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introduction to Cast Iron


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Cast iron ,types of cast iron, white iron,Grey iron

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introduction to Cast Iron

  1. 1. 1 INTRODUCTION Iron, the silvery-whitish metal, is the most important of metals since it forms the basis of the spectrum of steels and cast iron. Today in industries steel and cast iron comprise well over 80% by weight of Cast iron and steel. Pure iron* is not an easy material to produce. Pure iron is quite soft, weak and expensive. If carbon is added in certain quantity in it, it will change its mechanical properties. According to carbon content we classified iron carbon alloys into two ways: 1. S t e e l ( L e s s t h a n 2 . 1 1 % ) 2. C a s t i r o n ( 2 . 1 1 - 6 . 6 7 % ) Cast irons are basically iron-carbon alloys having carbon between 2.11% and 6.67%. The industrial cast irons have carbon normally in the range of 2.11% to 4.0%, along with other elements like silicon, manganese, sulphur and phosphorus in substantial amounts. Why cast iron has its name? Higher carbon content makes them more brittle. Cast irons are brittle, and cannot be forged, rolled, drawn, etc. but can only be ‘cast’ into desired shape and size by pouring the molten alloy of desired composition into a mould of desired shape and allowing it to solidify. Due to presence of high carbon content in it machinability is poor so casting is the only and exclusively suitable process to shape these alloys, known as Cast iron. Cast irons is made by remelting pig iron( C-3.5%,Si-1.9%,S-0.06%, P-1.00%,Mn- 0.70%) often along with substantial quantities of scrap iron and scrap steel, and taking various steps to remove undesirable contaminants such as phosphorus and sulphur. The melting unit may be cupola, electric arc, and induction furnaces etc. The common cast irons are brittle and have lower strength properties than steels. *Pure Iron-Iron contains 99.98% alpha ferrite in it. Pure iron pillars were manufactured and situated in Delhi around 1200 AD.
  2. 2. 2 Cast iron are also classified according to metallurgical point of view • Hypo Eutectic cast iron (2.11-4.3% carbon) • Eutectic* Cast iron (4.3% carbon) • Hyper Eutectic cast iron(4.3-6.67% carbon) Eutectic Cast iron- In the eutectic cast iron, there is only one phase (liquid) of Eutectic composition at just before 1147oC. And this liquid phase will transformed into austenite and cementite phases at 1147o C by eutectic reaction. Cooling L(4.3%) Austenite (2.11% C) + Cementite (6.67% C) Heating Hypo Eutectic Cast Iron - In the hypo eutectic cast iron, there are two phases (i.e. austenite, liquid of Eutectic composition) at just before 1147oC. And only liquid phase will transformed into austenite and cementite phases at 1147oC by eutectic reaction. Austenite which is present above Eutectic temperature line is known as proeutectic or primary Austenite. Hyper Eutectic Cast iron - In the hyper eutectic cast iron, there are two phases (i.e. cementite, liquid of Eutectic composition) at just before 11470C. And only liquid phase will transformed into austenite and cementite phases at 11470C by eutectic reaction. Cementite which is present initially is known as proeutectic or primary cementite. Iron bridge, made of cast iron Cover of sewerage system *Eutectic comes from Greek word Eutectus which means “That can be easily melted”
  3. 3. 3 Development of Cast Iron Initially, there are two types of Cast iron called White cast iron and Grey cast iron. If carbon is in form of cementite then white cast iron forms and if carbon is in form of graphite then graphite cast forms. White cast irons have all the carbon in the combined cementite form (ferrite is assumed to possess negligible carbon). Cementite is a hard, brittle, white compound. The fractured surface of white cast iron looks silvery-white due to white cementite, and that is why the name white cast iron is given. Graphite is soft, brittle and gray, and thus, imparts gray colour to the fracture. Cast irons containing graphite (as flakes) are thus, called gray cast irons. Under microscopic graphite flakes appear as irregular strands such as ‘corn flakes’. As shown in Figureure.1 Gray cast iron: 1 (a) Space model of flake graphite 1(b) Unetched photo-micrograph of gray cast iron From two original cast irons, white cast iron is very brittle and unmachinable as it is very hard due to presence of hard and brittle cementite and thus finds very few applications. It is the gray cast iron, the common commercial variety most extensively used in industry; due to its certain specific properties. The compressive strength and hardness of gray cast iron are quite high and very close to the properties of the steel of similar composition and matrix structure. While developing graphitic cast irons of superior properties resulted in four more types of cast irons called meahanite iron, compacted iron, malleable iron and S.G. iron. The microstructure of gray irons consists of graphite flakes embedded in the steel matrix, i.e., in varying proportions of ferrite and pearlite. The properties of Gray iron are determined by the properties both of the matrix, and the amount, size,
  4. 4. 4 shape and distribution of graphite inclusions. Graphite flakes have weakening and embrittling effects, as graphite is soft, powdery, and brittle, and can be considered in approximation as voids or cracks, breaking the continuity of ductile matrix. The properties of gray iron are determined by the properties both of the matrix, and the amount, size, shape and distribution of graphite inclusions. According to there graphite flakes condition, gray cast iron is further divided into Meahanite cast iron (by making flakes finer), S.G Cast iron (round shaped flakes), Malleable Cast iron, Mottled Cast iron, Chilled Cast Iron etc.
  5. 5. 5 Types of Cast Iron The best method of classifying cast iron is based on type of microstructure: Two main types of Cast iron I. White Cast Iron: Carbon is in form of White cementite II. Grey Cast Iron: Carbon is in form of Graphite flakes These all other cast iron except gray and white cast iron are made by special treatment (heat treatment and by mixing chemical composition) to enhance its properties. III. Chilled Cast iron: Surface layers are of white cast iron with interior of grey cast iron. IV. Mottled iron(Mixed Iron): The transition layer between Grey cast iron and white cast iron in chilled iron is mottled cast iron and consists normally of graphite flakes V. Meahanite Iron: Cast iron has very fine flakes of graphite due to addition of calcium silicide as inoculant in melt in ladle otherwise it would has solidified as white cast iron. VI. Malleable Iron: These consists of structure of irregularly round graphite particles called temper carbon and structure is obtained by heat treatment VII. Spheroidal graphite Iron structure of nodules embedded in steel matrix, nodules are of more regular, shape and compact spheres. VIII. Compacted/Vermicular Cast Iron: The graphite here is intermediate between flakes and spheres numerous rods of graphite. Strength and ductility is greater than gray cast iron IX. Alloy Cast Iron: Properties and microstructure of cast iron or any orf these is modified by addition of alloying elements.
