Steel is an alloy of iron and carbon with content upto a maximum of 1.5%. The carbon present in the form of iron carbide, because of its ability to increase the hardness and strength of the steel. Other elements such as Silicon, Sulphur, Phosphorus and Manganese are also added to improve the various qualities of steels. Carbon present in the steel is in combined form.
Plain Carbon Steel
Plain 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 effect on steels when their extent does not exceed, 0.3-0.4 % Si, 0.5-0.8 % Mn, 0.08 % P and 0.04 % S. The properties of plain carbon steels are greatly influenced by an increase in carbon content. So, as the carbon content increases Tensile strength, Hardness and toughness, decrease Ductility. Plain carbon steels are classified according to their carbon content, as follows:
- Low carbon steel (Mild steel)
- Medium carbon steel
- High carbon steel.
Low Carbon Steel (Mild steel)
Mild Steel is having carbon 0.15 to 0.3 percent. It is having bright fibrous structure. It is tough and more elastic than wrought iron. It can be easily forged and welded. It is malleable and ductile. It absorbs shocks. Its tensile strength is better than cast iron and wrought iron but compressive strength is better than wrought iron but less than cast iron. It rusts readily. It’s melting point is 1400°C.
- Mild Steel Containing 0.15 to 0.20% Carbon. It is used in structure steels, universal beams, screws, drop forgings, case hardened steel, bars, rods, tubes, angles and channels, etc.
- Mild Steel Containing 0.20-0.30% Carbon. It is used in machine and structure work, gears, free cutting steels and forging, etc.
Medium Carbon Steels
Medium carbon steel is having carbon from 0.3 to 0.8 percent. They are usually produced as killed or semi killed steels and are hardenable by heat treatment. Hardenability is limited to thin sections or to the thin outer layer of the thick parts. Medium carbon steels in the quenched and tempered condition provide a good balance of strength and ductility.
- Medium Carbon Steels having Carbon % 0.30 to 0.45. Axles, special duty shafts, connecting rods, forgings, machinery steel, spring clips, turbine rotors, gear shafts, key stock, forks and anchor bolts.
- Medium Carbon Steels having Carbon % 0.45 to 0.60. Railway coach axles, crank pins, crankshafts, axles, shafts, loco tyres, rails.
- Medium Carbon Steels having Carbon % 0.60 to 0.80. Drop forging dies, die blocks, bolt heading dies, self-tapping screws, valve springs, lock washers, hammers, cold chisels, hacksaws, jaws for vices etc.
High Carbon Steels
These steels have carbon percentage from 0.8 to 1.5%. Because of their high hardness, these are suitable for wear resistant parts. Spring steel is also high carbon steel available in annealed and pre-tempered strips and wires. High carbon steel loses their hardness at temperature from 200°C to 250°C. Therefore they may only be used in the manufacture of cutting tools operating at low cutting speeds. These steels are easy to forge and harden.
- When Carbon % is from 0.80 to 0.90.Railway rails, rock drills, circular saws, punches, dies and leaf springs.
- When Carbon % is from 0.90 to 1.20. Punches and die, springs, bearing balls, pins, railway springs, mandrels, taps, tools.
Any straight carbon steel that has been crucible made can be called “Tool Steel” provided the percentage of carbon exceeds 0.7%. Shear steels and open-hearth steels can never be termed as tool steels, as they are unsuitable for tool making. Tool steels contain Carbon up to 1.4%. Lathe tools, milling cutters, metal saws, razors, drills etc. are made of tools steels. They are not suitable for welding.
The name cast steel is used to signify that the particular steel has been melted in a crucible and has been cast into ingots. It is often termed as ‘Crucible cast steel’. After being cast into ingots form, it is subsequently forged or rolled into required bars or sheets. Generally it is applied to high carbon steel. Carbon, Manganese, Silicon, Sulphur and Phosphorus are the elements usually present in cast steel. Nickel, Chromium, Vanadium, Tungsten and Molybdenum are the metallic elements more commonly used in making of alloy steels.
