When certain special properties are required, some alloying elements are added to carbon steels. The alloying element may be Ni, Cr, Mn, Va ,W etc. The steel thus obtained are called alloy steels. The purposes of alloying elements are to improve the following properties:
- Hardness, toughness, and tensile strength.
- Wear resistance.
- Corrosion resistance.
- Ability to retain shape at high temperature.
- Resistance to distortion at high temperature.
- Fine grain size to steel.
- Case hardening.
- Cutting ability.
Nickel. Nickel is added to steel to:
- Increase toughness.
- Increase tensile strength, yield strength.
- Improve response to heat treatment.
- Improve corrosion resistance.
- Improve forming(shaping) properties of stainless
Chromium. Chromium is added to steel for following purposes.
- Increases tensile strength, hardness and heat resistance.
- Provide forming properties of stainless steel.
- Decreases malleability of steel.
- Improves corrosion resistance (about 12% addition).
Tungsten. Tungsten is added to steel or steel alloys to produce high speed steels with special heat resisting qualities. Tungsten is a carbide forming element and quantities of 0.5% may be used for steels employed in production of dies, but the quantity added may be as high as 20% in the case of special alloy, i.e. high-speed steels. Magnet steel may be produced, from steels containing 5 to 7% Tungsten, but cobalt steel is more efficient for the purpose. Tungsten has the highest melting point of all metals, i.e. 6098ºF. When alloyed with steel causes a very dense and fine structure, increase hardenability, stabilizes the martensite and imparts the property of red hardness. Tungsten steel retains cutting edge even at 1300ºF.
Carbon. It increases tensile strength and hardness. It decreases ductility and weldability. It affects the melting point of the material.
Columbium. When added to steel, it minimizes the intergranular corrosion. It is added specially in stainless steel.
Lead. When added to steel, it imparts machineability.
Lithium. It is a powerful deoxidizer. It increases the fluidity of stainless steel.
Sulphur. It increases the machineability, but weakens the steel if the content is more than 0.06 percent and the steel becomes “red short”. Sulphur should not be present in steel above 0.06 percent, or the steel will become brittle at forging temperatures.
Phosphorus. Phosphorus is not desired in steel as it causes cold short, i.e. it is the condition of iron or steel in which it cannot be worked by hammering or rolling at or below dull red heat. But this impurity cannot be completely removed, hence the percentage is regulated to 0.06%. Small amount of phosphorous however increases the strength and also it helps in resisting corrosion.
Nitrogen. It forms hard nitrides with Aluminium. Therefore, nitrogen hardening is used for nitralloys.
Uranium. It is a good deoxidiser and increases the elastic limit and strength of the steel. It possesses good strength and a high corrosion resistance, poor thermal conductivity and a high coefficient of expansion.
Zirconium. It is a powerful deoxidiser and desulphuriser. Steels can be made without manganese by the use of it. It reduces fatigue in steels.
Nickel Steel. The percentage of Nickel (Ni) varies from 2 to 45. 2%Ni makes steel more suitable for rivets, boiler plates, bolts and gears etc. Ni from 0.3 to 5% raises elastic limit and improves toughness. Steel containing 20% Nickel has very high tensile strength. 25% Ni makes the steel particularly stainless and might be used for I.C. engine valves, turbine blade etc. If Ni is present up to 27%, it makes the steel non-magnetic and non-corrodible. 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. 45% Ni steel possesses expansion equal to that of glass, a property very important for making links between the two materials i.e. in electronic valves and bulbs.
Chrome Steel. Chrome steel contains C 0.36% Cr 0.57%. Chromium intensifies the effect of rapid cooling on steel. Therefore, chromium is used only in steels which are to be heat- treated. Chromium forms carbides and thus gives high hardness and good wear resistance. In addition, chromium increases tensile strength and corrosion resistance of low alloy steels.
Tungsten Steel. Tungsten low alloy steels are tool steels containing approximately 2% Tungsten, 1.70% Chromium and 0.50% Carbon. This is hard tough tool steel that is commonly used for making cutting tools. Tungsten forms carbides and prevents softening of the alloy at high temperatures. The Tungsten steel may contain up to 15% Tungsten. Tungsten steel is used for making high speed cutting tools and permanent magnets.
Stainless Iron and Steel. Structural steel with copper content of 0.2% is more resistant to atmospheric corrosion than structural steel with no appreciable copper content. Chromium is the most effective ingredient for making corrosion and heat resistant steel or iron. And it is especially effective if the Chromium content is 13% or more. The protection against corrosion is due to dense tough film of oxide formed over the surface of the metal. Steel or Iron with Chromium content 13% or greater is classed as stainless steel.
There are three types of stainless steels. Steels in the first group have Chromium content lower than 14% and Carbon content lower than 0.4%. They respond to heat treatment and are not excessively brittle. They may be machined by the use of specially designed tools, & they can be welded. They resist the effect of weather and water and can be used at temperature up to 1500ºF.
Steels in the second groups have a Chromium content of 14 to 16% and carbon content not lower than 0.4%. They do not respond to heat treatment and are brittle. They can be forged rolled or cold drawn and can be machined by the use of specially designed tools. They can be welded but some metals in the second group are very brittle near a weld. They are superior to the steels in the first groups in resistance to corrosion. They do not show grain growth at high temperature, and are superior to the steels in the first group in resistance to oxidation even about 800ºF.
The third group of stainless steel do respond to heat treatment with little success. They contain sufficient chromium to make them magnetic and austenitic. They are very tough, and can be rolled, forged or cold drawn but can be machined only with great difficulty. They can be welded. Above 16% Chromium, their resistance to corrosion is excellent. The metals in this group are better than the metals of the second group for service above 1000ºF temperatures.
Vanadium Steel. Vanadium when added even in small proportion to an ordinary low carbon steel considerably increases its elastic limit and improves the fatigue resistance property. Vanadium makes the steel strong and tough. When vanadium is added up to 0.25%, the elastic limit of the steel is raised by 50% and can resist high alternating stresses and severe shocks. It is widely used for making tools. It can also be used for shafts, springs, gears, steering knuckles and drop forged parts.
Cobalt Steel. Cobalt tool steels are used where high frictional heats are developed. Cobalt imparts additional red hardness to steel and cutting ability of tool is maintained at elevated temperatures.
Nitralloy. Aluminium is a deoxidizer and restricts grain growth by forming nitride and oxides. If it is added to steel alloy with small quantities of Chromium and Molybdenum and heated in contact with Nitrogen and steel, it becomes extremely hard. This is known as Nitralloy.
Straight Carbon Steel. Straight carbon steels are those which contain primarily iron and carbon. Silicon, Manganese, Sulphur and Phosphorus are present but these are considered as impurities. These constituents have negligible effects on steel due to very low percentage. The properties of plain carbon steel are greatly influenced by an increase in Carbon content.
Manganese Steel. Manganese increases hardness and tensile strength. A secondary effect is an increased resistance to abrasion. The steel also with stands the shock tests excellently. Manganese steels are used for making power shovel buckets, Grinding and crushing machinery, Railway tracks, etc.
Silicon steel. Silicon steel contains Carbon- 0.01% Manganese- 0.60% and Si-1.00%. Silicon imparts strength, fatigue and improves electrical properties of steel. Many bridges have been built by Silicon Structural Steel. This is stronger than carbon steel of equal ductility.