Page 1 : Dast Irons 465, , It will be clear from the above microstructures that transformed ledeburite is common in, all the white cast irons and therefore it is a characteristic feature of white cast irons., , Due to presence of all the carbon in the combined form, white cast irons contain large, amount of cementite and hence they are hard and brittle. The hardness of these cast irons, depend on the carbon content and increases with increase in carbon, making the cast irons, excessively brittle with higher amounts of carbon and unsuitable for engineering, applications. Therefore, majority of the white cast irons are hypoeutectic in carbon with the ~, following composition range., bn C- 2.3. to 3.0%, fered Si- 0.5 to 13%, ee S - 0.06 to 0.1%, , 'P-0.1 to 0.2%, N 0.5 to 1.0%, 0 to 500 BHN), strong in compression (140 to 175 kg/mm2?),, , , , , , , , , ite cast iron include wearing plates, road roller surface, pump, , dies and extrusion nozzles. They are also used for the, , ) and holding at this temperature for a long time (24, by cooling to room temperature. The above heat treatment, , r

Page 2 : 466 Material Science and Metallurgy, , Due to heating to 900°C (i.e. at point 1), the structure of cast iron consists of austenite, and cementite. Cementite being a metastable phase, decomposes to austenite and graphite, (i.e. temper carbon graphite) with a long holding time. The above graphitization of cementite, gives rise to rough, ragged irregular nodules or spheroids called as rosettes of temper carbon, graphite. Therefore, at point 2 the structure consists of rosettes of temper carbon graphite, jn a matrix of austenite. Cooling to room temperature with moderate cooling rate results in, the transformation of austenite to pearlite at eutectoid temperature and therefore at 3, the, microstructure shows rosettes of temper carbon graphite in the matrix of pearlite. However,, if the cooling rate is slow (or if the silicon in the white casting is more), the cementite from, pearlite may also decompose giving ferrite and graphite and the structure at room, temperature i.e. at 3’ may show rosettes of temper carbon graphite in the matrix of ferrite., For intermediate conditions, the matrix may be pearlite and ferrite., , ‘These rosettes disrupt the steel like matrix (pearlite or ferrite) much less as compared to, the flake graphite of gray cast irons and therefore they greatly diminish the internal notch, ‘effects. Due to this, these cast irons show some ductility, toughness and are bendable in, contrast to gray cast irons which are brittle. Hence, they are called as malleable cast irons., , In the ordinary sense, they are not malleable because they can not be rolled, forged, or, extruded to an appreciable degree. Their properties vary with the matrix and are as below:, T.S. — 25 to 70 kg/mm?, Elongation — 6 to 18%, Hardness —- 80 to 275 BHN, , Typical microstructures of pearlitic malleable and ferritic malleable cast irons are shown, Fig. 11.9 (a) and 11.9 (b)., , , , (a) Pearlitic (b) Ferritic, , Fig. 11.9 : Microstructures of malleable cast iron. 100 x

