Cast iron

Cast iron is a group of iron-carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its colour when fractured: white cast iron has carbide impurities.
History:
Cast iron was invented in China in the 5th century BC and poured into molds to make ploughshares and pots as well as weapons and pagodas. Although steel was more desirable, cast iron was cheaper and thus was more commonly used for implements in ancient China, while wrought iron or steel was used for weapons.
The cast-iron is manufactured by re-melting pig-iron with coke and limestone. This re-melting is done in a furnace known as the cupola furnace. It is more or less same as the blast furnace, but it is smaller in size. Its shape is cylindrical with diameter of about 1 m and height of about 5 m.
The working of cupola furnace is also similar to that of blast furnace. The raw materials are fed from top. The cupola furnace is worked intermittently and it is open at top. After the raw materials are placed, the furnace is fired and blast of air is forced through tuyeres. The blast of air is cold as the impurities in pig-iron are removed by the oxidation

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1. CAST IRON Presented by: G. Lahari (319106101007) G. Likhitha(319106101008) K. Aditya(319106101009) K. Gayathri(319106101010) K. Lavanya(319106101011)

2. Introduction: Cast iron  is a group of iron-carbon  alloys with a carbon content more than 2 %.  Its usefulness derives from its relatively low melting temperature . The alloy constituents affect its colour when fractured: white cast iron has  carbide  impurities. History: Cast iron was invented in China in the 5th century BC and poured into molds to make ploughshares and pots as well as weapons and pagodas. Although steel was more desirable, cast iron was cheaper and thus was more commonly used for implements in ancient China, while wrought iron or steel was used for weapons.

3. Manufacture of Cast-Iron : The cast-iron is manufactured by re-melting pig-iron with coke and limestone. This re-melting is done in a furnace known as the cupola furnace. It is more or less same as the blast furnace, but it is smaller in size. Its shape is cylindrical with diameter of about 1 m and height of about 5 m. The working of cupola furnace is also similar to that of blast furnace. The raw materials are fed from top. The cupola furnace is worked intermittently and it is open at top. After the raw materials are placed, the furnace is fired and blast of air is forced through tuyeres . The blast of air is cold as the impurities in pig-iron are removed by the oxidation

4. The impurities of pig-iron are removed to some extent and comparatively pure iron is taken out in the molten stage from the bottom of furnace. The slag is also removed from top of cast-iron at regular intervals. The molten cast-iron is led into moulds of required shapes to form what are known as the cast-iron castings .

5. Composition of Cast-Iron : The cast-iron contains about 2 to 4 per cent of carbon. In addition, it contains the various impurities such as manganese, phosphorus, silicon and sulphur . The manganese makes cast-iron brittle and hard. Its amount should therefore be kept below 0.75 per cent or so. The phosphorus increases fluidity of cast-iron. It also makes cast-iron brittle and when its amount is more than 0.30 per cent, the resulting cast-iron is lacking in toughness and workability. Its percentage is sometimes kept as about 1 to 1.5 to get very thin castings. The silicon combines with part of iron and forms a solid solution. It also removes combined carbon from graphite form. If its amount is less than 2.50 per cent, it decreases shrinkage and ensures softer and better castings.

6. Types of Cast-Iron : Following are the varieties of cast-iron: Grey cast-iron White cast-iron Mottled cast-iron Chilled cast-iron Malleable cast-iron Spheroidal graphite iron or ductile iron Tophened cast iron

7. Grey cast iron: This is prepared from grey pig. Its colour is grey with a coarse crystalline structure. It is soft and it melts readily. It is somewhat weak in strength. It is extensively used for making castings . One of the key characteristics of gray iron is its ability to resist wear even when lubrication supply is limited (e.g. the upper cylinder walls in engine blocks ). Gray iron is used to make engine blocks and cylinder heads , manifolds, gas burners , gear blanks , enclosures, and housings .

8. 2. White Cast-Iron: Its colour is silvery white. It is hard and it melts with difficulty. It is not easily worked on machine. It cannot be used for delicate casting . The chilling process used to make white iron results in a brittle material that is very resistant to wear and abrasions. For this reason, it is used to make mill linings , shot-blasting nozzles, railroad brake shoes, slurry pump housings, rolling mill rolls, and crushers.

9. 3. Mottled Cast-Iron: Cast iron or pig iron that is intermediate between white cast iron and grey cast iron and shows a mottled surface on fracture is known as mottled cast iron Iron which consists of a mixture of a variable proportions of gray cast iron and white cast iron, such a material has a mottled fracture.

10. 4 . Chilled Cast-Iron: The chilling consists of making some portion of cast-iron hard and other portion soft. This variety of cast-iron is hard to a certain depth from the exterior surface and it is indicated by white iron Obtained by casting against a chiller. Surface is White C.I. (faster cooling rates ). Harder / wear resistant surface. Depth depends on composition (C, Si decrease chill depth) Carbide forming elements like Cr, Mo increase chill depth) Applications : Railway car wheels, crushing rolls, heavy machinery Additions of Ce / Mg poisons the easy growth di

11. 5. Malleable cast iron Malleable cast iron exhibits good malleability and good ductility. Due to the lower silicon content compared to other cast irons, it exhibits good fracture toughness at low temperature. Malleable cast iron applications As a result of its good tensile strength and ductility , malleable cast iron is used for electrical fittings and equipment, hand tools, pipe fittings, washers , brackets, farm equipment, mining hardware , and machine parts. A common classification for malleable cast iron is   ASTM A47 .