  6. 6. 6 Graphitisation We discussed in earlier section that cast iron classified (white and gray) on the basis of carbon present in it. In this section we know that how cementite* and graphite** are formed and at what condition in iron carbon diagram. The process of direct precipitation of graphite from liquid or by decomposition of previously formed cementite-process called graphitisation. • In iron carbon diagram graphite (Equilibrium state) is more stable phase than cementite (Metastable state) but kinetically it is easier to form cementite* than graphite** (because 6.67% C should segregate to nucleate cementite whereas 100% segregation of carbon is needed to nucleate graphite. • The crystal structure of austenite (FCC) is relatively to that cementite (complex orthorhombic***), but differ substantially from graphite (Hexagonal layer structure). • Cementite forms more easily from austenite or from liquid because energy required for diffusion is much less than that for graphite. • As kinetically cementite can form more easily it is more probable to get in microstructure Ferrite + Cementite from austenite then Ferrite + Graphite also the liquid to form eutectically to austenite + Cementite and not austenite than ferrite + graphite. If kinetic factor are favourable then graphite can form because graphite has less free energy than Cementite. When graphite form directly from liquid is called primary graphitisation. The formation of graphite from liquid is takes place in a narrow range of temperature (1153-1147°C) and also formation of graphite from austenite between 738°C to 727°C which require slow cooling. This graphite is known as secondary graphite. The line Q’C’R’ (1153°C) in Figure. 1.2 is for Eutectic reaction Cooling L Austenite + graphite (Primary graphitisation) Heating The line is for the Eutectoid reaction Cooling Austenite Ferrite + graphite (Primary graphitisation) Heating * Cementite-It is an interstitial compound of fixed carbon percentage of 6.67% carbon ** Graphite and Diamond are purest form of carbon present in nature *** For Fe3C- Complex orthorhombic structure with 12 Fe atoms and 4 carbon atoms per unit cell at melting point 1227°C. Crystal structure= Radius of solute atom/radius of solvent atom=0.63
  7. 7. 7 Commercial cast irons contains fine particle of inclusion (like Si) which becomes centre of graphite crystallisation and promote graphite formation. When graphite forms from dissociation of cementite is called secondary graphitization. Metastable cementite above temperature 738°C decompose to Austenite+ graphite or Ferrite + graphite below 738°C. As we know that slow cooling of liquid cast iron leads to formation of graphite and fast cooling leads to cementite. This is so because the formation of graphite from liquid or austenite is very slow cooling process and takes place only at small under cooling. FACTORS EFFECTING FORMATION OF CAST IRONS The main factors effecting the formation of white or gray iron, i.e., whether carbon is present in the combined form or in the graphite form are: I. Chemical composition II. Cooling rate.
  8. 8. 8 I. COMPOSITION (a) Carbon: Higher is the carbon, more is graphite formed and lower the mechanical properties. Carbons lower the melting point of metal and act a graphitiser to favour the formation of gray cast iron. (b) Silicon: Silicon is a strong graphitiser and increases the fluidity. It controls the relative proportions of combined carbon and free graphite. If silicon is present during the solidification carbon precipitates as graphite flakes. Silicon content may vary between 1.0% to 3.5%.Silicon shifts the graphite-eutectic line upwards. Thus during cooling from liquid state, a larger degree of under cooling is possible with greater chance to form graphite before cementite formation becomes possible. (c) Sulphur and Manganese: Sulphur retards graphitisation and increases the size of the flakes, High sulphur tends to reduce fluidity and is often the cause of blowholes in castings. Sulphur is kept low in amount of .06 to .12%. Sulphur in cast iron is present either as FeS or MnS. FeS tends to promote cementite formation, i.e., white cast iron. Mn is a mild carbide forming element. The amount of Mn (one part of S to 1.72 part of Mn) which combines with sulphur to form MnS particles in liquid iron and rises to be top of melt to be removed, has not been able to have its own effect of cementite formation, nor the lost sulphur could exert its effect of cementite formation thus, indirectly helps to give gray iron. Manganese in excess of what has formed MnS, weakly retards primarily graphitisation. However, it has strong cementite stabilising effect on eutectoid graphitisation. (d) Phosphorus- Most cast iron contain phosphorus between .1 to . 3%.Its amount may be more than .9%, then it forms iron phosphide (Fe3P), which form a ternary eutectic with cementite and austenite. The ternary Eutectic is called steadite. Steadite is brittle and has a melting point of around 960 degree. This increase the fluidity also helps in giving good castability to the thin and intricate casting, where low melting fluid could easily flow. However for thick and high strength cast iron casting, brittle steadite can be avoided by maintaining phosphorus less than 0.3%, which shall be present in dissolve state in ferrite. (d) Carbon equivalent Value: Si, P has similar effect on the microstructure, their effect in term of carbon is important. The carbon equivalent value (CE) = Total C% + 1/3(Si %+P %)
  9. 9. 9 The carbon content of cast iron may be lower (than 4.3%), but if C.E is 4.3%, then, the cast iron is eutectic cast iron. Carbon equivalent value for a given cooling rate, determines how close is to given composition of cast iron to the eutectic and thus how much free graphite, it is likely to form. This determines probable strength of a section of casting. I I . T h e E f f e c t o f R a t e o f C o o l i n g on t h e S t r u c t u r e of C a s t Iron • A high rate of cooling during solidification tends to favour the formation of cementite rather than graphite. That is, the higher the rate of cooling for any given cast-iron composition the 'whiter' and more brittle the casting is likely to be. This effect is important in connection with the choice of a suitable iron for the production of castings of thin section. Supposing an iron which, when cooled slowly, had a fine grey structure containing small eutectic cells were chosen for such a purpose. In thin sections it would cool so rapidly that cementite would form in preference to graphite and a thin section of completely white iron would result. Such a section would be brittle and useless. • This effect is illustrated by casting a 'stepped bar' of iron of a suitable composition. Here, the thin sections have cooled so quickly that solidification of cementite has occurred, as indicated by the white fracture and high Brinell values. The thicker sections, having cooled more slowly, are graphitic and consequently softer. Due to the chilling effect exerted by the mould, most castings have a hard white skin on the surface. This is often noticeable when taking the first cut in a machining operation. Figure 3 Illustrating the effects of thickness of section, and hence rate of cooling on the structure of a grey iron. The thinnest part of the section has cooled quickly enough to produce a white iron structure, whilst the core of the thickest part has a grey iron structure. The relationships between sectional thickness and microstructure are similar to those indicated in Figure.. on the opposite page. Both micrographs x 300 and etched in 2% nital. Macro section x 3.
  10. 10. 10 cooled so quickly that solidification of cementite has occurred, as indicated by the white fracture and high Brinell values. The thicker sections, having cooled more slowly, are graphitic and consequently softer. Due to the chilling effect exerted by the mould, most castings have a hard white skin on the surface. This is often noticeable when taking the first cut in a machining operation. In casting thin sections, then, it is necessary to choose an iron of rather coarser grey fracture than is required in the finished casting. That is, the iron must have a higher silicon content than that used for the production of castings of heavy section. Figure 4 The effect of thickness of cross-section on the rate of cooling, and hence upon the microstructure of a grey cast iron. Now we discussed types of cast iron in detailed: I. WHITE CAST IRONS These are iron-carbon alloys having more than 2.11% carbon and all the carbon is present in the combined cementite form, which makes the fracture of these alloys to have dull and white colour, and that is the reason of their name as white irons. Typical white cast iron contains 2.5 – 3.5% C, 0.4 – 1.5% Si, 0.4 – 0.6 % Mn, 0.1 – 0.4%P, 0.15%S, and balance Fe. Figure. 3 illustrates changes occurring on cooling in hypoeutectic white cast iron. At room temperature white cast iron is mixture of pearlite and cementite.
