Question

4. Is it possible to produce cast irons that consist of martensitic matrix in which graphite is embedded in either nodule or flake form. Briefly describe the treatment necessary to produce each of these two microstructures.

Answer 1

Cast irons typically contain 2-4 wt% of carbon with a high silicon concentrations and a greater concentration of impurities than steels. The carbon equivalent (CE) of a cast iron helps to distinguish the grey irons which cool into a microstructure containing graphite and and the white irons where the carbon is present mainly as cementite. The carbon equivalent is defined as:

\begin{displaymath}CE \hbox{(wt\%)} =C + {{Si +P}\over{3}} \numeqn \end{displaymath}

note:- in the attached file

A high cooling rate and a low carbon equivalent favours the formation of white cast iron whereas a low cooling rate or a high carbon equivalent promotes grey cast iron.

During solidification, the major proportion of the carbon precipitates in the form of graphite or cementite. When solidification is just complete, the precipitated phase is embedded in a matrix of austenite which has an equilibrium carbon concentration of about 2 wt%. On further cooling, the carbon concentration of the austenite decreases as more cementite or graphite precipitates from solid solution. For conventional cast irons, the austenite then decomposes into pearlite at the eutectoid temperature. However, in grey cast irons, if the cooling rate through the eutectoid temperature is sufficiently slow, then a completely ferritic matrix is obtained with the excess carbon being deposited on the already existing graphite.

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Answer 2

Is it possible to produce metals that comprise the martensitic matrix when graphite is embedded in any nodule or flake form and the treatment required to produce each of these two sub-structures is as follows is:

• Spheroidal graphite cast iron usually has a pear lite matrix.
• However, the removal of the anneal causes the carbon in the pear lite to roll over to the existing graphite or to form other small graphite particles, leaving a ferric matrix.
• This gives the metal even greater ductility.
• As the name implies flake graphite has a clear or platy morphology.
• All graphite has a morphology dispersed to some degree, but in most cases flake-graphite has this structure regardless of particle size.
• The platy morphology of flake graphite is reflected in electron scanning and bright micro graphs.
• Micro structures are the result of a variety of heat treatments depending on the degree of cooling from the Austen range; are predicted using a TTT diagram with cooling curves at the top of the selected object.