Page 3 : iGastlirons, Types of malle, ‘, : able cast iron: ae, , classified a: . = tructure or a, le s below : ‘Ppearance of fracture, malleable cast irons are, , (i) Ferritic, ; malleable : Th., The matrix i __ + He structure of thi ae, era is ferritic due to slow cooli S type of cast iron is shown in Fig. 11.9, ature. Graphite is very soft (5 ing from malleablizing temperature to room, © cast iron is also soft (80 to ee and ferrite is also soft and therefore ferritic, HIN). The mechanical properties are essentially, , f the ferrite dhas: uc, Phase reduce i i, d by interruptions in structure by graphite. Since the, , areas are rounded : :, ee i oe, it has sufficient plasticity and, ee ee withstand a Reesonaule amount of impact in, ee ane » val ves, farm equipment, chains, bearing blocks,, components requiring some ductility and toughness, and not, , , , , , , , , , , , , , , , , , , , li : i, itic ao 5 tad cooling rate used from malleablizing temperature to, ture is moderate i.e. slightly more than that required to produce ferritic, , ire shows rosettes of temper carbon graphite in the matrix of pearlite at room, _ In this condition, the cast iron is harder (200 to 275 BHN), stronger and, ore brittle, and some what more difficult to machine than the ferritic (or ferritic —, alleable cast iron. A typical microstructure of pearlitic malleable cast iron is, , , , , , , , , , , , , , , 11.9(a)., , rate is increased,, jron becomes hard, ble cast iron — is, pearlitic matrix are never, , 6 the applications of p, links and ordnance parts. They are also used for ele, , gear parts, fittings for high and low voltage transmission and, railway electrification systems., ~ malleable : This is produced due to interm, , : tic malleable and pearlitic malleable cast irons., cure of which is shown in Fig. 11.10., , the matrix may result in martensite instead of pearlite, and brittle and its toughness — which is the major, lost. Therefore, cooling rates higher than needed to, purposely used in the production of malleable, , earlitic malleable cast iron are camshafts,, ctrical, , ediate cooling rate, One of these

Page 4 : 468, Material Science and Metallur,, , al, , , , Fig. 11.10 : Bull's eye malleable cast iron. 100 x, , The cooling rate used from malleablizing temperature to room temperature is slow, enough to graphitize all the procutectoid cementite and a part of the eutectoid cementite, Since carbon itself is a graphitizer, the cementite from pearlite adjacent to the existing, rosettes of temper carbon graphite decomposes rapidly without graphitizing cementite away, from these rosettes. Therefore, the microstructure at room temperature shows rosettes of, temper carbon graphite surrounded by an envelope of ferrite. The matrix is usually coarse, pearlite or slightly spheroidised due to slow cooling. They are intermediate in properties to, those of ferritic and pearlitic malleable cast irons., , (iv) Black heart malleable : This cast iron shows dark gray appearance in the central, region or core. This is due to decarburization at the surface. This decarburized ferritic aye", has no temper carbon and hence appears bright whereas the presence of graphite (which is, black in colour) makes the core or centre to appear dark gray. Because of its unusual, fracture, the cast iron is identified as black heart malleable. It is simply a ferritic (“', pearlitic) malleable cast iron in which a decarburized layer of observable thickness has bee?, formed., , (v) White heart malleable : This cast iron shows white fracture from centre to, surface. It is a cast iron completely free from temper carbon graphite with ferritic oe ‘, case. If the graphite from a malleable cast iron is completely removed by choca ae, becomes a white heart malleable cast iron. The structure no more appears like a cast 1", but is similar to steel., , The chemical composition of the alloy, particularly carbon and silicon conten', , nt role in production, heat treatment and final properties of these cast iro, , importa vee yun ment?, Paiactare of malleable cast irons, it is desirable that the amount of graphitizing ele, , ts, play”, , ns. In th?

Page 5 : Cast Irons 469), , should be as high as possible to minimise the malleablizing period required for complete, graphitization. However, if the amount of graphitizing elements are more, the melt will not!, idify as white but may solidify as mottled or gray and hence a proper balance of carbon|, con contents must be maintained to optimise the production of malleable castings, cal properties are superior when the carbon content is less because of Jess, ite formed (1% free carbon by weight corresponds to about 4% graphite by, reduction in carbon content raises the liquidus temperature and henc¢, , , , , , , , , , , , , , , , , , , , 1: Space models of flake graphite., , is formed during freezing, in contrast to the formation (, which forms during subsequent heat treatment. Th, el like matrix to a large extent and hence these cast iron, ion as compared to malleable cast irons. Howeve, Il the ferrous alloys and easiest to cast due to their hig, , int, good fluidity of melt and low shrinkage durin, nping capacity due to more internal discontinuitid, energy. They also have very low notch sensitivit, sharp notches (i.e. edges of graphite flakes, relatively ineffective. Over and above this, the