Gases and Non-Metallic inclusions of Steel

GASES IN STEEL:-
The gaseous elements commonly present in steels are oxygen, hydrogen and nitrogen. In steel ingots or castings, gases may be present in the form of gaseous blow holes, as inclusions of oxides, nitrides or hydrides, or dissolved in the solid solution.
Gases get into the metal from the following principle sources:-

1. Charge materials.
2. Furnace atmosphere and the blast given to the surface or into the depth of the metal.
3. Ferro-alloys and various additions introduced into the metal and slag during melting or teeming.
4. The atmosphere surrounding molten metal at tapping and teeming.

NON- METALLIC INCLUSIONS IN STEEL:-
Compounds of metals (such as iron, manganese, silicon) with non-metals (such as oxygen, sulphur, nitrogen, etc.) which may be present in steels, are termed as non-metallic inclusions.
According to their size, inclusions are divided into two groups:-

1. Microinclusions.
2. Macroinclusions.

According to their composition, they are usually classified as:
Sulphides (FeS, MnS)
Oxides (FeO, MnO)
Phosphides (Fe3P, Fe2P)
Nitrides (NiN, VN)
Carbides (Fe3C, Mn3C)
Complex Inclusions such as silicates, oxysulphides.

The principle sources of non-metallic inclusions in steel are as follows:

1. Inclusions present in the charge materials; pig iron and scrap.
2. Inclusions passed to the metal from the lining of the furnace, launders, ladles etc.
3. Inclusions formed in the steelmaking process; by interaction between metal and slag; on addition of deoxidants or alloying elements; by precipitation from solidifying steel.

EFFECT OF GASES AND INCLUSIONS ON STEEL PROPERTIES:-
The solubility of gases in steel decreases with lowering temperature, especially on the change from liquid to solid state of the metal in moulds, steel that has had a high gas content in molten state will have many blow holes on solidification and possess poor mechanical properties, which can make it unsuitable for application.
Elevated concentrations of oxygen in killed steel can cause the formation of blowholes due to the reaction with carbon: [O] + [C] ó COg. Besides, if oxygen is present in a high concentration in steel, it can form substantial quantities of low melting oxide and oxysulphides inclusions which solidify at grain boundaries, thus lowering the strength of metal at high temperatures (RED SHORTNESS EFFECT).
Nitrogen impairs the plastic properties of steel, increases its brittleness at low temperatures (COLD BRITTELNESS), and makes it liable to ageing.
Hydrogen is responsible for a number of defects in steel (flakes, fish scale fracture, etc.). These defects are associated with evolution of monoatomic hydrogen which has been dissolved in molten metal.