In Fe-Fe3C diagram, three important invariant (at a constant temperature) reactions take place as described below:
1. Peritectic Reaction:
A peritectic reaction, in general, can be represented by an equation:
where, L represents a liquid of fixed composition, S1 and S2 are two different solids of fixed composition each. Fig. 1.23 illustrates the peritectic region of Fe-Fe3C diagram.
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The invariant peritectic reaction in Fe-Fe3C diagram is given by:
Actually, Fe-0.17% C steel is a peritectic steel because only this steel undergoes above reaction completely. When cooled from molten state, this steel starts solidifying at point x and the first solid going to nucleate is of δ-ferrite. As cooling continues more of δ-ferrite solidifies [Fig. 1.23 (b)] and at any temperature, say T1, Lever rule helps to calculate the fraction of δ- ferrite (= FG/EG) and the liquid (= EF/EG).
The composition (i.e., % C) of δ-ferrite changes along EO and of liquid along GB with further fall of temperature (as per Lever Rule), so that, when this peritectic steel just attains the peritectic temperature, 1495°C and before the peritectic reaction occurs, the liquid has a composition of (point B) 0.53% C and δ- ferrite (point 0) has 0.09% C, and the amount of these phases are, as per Lever Rule with OPB as tie line with P as fulcrum.
Now, this alloy at the peritectic temperature, 1495°C undergoes the peritectic reaction completely, i.e., 81.82% of 8-ferrite (c = 0.09%) reacts completely with 18.18% of liquid (c = 0.53%) to give 100% solid austenite (c = 0.17%), i.e., for complete peritectic reaction, the ratio of 8-ferrite (c = 0.09%) to liquid (c = 0.53%) should be (81.81/18.18) : : 4.5:1 at 1495 temperature just attained. Steels having carbon between 0.09% and 0.17% are called hypo-peritectic steels.
These steels have more amount of 8-ferrite (0.09% C) than required for complete peritectic reaction, and thus, extra unreacted 8-ferrite (0.09% C) alongwith peritectically formed austenite (c = 0.17%) is present, after the peritectic reaction has been completed. For example, a steel having 0.15% C cooled to the peritectic temperature (before the peritectic reaction occurs), 1495°C, has (use Lever Rule with tie line OZB with fulcrum Z).
For complete peritectic reaction to take place at 1495°C, the amount of δ-ferrite (c = 0.09) required for 13.64 % liquid present in this alloy is 13.64 x 4.5 = 61.36%. Thus, 0.15% C steel has 86.36 – 61.36 = 25% wt. of extra δ-ferrite, which remains present unreacted after the peritectic reaction, along with the product phase austenite of weight % (13.64 + 61.36) = 75. This result can be verified by applying the Lever Rule for this alloy at a temperature just slightly below peritectic temperature with OP as the tie line with fulcrum at Z.
Steels having compositions between 0.17% C to 0.53 % are called hyperperitectic steels. These steels have extra liquid at the peritectic temperature (just attained) than required for complete peritectic reaction, and thus, after this reaction, have product phase austenite and extra unreacted liquid (c = 0.53). All steels having carbon between 0.09% and 0.53 undergo peritectic reaction. Steels with C less than .09% and carbon more than 0.53% do not undergo peritectic reaction.
Say, a steel with 0.77% C (Fig. 1.23) starts solidifying at point J with the formation of solid austenite (of composition given by point L). As the temperature drops, more austenite forms. The carbon content of solid austenite changes along line LK, till the steel is 100% solid austenite (c = 0.77%) at the point K.
Peritectic reaction is of some importance during freezing of steels (carbon from 0.09 to 0.53%) particularly under fast cooling conditions, when micro-segregation may result, otherwise no commercial heat treatment is done in this region, and if for some bad practice, these temperatures are attained during heating of steels for forging or rolling etc., then severe overheating and burning results in steels turning them to scrap form.
2. Eutectic Reaction:
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An eutectic invariant reaction in general can be represented by an equation:
Where, L represents liquid of eutectic composition and, S1 and S2 are two different solids of fixed composition each. Fig. 1.24 illustrates the eutectic region of Fe-Fe3 C diagram.