  11. 11. 11 Figure. 5 The metastable iron—iron carbide phase diagram. All white cast irons are hypoeutectic alloys. The cooling of a 2.50 percent carbon alloy will now be described. The alloy, at x2 in Figure. 5, exists as a uniform liquid solution of carbon dissolved in liquid iron. It remains in this condition as cooling takes place until the liquidus line is crossed at x2. Solidification now begins by the formation of austenite crystals containing about 1 percent carbon. As the temperature falls, primary austenite continues to solidify, its composition moving down and to the right along the solidus line toward point C. The liquid in the meantime is becoming richer in carbon, its composition also moving down and to the right along the liquidus line toward point E. At the eutectic temperature, 1147°C the alloy consists of austenite dendrites containing 2 percent carbon and a liquid solution, containing 4.3 percent carbon. The liquid accounts for (2.5—2.0)/ (4.3—2.0) or 22 percent of the alloy by weight. This liquid now undergoes the eutectic reaction isothermally to form the eutectic mixture of austenite and cementite known as ledeburite. Heating Liquid (4.3%) Austenite (2.11%) + Cementite (6.67%) Cooling Since the reaction takes place at a relatively high temperature, 1edeburite tends to appear as a coarse mixture rather than the fine mixture typical of many eutectics. It is not unusual for ledeburite to be
  12. 12. 12 separated completely, with the eutectic austenite added to the primacy austenite dendrites, leaving behind layers of massive, free cementite. As the temperature falls, between x3 and x4, the solubility of carbon in austenite decreases, as indicated by the Acm line CJ. This causes precipitation of proeutectoid cementite, most of which is deposited upon the cementite already present. At the eutectoid temperature, 727°C, the remaining austenite containing 0.8 percent carbon and constituting (6.67—2.5)/ (6.67—0.8), or 70 percent of the alloy, undergoes the eutectoid reaction isothermally to form pearlite. During subsequent cooling to room temperature, the structure remains essentially unchanged. The typical microstructure of white cast iron, consisting of dendrites of transformed austenite (Pearlite) in a white interdendritic network of cementite as shown in Figure. 9. Figure 6 Changes during cooling of hypoeutectic white cast iron
  13. 13. 13 Figure 7 Microstructure of white cast iron Figure 8. Changes on cooling, of white cast irons (schematic). (a) Dendrites of austenite font: which get broken by secondary cementite. Austenite changes to Pearlite at eutectoid temperature. (b) Complete ledeburite forms by eutectic reaction. Coarse ledeburite forms as temperature is high. Secondary cementite forms reducing size of austenite particles which at eutectoid temperature changes to Pearlite to result in complete transformed ledeburite. (c) Cementite being a compound, feasts as plate, as primary cementite. Amount of tertiary cementite in all these cases is negligibly small, thus. Microstructure is same after eutectoid reaction and at room temperature.
  14. 14. 14 Figure.9 Microstructure of white cast irons, (a) Microstructure of hypoeutectic white cast iron. Carbon is close to 2.11%, as it has major amount of broken dendrites of Pearlite and less transformed ledeburite, (b)Microstructure of hypereutectic white cast icon having more carbon than (a) as the amount of broken dendrites is lest,(c) eutectic cast iron having only transformed ledeburite,(4)Hypereutectic white iron. Presence of plates of primary cementite indicates this. Properties: Hard and wear resistant The hardness and brittleness increases as the carbon content increases. Hardness Brinell 375 to 600. Tensile strength 20000 to 70000 psi. Compressive strength 200000 to 250000.
  15. 15. 15 Limitations Because of extreme brittleness and lack of machinability, white irons find limited engineering applications. Application The parts where resistance to wear is the most important requirement such as liners of cement mixers, ball mills, pumps, wearing plates. Parts of sand-slingers, certain type of drawing dies, extrusion nozzles, grinding balls. Most parts are sand-cast and don’t require much machining, which can be done by grinding. A large tonnage of white cast irons is used as a starting material for the production of malleable cast iron parts. • Brake shoes • Shot blasting nozzles • Mill liners • Crushers • Pump impellers and other abrasion resistant parts. II. GRAY CAST IRON Iron-carbon alloys containing flakes of graphite embedded in steel matrix, which show a gray-blackish coloured fracture due to graphite’—the free foam of carbon, are called gray cast irons. The strength of gray iron depends on the strength of steel matrix and the size and character of graphite flakes in it. A typical feature of gray iron is that graphite is in the form of flakes in microstructure, Figure 10. This microstructure represents their appearance on a plane surface, but flakes are three dimensional plates, sometimes connected.
  16. 16. 16 Figure. – 10 Microstructure of gray cast iron COMPOSITION OF GRAY IRONS The gray cast irons are hypoeutectic cast irons, the total carbon content lies between 2.4% to 3.8%. The amount of carbon does not exceed 3.8%, as more the carbon, more the eutectic liquid, which yields more graphite as flakes, resulting in poor mechanical properties. Carbon is kept at least 2.4%. So that cast iron has good fluidity and castability. Silicon is kept between1.2% to 3.5%. It being a graphitiser controls along with carbon and the rate of cooling, the nature of steel matrix. In such iron, graphitisation of all the cementite except the eutectoid cementite takes place. The generalised range of composition of gray irons is: Total carbon : 2.4—3.8% Silicon : 1.2—3.5% Manganese : 0.5—1.0% Sulphur : 0.06—0.12% Phosphorus : 0.1—0.9% In manufacturing of gray cast irons, the tendency of cementite to separate into graphite and austenite or ferrite is favoured by controlling alloy composition and cooling rate. These alloys solidify by
  17. 17. 17 first forming primary austenite. The graphitization process is added by high carbon content, high temperature and the proper amount of graphitizing elements mostly silicon. Figure.11 Iron-graphite equilibrium diagram With proper control of above factors alloy will follow the stable iron- graphite equilibrium diagram (Figure.11) forming austenite and graphite at the eutectic temperature of 1154°C at any rate any cementite which is formed will graphitize rapidly During continuous cooling, there is additional precipitation of carbon because of the decrease in solubility of carbon in austenite .this carbon is precipitate as graphite Strength of gray cast iron depends almost entirely on the matrix in which the graphite is embedded. If the composition and cooling rate are such that the eutectoid cementite also graphitizes, then the matrix will be entirely Ferritic. If graphitization of the eutectoid cementite is prevented, the Matrix will be entirely pearlitic. The graphite-ferrite mixture is the softest and weakest gray iron, the strength and hardness increase with the in increase in carbide, reaching a maximum with the pearlitic gray iron.
  18. 18. 18 Pearlitic matrix is obtained by proper control of alloy composition rate of cooling or heat treatment. Properties of gray iron depend on the nature of matrix, the size, character and amount of graphite flakes. The classification of cast irons is based on the minimum tensile strength possessed by a cast iron. i.e., is based on property and not the composition. Figure 12 Microstructure of gray irons. (a) Pearlitic gray iron, (b) Ferreto pearlitic gray iron x 250. (c) Gray phosphoric cast iron (CE, = 4.2%) (C = 3.4%, Si = 2.4%, Mn = 0.45%, S = 0.02%, P = 1.0%. showing ternary phosphide eutectic. Steadite, (d) Characteristic Herring bone structure of pseudo-binary eutectic (of dark Fe3P and ferrite) Pearlitic gray iron having high phosphorus (0.3-0.5%) used for piston rings. High wear resistance is obtained in rings due to tine Pearlite and uniformly distributed phosphide eutectic with few flakes of graphite. Bearings mating with hardened (or normalised) steel shaft are of gray iron with around 85% Pearlite. (3.2-3.6 C, 1.6-2.4% Si. 0.6-0.9% Mn). If shaft has not been heat treated, then the composition of the bearing: (3.2-3.8% C, 1.7-2.6% Si 0.4-0.7% Mn, 0.1% Ti, 0.3-0.5% Cu).
  19. 19. 19 FORMATION OF FLAKES Normally commercial gray iron is either hypoeutectic or eutectic in nature. Neglecting the presence dendrites of primary austenite in hypoeutectic iron, which imposes constraints later on in the radial growth of the eutectic cell, Figure.13 illustrates the successive stages in the formation of graphite flakes from the eutectic liquid present. Once graphite has nucleated (it occurs within the interdendritic liquid and not on austenite dendrite arms), solidification takes place at nuclei Figure 13a, from each of which is formed a roughly spherical lump called the eutectic cell. It grows in an approximately radial manner, where there it simultaneous growth of austenite and graphite, the latter being in continuous contact with the liquid. The flakes bend, twist and branch as depicted in Figure13d.There is a continuous branched skeleton of graphite in each eutectic cell like a cabbage. When the rate of cooling is increased, there is more Figure.13 (a), (b), (C): Stages in the formation of graphite flakes, (d) Growth of flake graphite eutectic cell Under cooling, then the skeleton is branched more frequently with the rapid radial growth of the cell and thus, finer graphite flakes are observed. The diameter of the eutectic cell decreases as the number of cells per unit volume increase, and this results in higher tensile strength, though the soundness of the casting is affected adversely. The number of nuclei can be increased by inoculants as well as by sulphur (sulphur promotes constitutional supercooling, increasing the frequency of branching i.e., cell density as well as produces coarser flakes). Superheating or holding time of molten metal reduces the number of nuclei.
