STEEL

The Elements Used in Steel

Carbon (C): Carbon, a nonmetallic element, forms a number of organic and inorganic compounds and can be found in coal, petroleum and limestone. It is the principle strengthening element in carbon steels and low-alloy steels. Atomic number 6, atomic weight 12.01115.

Manganese (Mn): Manganese is a brittle, metallic element that exists in the ore of pyrolusite. When making steel, it reacts with sulfur and helps to increase the metal’s resistance to heat. Atomic number 25, atomic weight 54.9380.

Phosphorus (P): Phosphorus is a poisonous, nonmetallic element that helps protect metal surfaces from corrosion. Atomic number 15, atomic weight 30.9738.

Sulfur (S): Sulfur is a nonmetallic element found mainly in volcanic and sedimentary deposits. Sulfur, in the form of iron sulfide, can cause steel to be too porous and prone to cracking. Atomic number 16, atomic weight 32.064.

Silicon (Si): Silicon is the second most abundant element in the earth’s crust and can be found in rocks, sand and clay. It acts as a deoxidizer in steel production. Atomic number 14, atomic weight 28.086.

Nickel (Ni): Nickel is a hard, metallic element that found in igneous rocks. Without nickel, stainless steel would be less resistant to heat and corrosion. Atomic number 28, atomic weight 58.71.

Chromium (Cr): Chromium, a metallic element, is found in the earth’s crust. It is used in the production of stainless steel to make the steel resistant to oxidation and corrosion. Atomic number 24, atomic weight 51.996.

The Elements of Steel Composition (percent by mass)

Cast Iron
Carbon 3.5%
Manganese .5%
Phosphorous .13%
Sulfur .13%
Silicon 1.2%
Cast iron contains high levels of carbon, which makes it a hard, brittle metal. Cast iron was commonly used throughout Europe to make church bells and, in colonial America, pots and pans.

Wrought Iron
Carbon .035%
Manganese .075%
Phosphorous .075%
Sulfur .1%
Silicon – .1%
Wrought iron is a strong, durable metal with a low carbon content. Items such as locks, bolts, tools, and fences are crafted out of this metal. Wrought iron bars were also sold and traded to be later converted into steel or cast iron.

Plain Steel
Carbon 1.35%
Manganese 1.65%
Phosphorous .04%
Sulfur .05%
Silicon .06%
During the early 20th century, new processes in steel production allowed steel to surpass iron as the most widely used structural metal. Its great strength and affordability allowed craftsmen to construct sturdier bridges and higher buildings.

High Strength Steel
Carbon .25%
Manganese 1.65%
Phosphorous .04%
Sulfur .05%
Silicon .12%
Nickel 2.5%
Chromium .8%
Adding alloys to steel yield higher strength, more wear-resistant metals. James Eads used alloy steel in the construction of a bridge across the Mississippi River — the first steel bridge built in America.

Stainless Steel
Carbon .08%
Manganese 2%
Phosphorous .04%
Sulfur .03%
Silicon .75%
Nickel 8%
Chromium 18%
From spoons to blenders, cars to trains, stainless steel, with its sleek, shiny surface, can glorify even the most simple of gadgets. In addition to its aesthetic appeal, the light weight and strength of stainless make it ideal for transportation.

MECHANICAL TREATMENT OF STEEL

The purpose of giving mechanical treatment to the steel is to give desired shape to the ingots so as to make steel available in market forms. The mechanical treatment of steel may be hot working or cold working. The hot working is very common.

Following are the operations involved in the mechanical treatment of steel:

(1) Drawing

(2) Forging

(3) Pressing

(4) Rolling.

Each of the above operation will now be briefly described.

(1) Drawing: This operation is carried out to reduce the cross-section and to increase the length proportionately. In this operation, the metal is drawn through dies or specially shaped tools. The drawing is continued till wire of required diameter or cross-section is obtained. This process is used to prepare wires and rods.

