Diffusion Coefficient
A temperature-dependent proportionality factor describing the migration rate of atoms or ions through a crystal lattice according to Fick's laws. It determines the kinetics of diffusion processes.
The diffusion coefficient describes how fast atoms move in the material – in welding, it determines whether alloying elements distribute cleanly or whether unwanted microstructures form. If the temperature isn't right, diffusion either runs too sluggishly, then the material doesn't weld properly, or too hot, then you get a brittle structure with cracks. So you have to control the heat input and cooling time, and choose the preheat and interpass temperatures suitable for the material. Especially with high-alloy steels, this is more critical than with structural steel, because carbon and chromium quickly cause problems.