  20. 20. 20 HEAT TREATMENT OF GRAY IRON The stress-relieving is probably the most frequently applied heat treatment to gray irons. In the as-cast state, castings have residual stresses developed due to differential cooling and differential contraction, especially in non-uniform cross-sectioned castings. These stresses are completely removed by soaking at 650°C, but grain growth is serious at and above 600°C. Annealing of gray iron is done to graphitise carbide, and to homogenise the castings. It softens, increases ductility and machinability of gray iron. Castings are soaked for up to 10 hour at 850-950°C. Normalising may be done to increase the strength and hardness of cast iron by heating at 900-930°C for a soaking time of 2.5 m/min of maximum thickness of casting and then air cooling. Hardening can be done by heating to and soaking at 800-850°C, and then quenching in water, oil, hot salt bath, though for through- hardening, oil is commonly used as water quenching may cause distortion and cracking. Tempering is done at 150 to 650°C. Table 1 illustrates hardness of cast irons based on the microstructure. Table 2 illustrates composition of some gray irons with some applications. Table 1 Hardness of Gray Iron based on Matrix Microstructure Nature of Ferritic Soft Pearlitic Low Austenitic Martensitic Tempered matrix Low Iron Alloy Martensite grade Pearlitic Hardness 110-140 140-160 160-220 200-250 140-160 350-450 260-350 BHN Table 2 Composition of Gray Irons with Applications Applications C Si Mn P S Ni Cr Tensile Strength(MPa) Break Drum 3.30 1.9 0.65 0.15 .08 1.25 0.5 150 Piston Ring 3.50 2.9 0.65 0.50 .06 Cylinder and 3.25 2.25 0.65 0.15 .10 To Pistons Heavy Castings 3.25 1.25 0.50 0.35 .10 350 Clutch Casting 3.20 2.10 0.80 0.17 .05 0.32 275
  21. 21. 21 Properties of Grey Cast iron: 1. Low cost of production: I n f a c t , g r a y i r o n , b e i n g t h e l e a s t e x p e n s i v e casting material, is always considered first when a cast metal is being chosen for a product, unless mechanical and physical properties of gray iron are inadequate. 2. Low melting point: ( 1 1 5 0 ° — 1 2 5 0 ° C ) o f c a s t i r o n s , s e v e r a l hundred degrees less than steel, requires simple furnaces like pit furnace, crucible furnace, cupola, etc. which are simple, inexpensive to fun and maintain. The control of impurities is not critical here as in steel melting. 3. Good Castability: C a s t i r o n s h a v e e x c e l l e n t f l u i d i t y a n d t a k e g o o d mould-impressions easily. Cast irons; as compared to steels solidity mainly at the constant eutectic temperature—a criterion used for choosing alloy compositions having best castability. Graphite having low density is voluminous. Its large volume compensates for the shrinkage. Gray iron, thus, does not need shrinkage allowance at all to take almost exact casting impressions. 4. Good machinability of gray cast iron i s d u e t o e a s y a n d d i s c o n t i n u o u s c h i p formation due to brittle graphite flakes. Graphite serves as a solid lubricant decreasing coefficient of friction. It smears the cutting tool allowing free sliding of chips increasing thus, tool life too. (White cast irons, due to high hardness, are unmachinable). 5. Good wear resistance of gray i r o n i s d u e t o g r a p h i t e a c t i n g a s s o l i d lubricant layer, avoiding thereby metal to metal direct contact. On other hand, white cast irons are wear resistant due to’ their high hardness. 6. High damping capacity i s d u e t o t h e g r a p h i t e f l a k e s , w h i c h b r e a k s t h e continuity of the metallic matrix, and thus, vibrations are not allowed to transfer from one side of flake to other, i.e., graphitic cracks quickly dampen the vibrations and resonance oscillations. Gray iron suits thus the machine beds as compared to steels. 7. High compressive strength of g r a y i r o n - a l m o s t 3 t o 5 t i m e s o f i t s t e n s i l e strength (110-350 N/mm2), and almost equal to that of steels makes it suitable for applications, where components are subjected to compression such as machine beds, etc. 8. High thermal conductivity, a n d h a v e a b i l i t y t o w i t h s t a n d t h e r m a l s h o c k s .
  22. 22. 22 9. Good resistance to atmospheric corrosion d u e t o h i g h s i l i c o n a n d p e r h a p s other factors, than mild steels. 10. Notch-insensitive: L a r g e n u m b e r o f f l a k e s i n g r a y i r o n a c t s a s n o t c h e s in spite of these notches, if gray iron has the required strength, then additional notch or notches shall have minor, or no effect, i.e., gray iron is notch-insensitive; whereas in steels. A notch has quite a damaging effect as it acts as stress-raiser to make the steel even brittle. Table 3 Properties of Grey Cast Iron Some other properties of Grey cast iron ASTM Chemical composition: C=2.7-4%, Mn=0.8%, Si=1.8-3%, S=0.07% max, P=0.2% max Property Value in metric unit Value unit Density 7.06 *10³-7.34 *10³ kg/m³ 441-458 lb/ft³ Modulus of elasticity 124 GPa 18000 ksi -6 -6 Thermal expansion (20 ºC) 9.0*10 ºCˉ¹ 5.0*10 in/(in* ºF) Specific heat capacity 840 J/(kg*K) 0.2 BTU/(lb*ºF) Thermal conductivity 53.3 W/(m*K) 370 BTU*in/(hr*ft²*ºF) -7 -5 Electric resistivity 1.1*10 Ohm*m 1.1*10 Ohm*cm Tensile strength 276 MPa 40000 psi Elongation 1 % 1 % Shear strength 400 MPa 58000 psi Compressive yield strength Min. 827 MPa Min. 120000 psi Fatigue strength 138 MPa 20000 psi Hardness (Brinell) 180-302 HB 180-302 HB Wear resistance Low Corrosion resistance Low Weldability Low Machinability Good Castability High Limitations: Apart from low ductility and toughness, gray irons are section sensitive, i.e., depending on the section thickness of the casting, the microstructure and thus, the properties vary. Thick sections have low strength (due to ferritic matrix) and care has to be taken, when designing the castings. Applications: Gray cast irons have extensive applications. The high damping capacity
  23. 23. 23 and high compressive strength make them suitable for the beds and bases of powerful machines and frames. Good wear resistance, good machinability and damping capacity make them suitable for applications like locomotive and internal combustion engine cylinder blocks and heads, pistons rings, cylinders. The ease of casting and low cost makes them suitable for counter-weights for elevators, industrial furnace doors; • Fly wheels • Guards and frames around hazardous machinery • Gear housings • Pump housings • Steam turbine housings • Motor frames • Sewer covers • Enclosures for electrical equipments. III. CHILLED CAST IRON Chilled-iron castings are made by casting the molten metal against chillers which result in a surface of white cast iron. Chilled iron has surface layers of white iron, while the structure of the core is that of gray iron. Normally, chilled iron castings are obtained by casting the molten alloy in metal mould. Chilling to certain depth (12 to 30 mm) is because of the fast cooling (chilling) obtained due to high thermal conductivity of metal mould, The composition of molten alloy is so chosen that normal cooling results in gray iron in the whole section, but fast cooling of the whole surface, or a part of the surface yield white iron there. The fast cooling obtained by employing metal or graphite plates—called chills in the sand mould. A chilled cast iron of following composition can get chilled easily: C = 2.8—3.6%; Si =0.5 to 0.8%; Mn = 0.4 — 0.6% Where the deeper chill is needed can be increased by increasing the thickness of the chill plates. It is possible to choose the composition of the cast iron so that the normal cooling rate at the surface is just fast enough to yield white iron there, and the slower cooling rate below this surface produces mottled or mottled and gray iron. Presence of graphitiser decreases the chill depth and the carbide forming elements increase the chill depth.
