alloy steels depending on thickness, carbon equivalent and hydrogen level, and higher for some alloys. It slows cooling, lets hydrogen diffuse out and lowers HAZ hardness. It is checked with temperature-indicating crayons or contact thermometers at a set distance from the joint, usually on the side away from the heat source.
Two-inch A572, preheat 150 F minimum, check it with the crayon three inches back from the bevel.
Preheat the full thickness, not just the skin: a torch waved over the bevel for a minute fools the crayon while the core of the plate stays cold.
Also heard as
- preheating
- preheat temperature
Related terms
Interpass temperature
The temperature of the weld area just before the next pass starts, with minimum and maximum limits.
Hydrogen cracking
Delayed cracking in steel welds caused by hydrogen, hard microstructure and stress acting together.
HAZ
Heat-affected zone: base metal next to a weld whose structure changed from the heat but did not melt.
WPS
Welding procedure specification: the written instructions that tell welders exactly how to make a weld.