The Ordinary cast Iron is hard and brittle. It is therefore, unsuitable for articles which are thin, light and subjected to shock and vibration or for small castings used in various machine opponents.
The application of the term "malleable" to castings is rather a misnomer because they are not very malleable when compared with the standards of malleability. When compared with grey iron castings, however , which are fairly brittle, malleable castings do possess a degree of toughness, and this is probably why they have been so named.
Malleable Castings are first made from an iron having all of its carbon in the combined form. i.e., from white cast iron. Two methods are then used for malleablizing the castings : (1) White heart, and (2) black heart. The names refer to the color of the fracture given by castings produced by each method. The approximate chemical compostions are
White Iron
Carbon - 3.2% , Silicon - 0.4 -1.1% , Manganese - 0.1to 0.4 % , Sulphur - 0.10 to 0.3 % , Phosphorous - 0.1%.
Black iron
Carbon - 2.2-2.8% , Silicon -0.7-1.1 % , Manganese - 0.3 - 0.4 % , Sulphur - 0.03 to 0.1 % , Phosphorous - 0.1 %.
In white heart Process the castings , composed of cast Iron with most of its carbon in the combined state , are packed in Iron or steel boxes and surrounded with a mixture of used and new hematite ore. The boxes are slowly heated to a temperature of 950* to 1000*C and maintained at that temperature for about 5 days. After heating for several days in such a way, part of the carbon is oxidized out of the castings , and the remainder is dispersed in finely divided forms throughout the structure. This process of decarburization is known as malleablizing. The heating period is followed by a very slow cooling which occupies several more days and the result is a casting which is tough and which will stand heat treatment without fracture. It is primarily used for thin - walled components. The weight of which ranges from a few grams to about 20kg.
In the black heart process, the white castings are heated to a temperature of about 1000*C for 2 to 3 days in a Neutral atmosphere which does not give off oxygen, e.g., Quartz sand or protective gas. The temperature is so controlled that the whole iron carbide in white cast Iron is decomposed into what is known as temper carbon. The microstructure of the graphite formed will appear in a ball-like form. The separation of carbon in the form of temper carbon from the combined state makes the final castings less brittle or, in other words, tough . It is used for thick - walled workpieces weighing up to 100 kg, Especially those which have to be machined by means of cutting tools.
It is therefore, seen that malleable cast iron, both white heart and black heart, compared with ordinary cast iron, is less brittle and therefore, stronger and tougher. It has the advantage of being more fluid. The maximum thickness of castings suitable for making this malleablizing process is about 20mm for the white heart and 25 to 35 mm for black heart process. The tensile strength of malleable castings is about 18 kg-f per mm3 (180 Newton per mm2).
On account of the special manner in which malleable cast Iron is produced, It is employed mainly for thin-walled and small workpiece, Such parts are , for example , hinges , door-keys , spanners , mounting of all sorts, gear wheels, cranks, levers, thin-walled components of sewing machines, textile machines, and others.
Malleable Cast iron Which has been a very suitable material for small and medium size mass produced castings of high strength for a long time, has recently been superseded in several fields by spheroidal graphite iron. This is commonly called ductile Iron.
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