6.0 SOLIDIFICATION OF METALS
6.1 The solidification process of the metal: -
The metal begins to solidify when the temperature of liquid metal drop below a certain critical value. As a result the metal is transformed into crystalline state. Refer fig 6.1
The figure shows the solidification process based on freezing or cooling curve. The first center of crystallization or nuclei appears at point ‘m’ at temperature t. As the liquid metal continues to cool, new nuclei form within the solidified metal and the already formed crystal grow in size. Since the freezing process is accompanied by the evolution of latent heat of fusion solidification occurs at constant temperature at point ‘n’. This is shown by horizontal line in the diagram. A further drop in temperature is observed, when all the metals solidify.
Depending on the rate of cooling the amount of impurities in the melt, the crystal, which forms in the process of solidification, may have dendritic, laminar needle type structure. A single crystal can be obtained, if the temperature of the melt is reduced slowly and material is of high degree of purity. In major cases, tree like crystal are obtained, which are called as dendrites. The nucleus has a number of equivalent crystallographic direction along which crystal growth may occur. The nucleus develops in form a dendritic crystal chiefly along the direction of maximum linear rate of growth. This results in the formation of long branches and they branch out in various direction of the initial nucleus. Finally the spaces between the branches are filled with the solidifying metal. Further freezing of the metal and the development of dendrite crystal, all the liquid metal in the spaces are solidified.
For example in a casting, the entire melt is not cooled at a uniform rate. This is due to heat removed through the walls of the mould. This causes the melt in contact with the wall to freeze first. Therefore solidification moves towards the center from the nuclei formed near to the wall. The crystals grow in the direction of heat removal i.e. normal to mould walls. This results in the formation of long grains perpendicular to the walls of the mould when the freezing is complete and thus a columnar grain structure is formed.
6.2The structural change in the metal with the rate-cooling curve –
Cooling curve helps to determine the temperature at which the phase changes, i.e from liquid to solid or vice versa will occur in an alloy system . It consists of following the temperature as the function of time as different alloys in the system are cooled slowly.Refer Fig 6.2
Consider the cooling curve for pure iron. Iron has many allotropic forms such as α,β,γ,δ
in the solid state. Existence of one form to other depends upon the temperature to which
iron is heated. Referring the figure the melting point of pure iron is 1535°C. At 1535°C, first horizontal steps appear on the curve, It shows transformation from the liquid state takes place at the constant temperature. On freezing the melt, delta iron is formed and this has body centered lattice with constant a=2.93A°. The second effect occurs at 1400°C and corresponds to the transformation of delta iron into gamma iron which has face centered cubic lattice with constant a=3.63 A°. Gamma iron is paramagnetic. The third effect starts effect starts at 910°C forming alpha iron from gamma-iron with body centered cubic lattice having a constant a=2.9A° and it is non magnetic. The last effect occurs at 768°C and corresponding to alpha iron, which is highly magnetic and exist at room temperature can dissolve very little amount of carbon. The changes said above are reversible.
Iron Carbon Equilibrium Diagram –
An equilibrium diagram is a graphical representation of the effects of a temperature and composition upon the phases present in an alloy. It is constructed by plotting temperature along Y-axis and percentage composition of the alloy along X-axis.
Iron carbon equilibrium diagram indicates the phase changes that occur during the heating and cooling, the nature and amount of the structural components that exist at any temperature. By referring to this diagram, one may get proper quenching temperatures for any carbon steel. The critical points in figure, on the line AFEX are denoted by A1 or Ae, those on line BE by A3, and those of line EC by Acm.
Consider an example of a piece of 0.2%Carbon steel which has been heated to a temperature around 850°C. Above A3 this steel is a solid solution of gamma- iron called austenite. It has a face centered cubic lattice and is non –magnetic. Plain austenite contains up to 2%carbon at temperature of 1148°C. On cooling this steel, the iron atoms start to form a body centered cubic lattice below the point A3 i.e. BE line, which is known as ferrite or alpha iron and is a solid solution of carbon in alpha iron containing up to 0.008% carbon at room temperature.
As the steel is cooled to A1 more ferrite is formed. At the A1 line the austenite that remains is transformed to pearlite. Pearlite may be fine or coarse, lamellar or granular structure. The pearlite constituent in steel is machinable.
As carbon content of steel increases above 0.2%, temperature at which ferrite is first rejected from austenite drops till 0.8% carbon and after this percentage (i.e. point E) no free ferrite is rejected. This steel is called “eutectoid Steel” and is 100% pearlite. Eutectoid point is the lowest temperature at which changes occur in solid solution.
If carbon content of steel is greater than eutectoid (0.8% carbon), new line Acm is seen, denoting the temperature at which iron carbide is first rejected from austenite, known as cementite.
A point ‘D’ the eutectic mixture containing 4.3% carbon is known as ‘Ledeburite’. Steels containing less than 0.8% carbon is called Hypereutectoid and which contain more carbon are called Hypereutectoid steels.
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