A study of the microstructure of all steels usually starts with the metastable iron-carbon (Fe-C) binary phase diagram (Figure 1). It provides an invaluable foundation on which to build knowledge of both carbon steels and alloy steels, as well as a number of various heat treatments they are usually subjected to (hardening, annealing, etc).

Figure 1. Iron Carbon Phase Diagram

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 (Fe3C). 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 A3 point), when it transforms into FCC austenite. It reverts to ferrite at 1394 °C (the A4 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) + 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 c, e, 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.

If alloying elements are added to an iron-carbon alloy (steel), the position of the A1, A3, and Acm boundaries, as well as the eutectoid composition, are changed. In general, the austenite-stabilizing elements (e.g., nickel, manganese, nitrogen, copper, etc) decrease the A1 temperature, whereas the ferrite-stabilizing elements (e.g., chromium, silicon, aluminum, titanium, vanadium, niobium, molybdenum, tungsten, etc) increase the A1 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 A1.

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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