Thermal gradient

A thermal gradient is simply how fast temperature changes as you move away from a heat source — steep near the arc, shallow far from it. It is not the temperature itself but the rate at which temperature falls off with distance, and that rate is what actually drives the mechanical consequences: a region that cools or heats faster than its neighbour expands or contracts by a different amount, and the mismatch is what produces thermal stress.

Welding creates some of the steepest thermal gradients found in ordinary fabrication, because the arc concentrates a large fraction of the heat input into a very small area for a very short time. The material closest to the weld reaches or exceeds its melting point while material a short distance away barely warms, and the gradient between those two points governs the microstructure across the heat-affected zone, the distortion the part develops as it cools, and the residual stress locked in once it has finished cooling. A shallower gradient — from preheat, a slower travel speed, or a controlled interpass temperature — reduces all three effects, generally at the cost of a larger heat-affected zone overall.

In processes that build up material layer by layer, such as wire arc additive manufacturing, the thermal gradient does not reset between passes the way a single weld’s does: each new layer reheats the ones below it, and the gradient across the growing part changes shape as the build gets taller. Thermography is the practical way to see a gradient directly, because it renders temperature as a function of position across the whole field of view rather than as a single number, which is exactly the information a gradient is.