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Friday, 12 February 2016

Special Alloy Steels

(iii) Special Alloy Steels:

(a) Manganese steels:
          All steels contain small amounts of manganese to mitigate the bad effects of sulphur. The true manganese alloy steels contain much larger amounts of Mn. More manganese reduces strength and ductility. Manganese steels show high percentage of elongation. Specific gravity = 7.92. Melting point = 1343°C. They have work hardening properties. Manganese steels can be forged but special care is necessary to avoid degrading the steel. After forging the steel should be heat-treated by raising it to temperature of 1010 and quenching in water. They are used for railway points and crossings, and with usage, they become more wear-resistant.

(b) Nickel steels:
         Nickel can be added in steels up to 50%. Nickel makes the steel highly resistant to corrosion, non-mgnetic, and having very low coefficient thermal expansion. Invar (Ni=36%) and super invar (Ni=31%) are the popular materials for least co-efficient of expansion and are used for measuring instruments, surveyor tapes and clock pendulums. Such steels are used for turbine blades, internal combustion engine valves etc.,

(c) Chromium steels:
         Chromium makes steel corrosion resistant, and increases its UTS. And IZOD strength. Very often alloy steels are used with both chromium and nickel being added. Ni-Cr steel wires are often used in furnaces, toasters and heaters.

(d) Silicon steels:
        A steel containing 0.05% carbon, about 0.3% Mn and 3.4% of silicon possesses extremely low magnetic hysteresis and is used widely for making laminations of electrical machines. Silico-manganese steels are also used frequently for making springs.

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