Refining Mechanism of Crystal Grains in the Weld in Upset Butt Welding

Abstract

Refining mechanism of crystal grains in the upset butt weld is studied by some model experiments under various conditions.
According to the experiment on the relation between the temperature of the weld interface and the behavior of crystal grains under constant upset pressure, it is clear that if the temperature of the weld, interface is above a certain value in the upset butt welding process, grains of the weld become fine, and so the boundary between fine grains and coarse grains makes nearly a line, and grains change abruptly from fine to coarse at this boundary. Fig. 3(c), 6. These phenomena show that there is a critical temperature for refining the crystal grains in the weld. Also this critical temperature is mainly determined by material, and is between 420-620°C for pure iron (0.003%) and 620-800°C for carbon steel (S15C). From these values, these critical temperatures seem to be recrystalization temperatures.
Next, the stage of grain refining in welding process is investigated by stopping the platen movement at maximum temperature of weld interface. As the result, it is made clear that grain refining occurs when the weld is pressed continuously in the stage while the interface is being cooled, and that upset pressure influences the degree of grain refining. Furthermore, it is shown that if the weld is not deformed, grain refining does not occur. (Fig.13).
From the above results, refining mechanism of crystal grains is as follows. In upset butt welding, the weld is deformed continuously by upset pressure until the temperature of the weld becomes low. At this cooling stage, crystal grains become fine by successive recrystalization without giving no chance of grain growth, because the deformation takes place up to the recrystalization temperature.