Investigations on Fume Generating Phenomena of Shielded Metal Arc Welding

Abstract

As a research of the essential characteristics of welding fumes, variations of elemental percentages in fumes have been studied as functions of respective percentages in the coverings, using various grades of commerical covered electrodes and some test electrodes. Together with observation of welding arc by means of high-speed camera, fume generating phenomena and mechanisms have been considered.
In general, the content of an element in fume is linearly increased with increasing content in covering. The higher the vapor pressure, the larger the regression coefficient. In the cases of iron-oxide and alkaline, the correlation between contents in fume and in covering is much different according to the type of covering, lime or non-lime. Iron-oxide occupies almost 50 % in fumes from non-lime covering, while it is always lower than 25 % in fumes from lime covering. Alkaline content is much higher in lime fumes than non-lime even if they have the same content in covering. Intermediate covering of a test electrode produces fumes with intermediate characteristics in terms of iron-oxide and alkaline. Calcium is a major cause of the lime type behavior, and it is more effective when more calcium fluoride is contained.
The arc of non-lime electrode generates from molten metal at the tip of the electrode, while that of lime electrode comes through the molten slag which covers the hanging drop at the tip. Therefore, the former fume composition is regulated by the molten metal composition, and the latter by the molten slag. Thus the mechanism of the difference between lime and non-lime can be explained.