In this article we will discuss about:- 1. Meaning of Malleable Castings 2. Classification of Malleable Castings 3. Mechanical Properties.
Meaning of Malleable Castings:
Malleable castings are made to the desired shape by pouring molten metal in sand mould which on solidification gives white cast iron castings. If these castings are used as they are, they will have poor strength, poor ductility and poor machinability and hence will be less important from commercial point of view.
In order to utilise the white cast iron castings, it is given an annealing treatment for a long time. It has been found that if white cast iron is annealed continuously at a temperature of 950 to 1000°C, carbon precipitates in the massive form.
Graphite originally present in the form of flakes is a source of weakness and causes brittleness but when converted in a very finely divided form, it is not so harmful and makes the iron malleable.
Several types of furnaces are used such as rotary furnace, cupola, air furnace and electric arc furnace. But the most commonly used furnace is an air furnace as it provides accurate control of carbon and other elements in the metal. The charge consists of 40% pig iron, 50% scarp and the rest malleable iron and steel scrap. The temperature of molten metal before it is poured into the mould is 1600°C.
The moulds used for obtaining cast iron castings are made of green sand mould of high refractory material. Since the white cast iron cools rapidly and casting needs greater allowance for contraction, the mould should be such as to provide free contraction of the metal.
The castings are cleared- off and also gates and risers are cut out. They are then packed in cast iron boxes containing sand and powdered slag. Packing helps in reducing the tendency of warping of the casting during annealing treatment.
Classification of Malleable Castings:
Depending upon the purpose for which the castings are to be used, they are classified in two ways:
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(а) Black heart malleable castings.
(b) White heart malleable castings.
(a) Black Heart Malleable Castings:
The castings are taken out of the mould and put into a drum having a neutral media, consisting of sand and powdered slag. The drum is closed and is kept in the air furnace and heated slowly. The temperature is raised to 922°C in two days’ time, kept at this temperature of nearly 50 to 80 hours and then the drum is allowed to cool in the furnace till it reaches the room temperature.
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The whole of cycle takes about one week. During this treatment, combined carbon separates out and all the carbon does not change into graphite state but in another form of free carbon called tempered carbon.
Fe3C —— > 3 Fe + C
This makes the casting less brittle or in other words malleable. The fracture portion of such a casting is dark grey or black in appearance. Black-heart malleable castings are specially used in the automobile industry.
(b) White Heart Malleable Castings:
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The castings are packed in a drum of oxidising media, which is generally powdered iron ore or powdered scale (film of Fe3O4 on surface). The closed drum is kept in the furnace and heated to 900°C and kept at this temperature for nearly 60 to 80 hours and allowed to cool slowly in the furnace itself. The castings so obtained are supposed to be malleable like white heart cast iron.
The typical composition used for malleable casting is given below:
Carbon 2.0 to 2.5%, Silica 1% (max.), Manganese 0.5 to 0.8%.
Phosphorus and sulphur (as low as possible).
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The percentage of carbon and silicon should be so selected that it promotes the development of free carbon when the castings are annealed. If thin section castings are to be obtained, then amount of silicon should be high and for large castings it should be low.
Mechanical Properties of Malleable Castings:
(a) Tensile strength of the order of 2500 to 3000 kg/ cm2.
(b) Elongation value = 12%
(c) Hardness = 170 to 230 BHN
(d) Good machinability.
Such castings are used for many industrial purposes such as agricultural implements, automobile parts, boilers, tanks, engines, conveyors and elevators, electrical and industrial power equipment, household appliances, pipe-fittings, plumbing supplies and specially for those parts which are subjected to impact and vibration.