The invariant eutectic reaction in Fe-Fe3 C diagram is given by:
The Fe-4.3% C alloy is called eutectic cast iron as it is the lowest melting alloy, which is single phase liquid (100%) of 4.3% carbon at the eutectic temperature, 1147°C just attained and undergoes eutectic reaction completely at this constant eutectic temperature to give a mixture of two different solids, namely austenite (c = 2.11 %) and cementite, solidifying simultaneously. This eutectic mixture is called Ledeburite. Lever Rule is used to calculate the amount of austenite and cementite in the eutectic alloy, just after the eutectic reaction, i.e. just below 1147°C (also in Ledeburite).
As Fe-C alloys having more than 2.11% carbon are classed as cast irons, the Fe-C alloys having carbon between 2.11 and 4.3% are called hypoeutectic cast irons, whereas those having carbon between 4.3% and 6.67% are called hypereutectic cast irons. Alloy of Fe with 4.3% carbon is called eutectic cast iron. A hypoeutectic cast iron, say, having carbon 3.3%, starts solidifying on cooling from molten state (Fig. 1.24), at point H and the first solid to nucleate is austenite of composition given by point I.
As cooling proceeds, more austenite, called proeutectic austenite, solidifies. Pro-eutectic austenite is the austenite formed from liquid alloy before the eutectic reaction takes place of the remaining liquid in the alloy. During this period, the composition of the solidified austenite changes along the line IQ and of the liquid along the line HC till at the eutectic temperature (1147°C) just attained, the solid austenite has a carbon 2.11% (point Q) and the liquid has a carbon 4.3%. Lever Rule is used to calculate the amount of these phases at this moment (QC is tie line).
This liquid in amount 54.34 wt% has a composition Fe-4.3% carbon and is at the eutectic temperature, 1147°C, and thus, eutectic reaction takes place i.e., 54.34 wt % liquid transforms to 54.34 wt% of the mixture consisting of austenite (c = 2.11%) and cementite, which is called, as said before, ledeburite of amount 54.34 wt % (weight of the liquid = weight of the ledeburite).
It can be verified by using Lever Rule at a temperature slightly below 1147°C. Remember the tie line shall extend now with an arm ending at the phase boundary corresponding to austenite, i.e., Q and the other end of the lever arm extends up to composition of 100% eutectic mixture i.e., point C with fulcrum at the composition of alloy, i.e. 3.3%C, thus,
Which match with results in equations 1.14 and 1.15.
A hypereutectic cast iron, say, having carbon 5.0%, when cooled from molten state starts solidifying at point M (Fig. 1.24) and the first solid to form is cementite of fixed carbon 6.67%. As cooling proceeds with more cementite forming till up to 1147°C temperature, the total cementite solidified up to 1147°C (eutectic temperature) is called primary cementite, or proeutectic cementite.
The amounts of phases present now are:
This liquid then undergoes eutectic reaction to give a mixture of austenite and cementite, called Ledeburite whose amount is 70.47%.
In Fig. 1.24, the horizontal line QCR signifies the eutectic reaction, that is, whenever an alloy on cooling from molten state crosses this line, the eutectic reaction must take place at this line (i.e. at 1147°C).
Any amount of liquid that is present when this line is reached has a composition of Fe-4.3% carbon, and must now solidify into the very fine intimate mixture of cementite and austenite (c = 2.11%) called ledeburite. Thus, Fe-C alloys having carbon between 2.11% to 6.67% undergo eutectic reaction at the eutectic temperature, 1147°C.
As austenite is not stable at room temperature in common alloys, Ledeburite is not usually seen in the micro-structure. First, because the solid solubility of carbon decreases in austenite from a maximum of 2.11% at 1147°C to 0.77% at 727°C, the extra carbon precipitates out in the form of secondary cementite till the carbon dissolved is 0.77% in austenite at 727°C, and this then by eutectoid reaction changes to pearlite. The ledeburite in which austenite has been transformed to pearlite is called transformed ledeburite.