This term applies to low carbon steels, which are cut into different shapes with considerable success. In recent years there has been a great development in the production of steel parts for machines and structures by direct castings. This is especially noticeable in the production of railway equipment such as locomotive frames, car couplers, etc. Steel castings may be made either from open hearth steel or Bessemer steel, but the open-hearth steel is much more common. The molten steel from the furnace or converter is poured directly into the moulds. Steel castings may be made of almost any desired carbon content. There are a large number of complex foundry problems involved in producing steel castings. To ensure soundness of steel in casting and freedom from cavities formed during pouring, it is usually necessary to pour the castings with large masses of steel called sink heads so placed in the mould that molten steel may flow from them to any part of the casting where there is a tendency towards the formation of cavities due to quick cooling.
The material of steel casting is not so strong or so tough as forged steel of the same chemical composition. As they come from moulds, steel castings usually have severe internal stresses set up in them by uneven cooling. These internal stresses may be greatly relieved, and the quality of material in various parts of the castings made more nearly uniform by annealing the finished casting and this is very frequently done. Annealing remarkably refines the grain structure of steel castings. For many purposes today, steel castings are replacing forgings and grey cast iron castings for its strength and toughness and ease of manufacture.
High Speed Steel
These are alloy steel and are so called because of the high-speed cutting qualities. They are capable of cutting at a much higher speed and greater depth than straight carbon steel. These steels retain the hardness even at red heat temperatures. The chief elements alloyed are Chromium, Manganese, Vanadium, Cobalt and Molybdenum.
High Tensile Steel
This name is given to steel that has tensile strength more than 50 tons per square inch. For aircraft use, this name usually refers to an alloy containing Nickel and Chromium. Apart from adding these elements, the high tensile properties are imparted in some cases by cold rolling or other work hardening process.
This is the name given to that steel particularly suitable for spring making, provided it is correctly heat-treated. It is usually straight carbon steel 0.7 to 0.9% carbons, but Silicon and Vanadium are often added for high-class springs giving greater resistance to fracture.
Case Hardening Steel
It is a low carbon steel with carbon content not exceeding 0.2%. If Nickel is added from 3 to 5% with a minor percentage of Chromium, this steel has the quality of greater toughness and ability to dissolve more Carbon and promotes the depth of hardness on case hardening. A special alloy steel (Nitralloy) is used for nitrogen hardening.
High Chromium Stainless Steel
This class of steel is used particularly for parts subjected to heavy, dirty, corrosive conditions, and where physical properties similar to those of high-grade alloy steel are required. The chief alloying element is 12 to 20% with small percentage of Nickel. In addition to rolling and forging, these stainless steel can be compressed, machined and hammer welded. The carbon content is usually about 0.3%. Fusion welding of stainless steel may in certain cases give rise to weakness and corrosion within the zone of the weld and such steels should not be welded without authority.
It is an alloy containing 12 to 14%, Chromium. The Carbon content is limited to 0.15% and if this percentage is increased, then the steel falls into the category of stainless steel. This metal is extensively used for turbine blades, tanks, tubes, rivets, nuts, bolts, cooking utensils etc. The tensile strength is about 30 to 40 tons per square inch.
Austenitic Stainless Steel
This class of steel is non-magnetic and can’t be hardened by heating and quenching. To soften after cold working, it is quenched in water from white heat. The steel resist corrosion better than high chrome stainless steel. Both 18-8 and 12-12 variety is austenitic stainless steel. These steels are very tough, have an ultimate tensile strength of 60 tons per square inch and can’t be easily machined. These steels are normally used for exhaust valves in aero engines.
This type of steel contains Nickel 36%. This does not scale on heating and posses the least co-efficient of expansion. It is used for making precision instruments.
It contains about 80% Nickel, 14% Chromium, and 6% Iron with Carbon.
This is an alloy containing Aluminium, Nickel and Cobalt, and is used for modern magnets.