(2) Forging: This operation is carried out by repeated blows under a power hammer or a press. The metal is heated above the critical temperature range. It is then placed on anvil and subjected to blows of a hammer. This process increases the density and improves grain size of metal. The riveting belongs to forging operations. The process is used for the manufacture of bolts, camps, etc.

The steel may be either forged free or die-forged. In the former case, the steel is free to spread in all directions as it is hammered. In the latter case, the steel flows under the blows of a hammer to fill the inside of a die and the excess material is forced out through a special groove and then it is cut off. The die-forged parts have very accurate dimensions.

(3) Pressing:

This process is useful when a large number of similar engineering articles are to be produced.

(4) Rolling:

This operation is carried out in specially prepared rolling mills. The ingots, while still red hot, are passed in succession through different rollers until articles of desired shape are obtained. The various shapes such as angles, channels, flats, joists, rails, etc. are obtained by the process of rolling. It is possible to prepare joint less pipe with the help of this process. The solid rod is bored by rollers in stages until the pipe of required diameter and thickness is obtained.

HEAT TREATMENT OF STEEL

Heat Treatment of Steel Steels can be heat treated to produce a great variety of microstructures and properties. Generally, heat treatment uses phase transformation during heating and cooling to change a microstructure in a solid state. In heat treatment, the processing is most often entirely thermal and modifies only structure. Thermomechanical treatments, which modify component shape and structure, and thermochemical treatments which modify surface chemistry and structure, are also important processing approaches which fall into the domain of heat treatment. The iron-carbon diagram is the base of heat treatment. Typical heat treatment operation is presented in Fig. 1. Fig. 1. Thermal history of heat treatment operation. According to cooling rate we can distinguish two main heat treatment operations: • annealing – upon slow cooling rate (in air or with a furnace) • quenching – upon fast cooling (in oil or in water) annealing – produces equilibrium structures according to the Fe-Fe3C diagram quenching – gives non-equilibrium structures Among annealing there are some important heat treatment processes like: • normalising • spheroidising • stress relieving Normalising The soaking temperature is 30-50°C above A3 or Acm in austenite field range. The temperature depends on carbon content. After soaking the alloy is cooled in still air. This cooling rate and applied temperature produces small grain size. The small grain structure improve both toughness and strength (especially yield strenght). During normalising we use grain refinement which is associated with allotropic transformation upon heating γ→α (Fig. 2). Fig. 2. Influence of temperature on an eutectoid steel grain size Important: austenite does not change grain size during cooling!! Spheroidising The process is limited to steels in excess of 0.5% carbon and consists of heating the steel to temperature about A1 (727°C). At this temperature any cold worked ferrite will recrystallise and the iron carbide present in pearlite will form as spheroids or “ball up”. As a result of change of carbides shape the strength and hardness are reduced. Quenching Soaking temperature 30-50°C above A3 or A1, then fast cooling (in water or oil) with cooling rate exceeding a critical value. The critical cooling rate is required to obtain non-equilibrium structure called martensite. During fast cooling austenite cannot transform to ferrite and pearlite by atomic diffusion. Martensite is supersaturated solid solution of carbon in α-iron (greatly supersaturated ferrite) with tetragonal body centered structure. Martensite is very hard and brittle. Martensite has a “needle-like” structure. Kinetics of martensite transformation is presented by TTT diagrams (Time-Temperature-Transformation). With the quenching-hardening process the speed of quenching can affect the amount of marteniste formed. This severe cooling rate will be affected by the component size and quenching medium type (water, oil). The critical cooling rate is the slowest speed of quenching that will ensure maximum hardness (full martensitic structure). Tempering This process is carried out on hardened steels to remove the internal stresses and brittleness created by the severe rate of cooling. The treatment requires heating the steel to a temperature range of between 200 and 600°C depending upon the final properties desired. This heat energy allows carbon atoms to diffuse out of the distorted lattice structure associated with martensite, and thus relieve some of the internal stresses. As a result the hardness is reduced and the ductility (which was negligible before tempering treatment) is increased slightly. The combined effect is to “toughen” the material which is now capable of resisting certain degree of shock loading. The higher the tempering temperature the greater the capacity for absorbing shock.

