Steel Haseeb Ullah Khan Jatoi Department of Chemical Engineering UET Lahore

Recap Eutectic phase diagram

Eutectic isotherm Invariant point Eutectic Reaction Compositions of components at invariant point Effect of solutes additions on phases

Microstructure development of Sn-Pb system

Iron and Steel After iron extraction –Pig Iron –Wrought Iron –Cast Iron

Pig Iron Very high carbon content (3.5–4.5%) This makes it very brittle with low melting points and not useful directly as a material except for limited applications Pig iron is used to make steel where molten pig iron (hot melt) is charged into BOF or Electric Arc Furnace to make steel

Cast Iron It contains carbon contents above 2.14%. Most contains between 3 – 4.5% It is brittle, wear and tear resistant, cannot be welded, neither malleable nor ductile, does not rust, lack plasticity and has high compressive strength, excellent machineability and M.P is 1150 – 1300 ˚C Used for manufacturing of parts of cars, man hole covers, and railings, etc

Types of Cast Iron Gray Cast Iron Ductile Cast Iron White Cast Iron Malleable Cast Iron Compacted Graphite Iron

Gray Cast Iron The carbon and silicon contents of gray cast irons vary between 2.5 and 4.0 wt% and 1.0 and 3.0 wt%, respectively Factors that promote the formation of gray cast iron are: –slow rate of cooling –slow rate of solidification Mechanically, gray iron is comparatively weak and brittle in tension as a consequence of its microstructure the tips of the graphite flakes are sharp and pointed, and may serve as points of stress concentration when an external tensile stress is applied

Ductile (or Nodular) Iron Adding a small amount of magnesium and/or cerium to the gray iron before casting produces a distinctly different microstructure and set of mechanical properties Graphite still forms, but as nodules or sphere-like particles instead of flakes. The resulting alloy is called nodular or ductile iron Typical applications for this material include valves, pump bodies, crankshafts, gears, and other automotive and machine components

a)Gray iron b)Nodular (ductile) iron the dark graphite flakes the dark graphite nodules

White Cast Iron When pig iron converts to cast iron, if cooling is done rapidly then pig iron is changed in to white cast iron It is hard and brittle. Its use is limited to applications that necessitate a very hard and wear-resistant surface, without a high degree of ductility—for example, as rollers in rolling mills

Malleable Cast Iron Heating white iron at temperatures between 800 to 900˚C for a prolonged time period and in a neutral atmosphere (to prevent oxidation) causes a decomposition of the cementite, forming graphite, which exists in the form of clusters Representative applications include connecting rods, transmission gears, and differential cases for the automotive industry, and also flanges, pipe fittings, and valve parts for railroad, marine, and other heavy-duty services

(c) White iron (d) Malleable iron Pearlite dark graphite rosettes

Compacted Graphite Cast Iron Silicon content ranges between 1.7 and 3.0 wt%, whereas carbon concentration is normally between 3.1 and 4.0 wt% Important applications are in diesel engine blocks, exhaust manifolds, gearbox housings, brake discs for high-speed trains, and flywheels

Wrought Iron It is tough, can bear shocks and impact, can be welded, M.P is 1500˚C Corroded easily Purest form of Iron in which impurities does not exceed from 0.5% Used for making sheets, for rod making, making of gas pipelines, boiler tubes and frames of windows

Steel When carbon in small quantities is added to iron, ‘Steel’ is obtained The atomic diameter of carbon is less than the interstices between iron atoms and the carbon goes into solid solution of iron As carbon dissolves in the interstices, it distorts the original crystal lattice of iron

Steel This mechanical distortion of crystal lattice interferes with the external applied strain to the crystal lattice, by mechanically blocking the dislocation of the crystal lattices (In other words, they provide mechanical strength) Adding more and more carbon to iron (upto solubility of iron) results in more and more distortion of the crystal lattices and hence provides increased mechanical strength

Steel However, solubility of more carbon influences negatively with another important property of iron called the ductility’ Carbon contents may vary between 0.2 – 2.1% by wt, depending on the grade. Purest form of iron and the impurities present in it are less than 0.5%

Steel Hence we see that when more carbon is added, enhanced mechanical strength is obtained, but ductility is reduced Increase in carbon content isnot the only way, and certainly not the desirable way to get increased strength of steels. More amount of carbon causes problems during the welding process Phase diagram for different steel types

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