Weld Pool Solidification Rate
The local velocity of the phase boundary during the crystallization of the weld pool. It decisively influences the dendritic microstructure and segregation tendency in the weld metal.
The solidification speed of the weld pool determines how the weld structure looks. If the metal cools too fast, the structure becomes hard and brittle, increasing the risk of cracks. If it cools too slowly, you get a coarse grain that is softer and picks up more stress. You have to adjust the cooling so that the weld becomes tough enough. You control this via heat input, i.e., amperage and travel speed, and for thick parts, if necessary, preheating or post-heating. You can estimate the exact time in the critical range with simple rules of thumb, but when in doubt, it's better to let it cool a bit slower, then the weld won't be as hard.