Iron-Carbon Diagram and Its Reactions

Iron-Carbon Diagram and Its Reactions | Metallurgy 

Three major invariant reactions occur in Fe-Fe3C diagram (at constant temperature), as described below:

Peritectic Reaction:

In general, a peritectical reaction can be shown with an equation:
S1 and S2 are two different solids of fixed composition, where L represents the fluid of the fixed composition. Picture. 1.23 shows the Fe-Fe3C diagram's peritectical region.
The Fe-Fe3C diagram provides an invariant peritectic reaction:
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 stones have 8 ferrite (0,09% C) more than needed in the peritex reaction, and so, after the peritex reaction is completed, there is an additional, unreacted 8 ferrite (0,09% C) with peritestatically formed austenite (c= 0,18%). For instance, 0.15 percent C cooled stainless steel has a temperature of 1495 ° C (use Lever Rule with a tight line OZB and anchor 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.
Peritectical reactions in steels freeze (carbons from 0,09 to 0,53 percent) are important, especially in fast cooling conditions, when micro-secregation can occur otherwise commercial thermal treatment is done in this area, and when these temperatures are achieved in the case of bad practice, they are then overheated and burned during heating of forging or rolling steels, etc.

Eutectic Reaction:

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 known as euthetic cast iron, as it is the lowest melting alloy that, at eutetic temperatures (100%), is a single-phase liquid (100%) of 4,3% carbon and at this constant eutectical temperature is just at 1147 ° C eutectic reaction to give an austenite (c=2,10%) and a cement solidifying silicon mixture. This eutectic combination is called Ledeburite. Ledeburite. The austenite and cement in the Eutectic alloy are measured using Lever Rule immediately after the eutectic reaction, i.e. just below 1147 ° C (in Ledeburite as well).
Because Fe-C alloy with over 2.11% carbon has cast iron, fe-c alloys with carbon of 2.11 to 4.3% are called hypoeutetic cast iron, while Fe-C iron is classified as hypoeutectic cast iron between 4.3 and 6.67%. Fe alloy is called eutectical cast iron with 4.3% carbon. Hypoeutectic cast iron, say, carbon 3.3 percent, starts to solidify with the molten state cooling (Fig. 1.24) and is a compositional austenite at point H and the first solid to nucleate.

More austenite, known as proeutectic austenite, solidifies during cooling. Pro-eutectic austenite is the austenite formed from liquid alloy before the remaining liquid in the alloy takes place with the eutectic reaction. The composition of the solidified austenite in line IQ and liquid in line HC varies during this time to the just eutectic atmosphere (1147 ° C), the solid austenite is 2.11% carbon (point Q) and the liquid has a carbon of 4.3%. The height of these stage (QC is the tie line) is now calculated using the Lever Rule.
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.

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 8:1 ratio is thus the weight percentage of these two phases. The ferrite and cement densities are equivalent respectively at 7,87 g / cm3 and 7,70 g / cm3. There are therefore also approximately 8:1 volumes percent s of ferrite and cementitis in Perlite. So the lamilla of ferrite is 8 times the lamilla of concrete. Once grafted with nital, all phases of the grain grain etched and white appear within microscopic limits, with alcohol's dilute solution of nitric acid.

Since the two limits of the cement plate are close to each other, they can not be solved as separate lines, so that cement often appears as a single dark line. Cementite boundaries can appear as separate lines in higher magnifications than in coarsely perlite.

Carbon steels are called hypoeutectoid stones between 0.02 percent and 0.77 percent. For instance, Fe-0.4% C steel has solid austenite of 100% at 1000oC and there is no change until it is cooled at point N (Fig. 1.25), where ferrite nucleates are at the austenite grain boundary. The schematic micro structure is shown in the Fig at a lower temperature. 1.25, made up of ferrite and austenite.

As this alloy is cooled to eutectoid temperature, the amounts of phases are:

This 50.67% austenite (0.77%) at the temperature of the eutectoid must undergo the eutectoid responsiveness to produce a very fine mixture of ferrite and cement called 50.67% pearlite. There is, therefore, around 50% perlite and 50% ferrite in carbon steel. This ferrite is known as ferrite pro-eutectoid.

Carbon steels are called hypereutectoid steels between 0.77 and 2.11 percent. A 1.2 percent of carbon steel is all austenite at point w cooling (Fig. 1.25), but as the temperature drops further and the solid carbon solubilities in austenite decline with the temperature drop, the carbon comes out as secondary cement plumage, more commonly referred to as proeutectoid cement in hyperechtectoid steels. (protectuary cement formations on grain limits of austenite as a network) The number of phases in the 1,2% CO2 carbon steel at a eutectoid temperature that is just met (before the eutectoid response occurs),

This 92.71% (from 0.77% C) austenite now undergoes an eutectoid reaction in order to produce a fine ferrite and cement mixture, as pearlite, at 92.71%. There is 7.29 percent of proeutectoid cement and 92.71 percent of Perlite in a 1.2 percent carbon steel below the eutectoid level.

The TUT horizontal line (Fig. 1.25) is the eutectoid reaction and the eutectoid reaction needs to take place when an alloy on this cooling line crosses the line. When this line is crossed, every volume of austenite must be converted into equivalent perlite. Fe-C carbon alloys between 0.02 and 6.67% have an eutectoid reaction at 727 ° C, which means virtually all iron-carbon industrial alloys.

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