Malleable cast iron
Malleable cast iron: The ordinary cast iron is very hard and brittle. Malleable cast iron is unsuitable for articles which are thin, light and subjected to shock. It can be flattened under pressure by forging and rolling. It is an alloy in which all combined carbon changed to free form by suitable heat treatment. Graphite originally present in iron in the form of flakes which is the source of weakness and brittleness. Carbon in this cast iron is dispersed as tiny specks instead of being flaky or in combined form. The tiny specks have not such weakening effect and casting would not break when dropped. The tensile strength of this cast iron is usually higher than that of grey cast iron. It has excellent machining quality and is used for making machine parts for which the steel forging and in which the metal should have a fair degree of machining accuracy e.g., hubs of wagon, heels small fittings for railway rolling brake supports, parts of agricultural machinery, pipe fittings, hinges, locks etc.
It can be obtained by annealing the castings. The cast iron castings are packed in an oxidizing material such as iron ore or in an inert material such as ground fire clay depends upon the process used either white heart or black heart. The packed casting is put into an oven and is heated around 900°C temperature and is kept at that temperature for about two days and it is then allowed to cool slowly in the furnace itself. Iron ore acting as an oxidizing agent reacts with C and CO2 escape. Thus annealed cast product is free from carbon. If the castings are packed in an inert material then slow cooling will separate out the combined carbon to temper carbon. To produce malleable casting, first casting is produced which has all combined carbon. The produced castings are then heat-treated in a special manner according to white heart method or black heart method.White heart malleable iron casting The castings taken out of the mould are put into a drum having sand and powdered slag. The drum is then closed and kept in the air furnace and it is raised to highly temperature slowly. The temperature is raised to 920°C in two days time, kept at this temperature for nearly up to 50 to 80 hours then the drum is allowed to cool in the furnace (generally air furnaces) at the rate 5 to 10°C per hour till it reaches to room temperature. The whole cycle takes about one weak. During this treatment combined carbon separates out and all the carbon does not change into graphite state but change in other form of free carbon called tempered carbon.
Fe3C ——→ 3Fe C
This makes the casting less brittle and malleable. The fracture portion of such a casting is dark grey or black in appearance. These castings are specially used in automobile industries.
Black heart malleable iron casting: The castings packed in a drum of oxidizing media which is generally powdered iron ore or powered scale (film of Fe3O4 on surface). This close drum is kept in the furnace and heated to 900°C. It is then maintained at this temperature to nearly 40 to 70 hours and allowed to cool slowly in a furnace itself. The castings become malleable like white heart cast iron. The percentage of carbon and silicon should be so selected that it can promote the development of free carbon when these castings are annealed.
- Malleable cast iron is like steel than cast iron.
- It is costly than grey cast iron and cheaper than softer steel.
Applications:Malleable cast iron are generally used to form automobile parts, agriculture implementation, hinges, door keys, spanners mountings of all sorts, seat wheels, cranks, levers thin, waned components of sewing machines and textiles machine parts.
Meehanite cast iron: Meehanite cast iron is an inoculated iron of a specially made white cast iron. The composition of this cast iron is graphitized in the ladle with calcium silicide. There are various types of meehanite cast iron namely heat resisting, wear resisting and corrosion resisting
kind. These materials have high strength, toughness, ductility and good machinability. It is highly useful for making castings requiring high temperature applications. 188.8.131.52 Alloy cast iron
The cast irons as discussed above contain small percentages of other constituents like silicon, manganese, sulphur and phosphorus. These cast irons may be called as plain cast irons. The alloy cast iron is produced by adding alloying elements like nickel, chromium, molybdenum, copper and manganese in sufficient quantities in the molten metal collected in ladles from cupola furnace. These alloying elements give more strength and result in improvement of properties. The alloy cast iron has special properties like increased strength, high wear resistance, corrosion resistance or heat resistance. The alloy cast irons are extensively used for automobile parts like cylinders, pistons, piston rings, crank cases, brake drums, parts of .crushing and grinding machinery etc. 184.108.40.206 Effect of impurities on cast iron The cast iron contains small percentages of carbon, silicon, sulphur, manganese and phosphorus. The affect of these impurities on the cast iron are as follows:
(1) Carbon. Carbon is one of the important elements in cast iron. It reduces melting point of iron. Pure iron has a melting point of about 1500°C but iron with 3.50% C has melting point of about 1350°C. When carbon is in free form i.e. as graphite form, the resulting cast iron is known grey cast iron. On the other hand, when the iron and carbon are chemically combined form of cementite, the cast iron will be hard and known as white cast iron.
(2) Silicon. Presence of silicon in cast iron promotes the decomposition of cementite into graphite. It also helps to reduce the shrinkage in cast iron when carbon is changed to graphite forms.
(3) Sulphur. It makes the cast iron hard and brittle. Since too much sulphur gives unsound casting, therefore, it should be kept below 0.1% for most casting purposes. It is often responsible for creating troubles to foundry men. It will make cast iron hard thereby counteracting the softening influences of silicon. It decreases strength and increases brittleness. It also promotes oxidation of cast iron. Hence, it is kept as low as possible in cast iron.
(4) Manganese. It makes cast iron white and hard. It is often kept below 0.75%. It helps to exert a controlling influence over the harmful effect of sulphur. It reduces the harmful effects of the sulphur by forming the manganese sulphide which is not soluble in cast iron.
(5) Phosphorus. It increases fusibility and fluidity in cast iron but induces brittleness. It is rarely allowed to exceed 1 %. Phosphorus in irons is useful for casting of intricate shapes and for producing very cheap and light engineering castings. Phosphorus has no effect on the carbon as well as on shrinkage in the cast iron.