  24. 24. 24 Element Chill depth Hardness of chill Other Facts depth C Decrease Increase Si Decrease Slightly Increase Mn as Decrease Increase MnS Increase Slightly Increase Dissolved Decrease Increase Added unto 0.1 % P decrease chill depth Mn 5% by 2.5% for constant C P Decrease and Si Increase Ni Increase Refines Carbides Chilled Increase structure and core. Helps to Cr pearlitic structure in thick Increase sections. Increase Mo Decrease unto 4% then 1-4% Cr as chromium increase Increase carbide increases hardness Cu and wear resistance 12-35% for corrosion and oxidation resistance at high temperatures Chilled layer has more resistance to spalling, heat checking, Chipping It decrease mottled layer Table 4 Effect of Elements on Chill Depth, etc. of Chilled Iron General, chill depth is increased by increasing carbide forming elements and decreasing carbon and silicon. Cast iron melt is allowed so solidify in mould of shape of wedge. Figure. (14b). The Cooling rate is faster at mold walls, which prevents graphitisation to yield white cast iron. The cooling rate decreases as the centre of the casting is approached, allowing graphitisation to take place to yield gray iron. Figure. (14a) illustrates changes in hardness of the step-bar test piece which is due to changes in microstructures. The depth of chill decreases and the hardness of the chilled zone increases with increasing carbon content. The depth of chill is decreased with increasing silicon content. Phosphorus decreases the depth of chill. With carbon and silicon constant, an increase of 0.1 % Phosphorus will decrease the depth of chill about 0.1 in. Nickel reduces the chill depth and refines the carbide structure. Chromium is used in small amount to control chill depth. Manganese decreases the depth of chill until the formation of Manganese sulphide after that increases chill depth and hardness. Molybdenum improves the resistance of the chilled face to spalling, pitting, chipping and heat checking.
  25. 25. 25 Figure. 14(a) Effect of elements on chill depth. (b) Step bar test piece for chill depth cast iron having 3-3.3% C. Properties: C a s t i n g s h a v e s o m e g o o d p r o p e r t i e s d u e t o w h i t e i r o n surface which are high wear and abrasion resistance, and some good properties due gray iron core which are damping capacity, low notch sensitivity. Application: • Chilled cast irons used as • Rail-freight car wheel • Cane-crushing rolls • Road rollers • Grinding balls • Liners • Stamp shoes and dies • Sprockets • Ploughshares many other heavy-duty machinery parts
  26. 26. 26 IV.MOTTLED IRON In a chilled cast iron casting, surface layers are of white iron and the core is of gray iron, but in the Figure. 15 Microstructure of mottled cast iron transition region, the structure consists both of gray and white iron, i.e., has graphite flakes, Pearlite and secondary free cementite, i.e., mixed iron or called mottled iron, The intermediate cooling rate for certain carbon and silicon contents could not graphitise the free secondary cementite, Due to incorrect foundry control for certain compositions, The non uniform flakes increase brittleness of the castings, apart from the extra brittleness due to the presence of secondary cementite. Mottled cast irons, thus, don’t find applications. If carbon and silicon content of the cast iron is increased, then the casting shall solidify as gray iron. The thickness of the mottled zone in chilled iron can be reduced by increasing both the graphitiser and the carbide forming elements in the cast iron. Figure. 15 illustrates microstructure of mottled cast iron.
  27. 27. 27 V. MEAHANITE CAST IRON The molten cast iron is treated with calcium silicides as inoculants to produce a fine graphitic structure. The flakes are uniformly distributed to give high mechanical properties (Tensile strength = 25 — 40 Kg/mm2). The composition is so chosen that white fracture is obtained in the absence of any treatment, i.e., the cast iron is low in silicon content, moderately low in carbon content about 2.5-3%. Calcium silicides act as graphitiser, so that resulting casting is gray and merchantable. Meahanite cast iron finds applications as a gray iron with high mechanical strength, such as for heavy machine beds and frames. VI.MALLEABLE CAST IRON Cementite (iron carbide) is actually a metastable phase. There is a tendency for cementite to decompose into iron and carbon, but under normal conditions it tends to persist indefinitely in its original form. Up to this point, cementite has been treated as a stable phase; however, this tendency to form free carbon is the basis for the manufacture of malleable cast iron. The reaction Fe3C3Fe + C is favoured by elevated temperatures, the existence of solid non metallic impurities, higher carbon contents, and the presence of elements that aid the decomposition of Fe3C On the iron—iron carbide equilibrium diagram for the metastable system, shown in Figure. 16, are superimposed the phase boundaries of the stable iron-carbon (graphite) system as dotted lines. The purpose of malleabilization is to convert all the combined carbon in white iron into irregular nodules of tamper carbon (graphite) and ferrite. Commercially, this process is carried out in two steps known as the first and second stages of the anneal. White irons suitable for conversion to malleable iron are of the following range of composition: Components Percentage Carbon 2.00-2.65 Silicon 0.90-1.40 Manganese 0.25-0.55 Phosphorus Less than 0.18 Sulphur 0.05
  28. 28. 28 Table 5 Composition of Malleable Iron In the first-stage annealing, the white-iron casting is slowly reheated to a temperature between 1660 and 1750°F. During heating, the pearlite is converted to austenite at the lower critical line. The austenite thus formed dissolves some additional cementite as heated to the annealing temperature Figure. 16 The stable iron-Graphite system (dotted lines) superimposed on the metastable iron—iron carbide system. Figure 16 show that the austenite of the metastable system can dissolve more carbon than can austenite of the stable system. Therefore, a driving force exists for the carbon to precipitate out of the austenite as free graphite. This graphitization starts at the malleabilising temperature. The initial precipitation of a graphite nucleus depletes the austenite of carbon, and so more is dissolved from the adjacent cementite, leading to further carbon deposition on the original graphite nucleus. The graphite nuclei grow at approximately equal rates in all directions and ultimately appear as irregular nodules or spheroids usually called temper carbon (Figure. 17). Temper carbon graphite is
  29. 29. 29 formed at the interface between primary carbide and saturated austenite at the first-stage annealing temperature, with growth around the nuclei by a reaction involving diffusion and carbide decomposition. Nucleation and graphitization are accelerated by the presence of sub microscopic particles that can be introduced into the iron by the proper melting practice. High silicon and carbon contents promote nucleation and graphitization, but these elements must be restricted to certain maximum levels since the iron must solidify as white iron. Therefore, graphitizing nuclei are best provided by proper annealing practice Figure. 17 Malleable iron, unetched. Irregular nodules of graphite called temper carbon, box. (b) Ferritic malleable iron, temper carbon black) in a ferrite matrix. The rate of annealing depends on chemical composition, nucleation tendency, and temperature of annealing. The temperature of first-stage annealing exerts considerable influence on the number of temper- carbon particles produced. Increasing annealing temperature accelerates the rate decomposition of primary carbide and produces more graphite particles per unit area. However, high first-stage annealing temperatures result in excessive distortion of castings during annealing and the need for-straightening operations after heat treatment Annealing temperatures are adjusted to provide maximum practical annealing rates and minimum distortion and are therefore controlled between 1650 and 1750°F. The white-iron casting is held at the first- stage annealing temperature until all massive carbides have been decomposed. Since graphitization is a relatively slow process, the casting must be soaked at temperature for at least 20 h, and large loads may require as much as 72 h. The structure at completion of first-stage graphitization consists of temper-carbon nodules distributed throughout the matrix of saturated austenite. After first-stage annealing, the castings are cooled as rapidly as practical to about 1400°F in preparation for the second stage of the annealing treatment. The fast cooling cycle usually requires 2 to 6 h, depending on the equipment used. In the second-stage annealing, the castings are cooled slowly at a rate of 5 to 15°F/h through the critical range at which the eutectoid reaction
  30. 30. 30 would take place. During the slow cooling, the carbon dissolved in the austenite is converted to graphite on the existing temper-carbon particles, and the remaining austenite transforms into ferrite. Once graphitization is complete, no further structural changes take place during cooling to room temperature, and the structure consists of temper-carbon nodules in a ferrite matrix (Figure. 17). This type is known as standard or Ferritic malleable iron. The changes in microstructure during the malleabilising cycle are shown schematically in Figure. 18. . Figure. 18 The changes in micro structure as a function of the malleabilising cycle resulting in temper carbon in a ferrite matrix. TYPES OF MALLEABLE CAST IRONS 1. Ferrite malleable iron: T h e s t r u c t u r e c o n s i s t s o f n o d u l e s o f temper carbon embedded in ferrite matrix (due to slow cooling in eutectoid temperature range). As these nodules break the continuity to lesser damaging extent of tough ferrite. The castings are cooled at slowly at the rate of 5 to 15 °F/hr. Through the critical range at which the eutectoid reaction would take place. During the slow cooling the carbon dissolved in the austenite is converted to graphite on the existing temper –carbon particles & remaining austenite transforms into ferrite. Once graphitization is complete no further structural changes takes place during cooling to room temperature and the structure consist of temper- carbon nodules in a ferrite matrix. This known as ferrite malleable C.I.
