Steel Structure

Steel Structure


It is an alloy of iron and carbon, wheat carbon content up to a maximum of 1.5 per cent. The carbon occurs in the form of iron carbide, because of its ability to increase the hardness and strength of the Steel. Other elements e.g. Sulphur, silicon, Phosphorus and manganese are also present to a greater or lesser amount to import certain desired properties to it. Most of the Steel nowadays is plain carbon steel or simply Carbon Steel. Carbon steel is defined as steel that has its properties mainly due to its carbon content and does not contain more than 0.5 % of silicon and 1.5% of Manganese. The plain carbon steel bearing from 0.06% carbon point 1.5% carbon is divided into the following types depending upon the carbon content.

Dead mild steel- up to 0.15% carbon.
Low carbon for mild steel – 0.15% 0.45% carbon.
Medium Carbon Steel – 0.45% to 0.8 % carbon.
High Carbon Steel – 0.8% to 1.5 % carbon.

According to Indian standard(IS: 1762(part-I) – 1974), a new system of designated Steel is recommended. According to the standard Steel are designated on the following two bases.

a) on the basis of Mechanical properties
b) on the basis of chemical composition

Mechanical properties of steel

These Steel are carbon and low alloy Steel wire the main criterion selection and inspection of steel is the tensile strength for yield stress. According to Indian standard IS: 1570( part 1) – 1978, these Steel are designated by a symbol ‘Fe or Fe E’ depending on whether the Steel has been specified on the basis of minimum tensile strength or yield strength, followed by the figure indicating the minimum tensile strength or and stresses in N/mm2.

Impurities of Steel::-

The following are the effects of impurities like silicon, sulphur, Manganese and phosphorus on steel.

Silicon:- the amount of silicon in the finished Steel usually range from 0.05 to 0.30%. Silicon is added in low carbon Steels to prevent them from becoming porous. It removes the gases and oxides, prevent below the hole and thereby makes the Steel tougher and harder.

Manganese::-. It serves as a valuable oxidizing and purifying agent in steel. Manganese also combines with Sulphur and thereby decreases the harmful effect of this element remaining in the Steel. When used in ordinary low Carbon Steel, manganese makes the metal ductile and of good bending qualities. In high-speed Steel(HSS), it is used to together the metal and to increase and critical temperature.

Sulphur::-  it occurs in the steel either as iron sulphide Or manganese sulphide. Iron sulphide because of its low melting point produce red shortness, manganese sulphide does not affect so much. Therefore, manganese sulphide is less objectionable in steel than sulphide.

Phosphorus::-  it makes the Steel brittle. It also produces cold shortness in steel. Low carbon steel heat Rises the yield point and improves the resistance to atmospheric corrosion. The sum of carbon and phosphorus usually does not exceed 0.25%.

Free cutting Steel::-
The free cutting Steel contains Sulphur and phosphorus. These Steel have higher sulphur content than other carbon Steels. In general, the carbon content of 7 Steel varies from 0.1 to 0.45% and sulphur from 0.08 to 0.3%. These Steel are used where Rapid machining is the prime requirement. It may be noted that the presence of Sulphur and phosphorus causes long cheap in the machine to be easily broken and does prevent cooling of the machine. Nowadays, lead is used from 0.05 to 0.2% instead of sulphur, because lead also greatly improves the machinability of Steel without the loss of toughness.

According to Indian standard IS: 1570( part-III) – 1979, carbon and carbon-manganese free cutting Steel are designated in the following order,…
1) figure indicating 100 times the average percentage of carbon.
2) letter ‘C’
3) figure indicating 10 times the average percentage of Manganese.
4) symbol ‘S’ followed by the figure indicating 100 times the average contents of sulphur. If instead of sulphur, lead(Pb) is added to make the Steel free cutting, then the symbol ‘Pb’ may be used.

Alloy Steel::-

An alloy Steel may be defined as steel to which elements other than carbon are added in sufficient amount to produce an improvement in properties. The alloying is done for a specific purpose to increase wearing resistance, corrosion resistance and improve electrical and magnetic properties, which cannot be obtained in plain Carbon Steels. The chief alloying element used in steel Nickel, chromium, molybdenum, cobalt, vanadium, manganese, silicon and tungsten. Each of these elements confirms certain qualities upon the Steel to which it is added. Elements may be used separately or in combination to produce the desired characteristic in steel. Following are the effects of alloying elements on steel.

Nickel::- it increases the strength and toughness of Steel. Digital content 2 to 5% Nickel and from 0.1 to 0.5 % carbon. Hindi syringe Nikhil contributes great strength and hardness with high elastic limit, good deal ATI and good resistance to corrosion. An alloy containing up to 5% Nickel process Seema toughness and offers the greatest resistance to rusting, corrosion and burning at high temperature. It has proved to be an advantage in the manufacturing boilers tubes, valves for use with superheated steam, valves for IC engines and spark plug for petrol engines. A Nickel Steel alloy containing per cent of Nickel is known as invar. It has a nearly zero coefficient of expansion. So it is in great demand for measuring instrument and standard of length for everyday use.

Chromium::- it is in steel as an alluring element to combine hardness with high strength and high elastic limit. It also imparts corrosion resistance properties to Steel. The most common Chrome Steel content from 0.5 to 2% chromium and 0.1 to 1.5 % carbon. The Chrome steel is used for Balls, rollers and Race forbearing. A Nickel Chrome Steel containing 3.25%Ni, 1.5% chromium and 0.25% carbon is much used for armour plates. Nickel steel is extensively used for motor car crankshaft, Excellent dear requiring great strength and hardness.

Tungsten::- It prohibits green growth, increases the depth of hardening of quenched steel and confers the property of remaining hard even when heated to red colour. It is usually used in conjunction with other elements. Steel containing 3 to 18 per cent tungsten and 0.2 to find 1.5% carbon is used for cutting tools. The principal use of tungsten Steel for cutting tools dies, valves, taps and permanent magnets.

Vanadium::- it aids in obtaining a fine grain structure in tool Steel. The addition of a very small amount of vanadium( less than 0.2%) produces a marked increase in tensile strength and elastic limit in low and medium carbon Steels without a loss of ductility. The Chrome Vanadium Steel containing about 0.5 to 1.5% chromium, 0.15 to 0.3% vanadium, and 0.13 to 1.1 % carbon have extremely good tensile strength. These Steel are frequently used for parts such as spring, shafts, gears, pins and made drop-forged parts.

Manganese::- It improves the strength of the Steel in both the hot rolled and heat condition. Is alloy Steel containing over find five per cent manganese with a carbon range of 0.40 to 0.55% are used extensively in gears, axles, shafts and other parts where high strength combined with hair ductility is required? The principal use of Manganese Steel in part subjected to wear. These Steels are all cast and ground to finish.

Silicon::- the Silicon Steel behave like Nickel Steels. These girls have a higher elastic limit as compared to ordinary Carbon Steel. Silicon Steel containing from 1 to 2% silicon and 0.1 to 0.4 % carbon and other alloying elements are used for Electrical machinery, valves in IC engines, Tin and corrosion-resistant military materials.

Cobalt::- it gives red hardness by retention of hard carbide at high temperature. It tends to Describes Steel during heat treatment. It increases hardness and strength and also decidual magnetism and magnetic force in steel or magnets.

Molybdenum::- a very small quantity of molybdenum is generally used with chromium and manganese to make molybdenum Steel. Digitals extra tensile strength and are used for aeroplane and Automobile parts it can replace tungsten in high-speed Steel.

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