In this research, two-color pyrometry has been conducted to obtain the surface temperature of the weld pool, in which the weld pool is photographed by a high speed camera during arc welding. Two wave-lengths (950 and 980 nm) of light in the infrared range were selected from the thermal radiation light emitted from the weld pool at the instant when the arc was extinguished, by using an imaging spectroscope. The temperature is obtained from the intensity ratio of two waves of light. Consequently, in GTA welding, it is shown that the surface temperature distribution of a weld pool is affected by the sulfur content in the base metal. It is thought that this surface temperature distribution is determined by the balance between the driving forces of viscous drag from the cathode jet of plasma and Marangoni surface tension. In GMA welding, it is seen that the surface temperature distribution of a weld pool becomes uniform and the temperature is 1715–1845K, which is obviously lower than that of the metal droplet. For this reason, it is considered that the convection velocity on the weld pool surface and in the weld pool is high and, consequently, the heat input from the arc is transported to the back of the weld pool due to this convection. It is supposed that the edge of the pool below the arc has a high temperature compared with the central part of it due to the influence of high speed convection.
