Preheat
Preheat turns up in 2 fields: thermal spray, welding. Here is what it means to the people in each one.
Preheat in thermal spray
Warming the substrate before spraying to drive off moisture and reduce stress at the coating interface.
Heating the part, often to about 90 to 150 °C, just before the first coating pass, usually with the gun itself making passes without powder or with a separate torch or oven. It removes condensed moisture and adsorbed contaminants and narrows the temperature gap between hot splats and cold metal, reducing interface stress. Too much preheat oxidizes freshly blasted surfaces, so it is kept short and controlled.
"Two preheat passes with no powder, check it's at 200 F on the gun, then start the bond coat."
Preheat in welding
Heating base metal before welding to slow cooling and drive off moisture, preventing hydrogen cracks.
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."