In this article we will discuss about:- 1. Manufacture of Cast-Iron 2. Composition of Cast-Iron 3. Types 4. Properties 5. Uses 6. Castings 7. Types of Castings 8. Characteristics of a Good Casting 9. Defects in Casting.
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. Fig. 10-2 shows a typical cupola furnace.
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.
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.
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.
The sulphur makes cast-iron brittle and hard. It also does not allow smooth cooling in sand moulds. Its presence causes rapid solidification of cast-iron and it ultimately results in blow-holes and sand-holes. The sulphur content should be kept below 0.10 per cent.
Types of Cast-Iron:
Following are the varieties of cast-iron:
(1) Grey cast-iron
(2) White cast-iron
(3) Mottled cast-iron
(4) Chilled cast-iron
(5) Malleable cast-iron
(6) Spheroidal graphite iron or ductile iron
(7) Toughened cast-iron.
(1) 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.
(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.
(3) Mottled Cast-Iron:
It is an intermediate variety between grey cast-iron and white cast-iron. The fracture of this variety is mottled. This variety is used for small castings.
(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. The interior portion of the body of casting is soft and it is made up of grey iron. It is used to provide wearing surfaces to the castings.
(5) Malleable Cast-Iron:
The composition of this variety of cast-iron is so adjusted that it becomes malleable. It is done by extracting a portion of carbon from cast- iron, which makes it less brittle than cast-iron. The process of manufacturing is known as malleable cast-iron process. In this process, the individual castings are first cast and cooled as ordinary white cast-iron.
Then it is heated to about 1050°C and soaked for several hours or days followed by slow cooling. During whole process the combined carbon is reduced and graphite is precipitated as temper carbon. It is used for railway equipment, automobiles, pipe fittings, agricultural implements, door fastenings, hinges, etc.
Malleable cast-iron can be machined easily. It gets deformed without rupture. Also, it has better resistance to corrosion.
(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.
(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.
Following are the properties of cast-iron:
(i) If placed in salt water, it becomes soft.
(ii) It can be hardened by heating and sudden cooling, but it cannot be tempered.
(iii) It cannot be magnetised.
(iv) It does not rust easily.
(v) It is fusible.
(vi) It is hard, but it is brittle also.
(vii) It is not ductile and hence it cannot be adopted to absorb shocks and impacts.
(viii) Its melting temperature is about 1250°C.
(ix) It shrinks on cooling. This fact is to be considered while making patterns or moulds for foundry work.
(x) Its structure is granular and crystalline with whitish or greyish tinge.
(xi) Its specific gravity is 7.5.
(xii) It lacks plasticity and hence it is unsuitable for the forging work.
(xiii) It is weak in tension and strong in compression. The tensile and compressive strengths of cast-iron of average quality are respectively 150 N/mm2 and 600 N/mm2.
(xiv) 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.
The use of cast-iron is not recommended in horizontal direction either for heavy or variable loads or at places where there are chances for the slightest shock to exist. The cast-iron cracks and snaps suddenly when subjected to the shocks, overloading or fire without giving any warning of approaching failure under such stresses.
The cast-iron to be used on the works should be tough, close-grained grey metal, free from air holes, sand holes, flaws and with an even surface. It should be sufficiently soft to admit of being easily cut either by a chisel or a drill.
Following are the important uses of cast-iron:
(i) For making cisterns, water pipes, gas pipes and sewers, manhole covers and sanitary fittings.
(ii) For making ornamental castings such as brackets, gates, lamp posts, spiral staircases, etc.
(iii) For making parts of machinery which are not subject to heavy shocks.
(iv) For manufacturing compression members like columns in buildings, bases of columns, etc.
(v) For preparing agricultural implements.
(vi) For preparing rail chairs, carriage wheels, etc.
For making an ordinary sand casting, the following procedure is generally adopted:
(i) A pattern resembling the product to be casted is prepared. It is generally made from hard wood. Its inside surface is painted or waxed to give smooth surface to the finished product. If pattern is to be used several times, it may even be made of aluminium, brass or cast-iron. The dimensions of pattern are kept slightly more, about 10 per cent in each direction, to allow for shrinkage.
