Chromium is the main constituent of stainless steel. It forms a very thin protective and stable oxide film on the surface of the stainless steel.
This film is continuous, impervious and passive in nature. It checks the reaction between the stainless steel and the surrounding atmosphere.
Chromium also imparts strength, toughness and pleasing appearance to stainless steel. It helps in corrosion resistance and oxidation resistance.
It also provides good creep strength. It provides high resistance to scaling and oxidation at elevated temperature. Due to this character,
stainless steel is used in reactors, storage tanks, heat exchange pipes and underwater workings.
The stainless steel is divided into four groups based upon presence of chromium.
a) Austenitic stainless steel (Cr-18-25%)
b) Martensistic stainless steel (Cr-12-17%)
c) Ferritic stainless steel (Cr-20-25%)
d) Duplex stainless steel (Cr-25-35%)
The major portion of chromium goes into the production of alloy steel. Addition of chromium to steel increases its hardness. The increase in hardness is due to the carbide stabilization character of chromium. The carbide is harder than ordinary cementite. In low carbon steel, the addition of chromium increases its strength. But there is loss of ductility in steel. Care is to be taken to avoid overheating of the chrome steel. Low chromium, low carbon steels are popular for case hardening. This type of steel is used for automobile axles, connecting rods, gears, ball-bearings and parts of grinding machines. Low chromium, medium carbon steel is used for axles and connecting rods for heavy vehicles. Addition of chromium in large quantity up to 25% has improved the corrosion resistance of alloy steel. This is possible due to development of protective layer on the surface of the alloy steel. To further improve the corrosion resistance, a small amount of titanium and niobium may be added. Ferritic stainless iron contains around 13% of chromium. The amount of carbon is kept very low. Due to its high corrosion resistance, it is used in chemical and engineering industries. Low carbon stainless steel is used as sink, food container, refrigerator part, bear barrel, cutlery and table wire. With 13% of chromium and 0.3% of carbon, an improved version of steel is produced and named as cutlery-blade-steel. Its improved version is used in razor blades, hand tools, garden tools, knife blades and food processing equipments. Molybdenum is also added to chrome steel to provide improvements in machinability and mechanical properties. The nickel-chrome-molybdenum steel is a best combination of alloy steel. It is used for good ductility and high tensile strength to produce large components of engineering goods.
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.