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WHAT ARE THE SPECIAL ALLOY STEELS ?

In service situations where steels must resist high temperatures,. corrosion, shock, etc. special alloy steels are invaluable. The most important groups of special alloy steels are described in the following discussions,

MAGNET STEELS :

High Cobalt Steel, when correctly heat treated, are frequently used in the making of permanent magnets for magnetos, loud speakers and other electrical machines and instruments. Steels having compositions 15 to 40 percent cobalt 0.4 to 10 per cent tungsten possess improved magnetic properties.


HEAT RESISTING STEELS :

Heat Resisting steels are those which are particularly suitable for working at high temperatures. Such Steels must resist the influences which lead to the failure of ordinary steels when put to work under high temperature. A steel containing chromium, nickel and tungsten, with the carbon content suitably controlled (developed for the stainless series) provides a useful combination of non-scaling and strength - retaining properties together with resistance to acid corrosion comparable with that of stainless steels.



 

Alloy steels containing 23 to 30 per cent chromium with carbon less than 0.35 per cent are used principally for service at temperatures between 815*C and 1150*C. Furnace parts, annealing boxes and other equipment requiring resistance to high temperature are often made of these steels.

SHOCK RESISTING STEELS :

Shock resisting steels are those which resist shock and severe fatigue stresses. One grade of steel for this purpose  contains 0.50 per cent carbon, 2.25 per cent tungsten, 1.50 percent chromium and o.25 per cent vanadium. Another grade of shock resisting steel , known as silicon manganese steels, contains 0.55 per cent carbon, 2.00 per cent silicon, 0.80 per cent manganese, and 0.30 per cent molybdenum. This kind of steel is mainly used for leaf and coil springs. 




STAINLESS STEELS :

Stainless steels are essentially those containing chromium, together with other elements such as nickel , and are grouped as under.

a) Austenitic Stainless steels 

b) Martensitic Stainless steels

c) Ferritic Stainless Steels 




MARAGING STEELS :

They are Iron based alloys containing 18 Ni 8 Co 5 Mo with small amounts of Al and Ti and less than 0.03 per cent C. The strength is maintained with increase in section thickness and also up to 350*C, These steels are used for air frame and engine components, injection molds and dies. 



On cooling from austenitic condition the alloy transforms to a fine lath type martensite, and precipitation hardening is induced by maraging at 480*C.

The steels have high fracture toughness due to a combination of fine grain size of the martensite and high dislocation density, leading to fine precipitation.

HIGH - SPEED STEELS : 

High - Speed Steels (HSS) get their name from the fact that they may be operated as cutting tools at much higher cutting speeds than is possible with plain carbon tool steels. High - Speed Steels operate at cutting speed 2 to 3 times higher than for carbon steels. When a hard material is machined at high speed with heavy cuts, sufficient heat may be developed to cause the temperature of the cutting edge to reach a red heat. This temperature would soften carbon tool steel containing even up to 1.5 per cent carbon to the extent of destroying their cutting ability. Certain highly alloyed steels , designated at high - speed steels, therefore, have been developed which must retain their cutting properties at temperatures up 600*C to 620*C. There are four general types of high speed steels  :

a.) 18-4-1 High-speed Steel . 

b.) Cobalt high - speed steel. 

c.) Vanadium high - speed steel.

d.) Molybdenum high - speed steel.




 

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