Sources of inclusion formation

Inclusions can form either (a) during transfer of molten steel from one reactor to other or (b) during solidification of steel  (lecture 27) or during solid state processing by any of the following mechanisms:

i)  Reaction between rejected solute elements during solidification, for example, reaction     between sulphur and manganese, and between oxygen and aluminium etc.

ii)  Mechanical and chemical erosion of refractory and other materials.

iii)  Oxygen pickup by teeming stream and consequent oxide formation.

iv)  Chemical reactions.

Inclusions produced by mechanism (i) are called endogenous, whereas mechanisms (ii) and (iii) produce exogenous inclusions. Inclusions can form during

a)  Tapping of molten stream from BOF/EAF to ladle. Erosion of launder refractory is the       possible source. Pick up of oxygen from atmosphere and formation of FeO.

b)  Treatment of steel in ladle. Here molten steel is in contact with the refractory. Also      during deoxidation and synthetic slag treatment  oxide/sulphide inclusions may form

c)  Teeming of molten stream. Molten steel is in contact with stopper and nozzle refractory      and elements like Ti, Mg etc., which can form oxides. Air entrainment into molten steel      stream brings oxygen and FeO formation is initiated.

d)  Solidification in mould due to precipitation of excess solute elements.

e)  Final finishing operations like heat treatment and deformation processing. Here steel is      heated to high temperature which may cause surface oxidation, surface sulphurization,      inner oxidation,etc.

f)  Fusion welding. Oxidation of weld pool, electrode coatings are the possible source of     inclusions in fusion welding processes.