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  Figure 1.  The Fe-C phase diagram shows which phases are to be expected at metastable equilibrium for different combinations of carbon content and temperature. The metastable Fe-C phase diagram was calculated with Thermo-Calc, coupled with PBIN thermodynamic database. At the low-carbon end of the metastable Fe-C phase diagram, we distinguish ferrite  (alpha-iron), which can at most dissolve 0.028 wt. % C at 738 °C, and austenite  (gamma-iron), which can dissolve 2.08 wt. % C at 1154 °C. The much larger phase field of gamma-iron  (austenite) compared with that of alpha-iron  (ferrite) indicates clearly the considerably grater solubility of carbon in gamma-iron (austenite), the maximum value being 2.08 wt. % at 1154 °C. The hardening  of carbon steels, as well as many alloy steels, is based on this difference in the solubility of carbon  in alpha-iron (ferrite) and gamma-iron (austenite). At the carbon-rich side of the metastable Fe-C phase diagram we find cementite  (Fe 3 C). Of less interest, except for highly alloyed steels, is the delta-ferrite  at the highest temperatures. The vast majority of steels rely on just two allotropes of iron: (1) alpha-iron , which is body-centered cubic  (BCC) ferrite, and (2) gamma-iron , which is face-centered cubic  (FCC) austenite. At ambient pressure, BCC ferrite is stable from all temperatures up to 912 °C (the A 3 point), when it transforms into FCC austenite. It reverts to ferrite at 1394 °C (the A 4  point). This high-temperature ferrite is labeled delta-iron , even though its crystal structure is identical to that of alpha-ferrite. The delta-ferrite remains stable until it melts at 1538 °C. Regions with mixtures of two phases (such as ferrite + cementite, austenite + cementite, and ferrite + austenite) are found between the single-phase fields. At the highest temperatures, the liquid phase field can be found, and below this are the two-phase fields (liquid + austenite, liquid + cementite, and liquid + delta-ferrite). In heat treating of steels, the liquid phase is always avoided. The steel portion of the Fe-C phase diagram covers the range between 0 and 2.08 wt. % C. The cast iron  portion of the Fe-C phase diagram covers the range between 2.08 and 6.67 wt. % C. The steel portion of the metastable Fe-C phase diagram can be subdivided into three regions: hypoeutectoid  (0 < wt. % C < 0.68 wt. %), eutectoid  (C = 0.68 wt. %), and hypereutectoid (0.68 < wt. % C < 2.08 wt. %). A very important phase change in the metastable Fe-C phase diagram occurs at 0.68 wt. % C. The transformation is eutectoid, and its product is called pearlite  (ferrite + cementite): gamma-iron (austenite) — > alpha-iron (ferrite) + Fe 3 C (cementite). Some important boundaries at single-phase fields have been given special names. These include: ã A 1   —  The so-called eutectoid temperature , which is the minimum temperature for austenite. ã A 3   —  The lower-temperature boundary of the austenite region at low carbon contents; i.e., the gamma / gamma + ferrite boundary. ã A cm   —  The counterpart boundary for high-carbon contents; i.e., the gamma / gamma + Fe 3 C boundary. Sometimes the letters c , e , or r  are included: ã Ac cm   —  In hypereutectoid steel , the temperature at which the solution of cementite in austenite is completed during heating. ã Ac 1   —  The temperature at which austenite begins to form during heating, with the c  being derived from the French chauffant  . ã Ac 3   —  The temperature at which transformation of ferrite to austenite is completed during heating. ã Ae cm , Ae 1 , Ae 3   —  The temperatures of phase changes at equilibrium. ã Ar  cm   —  In hypereutectoid steel, the temperature at which precipitation of cementite starts during cooling, with the r  being derived from the French refroidissant  . ã Ar  1   —  The temperature at which transformation of austenite to ferrite or to ferrite plus cementite is completed during cooling. ã Ar  3   —  The temperature at which austenite begins to transform to ferrite during cooling.  ã Ar  4   —  The temperature at which delta-ferrite transforms to austenite during cooling. If alloying elements are added to an iron-carbon alloy (steel), the position of the A 1 , A 3 , and A cm boundaries, as well as the eutectoid composition, are changed. In general, the austenite-stabilizing elements  (e.g., nickel, manganese, nitrogen, copper, etc) decrease the A 1  temperature, whereas the ferrite-stabilizing elements  (e.g., chromium, silicon, aluminum, titanium, vanadium, niobium, molybdenum, tungsten, etc) increase the A 1  temperature. The carbon content at which the minimum austenite temperature is attained is called the eutectoid carbon content  (0.68 wt. % C in case of the metastable Fe-C phase diagram). The ferrite-cementite phase mixture of this composition formed during slow cooling has a characteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite lamellae that coarsens (or spheroidizes ) into cementite particles  dispersed within a ferrite matrix  after extended holding at a temperature close to A 1 . Finally, we have the martensite start temperature , M s , and the martensite finish temperature , M f  : ã M s   —  The highest temperature at which transformation of austenite to martensite starts during rapid cooling. ã M f    —  The temperature at which martensite formation finishes during rapid cooling.
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