—An equilibrium, phase or constitutional diagram is a graphic representation of the effects of temperature and composition upon the phases present in an alloy.

—An equilibrium diagram is constructed by plotting temperature along they-axis and percentage composition of the alloy along the x-axis. This diagram shows ranges of temperatures and compo­sitions within which the various phase changes are stable and also the boundaries at which the phase changes occur.

Iron-carbon equilibrium diagram (refer Fig. 40.4) indicates the phase changes that occur during heating and cooling and the nature and amount of the structural components that exist at any temperature. Besides, it establishes a correlation between the

 

microstructure and properties of steel and cast irons and provides a basis for the understanding of the principles of heat-treatment.

An iron-carbon equilibrium diagram forms a basis for differen­tiating among iron (0.008% C or less), hypoeutectoid steels (0.008 to 0.8%C), hypereutectoid steels (0.8 to 2.0% C), hypoeutectic cast irons (2 to 4.3% C) and hypereutectic cast irons (above 4.3% carbon).

— The iron carbon equilibrium diagram has a peritectic (point J) an eutectic (point C) and an eutectoid (point S).

Peritectic reaction equation may be written as

cooling

(δ)delta + liquid        ↔              Austenite

heating

 

The horizontal line at 2720°F shows the peritectic reaction.

—            The eutectic reaction takes place at 2066°F and its equation may be written as

cooling

liquid            ↔            austenite +cementite(eutectic mixture{ledeburite})

heating

Eutectic point is at 4.3% carbon. Eutectic mixture is not usually seen in the microstructure, because austenite is not stable at room temperature and must undergo another reaction during cooling.

—                The eitlectaid. reaction is represented by the horizontal line of 1333°F and (point) S marks the eutectoid point. The eutectoid equation may be written as

cooling

solid         ↔                 Ferrite + cementite(eutectoid mixture{pearlite})

heating

 Transformation which takes place in the structures of steels

containing 0.4%, 0.83% and 1.2% carbon respectively (refer Fig. 40.4) when heated to a temperature high enough to make them austenitic and then allowed to cool slowly (under equilibrium conditions), have been explained below.

(0 Steel containing 0.4% carbon is a hypoeutectoid steel and is completely austenite above j43, i.e., upper critical temperature line. As it is cooled below A3 line the iron begins to change from F.C.C. to B.C.C. As a result, small crystals of body centered cubic (B.C.C.) iron begin to sepa­rate out from the austenite (F.C.C).

The B.C.C. crystals retain a small amount of carbon (less than 0.03%) and are referred as crystals of ferrite.

As the cooling proceeds, ferrite crystals grow in size at expense of austenite

By the time the steel has reached Ax line, i.e., 1333°F (called lower critical temperature) it is composed of approximately half ferrite and half austenite. At this stage the austenite contains 0.83% carbon and since austenite can hold no more than 0.83% carbon in solid solution at 1333°F (or 723°C) thus as the temperature drops further, carbon begins to preci­pitate as cementite.

This cementite and still separating ferrite form alternate layers until all the remaining austenite is consumed. The lamellae structure, i.e., eutec­toid of ferrite and cementite contains 0.83% carbon and is known as Pearlite (refer Fig. 40.5).

All hypoeutectoid steels when cooled from austenite state will trans­form into ferrite and pearlite in the same way as explained above

(ii) Consider the transformation of an eutectoid steel containing 0.83% carbon. It will remain austenite up to the point S. The transfor­mation will begin and end at the same temperature, i.e., 1333°F (or 723°C). Since eutectoid steel contains 0.83% carbon initially, it follows that the final transformed structure will be completely pearlite (see Fig. 40.5). For details refer section 40.7.

(iii) Consider the transformation of a hypereutectoid steel (say con­taining 1.2% carbon).

As the temperature drops and steel crossesv4cm (i.e., upper critical temperature) line at point d and moves towards e, the excess carbon above the amount required to saturate austenite (i.e. 0.83%) is precipitated as cementite primarily along the grain boundaries (Fig. 40.4).

Thus above 1333°F, i.e., lower critical temperature line, the structure consists of austenite and cementite.

As the temperature drops below 1333°F, the austenite has become less rich in carbon (because of cementite precipitation), it contains only 0.83% carbon and it transforms to pearlite as it does so in the cases of hypoeutectoid and eutectoid steels explained earlier.

The structure of a hypereutectoid steel at room temperature consists of cementite and pearlite (Fig. 40.4).

So far the discussions were only with regard to structures produced in steels by slow cooling from austenite under equilibrium conditions.

In normal foundry practice, the rate of cooling is slightly faster and as a result more cementite plates are nucleated and individual lamellae of pearlite become thinner and the structure is called fine pearlite.

If castings are cooled at still faster rate to prevent transformation of austenite above (approximately) 600°F, martensite forms on further conti­nuous cooling. Martensite is a hard, strong and brittle constituent. It is a super-saturated solution of carbon in ferrite and the presence of excess carbon distorts the normally cubic ferrite to a body-centered tetragonal structure which is produced by a shear mechanism and is strained.

Transformations which take place in the structure of a cast iron containing 3% carbon is shown in Fig. 40.4 and explained as under:

Case (a): Cast iron containing 3% carbon is when cooled under rapid rate as a thin section of a sand casting, from a temperature of about 2500?F, it begins to solidify with the formation of grains of austenite. Austenite continues to solidify until the cast iron reaches the temperature of 2066°F. At this stage the alloy consists of 50% austenite and 50% liquid of eutectic composition (austenite and cementite, i.e., ledeburite).

As the alloy cools below solidus, i.e., 2066°F, ledeburite (a form of eutectic consisting of spheres of austenite embedded in cementite) freezes and cementite precipitates from austenite because of the decreasing solu­bility of carbon in the austenite. This occurs between 2066 and 1333°F.

Cooling of the alloy below 1333°F* involves the transformation of remaining austenite of eutectoid composition (i.e., 0.83% C) to pearlite as explained earlier for steels.

Thus, the structure of alloy at room temperature consists of cementite, pearlite and transformed ledeburite.

Cast iron of any composition between 2.0 to 4.3% carbon will solidify in exactly the same way as has been described above.

Case (b): If the above very cast iron is cooled at a slow rate, as usual, austenite will first form from the melt (i.e., liquid) but eutectic freezing being slow, products of eutectic reaction will be austenite and graphite [Fig. 40.6 (a)]. This is between 2066 and 1333°F.

As cooling continues, austenite gets depleted in carbon content and graphite flakes grow. At 1333°F, remaining austenite transforms to pear­lite and the structure of the alloy at room temperature looks as shown in Fig. 40.6 (b). It is pearlitic gray cast iron.

Case (c): Phase changes in the same alloy when cooled at a very slow rate will be similar to case (b) above, except that, at the eutectoid, (i.e., 1333°F) cooling will be sufficiently slow to permit graphite to precipitate rather than pearlite. Although no new graphite flakes^form, the one present, grow in size. Instead of pearlite as in case (b), the matrix of the alloy solidified in this case is ferrite and graphite flakes are embedded in it