MARKET FORM OF STEEL

Following are the standard shapes in which the rolled steel sections are available in the market:

(1) Angle sections

(2) Channel sections

(3) Corrugated sheets

(4) Expanded metal

(5) Flat bars

(6) I-selections

(7) Plates

(8) Ribbed-torsteel bars

(9) Round bars

(10) Square bars

(11) T-sections.

(1) Angle sections: The angle sections ma be of equal legs or unequal legs. The equal angle sections are available in sizes varying from 20 mm x 20 mm x 3 mm to 200 mm x 200 mm x 25 mm. The corresponding weights per meter length are respectively 9 N and 736 N,

The unequal angle sections are available in sizes varying from 30 mm x20 mm x 3 mm to 200 mm x 150 mm x 18 mm. The corresponding weights per meter length are respectively 11 N and 469 N.

The angle sections are extensively used in the structural steelwork especially in the construction of steel roof trusses and filler joist floors.

(2) Channel Selections: The channel section consist of a web with two equal flanges. A channel section is designated by the height of web and width of flange. These sections are available in sizes varying from 100 mm x 45 mm to 400 mm x 100 mm. the corresponding weights per meter length are respectively 58 N and 494 N.

The Bureau of Indian standards has classified channel sections as junior channel, light channel and medium channel and accordingly they are designated as L.S.J.C., I.S.L.C. and I.S.M.C. respectively.

The channel sections are widely used as the structural members of the steel framed structures.

(3) Corrugated sheets: these are formed by passing steel sheets through grooves. These grooves bend and press steel sheets and corrugations are formed on the sheets. These corrugated sheets are usually galvanized and they are referred to as the galvanized iron sheets or G.I. sheets. These sheets are widely used for roof covering.

(4) Expanded metal: This form of steel is available in different shapes and sizes. It is prepared fro sheets of mild steel which are machine cut and drawn out or expanded. A diamond mesh appearance is thus formed throughout the whole area of the sheet.

The expanded metal is widely used for reinforcing concrete in foundations, roads, floors, bridges, etc. It is also used as lathing materia

(4) Expanded metal

(5) Flat bars

(6) I-selections

(7) Plates

(8) Ribbed-torsteel bars

(9) Round bars

(10) Square bars

(11) T-sections.

(1) Angle sections: The angle sections ma be of equal legs or unequal legs. The equal angle sections are available in sizes varying from 20 mm x 20 mm x 3 mm to 200 mm x 200 mm x 25 mm. The corresponding weights per meter length are respectively 9 N and 736 N,

The unequal angle sections are available in sizes varying from 30 mm x20 mm x 3 mm to 200 mm x 150 mm x 18 mm. The corresponding weights per meter length are respectively 11 N and 469 N.

The angle sections are extensively used in the structural steelwork especially in the construction of steel roof trusses and filler joist floors.

(2) Channel Selections: The channel section consist of a web with two equal flanges. A channel section is designated by the height of web and width of flange. These sections are available in sizes varying from 100 mm x 45 mm to 400 mm x 100 mm. the corresponding weights per meter length are respectively 58 N and 494 N.

The Bureau of Indian standards has classified channel sections as junior channel, light channel and medium channel and accordingly they are designated as L.S.J.C., I.S.L.C. and I.S.M.C. respectively.

The channel sections are widely used as the structural members of the steel framed structures.

(3) Corrugated sheets: these are formed by passing steel sheets through grooves. These grooves bend and press steel sheets and corrugations are formed on the sheets. These corrugated sheets are usually galvanized and they are referred to as the galvanized iron sheets or G.I. sheets. These sheets are widely used for roof covering.

(4) Expanded metal: This form of steel is available in different shapes and sizes.

SOURCE
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