  31. 31. 31 Properties • In the form of compact nodules, the temper carbon does not break up the continuity of the tough Ferritic matrix. This results in a higher strength and ductility than exhibited by gray cast iron. The graphite nodules also serve to lubricate cutting tools, which accounts for the very high machinability of malleable iron. • The Ferritic malleable iron shows higher strength and ductility than gray cast iron. • Graphite nodules lubricate the cutting tools leading to good machinability of malleable iron. Application Ferritic malleable iron has been used for pipe fittings, expansion joints, railing casting on bridges, n-hoist assemblies, bearing blocks, valves, farm equipment, chains, automobile parts, in general hardware, reducing gear housings, rear-axle housings, hubs, hooks, shackles, leads, yokes, nuts, mufflers, flanges, couplings. 2. Pearlitic malleable iron: T o o b t a i n p e a r l i t i c m a t r i x , 1 % m a n g a n e s e i s added to cast iron, or second-stage graphitisation is replaced by a quench, usually air, which cools the castings through the eutectoid range fast enough to retain combined carbon throughout the matrix. The amount of Pearlite formed depends upon the temperature at which the quench starts and rate of cooling. If the air quench produces a fast enough cooling rate through the eutectoid rang, the matrix will be completely pearlitic. A fully ferritic malleable iron may be converted into pearlitic malleable iron by reheating above the lower critical temperature, followed by rapid cooling. At higher temperature carbon will be dissolved from the graphite nodules and subsequent cooling retains the combined carbon. The cooling from temperature of first-stage graphitisation (curve II in Figure 19). Normally, after air cooling, the pearlitic malleable cast iron castings are heated to higher temperatures (called drawing process) at 550- 650°C or so to spheroidise the Pearlite to improve the machinability, ductility and toughness with slight drop in hardness and strength. Pearlitic malleable iron can be hardened and
  32. 32. 32 tempered. Welding of pearlitic malleable iron is rarely used due to the formation of brittle and low strength white iron under the weld bead. Presence of larger amount of silicon in white cast iron castings helps to graphitise it during malleable heat treatment. But a thick casting having higher silicon may result in gray iron in the centre while casting it. As the casting should he of white iron up to the centre before malleable heat treatment is given, silicon content has to be kept low in the composition, which makes graphitisation during malleable treatment a long and difficult process. Figure 19 Typical Malleabilising iron Application: Due to high strength and hardness, pearlitic malleable iron is used for cam shafts, crank-shafts, axles, differential housing in automobile industry, rolls, pumps, nozzles, gears, links, sprockets, elevator brackets in conveyer equipment, hammers, wrenches, shows, switch gear parts, fittings for high and low voltage transmission and distribution system, jaws of universal-joint shafts, links and rollers of conveyer chains, bushings, couplings brake-shoes. Malleable irons are
  33. 33. 33 used chiefly for thin walled castings because there are restrictions in section thickness. Properties: The malleable cast irons have reasonable ductility, high strength, toughness and even are bendable. The main reasons of using malleable irons are low cost and ease of machining with above properties. Malleable iron has limitations of section thickness, lower damping capacity and impact resistance. VII. SPHEROIDAL GRAPHITE IRON (S.G. IRON) This cast iron also known as nodular cast iron. In an ordinary grey cast iron graphite is present as 'flakes' which tend to have sharp-edged rims. Since these flakes have negligible strength they act as wide-faced discontinuities in the structure whilst the sharp-edged rims introduce regions of stress-concentration. In SG cast iron the graphite flakes are replaced by spherical particles of graphite (Figure. 20 a), so that the metallic matrix is much less broken up, and the sharp stress raisers are eliminated. Figure 20 a) A Spheroidal-graphite cast iron. Here the graphite has been made to precipitate in nodular form by adding a nickel-magnesium alloy b) A compacted graphite cast iron. Unetched to show the rounded edges of the graphite flakes, The formation of this Spheroidal graphite is effected by adding small amounts of cerium or magnesium to the molten iron just before casting. Since both of these elements have strong carbide-forming tendencies, the silicon content of the iron must be high enough (at least 2.5%) in order to prevent the formation of white iron (by chilling) in thin sections. Magnesium is the more widely used, and is usually added (as a nickel-magnesium alloy) in amounts sufficient to give a residual
  34. 34. 34 magnesium content of 0.1% in the iron. SG cast irons produced by the magnesium process have tensile strengths of up to 900 N/mm2 or even higher in some heat-treated irons. The term 4SG iron' really describes a family of cast irons, which include some alloy irons, but in all cases treatment by inoculants is employed to produce Spheroidal-graphite particles. Some SG iron produced by using the following substances instead of cerium or magnesium: calcium, calcium carbide, calcium fluoride, lithium, strontium, barium and argon. Those irons consisting of graphite nodules in a ferrite matrix will have high ductility and toughness whilst those consisting of graphite nodules in a pearlite matrix will be characterised by high strength. Some of these irons are heat-treated to give even better mechanical properties. Thus, American motor industry hardens some of their SG iron gears by the use of 'interrupted austempering'. This involves austenitising the gears at 9000C for 3.5 h in a nitrogen atmosphere followed by quenching to 235°C and holding at that temperature for 2 hour. Since transformation from austenite occurs isothermally at 235°C there is little distortion in shape. It is claimed that SG iron hypoid ring and pinion gears are comparable with those of steel in terms of fatigue and also have a greater torsional strength. Tensile strengths of the order of 1600 N/mm2 (with an elongation of. 1%) can be obtained by austempering SG iron at 2500C, following an initial austenitising at 9000C; whilst higher austempering temperatures up to 4500C will yield bainitic structures of lower strengths (900-1200 N/mm2) but elongations up to 14%. SG iron crankshafts cast to near final shape are less expensive and some 10% lighter than equivalent forged components. They are heat-treated in a similar way to the gears mentioned above. Properties of S.G. Iron: S.G. irons have higher mechanical properties, almost equal to cast carbon steels (thus used for pipes), such as tensile strength, ductility and toughness (Table 6), combined with favourable properties of gray cast irons, like good machinability, damping capacity, high wear resistance, reasonable castability, but do not suffer from the defects of gray irons such as growth and fire crazes, when used at elevated temperatures, and is less section-sensitive. Table 6 Properties of S.G. Iron Grade Minimum Minimum, BHN % Matrix Heat Tensile 0.2% Elongations Treatment strengthMN/m2 Proof
