List of Important Alloys of Steel | Metallurgy

List of important alloys of steel: 1. Silicon Steel 2. High Speed Tool Steels 3. Heat Resistant Steels 4. Spring Steels 5. Stainless Steel 6. Chrome Vanadium Steels 7. Nickel Steel 8. Vanadium Steel 9. Manganese Steel 10. Tungsten Steel 11. Magnet Steels.

1. Silicon Steel: 

A. Silicon improves considerably the electrical properties of steel.

B. Silicon imparts fatigue strength and resistance to steel.

C. Steel containing silicon is more ductile than a plain carbon steel.

D. Steel containing 3-5 per cent silicon has very low magnetic hysteresis.

Uses:

1. Steel with Mn = 1%, Si = 2%, C = 0.4 to 0.6% has very high elastic limit and is used for springs.

2. With Cr = 5 to 7%, Si = 2 to 4%, C = 0.4 to 0.5%, steel retains its hardness and resistance to oxidation even at red heat. Such steel is used for internal combustion engines.

3. Silicon steel containing 2.5 to 4% silicon and low percentage of carbon and manganese is employed for making laminations of electrical machines.

4. 13% silicon content steel has a very high corrosion resistance and so is used in chemical industries.

5. Steel containing 0.6 to 1% silicon and 0.75 to 0.95% manganese is used for structural purposes.

2. High Speed Tool Steels:

A. High speed tool steels are widely used for cutting of metals where hardness must be retained at elevated temperatures.

B. These steels are obtained by alloying tungsten, chromium, vanadium, cobalt and molybdenum with steel. This alloying produces metals which remain hard at temperatures at which normal steels become quite soft. A common analysis is 18% tungsten, 4% chromium and 1% vanadium with a carbon content of 0.6 to 0.7%.

This alloy is termed as 18:4:1 while an increase of vanadium to 2% produces 18:4:2 steels. Cobalt is often added to 18:4:1 and 18:4:2 steels as it improves the red hardness and cutting ability; from 5 to 10% is used.

C. Another class of high speed steel contains a lower percentage of tungsten, this being compensated by the addition of molybdenum; steel of this type is being increasingly used today.

D. In addition to heat resistance, high speed steels have the desirable properties of high hardness, high compressive strength and outstanding wear resistance. They are close competitors to carbides for metal cutting-tool material such as drills, reamers, milling cutters etc.

3. Heat Resistant Steels:

A. Steels which must be resistant to creep at high temperatures must contain molybdenum. Silicon and chromium impart resistance to oxidation and scaling.

B. Steels which are satisfactory upto about 700°C operating temperature are- C = 0.15% max., Si = 0.5 to 20%, Mn = 0.5% max., Ni = none, Cr = 1.0 to 6%, Mo = 0.5%.

Uses:

These are used in the as-rolled or as-forged condition, particularly for the valves of internal combustion engines. For higher temperatures upto 1000°C, steels containing upto 22% nickel and 26% chromium are used.

4. Spring Steels:

(i) Carbon-Manganese Spring Steels

A. C = 0.45 to 0.65%

Si = 0.1 to 0.35%

Mn = 0.5 to 1.0%

B. These steels are quenched and tempered to give a Brinell hardness of about 350.

Uses:

They are widely used for laminated springs for railway and general purposes.

(ii) Hyper-Eutectoid Spring Steels:

A. C = 0.9 to 1.2%

Si = 0.30% max.

Mn = 0.45 to 0.70%

B. These steels are oil quenched and tempered at a low temperature.

Uses:

They are used for volute and helical springs.

(iii) Silicon-Manganese Spring Steels:

A. C = 0.33 to 0.6%

Si = 1.5 to 2%

Mn = 0.6 to 1.0%

B. These steels are hardened and tempered to give a Brinell hardness of about 450.

Uses:

Employed for the manufacture of railway and road springs generally.

5. Stainless Steel:

The stainless steels are classified in two groups:

1. Plain chromium and high chromium low nickel steel.

2. Chromium nickel steel.

(i) Plain Chromium and High Chromium Low Nickel Steel:

A. Out of this group the former has C = 0.8% and Cr = 12 to 20%, whilst the latter has C = 0.1 to 0.2%; Cr = 12 to 20% and Ni = 2%.

B. These steels can be heat-treated.

C. Used for dies, valves and cutlery,

(ii) Chromium Nickel Steel:

A. These steels are non-magnetic and cannot be hardened.

B. They have varieties due to the varying contents of chromium and nickel respectively, e.g., 18 : 8, 12 : 2 and 18 : 9 (called stay bride). Small quantity of copper, tungsten and molybdenum is also added to these steels.

C. They have high resistance to corrosion and may be cold or hot worked, pressed welded, brazed or soldered.

D. The percentage of carbon in these steels is kept upto 0.5.

E. These steels are poor conductors of heat and electricity.

6. Chrome Vanadium Steels:

A. These steels usually contain 0.8 to 1.1% Cr, 0.25 to 0.33 C and less than 0.25% V. Even such a small amount of V has a marked effect on the properties of steel. It helps in producing cleaner steel because of the deoxidizing action. Vanadium is distributed both between the ferrite and the carbide involving both Cr intensifies the effect of V more than that of Ni.

B. Chrome-vanadium steels are used where great strength, toughness and resistance to fatigue are required such as axles and shafts of aeroplanes, automobiles and locomotives.

