Effects of Welding Speed and Micro-Alloy Element on the Weld Solidification Structures of Steels

Abstract

Effects of welding speed, micro-alloy element and amount of C on the weld solidification structures have been investigated using TIG welds of mild steels and carbon steels (0.36 wt% C) with a micro-alloy element, such as Ti, Nb and Zr. The results obtained are as follows. (1) The weld solidification structures are changed by welding speed, that is, axial crystal is developed at slow welding speed, stray crystal and columnar crystal at moderate and equiaxed dendrite at high. (2) The weld solidification structures are also changed by addition of the micro-alloy element. For specimens with the micro-alloy element, axial crystal is not obtserved and equiaxed dendrite is developed at lower welding speed in comparison with specimens without the micro-alloy element. (3) The formation of equiaxed dendrite is influenced by kind and amount of the micro-alloy element and G/R (G: temperature gradient, R: growth rate). When TiN and TiC etc., these disregistries are small, exist in molten pool, heterogeneous nucleation occurs easily and the ratio of equiaxed dendrite zone to the bead width becomes larger.