07-05-2012, 02:44 PM
inoculation
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GRAY CAST IRON:
The iron carbon diagram reveals that irons containing more than 2% carbon is classified as cast irons and if most of the carbon is in the form of free graphite and some in combined form as in cementite the irons will be known as gray. Gray cast irons are the most widely used cast irons because of the ease with which they can be cast into complicated shapes and the relatively low melting temperature and high fluidity (about 1150-12000C). The presence of free graphite has inherently the problems of lower tensile strength, low notch sensitivity but of high compressive and damping properties. Graphite further reduces elastic limits, plastic limits and modulus of elasticity. With increased insight the microstructure property relationship it is possible now to produce gray cast irons with improved properties of tensile strength and impact strength by controlling the size, shape and distribution of graphite and also changing the matrix structure by alloy additions.
HOW CHEMISTRY AFFECTS THE PROPERTIES OF GRAY CAST IRONS:
Primary structure of gray cast irons on the normal solidifications shows austenite and free flaky graphite. Graphite also comes out during eutectic transformation of austenite on cooling producing ferrite as well, which is base centered cubic in structure. Slower cooling in an insulated mould gives more fraction of free graphite. Thus finally the structure would show ferrite and graphite. The volume fraction of ferrite, graphite and other phases can be found out by applying the well known lever rule to the phase diagram. On the other hand very fast cooling during solidification can suppress the separation of graphite and facilitate producing fully cementite phase, which is very hard and extremely brittle and normally undesirable in engineering applications. The role of various additions to gray cast irons is given in Table 1.
Effects on hardness and strength Mn.
High sulphur promotes dross and tendency for manganese sulphide blowholes.
Levels less than 0.05% reduce response to inoculation treatment.
Phosphorus 0.02-1.2 Increases hardness and brittleness.
Increases tendency for internal unsoundness defects.
At low levels (0.04%), increases metal penetration and finning.
Table 1 The Principal effects of the five main elements in gray cast irons8.
INOCULATION2:
In order to achieve the desired mechanical properties in iron castings, the liquid iron must have the correct composition and it must also contain suitable nuclei to induce the correct graphite structure to form on solidification. The liquid iron must have a suitable graphitisation potential; this is determined mainly by its carbon equivalent value, and in particular by the silicon content. It is normal practice to adjust the graphitisation potential by controlling the silicon content. However, the effect of other elements must also be considered.
Even if iron of the correct chemical analysis is made in the melting furnace, castings having the desired graphite structure will not be produced without the addition of inoculants, fig.1. Inoculants are alloys added in small amounts to induce eutectic graphite nucleation. Without the presence of suitable nuclei, liquid iron will 'undercool' below the eutectic temperature. Uninoculated gray iron castings will contain: