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7/27/2019 UNIT7 EMgg

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UNIT7 (CAST IRON AND STEELS)

Plain Carbon Steels Low carbon steels(mild steels) : 0.1-0.25%

high formability, high ductility: elongation: ~30%relatively low strength: yield strength:

250~400MPaexcellent weldability cannot be strengthened by heat treatment usually strengthened

by cold working typical applications: pipes, panels, sheets, wires, I-beams etc.

Medium-carbon steels (structural steels)0.25-0.55%CGood combination of strength and ductility

Yield strength: 300~600MPa

Tensile strength: 400~800MPa

Elongation: ~25%

Strengthenable by heat treatment Weldable; weldability deteriorates with increasing C%


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Used for load-bearing applications, crankshaft, bolts, gears, heavy-duty machinery, mining

equipment, cranes.

(0.4% carbon steel)

High strength low alloy steels (HSLA):Medium carbon steels have desired mechanical properties for structural applications, but suffer

from welding-induced embrittlement due to the formation of martensite. To overcome this

problem, C content in these steels is reduced (1%) and by micro alloying with Nb, V, Ti, Cr and Cu. This leads to the

development of HSLA steels. These steels are widely used for manufacturing large welded

structures, such as Sydney harbor bridge, ocean liners and cargo ships, oil drilling rigs and


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platforms, large mining and earth moving equipment, and pressure vessels and

storage tanks.

High carbon steels (Spring steels): 0.6~0.8%Cpredominately eutectoid pearliteat room temperature often strengthened and hardened by heat treatment high strength and

moderate toughness

Tool steels : 0.8~1.2%Cproeutectoidcementite+ pearlite

very high hardness, low toughness, very difficult to machine used for chisels, hammers, knives,

saw blades, drills, dies, punches, cutlery, chine tools and wear resistant applications High carbon

steels have poor weldability and poor machinability

Tool Steels


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High alloy tool steels are often alloyed with Mo, V, Cr and/or Ni.

Cast Irons

On the Fe-C system, these are to the right of steels, with carbon between 2 & 5.3 %,

but more usual 2.5 to 4%.

BASIS Cast irons have carbon beyond the limit of solubility of C in . The different types of cast

iron are merely the different forms the carbon takes. The carbon can be in the form of cementite,

or whitecast iron. If some silicon is added (1 to 2%, maybe 3 %) the carbon will tend to

graphitize, and there are various forms the graphite can have. Then, the carbon can also be in

the form of graphite (gray, malleable and nodularcast iron)


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White cast iron:Rapid solidification of gray iron results in white cast iron. It has white fracture surface.Graphite flakes are not present in this type of cast irons rather; an iron carbide network is

present that gives a white fracture surface. The Si content is lower to minimize thegraphitizing effect. They are hard and have excellent abrasion resistance. But they also

have excessive brittleness and poor machinability. To enhance wear resistance generallyMo, Cr, Ni are added to it.

Malleable cast iron White cast iron + annealing treatment

During annealing treatment graphite

nucleates and grows from the Fe3C to form nodules

Malleable cast iron:It is produced by heat treatment of white cast iron in which the iron carbide network decomposes or breaks down into temper carbon . This process is called malleablizationwhich involves two stages of annealing as the first stage of annealing and the secondstage of annealing. Because of the absence of hard and brittle carbide phase, iron

becomes malleable. Disadvantage of this type of cast iron is its limited section thicknessand prolonged annealing cycles.

Gray cast iron During slow solidification carbon in Feseparates or graphitizes to form separate graphite flakesIt is the most common type of cast iron found. It has a gray fracture surface due to highvolume of graphite flakes. Carbon in graphite form is more stable than carbide form.During cooling if it is subjected to a controlled cooling rate and adequate alloyingaddition then carbon gets precipitated out as graphite flakes. It has high Si content because it

promotes the formation of graphite during solidification. Gray cast irons havenegligible ductility but are useful because they can be easily casted to complex shapesand are inexpensive. These also have very low impact resistance

General characteristics/advantagesof gray cast iron Cheap

Low melting point Fluid easy to cast, especially advantageousinto large complex shapes Excellent machinability Excellent bearing properties Excellent damping properties Excellent wear resistance (hi C)


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Can be heat treated (surface hardened etc.) Can be alloyed etc.Essentially steel + graphite

Spheroidal graphite cast iron or ductile iron:It is produced by adopting special alloy addition and proper cooling rates so that thecarbon can be converted to spherical forms which can be used in those fields wherecarbon in flake form or temper form cant be used. The nodules are formed duringsolidification and not during heat treatment. It can be of three types namely, ferritic,

pearlitic/ferritic, martensitic. It has excellent mechanical properties which can becompared to steels.There is a subclass of ductile iron named as Austempered ductile iron . It has the samenodular or spherical graphite as in ductile iron but the matrix is a combination of bainiteand stabilized austenite. Austempering is necessary to get this type of cast iron structure.Here graphite is present in compact form and shape of graphite is controlled by minor

alloying addition. Austemperd ductile irons have excellent mechanical properties suchhas tensile strength, ductility and wear resistance


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White Cast Iron

It contains r

Vrelativelyless C andSi.If we cooled rapidly and we do not addenough Si, the cast ironsolidifies asWhitein the Fe-Fe 3C System.We can have hypoeutectic andhypereutectic white cast irons.Cementitemakes the alloy hard andbrittle and it is practically useless asstructural material.The high hardness renders them high

resistant to abrasive wear.White irons are produced mainly for two purposes:(a) Intermediate product for producing malleableirons and (b) As abrasive wear resistant


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components, such as ball mill lining tiles, slurry pipeelbows, slurry pump bodies.

Gray Cast Iron

If we put in2 to 3%Silicon,and cool the iron reasonably slowly (dontquench it) the Si will cause the carbon toform as graphite flakes Gray Cast Iron.If we put in more Silicon and cool slowly

we can get virtually all the carbon out of theausteniteand into the flakes, so the matrix is ferrite,or we have a ferriticgray CI.


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If we dont cool as slowly,or we add less silicon,wed have

somecarbon leftin .When the austenite hits the eutectoidtemperature it would form pearlite,just like it did when we talked about steels pearliticgrayCI.

Grey Irons -ApplicationGrey irons are by far the most produced amongall cast irons. Grey irons are used primarily for

their low cost and excellent castability. Typicalapplications include:engine cylinders, pistons, gear box casing,transmission casing, machine tool bases, balance


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weight of large cranes, large diameterunderground pipework.

They are used always under compressiveloading conditions. They are unsuitable fortaking tensile loads


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Cheap to produce, excellent castability, highdamping capacity, good metal-metal wearresistance when lubricated, strength muchhigher in compression than in tension, brittle

in tension.

Malleable Cast Iron

If we heat white CI above its critical line,normally between 900 to 1000 oC for 20

hours well make the carbide convert tographite,and it will produce a rough clump of graphite,kind of between aflake and a nodule(agglomerate), -malleable CI


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These cast irons are stronger, tougher andmuch more ductile than grey irons,compatible to nodular irons.

They have certain capacity to takeshock loading, bending and tension.They are suitable for castings of thinthickness.They are expensive to produce, largelydue to the heat treatment.Typical applications include gear box casing,transmission casing, differential casing.

Nodular or Ductile Cast Irons

we add some magnesium or rare elements

to the molten liquid justbefore casting(inoculation)and do things aswe did with thegray CIwe canproducenodular or ductile CI.We will produce nodules of graphite instead of flakes,


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but as before we can end up with amatrix of either ferrite or pearlite.These irons are much stronger and

tougher than grey irons.They are produced and used for highspecification applications.They are more expensive than grey irons. Typicalapplications include: gears, crankshafts, pumpbodies, pressure valves, rollers.


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