one, but we will only consider the steel part of the diagram, Fe. 3. C (iron carbide or cementite). • This intermetallic compound is metastable, it remains as a. slow. In most of the common grades iron – carbon alloy excess carbon is present as cementite. Let us first look at the Fe-Fe3C meta-stable phase diagram and in. Following phases exist on Fe-Fe3C diagram: liquid solution of iron and carbon ( L). - ferrite (α) – an interstitial solid solution of carbon in Feα (bcc). At room.
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Phase Diagram Evaluations: Section II. The C-Fe (Carbon-Iron) System by Ho Okamoto. ASM International. Equilibrium. Diagram. The number of experimental . IRON IRON-CARBON DIAGRAM. Ferrite. Austenite. Steel. Cast iron. Pearlite Various phases that appear on the Iron-Carbon equilibrium phase diagram are. continuous cooling transformation diagrams for plain carbon and alloy steels. Iron- Carbon diagram shows. 1- the type of alloys formed under very slow cooling .
The latter decomposes by eutectic mechanism to a fine mixture of austenite and cementite, called ledeburite.
The eutectoid concentration of carbon is 0. Critical temperatures Upper critical temperature point A3 is the temperature, below which ferrite starts to form as a result of ejection from austenite in the hypoeutectoid alloys. Upper critical temperature point ACM is the temperature, below which cementite starts to form as a result of ejection from austenite in the hypereutectoid alloys.
Lower critical temperature point A1 is the temperature of the austenite-to-pearlite eutectoid transformation. Below this temperature austenite does not exist. Hypoeutectoid steels carbon content from 0 to 0. It may be noted that it is only austenite which is changing at A1 line.
Therefore, when the reaction is complete the microstructure will show approximately 25 percent Pearlite and 75 percent ferrite. The changes just described would be the same for any hypoeutectoid steel.
The only difference would be in the relative amount of ferrite and pearlite.
Above figure shows microstructure of a hypoeutectoid steel. Formation and Growth of Pearlite At eutectoid point, austenite is an interstitial solid solution having 0.
Ferrite, however, is b. So, the change in crystal cannot occur until the carbon atoms come out of solution.
Therefore, the first step is the precipitation of the carbon atoms to form plates of cementite iron carbide. In the area immediately adjacent to the cementite plate the iron is depleted of carbon, and the atoms may now rearrange themselves to form b. Thin layers of ferrite are formed on each side of the cementite plate.
The process continues by the formation of alternate layers of cementite and ferrite to give the fine fingerprint mixture known as pearlite. As shown in the above figure, the reaction usually starts at the austenite grain boundary, with the pearlite growing along the boundary and into the grain by nucleation of new cementite and ferrite and growth of existing ferrite and cementite by diffusion of carbon from austenite to cementite.
Hypereutectoid steel Alloy 2 shown in above figure is a hypereutectoid steel containing 1.
In the austenite range, this alloy consists of a uniform f. Upon slow cooling, nothing happens until the line CJ is crossed at point x3. This line is called upper-critical-temperature line on the hypereutectoid side and is labeled Acm. The Acm line shows the maximum amount of carbon that can be dissolved in austenite as a function of temperature. Above the Acm line, austenite is an unsaturated solid solution.
At the Acm line, point x3, the austenite is saturated in carbon. As the temperature is decreased, the carbon content of the austenite, that is, the maximum amount of carbon that can be dissolved in austenite, moves down along the Acm line towards point J.
Therefore, as the temperature decreases from x3 to x4, the excess carbon above the amount required to saturate austenite is precipitated as cementite primarily along the grain boundaries.
Finally, the eutectoid line is reached at x4. This line is called the lower-critical-temperature line on the hypereutectoid side and is labeled A3,1. Just above the A3,1 line the microstructure consists largely of austenite, with the excess proeutectoid cementite as a network surrounding the austenite grains.
The remaining austenite containing 0. Above figure shows microstructure of a hypereutectoid steel. The difference in signification of the upper-critical-temperature lines, A3 and Acm may be noted. The line A3 involves an allotropic change where as Acm involves only a change in carbon solubility. Effect of Carbon on Mechanical Properties of Steel The mechanical properties of an alloy depend upon the properties of the phases and the way in which these phases are arranged to make up the structure.
As was pointed out earlier, ferrite is relatively soft with low tensile strength psi , while cementite is hard with very low tensile strength psi. However, the combination of these two phases in the form of the eutectoid pearlite produces an alloy of much greater tensile strength psi than that of either phase.
Above figure shows effect of carbon on mechanical properties of hot-worked steel. Since the amount of pearlite increases with an increase in carbon content for hypoeutectoid steels, the strength and Brinell hardness number will also increase up to the eutectoid composition of 0. The ductility and impact strength decreases with increasing carbon. Beyond the eutectoid composition, the strength levels off and may even show a decrease due to the brittle cementite network.
The Brinell hardness, however, continues to increase due to the greater proportion of hard cementite. The Critical-temperature Lines. So let's start with a phase diagram that contains maximal information:. The important boundaries the lines separating phases have some universally used abbreviations: The temperature where iron looses its magnetism so-called Curie temperature.
Note that for pure iron this is still in the a -phase.
The point in this case where a changes to d at high temperatures. Why would anybody abbreviate a temperature with the letter "A"? Well, it stands for "arrest", something that happens in the slope of dilatometric or thermal curves recorded whenever phase diagrams where first measured.
Statements like "the addition of x lowers A 3 " are now clear. The circular insets give a schematic idea of what the structure would like at the compositions and temperatures indicated. The next thing to know is that the phase diagrams above is actually not the true iron-carbon phase diagram. I lied to you.
Some mixture of cementite and iron is not the configuration that allows the system to achieve total nirvana.
That would be a iron - graphite mixture. All the cementite forming is just a transient phase on the way to nirvana; it will decay into pure carbon graphite and iron in due time.