Effect of S and Si on the Formation of Intragranular Ferrite and Inclusions in Ultra-low Oxygen Weld Metal of Low Carbon Steel

Abstract

It is well-known that the toughness of carbon steel weld metal can be improved when the oxygen content is generally about 300 to 400 ppm in arc weld metal, because of high fraction of intragranular ferrite (α) nucleated and formed on oxides. It is generally difficult to nucleate intragranular α from oxides in electron beam weld metal which is extremely low oxygen content at about 10 ppm. In this study, the influence of S and Si contents on inclusions and intragranular α formation in the extremely low O content weld metal are investigated. Specifically, inclusions as nucleation sites of intragranular α are examined and their role in the nucleation mechanism is discussed. Adding S and Si is very effective in promoting the nucleation of the intragranular α in weld metals. The intragranular α is nucleated by complex inclusion of Si–Mn oxide and MnS. The rise of the A_e3 transformation temperature around the inclusions along with the formation of the manganese-depleted zone (MDZ) is considered as a principal intragranular α nucleation mechanism. The S addition has the effect of increasing the α nucleation ability of the inclusions by coarsening the inclusions and forming MDZs around the inclusions. The Si addition has the effect of increasing the SiO₂ content of the inclusions and increasing the intragranular α nucleation probability of the inclusions.