Metastable Iron-Carbon (Fe-C) Phase Diagram

Metastable Iron-Carbon (Fe-C) 

Phase Diagram

The study of the iron-carbon (Fe-C) binary phase diagram (Figure 1) is usually the start of a study for the microstruction of all steels. It provides an invaluable basis upon which both carbon steels and alloy steels as well as various heat therapies are usually subject to (hardening, clothing, etc.) can be acquired.

Figure 1. The Fe-C phase diagram shows that phases of different combinations of carbon content and temperature should be required in metastable equilibrium. Thermo-Calc and the PBIN thermodynamic database are computed for the metastable Fe-C phase diagram.

We distinguish ferrite (alpha-iron) at the low-carbon end of a metastable Fe-C phase chart, which can at most 0.028 wt dissolve. The percentages of C will dissolve 2.08 wt at 738 ° C and austenite (Gamma-iron). At 1154 ° C, percent C. The considerably larger gamma-iron (austenite) phase field as opposed to that of ferrite clearly indicates that the gamma-iron (austenite) produces significantly higher solubility of carbon, with a maximum value of 2.08 wt. At 1154 ° C percent. The hardening of carbon steels and various alloy stones is based on this disparity in alpha-iron (ferrite) and gamma-iron (austénite) solubility of metal.

Cementite (Fe3C) is found on the carbon-rich side of the metastable Fe-C phase diagram. The delta-ferrite is of less concern at the highest temperatures, except for heavily alloyed steels.

The vast majority of stains rely on only two iron allotropes: (1) body centered cubic ferrite iron (BCC) and (2) face-centered cubic austenite iron (FCC) iron. When it becomes FCC austenite, BCC ferrite is stable at ambient pressure from all temperatures up to912 ° C (A3 point). At 1394 ° C (A4 point), it returns to ferrite. Although its crystal structure is identical to that of alpha-ferrotum, this high-temperature ferrite is labeled delta-iron. The ferrite remains stable until it melts at 1538 degrees Celsius.

Between single-phase fields regions are found with two-phase mixtures (such as ferrite + cement, austenite+ cement, and ferrite + austenite). The fluid phase field can be found at the highest temperature, and the two stage fields below are: fluid + austenite, fluid + cement and liquid + delta-ferrite. The liquid phase is always avoided in the heat treatment of steels.

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) + Fe3C (cementite).

Some important boundaries at single-phase fields have been given special names. These include:

• A1 — The so-called eutectoid temperature, which is the minimum temperature for austenite.
• A3 — The lower-temperature boundary of the austenite region at low carbon contents; i.e., the gamma / gamma + ferrite boundary.
• Acm — The counterpart boundary for high-carbon contents; i.e., the gamma / gamma + Fe3C boundary.

Sometimes the letters ce, or r are included:

• Accm — In hypereutectoid steel, the temperature at which the solution of cementite in austenite is completed during heating.
• Ac1 — The temperature at which austenite begins to form during heating, with the c being derived from the French chauffant.
• Ac3 — The temperature at which transformation of ferrite to austenite is completed during heating.
• Aecm, Ae1, Ae3 — The temperatures of phase changes at equilibrium.
• Arcm — In hypereutectoid steel, the temperature at which precipitation of cementite starts during cooling, with the r being derived from the French refroidissant.
• Ar1 — The temperature at which transformation of austenite to ferrite or to ferrite plus cementite is completed during cooling.
• Ar3 — The temperature at which austenite begins to transform to ferrite during cooling.
• Ar4 — The temperature at which delta-ferrite transforms to austenite during cooling.

The location of the A1, A3, and Acm boundaries as well as the eutectoid composition are modified when alloying elements are added into an iron-carbon alloy (steel). A1 generally decreses the austenite-stabilizing elements (for example, chromium, silicon, aluminum, titanium, vanadium, niobium, molybdenum, tungsten, etc) while the ferrite-stabilizing elements (for example, chromium, silicon, titanium, vanadium).

Carbon content is referred to as eutectoid carbon content at which the minimum austenite temperature is reached (0.68% C for the metastable Fe-C-phase diagram). The phases of ferrite cement mixture produced during low cooling are pearlite, and can be viewed as microstructures or microcontractants. The ferrite cement mixture is distinctive in appearance. A combined ferrite and cement lamellas are alternated which, after extensive holding at temperature near to A1, spheroidize into cement particles dispersed into a ferrite matrix.

Finally, we have the martensite start temperature, Ms, and the martensite finish temperature, Mf:

• Ms — The highest temperature at which transformation of austenite to martensite starts during rapid cooling.
• Mf — The temperature at which martensite formation finishes during rapid cooling.

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