12. 6. Spheroidal Graphite Iron or Ductile Iron: Malleable cast-iron is being replaced by spheroidal graphite iron or ductile iron now-a-days. Its manufacturing process is much easier than malleable cast-iron. Manganese treatment is given to the cast-iron. Manganese increases the content of carbon in iron and opposes the formation of free graphite in flaky form. This type of iron is used for manufacturing of pipes for carrying water and sewage due to its high strength, ductility and good resistance to corrosion.

13.   7. Toughened Cast-Iron: This variety of cast-iron is obtained by melting cast-iron with wrought-iron scrap. The proportion of wrought-iron scrap is about to ¼th to 1/7th of weight of cast-iron.

14. Name Nominal composition [% by weight] Form and condition Yield strength [ ksi  (0.2% offset)] Tensile strength [ksi] Elongation [%] Hardness [ Brinell scale ] Uses Grey cast iron ( ASTM  A48) C 3.4, Si 1.8,  Mn  0.5 Cast — 50 0.5 260 Engine  cylinder blocks,  flywheels ,  gearbox cases , machine-tool bases White cast iron C 3.4, Si 0.7, Mn 0.6 Cast (as cast) — 25 450 Bearing  surfaces Malleable iron (ASTM A47) C 2.5, Si 1.0, Mn 0.55 Cast (annealed) 33 52 12 130 Axle  bearings, track wheels, automotive  crankshafts Ductile or nodular iron C 3.4, P 0.1, Mn 0.4,  Ni  1.0, Mg 0.06 Cast 53 70 18 170 Gears,  camshafts , crankshafts Ductile or nodular iron (ASTM A339) — Cast (quench tempered) 108 135 5 310 —

15. Properties of Cast-Iron : If placed in salt water, it becomes soft. It can be hardened by heating and sudden cooling, but it cannot be tempered. It cannot be magnetized. It does not rust easily. It is fusible. It is hard, but it is brittle also. It is weak in tension and strong in compression. The tensile and compressive strengths of cast-iron of average quality are respectively 150 N/mm 2  and 600 N/mm 2 .

16. It is not ductile and hence it cannot be adopted to absorb shocks and impacts. Its melting temperature is about 1250°C. It shrinks on cooling. This fact is to be considered while making patterns or moulds for foundry work. Its structure is granular and crystalline with whitish or greyish tinge. Its specific gravity is 7.5. It lacks plasticity and hence it is unsuitable for the forging work. The two pieces of cast-iron cannot be connected by the process of riveting or welding. They are to be connected by nuts and bolts which are fixed to the flanges. The holes for bolts, etc. are either drilled out or cast in the casting.

17. A few common mechanical properties for cast iron include: Hardness– material’s resistance to abrasion and indentation Toughness – material’s ability to absorb energy Ductility – material’s ability to deform without fracture Elasticity – material’s ability to return to its original dimensions after it has been deformed Malleability – material’s ability to deform under compression without rupturing Tensile strength – the greatest longitudinal stress a material can bear without tearing apart Fatigue strength – the highest stress that a material can withstand for a given number of cycles without breaking

18. Uses of Cast-Iron :  For making cisterns, water pipes, gas pipes and sewers, manhole covers and sanitary fittings. For making ornamental castings such as brackets, gates, lamp posts, spiral staircases, etc. For making parts of machinery which are not subject to heavy shocks.  For manufacturing compression members like columns in buildings, bases of columns, etc. For preparing agricultural implements. For preparing rail chairs, carriage wheels, etc.

19. Types of Castings :  Centrifugal casting Chilled casting Die casting Hollow casting Sand casting Vertical sand casting.

20. Characteristics of a Good Casting : A good casting should possess the following qualities or characteristics: Its edges and corners should be sharp, perfect and clean. Its fresh fracture should exhibit fine grained texture with bluish grey colour. It should be free from air bubbles, cracks, etc. It should be soft enough for drilling or chiselling. It should be uniform in shape and it should be consistent with the requirements of the design. Its outer surface should be smooth.

21. Production and processing:   To produce cast iron, the iron must be extracted from iron ore. The ore is smelted in a blast furnace where it separates into pig iron and slag. The furnace is heated to around 1800 degrees Celsius in an oxygen atmosphere and the slag formed rises to the top and can be removed. The molten pig iron below contains around 3 wt.%  – 5 wt.% carbon. This is then combined with iron, steel, coke and limestone. Once impurities are removed selectively from this iron, the carbon content is reduced. At this point, silicon may be added to convert the carbon content to graphite or cementite. The iron is then cast into various forms.

22. Cast iron and the future: From its early use over 3,000 years ago, iron has remained an integral part of human society. Iron production has come a long way since the centuries of working iron by blacksmiths to the invention of cast iron in the industrial age. Since then, wrought iron has become largely obsolete except for decorative uses. Contrastingly, cast iron is still progressing in terms of composition, microstructure, and mechanical properties—continuing to make its mark in the modern world.

23. Disadvantages of cast iron: However, it has low wear resistance, mobility and low shake-suction. Further, it has bad casting ability and machinability which is a distinct disadvantage. The  cost  of casting steel alloys is also more than cast iron . It is Prone to rusting. It has poor tensile strength. Its parts are section sensitive, this is due to slow cooling of thick sections. failure of Its parts is sudden and total, it does not exhibit yield point. It has poor impact resistance. Compared to steel it has poor machinability.

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