  35. 35. 35 stress MN/m2 350/22 350 220 130 22 Ferrite Annealed 420/12 420 270 150 12 Mainly Ferritic Annealed 500/7 500 320 275 7 Ferrite+Pearlite Annealed 800/7 800 480 320 2 Tempered Quenched martensite and Tempered 400/40 400 200 130 40 Austenite Cast STEPS IN PRODUCTION OF S.G. IRON 1. D e s u l p h u r i s a t i o n : Sulphur helps to form graphite as flakes. Thus, the raw material for producing iron should have low sulphur (less that 0. 1%), or remove sulphur from iron during melting, or by mixing iron with a desulphurising agent such as calcium carbide, or soda ash (sodium carbonate). 2. N o d u l i s i n g : Magnesium is added to remove sulphur and oxygen still present in the liquid alloy and provides 0.04% magnesium, which causes growth of graphite to be Spheroidal. Magnesium treatment desulphurises the iron to below 0.02% S before alloying with it. Magnesium and such elements have strong affinity for sulphur, and thus scavenge sulphur from the molten alloy’s an initial step or, producing S.G. iron. These additions are expensive to increase the cost of S.G. iron produced. Thus, sulphur S molten alloy (or the raw material used), before nodulising, should be kept low. Magnesium is added when melt is near 1500°C but magnesium vaporises at 1 150°C. Magnesium, being lighter floats on the top of the bath, and being reactive burn off at the surface. In such cases magnesium is added as Ni-Mg, Ni-Si-Mg alloy or magnesium coke to reduce violence of reaction and to have saving in magnesium. Magnesium metal can be added as metal itself The method of addition include ladle transfer, covered ladle technique, porous plug stirring, and in-mould technique. Addition of magnesium and ferrosilicon is done shortly before casting. 3 . Inoculation : A s m a g n e s i u m i s c a r b i d e f o r m e r , f e r r o s i l i c o n i s added immediately as inoculant. Remelting causes reversion to flake graphite due to the loss of magnesium. Stirring of molten alloy after addition of nodulising element evolves a lot of gas, which gets dissolved in liquid alloy, and forms blow-holes in solid casting. The contraction during solidification of nodular cast iron castings is much
  36. 36. 36 greater than of gray iron castings, which needs careful design of moulds to avoid shrinkage cavities in solidified castings. In spite of these drawbacks, nodular cast iron is replacing gray iron and steels in applications. A nodule of graphite (having minimum surface area per unit volume) weakens the steel matrix to a lesser extent than gray iron flakes. The nodules don’t act very much as stress-raisers. Figure.19 Illustrates range of carbon and silicon in S.G. iron. One of the composition of S.G. iron can be: C = 3.7%, Si = 2.5%, Mn = 0.3% S = 0.01%, P = 0.01%, Mg = 0.04% Range of carbon and silicon for S.G irons. Figure 21 Helpful Neutral Inhibitors Mg,Ce,Ca,Ba,Li,Zr Fe,C,Ni,Si,Mo Al,Ti,Sb,As,Pb,Bi Table 7 illustrates effect of some elements in the production of S.G. iron.
  37. 37. 37 Figure. 22 illustrates various C.E.V. depending on the maximum thickness of the casting in sand moulds and in metal moulds. The matrix of as-cast S.G. iron depends on the composition and the rate of cooling. Complete Ferritic or matrix having a maximum of 10% Pearlite, still called Ferritic matrix, possesses maximum ductility, toughness and machinability, Figure. 23(a). A largely pearlitic matrix 23(b) , obtained in as-cast, or by normalising (air cooling from 850 to 900°C) makes S.G. iron stronger but less ductile. Oil or water quenching from 900°-950°C yields martensite matrix, which is tempered to desired strength, hardness and toughness. Austenitic ductile matrix, Figure.21 (d) (which can be retained up to 25°C) is obtained by alloying (15-36% Ni, 1.8-6% Cr) the cast iron to have high corrosion resistance and good creep resistance at high temperatures. Figure.21(c) illustrates, bulls eye S.G. iron, where ferrite in immediate vicinity of graphite is present in mainly Pearlite matrix. Figure.23 Microstructures of S.G. irons. (a) Ferrite S.G. iron. x 250, (b) Pearlitic S.G. Iron. x 500, (c) Bull’s eye S.G. Iron. x 100, (d) Austenitic S.G. iron (Ni-Resist 21.06% Ni, 2.20% Cr,0.06% Mg) as cast. X 500 (nital)
  38. 38. 38 Application of S.G. Iron: S.G. iron is used for gear pumps for processing and transport of sulphuric acid, pumps and valves in sea water applications, components used in steam services, and in the handling of alkali, caustic and ammonia-cal solutions, and for pumping and handling of sour crude oils in petroleum industry. Other wide applications are- • Crank-shafts • Pistons and cylinder heads in automobile and diesel engines • Pressure castings like gears and roller slides • Steering knuckles • Rocker arms • Paper mill dryer rolls • Bearing VIII. COMPACTED/VERMICULAR CAST IRON This is the latest member to join the family of cast irons in which graphite occurs as worm-like blunt-edged stubby flakes (rounded rods, which are interconnected within eutectic cell); embedded in steel matrix, Figure.24 The formation of compacted iron depends on the chemical composition, section thickness. and the process used for production. Normally, 10-20% of the spheroidal graphite may be present, which requires C.E.V. of 4.00, and flake-graphite should be avoided. In one of the production methods, nitrogen (—0.015%) is added to liquid alloy in ladle by adding nitride Ferro-manganese
  39. 39. 39 Figure.24 Microstructure of compacted vermicular cast iron. (80% Mn, 4% N, rest Fe). This method gives non-uniformity of structure and unsoundness in castings. In another method, an alloy (4-5% Mg, 8.5-10.5% Ti, 4-5.5% Ca, 1-1.5% Al, 0.2-0,5% Ce, 48-52% Si, rest Fe) in amounts 0.6-1.6% is added, as additions are made to produce S.G. iron. Sulphur content of iron should not be more than 0.035%. This method is section-sensitive as spheroids get formed in thin sections. The compacted graphite permits strength. Stiffness and ductility that exceeds those of gray iron. Properties: Compacted cast iron to retain good damping capacity, and thermal conductivity. Its resistance to crazing, tracking and distortion is superior to both S.G. iron and gray iron. As the shrinkage during casting is less than in S.G. iron. This cast iron having inferior mechanical properties hut similar production costs as S.G. Iron has limited replacement potential to S.G. iron parts. However, because of greater strength and toughness, it can replace more expensive alloyed gray cast irons. Applications: Compacted cast iron is used for making thick sections.