(ii) The pattern is generally divided into two portions:
(a) Upper portion and
(b) Lower portion.
Each portion is placed in a rectangular frame made of wood or metal. This frame is known as the flask.
(iii) The space between the flask and pattern is filled with green sand or loam and it is packed with wet moulding sand.
(iv) The vertical holes are made in the sand to serve as vent pipes.
(v) When the sand has sufficiently dried, the pattern is carefully removed. The mould is thus prepared. It is cleaned and repaired to receive molten metal.
(vi) The two portions of mould are placed one above the other and the melted metal is poured in the mould.
(vii) After the metal has cooled down, the casting is taken out and the irregularities formed on the casting are carefully rubbed out or chipped off.
Following are the various types of castings:
(1) Centrifugal casting
(2) Chilled casting
(3) Die casting
(4) Hollow casting
(5) Sand casting
(6) Vertical sand casting.
(1) Centrifugal Casting:
In this type of casting, the molten metal is poured into moulds which are kept rotating. The quantity of metal should be carefully determined and accurately controlled. The moulds are cylindrical and made of metal. The molten metal is spread uniformly by the centrifugal force and it is held till it becomes solid. This method is generally used to prepare pipes and it is found that these castings are stronger and compact than ordinary castings.
(2) Chilled Casting:
In this type of casting, the outer surface is made hard by sudden cooling or chilling and the inner surface remains comparatively soft. The mould is either made of metal or is lined with metal. The hot molten metal is suddenly cooled or chilled as it comes into contact with metallic surface of the mould.
The outer surface thus becomes suddenly hard and inner surface becomes soft and tough due to pressure of contraction on the molten metal. This type of casting is adopted to produce wearing surface as in case of tyres and axle holes of railway carriage wheels, etc.
(3) Die Casting:
In this type of casting, the molten metal is poured into metal moulds under pressure. These castings are cheap, smooth and compact. They require no finishing treatment except the removal of surplus metal.
(4) Hollow Casting:
In this type of casting, the mould is made as usual and a solid core is suspended in the middle of mould to form cavity. The thickness of casting is represented by the space between the core and mould. The metal is poured into this annular space and when it has cooled down, the mould is removed and the core is withdrawn. This casting is used for preparing hollow columns, pipes, piles, etc.
(5) Sand Casting:
This is the ordinary type of casting and its procedure is described above.
(6) Vertical Sand Casting:
In this type of casting, the sand mould and solid core are held in a vertical position. This method of casting is used to prepare the cast-iron pipes for carrying water under pressure.
A good casting should possess the following qualities or characteristics:
(i) Its edges and corners should be sharp, perfect and clean.
(ii) Its fresh fracture should exhibit fine grained texture with bluish grey colour.
(iii) It should be free from air bubbles, cracks, etc.
(iv) It should be soft enough for drilling or chiselling.
(v) It should be uniform in shape and it should be consistent with the requirements of the design.
(vi) Its outer surface should be smooth.
Defects in Casting:
The sand castings are commonly adopted and if proper precautions are not taken during the process of sand casting, there are chances for the developments of the following defects:
(1) Cold Short:
This defect is formed at the junction where two streams of molten metal meet. If these streams do not unite properly, the cold short is formed at the junction point.
In this type of defect, the metal becomes solid before the mould is completely filled up. It may develop either due to insufficient fluid state of metal or due to insufficient passage for the entry of metal into the mould.
If vent holes are insufficient, the air and gases become entrapped and it ultimately results in porous casting with holes.
The fusing of surface sand causes this defect in the casting.
(5) Lifts and Shifts:
These are the external defects of casting and they are usually due to misplacement of core.
In this type of defect, the scales are seen on the casting. It occurs when sand is very heavy and sticks to the casting.
When moulds are improperly rammed, the swelling of casting takes place.