3. Eutectoid Reaction:
The eutectoid invariant reaction is a solid state version of eutectic reaction and, in general, can be represented by an equation:
where, S1, S2 and S3 are three different solids each of fixed composition. The Fig. 1.25 illustrates the eutectoid region of Fe-Fe3C diagram.
The invariant eutectoid reaction in Fe-Fe3C diagram is given by equation:
i.e. during cooling, austenite of 0.77%C at constant eutectoid temperature, 727°C undergoes eutectoid transformation to form a mixture of ferrite (e = 0.02%) and cementite i.e. there are alternate lamellae of ferrite and cementite. This eutectoid mixture of ferrite and cementite is called pearlite, because of its pearly appearance under optical microscope. Fe-0.77% C alloy is called the eutectoid steel as this alloy solidifies completely as a single phase austenite (c = 77%) at point K (Fig. 1.23) and remains as it is, on cooling up to the eutectoid temperature, 727°C (Fig. 1.25) and then, the eutectoid reaction takes place to form 100% pearlite (Fig. 1.25).
The amount of ferrite (0.02% C) and cementite in this pearlite at slightly below the eutectoid temperature, 727°C is given:
The weight % of these two phases are thus in ratio 8:1. The densities of ferrite and cementite are 7.87 g/cm3 and 7.70 g/cm3 respectively, which are quite comparable. Thus, the volume %s of ferrite and cementite in pearlite are also approximately in ratio 8:1. Thus, ferrite lamilla is 8 times thicker than cementite lamilla. When etched with nital (dilute solution of nitric acid in alcohol), both phases etch and appear white under microscope but boundaries etch black.
As the two boundaries of cementite plate are close together, they may not be resolved as separate lines and thus cementite often appears as a single dark line. At higher magnifications as in a coarse pearlite, cementite boundaries may appear as separate lines.
Steels having carbon between 0.02% to 0.77% are called hypoeutectoid steels. For example, Fe-0.4% C steel is 100% solid austenite at say 1000°C and no change occurs till it is cooled to point N (Fig. 1.25), where ferrite nucleates at the grain boundaries of austenite. At a lower temperature, the schematic micro-structure is illustrated in Fig. 1.25, which consists of ferrite and austenite.
As this alloy is cooled to eutectoid temperature, the amounts of phases are:
This 50.67% of austenite (of 0.77%) at the eutectoid temperature must undergo eutectoid reaction to give a very fine mixture of ferrite and cementite called pearlite of amount 50.67%. Thus, 0.4% carbon steel has approximately 50% pearlite and 50% Ferrite. This ferrite is called pro-eutectoid ferrite, or free ferrite.
Steels having carbon between 0.77% to 2.11% are called hypereutectoid steels. A 1.2% carbon steel is all austenite on cooling to point w (Fig. 1.25), but as the temperature falls further and because the solid solubility of carbon in austenite decreases with the fall of temperature, carbon comes out as precipitates of secondary cementite, which is more commonly called proeutectoid cementite particularly in hypereutectoid steels. The amount of phases in a 1.2% carbon steel at the eutectoid temperature just attained (before the eutectoid reaction takes place) are- (Proeutectoid cementite forms at grain boundaries of austenite as network),
This 92.71 % of austenite (of 0.77 % C) now at the eutectoid temperature must undergo eutectoid reaction to give a fine mixture of ferrite and cementite as called pearlite, in amount 92.71%. Thus, the 1.2% carbon steel below the eutectoid temperature has 7.29 % proeutectoid cementite and 92.71 % pearlite.
The horizontal line TUT’ (Fig. 1.25) represents the eutectoid reaction, and whenever an alloy on cooling crosses this line, the eutectoid reaction’ must take place. Any amount of austenite that is present when this line is reached must transform to the equal amount of pearlite. Fe-C alloys having carbon between 0.02 % and 6.67 % undergo eutectoid reaction at 727°C, that is, practically all commercial iron-carbon alloys.