Why does weld metal cool rapidly after an arc passes?

In fusion welding, the rapid cooling of the weld metal after the arc passes is primarily driven by the dissipation of heat through the surrounding base metal. Because the base metal remains relatively cool compared to the molten weld pool, it acts as a heat sink, drawing thermal energy away from the fusion zone through conduction. Additionally, some heat is lost to the environment through radiation.1

This rapid cooling is a critical factor in the metallurgy of the joint, as the rate at which the weld metal freezes significantly influences its final microstructure and mechanical properties. For instance, a one-pass weld typically develops a cast structure characterized by columnar grains that grow from the edges of the molten pool toward the center. Managing these cooling rates is essential, as the thermal cycle—along with the resulting contraction of the weld metal—can induce residual stresses that may lead to distortion or bowing in the finished structure.1