  40. 40. 40 • Hydraulic valves • Ingot moulds • Cylinder heads • Exhaust manifolds, • Brake drums • Discs and piston rings are made from this iron as it has good elevated temperature properties. Figure. 25 Types of cast irons (a) Gray iron, (b) White iron, (c) Malleable iron, (d) S.G. iron, compacted graphite iron. IX. ALLOYED CAST IRONS One or more of the elements like, Ni, Cr, Cu, Si, Mo, V etc. (> 3%) are added into graphite free, or graphite-bearing cast irons to improve corrosion, elevated temperature and wear and abrasion resistance properties. 1. Ni-hard: In the white iron composition, 3-5% Ni and 1-3% Cr are added, producing a microstructure consisting of massive continuous carbides in the matrix of martensite and some retained austenite on cooling after solidification. Martensite is obtained due to increased hardenability, due to the presence of these elements, which along with carbon lower the Mf temperature to below room temperature to retain
  41. 41. 41 some austenite. Hardness attained is 550-700 BHN. As nickel is half as powerful a graphitiser as silicon, the risk of graphitisation is prevented by adding carbide-former, chromium. The poor impact strength and fatigue resistance due to the continuous network of carbides can be improved by increasing Ni and Cr content. The modified Ni-hard having 4-8% Ni and 4-15% Cr, after heat treatment has a microstructure of discontinuous carbides in the matrix of tempered martensite and Bainite. F i g u r e . 2 6 N i - H a r d c a s t i r o n m i c r o gr a p h y Properties: • Very good usury strength until 700°C • These cast irons have excellent wear resistance. 2. Ni-Resist: N i ( 1 3 - 3 6 % ) a n d C r ( 1 . 8 - 6 % ) a r e a d d e d t o p r o d u c e austenitic matrix with flake or Spheroidal graphite, to get good corrosion resistance. The latter offers better mechanical properties but are more expensive. Ni being austenite stabiliser makes the matrix austenitic, and thus, these are called austenitic cast irons. The concentration of the elements depends on the nature of the corrosion environment. Chromium in combination with nickel forms an effective oxidation resistant scale. Ni-resists combine good corrosion resistance, excellent erosion resistance to the flow of liquids with heat resisting properties. Some Ni-resists contain 5.5-8.0% copper. Though, these alloys could be used up to 800°C, but after stabilisation at 950°C. These alloys could be used at temperatures higher than 800°C. Average properties are:
  42. 42. 42 T.S. = 247 — 485 MN&2 BHN = 120 - 250 %E =2-15% Important applications are gear pumps (for processing and transport of sulphuric acid), pumps and valves in sea-water applications, parts used in steam and for handling of alkali, caustic, for pumping and handling of sour crude oils in petroleum industry. Furnace parts, cylinder liners, exhaust manifolds, etc. F i g u r e . 2 7 N i - R e s i s t c a s t i r o n m i c r o g r ap h y 3. Silal and Nicrosilal: S i l a l i s t h e c h e a p e s t o x i d a t i o n a n d g r o w t h - resistant cast iron, particularly the low carbon cast iron resists up to 750°C. Thy composition on an average is: C = 2.3%; Si=5.5-7.0%; Mn=0.5-0.8%; S=0.06%; P=0.1-0.3% T.S. = 139—263 Nmm-2 BHN 220 — 255 The microstructure of Silal consists of ferrite and fine graphite ‘D’ type flakes. These cast irons are very brittle. Silicon increase oxidation resistance by forming a resist it oxide film, and with more silicon, an impermeable silicate film. Nicrosilal is Ni-Cr added. Silal which gives austenitic matrix reducing the brittleness, and can be used at 650-900°C. The composition is: C 1.5-2.0%; Si = 4.5-5.0%, Mn = 0.6-1.0%; S = 0.10%; p <0.1%,
  43. 43. 43 Ni = 18— 23%; Cr = 2-2.4% T.S. 139-247 Nmm-2 BHN = 150-200 Nicrosilal offers excellent corrosion resistance. Common applications are: Ingot-moulds. Cylinder heads exhaust manifolds, aluminium melting crucibles, retorts, glass-moulds, gas-turbine parts. Table (8) Comparative qualities of cast irons Comparative qualities of cast irons Name Nominal Form and Yield Tensile Elongation Hardness Uses composition condition strength strength [% (in [Brinell [% by ksi [ksi] 2 inches)] scale] weight] (0.2% offset)] Grey cast C 3.4, Si 1.8, Cast — 25 0.5 180 Engine iron Mn 0.5 cylinder (ASTM blocks, A48) flywheels, gears, machine-tool bases White cast C 3.4, Si 0.7, Cast (as — 25 0 450 Bearing iron Mn 0.6 cast) surfaces Malleable C 2.5, Si 1.0, Cast 33 52 12 130 Axle iron Mn 0.55 (annealed) bearings, (ASTM track wheels, A47) automotive crankshafts Ductile or C 3.4, P 0.1, Cast 53 70 18 170 Gears, nodular Mn 0.4, camshafts, iron Ni 1.0, crankshafts Mg 0.06 Ductile or — cast 108 135 5 310 — nodular (quench iron tempered) (ASTM A339) Ni-hard C 2.7, Si 0.6, Sand-cast — 55 — 550 High strength type 2 Mn 0.5, applications Ni 4.5, Cr 2.0 Ni-resist C 3.0, Si 2.0, Cast — 27 2 140 Resistance to type 2 Mn 1.0, heat and Ni 20.0, corrosion Cr 2.5
  44. 44. 44 HEAT TREATMENT OF CAST IRONS The common heat treatments given to cast irons are: I. STRESS-RELIEVING TREATMENT Residual-stresses develop during solidification and differential cooling and thus cause differential contraction. Thermal gradients and residual- stresses are more pronounced in castings with non-uniform cross- sections. Phase transformations accompanied with volume changes aggravate the situation further. Castings are slowly heated to a temperature 480-650°C, normally at 600°C and then furnace cooled to 200°C, followed by air cooling. II. ANNEALING The aim is to decompose carbides and Pearlite from the as cast- structure. This gives graphite in Ferritic matrix. Gray cast iron and S.G. irons get softened increasing ductility and machinability. White cast iron gets malleablised. A typical two stage process particularly for S.G. iron could be used: First austenitising at 900°C and then cool to transform to Pearlite to 675°C and then ferritization of Pearlite is done at 760°C. Air cooling may be done unless casting is susceptible to residual-stresses. III. NORMALISING It is heating the castings to temperatures above the critical range, soaking at it and cooling in still air as induced by large fans. Normalising gives higher hardness and strength by obtaining fine pearlitic matrix. Table 9 gives normalising temperature range for some cast irons. Malleable Iron High Strength Low strength S.G. Iron Gray Iron Gray iron Temperature 800-830°C 810-870°C 840-900°C 820-900°C Range
  45. 45. 45 Table 9 IV. HARDENING AND TEMPERING Hardening and tempering induce higher strengths, and good wear resistance. The time and temperature austenitising depends on the original matrix of the cast iron. The temperature is up to 50°C above critical temperature range, but time is important in low combined- carbon-matrix and thus, soaking is continued till desired amount of carbon has been dissolved in austenite from free graphite. High silicon cast irons are less responsive to quenching and prone to cracking as silicon reduces solubility of carbon in austenite necessitating high temperatures of austenitising, but which can cause cracking due to more severe quenching. Water quenching of castings (complex shapes and different sectioned) causes quench cracks. Oil quench is normally used or even air-quench, if large amounts alloying elements are present. Tempering improves tensile strength, reducing hardness, though depends on tempering temperature and type of iron. V. MARTEMPERING It reduces chances of distortion and cracks. Thin-walled cylinder liners for diesel engines (BHN needed 390-430) are martempered. The casting is quenched in a hot salt bath, or oil kept slightly above Ms, temperature (from austenitising temperature) till the centre of the casting too attains the bath temperature, and then air cooled. Tempering may be done as usual. VI. AUSTEMPERING Castings austenitised at 850-950°C, quenched into, salt or oil bath, kept at temperature 450-250°C for around 4 hrs. Lower S.G. iron is twice as strong with same toughness. As it approaches properties of steels, crank shafts, camshafts, gears of S.G. iron are used in austempered state. VII. SURFACE HARDENING
  46. 46. 46 It is an economical method to get wear resistance in selected areas. Excepting white and highly alloyed cast irons, most cast irons could be surface hardened by induction, flame, laser etc. Ferritic matrix is not used, necessitating a pearlitic (even bulls eye), or tempered martensitic matrix. Flame hardening requires combined carbon of 0.5-0.7% in matrix. Induction hardening is good for mass production. Electron beam, plasma and laser are increasingly used methods for surface hardening. General Motors have been using gray iron diesel engine cylinder liners, which are laser hardened.
  47. 47. 47 References 1. Physical Metallurgy by Vijendra Singh (Standard publications) 2. A Introduction to Physical Metallurgy by Sidney H Avner (Tata McGraw-Hill Publications) 3. Physical Metallurgy for Engineers by Clark Donald 4. Engineering metallurgy Raymond Higgins 5.