Foreign Inclusions

Slag inclusions

This is formed due to entrapment of oxides and non metallic solid material in the weld deposit or between the weld metal and base metal. Due to their low specific gravity, the slag normally floats over the molten metal unless they are restrained. Because of the stirring action of the arc, the slag may be forced down below the molten metal and the high viscosity of the weld metal, rapid solidification at a low temperature may prevent the release. Slag inclusions usually appear as a linear continuous or interrupted bands. Slag inclusions inside a weld would have rarely, resulted in service failures unless they occur along the surface or were of a sufficiently large section to reduce significantly the strength across the wall thickness.

Proper preparation of the groove before depositing further layers, preheating the base metal to have control on the rate of solidification and hence release of slag from molten metal, avoiding larger size electrode for root pass welding, thus preventing the slag flowing down into the root opening, are some of the ways of preventing slag inclusion.

Oxide films in fusion weld

Because of inadequate shielding, often encountered in MIG welding, a thin film of oxide forms on the globules of deposited weld metal. The presence of such films (defects) seriously impairs the ductility of the weld metal. This defect is rarely detectable except with the aid of a microscope.