CHROMIUM REACTION
The study of chromium reaction is important in case of melting of stainless steel scraps. The requirement for production of stainless steel is to have minimum oxidation of
chromium during the steel making process. The reaction between carbon and chromium depends upon the thermodynamics of oxidation of these two elements. The presence of carbon
and oxygen slows down the reaction of chromium. Some chromium oxides also joins the slag phase. The chromium presence in slag phase depends upon the solubility of chromium in
the metal bath. To produce high chromium low carbon steel, the temperature of the bath should be kept as high as possible. For making low carbon chromium steel, low chromium
contents scraps which are oxidized at high temperatures should be added. These scraps can oxidize carbon. Finally the metal bath may be enriched with low carbon ferro chrome.
Studies on kinetics of chromium transfer have been made and it is found that the rate of chromium distribution approaches equilibrium stage at the end of the heating period.
STAGE 1 : Chromium contents below 9%
FeCr2O4 + 4C = Fe + 2Cr + 4CO
STAGE 2 : Chromium contents above 9%
Cr3O4 + 4C = 3Cr + 4CO
EFFECT OF CHROMIUM ON ALLOY STEEL
Chromium is a strong carbide forming element. It is added to alloy steel to increase hardenability. It dissolves in alpha and gamma iron to increase strength and toughness.
The fine dispersion of carbide increases hardness and wear resistance. Chromium also increases the wear resistance of carbonized steel.
Due to this character chrome steels find wide applications in dies, rolls, files, tools, plates, cutlery and surgical equipments. When nickel is added to chrome steel,
its corrosion resistance, hardness and wears resistance increases. Such alloy steels are used for gears, piston pins, ball bearings and crushing machineries.
But such alloys suffer from tampered brittlement. Other alloying elements like molybdenum, vanadium, tungsten, cobalt and titanium are used to produce special chrome steel alloy.
Another type of steel known as trip steel is an alloy of iron, chromium, manganese, nickel, molybdenum, silicon and carbon. In trip steel, chromium is present up to
10% of the composition. Trip steel is cold rolled at the room temperature. It is used for production of wires, rods and sheets.
Chromium-Vanadium steel, with 0.2% of vanadium are widely used for structural purposes. Vanadium has strong carbide-stabilizing tendency. Heat resisting steels are produced by Chromium-Nickel-Aluminium-Silicon steel. These steels are used for exhaust valves for aero-engines. These steels are also used in furnace stove. Conveyor chains .furnace arch ,floor plate, recuperator tubes ,rotors, for gas turbines, annealing boxes, pyrometer sheaths, retorts and burner nozzles .The requirements of such steels are resistance to oxidation by gases in working temperature. These alloys should have sufficient strength at working temperature .These alloys should have freedom from structural changes at the working temperature.