7. Nickel Steel:

A. The percentage of nickel varies from 2 to 40.

B. Nickel from 3 to 5% raises elastic limit and improves toughness.

C. Steel containing 20% nickel has very high tensile strength.

D. If nickel is present upto 27% it makes the steel non-magnetic and noncorrodible.

E. Nickel containing 3.5% nickel and 0.15% to 0.45% carbon has good ductility, high elastic ratio and resistance to fatigue. It is used for long span bridges.

F. 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.

G. Nickel tends to retard the grain growth in steel, resulting in a wide range of heating, without damage to the steel, or it may be maintained above the critical range for long periods of time without great damage.

H. The addition of nickel to annealed carbon steel apparently strengthens the ferrite, with the result that the endurance ratio is raised. The full effect of nickel on the endurance ratio, however, is quenching and tempering.

8. Vanadium Steel:

A. Addition of vanadium even in small proportion to an ordinary low carbon steel considerably raises its elastic limit and improves the fatigue resistance property.

B. When vanadium is added upto 0.25%; the elastic limit of the steel is raised by 50%, becomes exceedingly tough and can resist high alternating stresses and severe shocks.

C. Vanadium makes the steel strong and more ductile.

D. Steel with C = 0.15% to 1.1%, Cr = 0.5 to 1.5% and V 0.15 to 0.3% has high tensile strength, elastic limit, endurance limit and ductility.

E. Improves response to heat treatment.

F. Provides control of structure.

Uses:

1. It is widely used for making tools.

2. It may be used for shafts, springs, gears and drop forged parts. .

9. Manganese Steel:

A. Manganese from 1 to 1.5% makes the steel strong and tough to withstand severe duty but manganese content from 1.5 to 5% renders the steel hard and brittle.

B. When manganese is from 11 to 14%, carbon 0.8 to 1.5%, steel becomes very hard, tough, non-magnetic and possesses considerably high tensile strength.

C. More manganese reduces strength and ductility.

D. Presence of manganese even upto 2% reduces the formation of sulphide and deoxidation of molten metal.

E. Manganese steels show high percentage of elongation.

F. It may be forged easily but difficult to machine and thus it is usually ground.

G. The austenitic manganese steel is practically non-magnetic having a permeability of less than 2.0, so that it has special applications for parts that have to combine non-magnetism with high strength and resistance to wear.

H. Heat treated cast manganese steel in bar form is so ductile that it can be bent double when cold, without fracture; similarly castings of this metal will deform when fracturing. This deformation, however, is not continuous owing to the fact that cold-working raises the yield point in the stressed areas.

I. Specific gravity = 7.9; melting point = 1.343°C.

J. Manganese steel can be forged but special care is necessary to avoid degrading of the steel. After forging the steel should be heat-treated by raising it to temperature of 1010°C and quenching in water.

K. Manganese steel is weldable, a nickel manganese welding rod being employed for repairing fractures, an alternative is an 18/8 stainless steel rod. Care is necessary in welding to avoid overheating, as this tends to cause degrading of the steel.

Uses:

1. The properties of work-hardening renders this steel eminently suitable for such parts as the jaws of stone and ore crushers, tramway and railway points and crossings, etc.

2. Manganese steel has been much used for helmets and shields; it is also a good material for “burglar proof” safes and vaults as it is too hard to be cut with ordinary explosives.

3. Manganese steel in the hot worked condition is much used for rails.

4. Manganese steel in the form of bars is now widely used for screening coke. Ordinary steel wears away rapidly under the impact of the coke, but with manganese steel it has a life about a hundred times as long.

5. Owing to its non-magnetic property it has an important used in the cover plates of lifting magnets for handling heavy iron steel articles, and also for ship structure situated near the magnetic compass, since it has no effect on the latter.

6. Other applications include agricultural implements such as shovels; these are made from rolled manganese steel plates.

10. Tungsten Steel:

A. Tungsten when added to steel improves its magnetic properties and hardenability.

B. Addition of tungsten to an extent of 6% to high carbon steel enables it to retain the magnetic properties to high degree and produce field more intense than an ordinary steel.

C. 8% tungsten attributes so much hardness in steel that it can scratch glass.

D. Steel containing 1.5% is too hard to be machined by ordinary methods.

E. High percentages of tungsten makes the steel to retain its hardness even at high temperatures.

F. Tungsten used in certain known percentages, imparts air hardening properties to the steel.

Uses:

It is used for making permanent magnets and high speed cutting tools.

11. Magnet Steels:

Carbon, chromium, tungsten and cobalt steels are used for permanent magnets. The superiority, as well as price, increases in the order given above. These steels are used in the hardened condition. The carbon steels contain from 0.80 to 1.20% carbon and are, in fact, carbon tool steels. The chromium magnet steels contain from 0.70% to 1.0% carbon and 2% to 3% chromium.

The tungsten steels contain about 0.70% to 1.0% carbon and 2% to 3% chromium. The tungsten steel contains about 0.7% carbon and 5% tungsten. The best and most expensive cobalt steels contain 35% cobalt together with several-percent of both chromium and tungsten.

An important permanent magnet alloy (Alnico) contains approximately 60% iron, 20% nickel, 8% cobalt and 12% aluminium. This alloy cannot be forged and is used as a casting hardened by precipitation heat treatments.