The Iron-Carbon (Fe-C) and Iron-Cementite (Fe-Fe3C) Systems

Because carbon is a ubiquitous element in both iron- and steelmaking processes due to its essential use as a reductant during the extractive process of iron from its ores, carbon has   a predominant role in  siderurgy  (i.e., the metallurgy of iron and its alloys). Although other  alloying elements may be added to produce steels for special purposes, usually the structure of iron and steels is determined first by the content of carbon, secondly by the type of other alloying elements, and finally by the rate of cooling from the molten state. For all the above reasons, a solid grasp of the iron-carbon system is a mandatory step for understanding iron and iron alloys (i.e., steels and cast irons). As for the phase diagram of pure iron, the major phases occuring in the Fe-C phase diagram (Figure 1) can be accurately characterized by means of X-ray diffraction, thermal analysis, and dilatometry techniques. In practice, the iron-carbon phase diagram is a graphical plot of phases existing in thermodynamic equilibrium as a function of temperature versus the mass fraction of total carbon in the iron. The diagram depicted in this book is only a detail of the entire diagram. Actually, the phase diagram extends on the abscissa axis at left from pure iron free of carbon to a content of total carbon reaching 6.70 wt.% C that corresponds to the theoretical composition of iron carbide or cementite (Fe3C), while temperatures range from 200°C to 1600°C, the temperature at  which the system is fully liquid. The binary phase diagram exhibits, in addition to the four  critical points of the allotropes of pure iron, three other important characteristics:

 

(i) a eutectic point at 4.30 wt.% C and 1148°C;

(ii) a eutectoid point at 0.77 wt.% C and 727°C;

(iii) a peritectic transformation occurring at 1495°C.

 

 

 

Moreover, experimentally the following solid phases were identified.

 

Alpha-ferrite (α-ferrite, bcc). Sensu stricto and historically, ferrite consists of a solid solution of carbon inside a body-centered cubic crystal lattice in alpha-iron. The solubility of carbon in alpha-iron is extremely low, ca. 0.01 wt.% C at ambient temperature, and reaches only 0.025 wt.% C at 723oC. Therefore, at room temperature under conditions of equilibrium, any carbon present in excess of that small amount will exsolute in the form of cementite. Due to this low carbon content, some textbooks treat the ferrite phase substantially as pure iron, but this view must be discontinued to avoid confusion.  Usually, the ferrite of an alloyed steel may contain in solid solution appreciable amounts of other elements; ab extenso, any solid solution of which alpha-iron is the solvent is called ferrite (i.e., a solid solution of any element in alpha-iron). Alloying elements that stabilize ferrite are listed in Table 1.
 

 

 

Beta-ferrite (b-ferrite, bcc). Like alpha-ferrite, beta-ferrite consists of a solid solution of  any element in body-centered cubic beta iron.

Delta-ferrite (δ-ferrite, bcc).
Like alpha-ferrite, delta-ferrite consists of a solid solution of  any element in body-centered cubic delta iron. In the case of carbon, its maximum solubility in delta-iron is only 0.1 wt.% at 1487°C.
Gamma-austenite (g-austenite, fcc). Austenite is a solid insertion solution of carbon into the crystal lattice of face-centered cubic gamma-iron. It has been definitively established that the carbon atoms in austenite occupy interstitial positions in the face-centered cubic space lattice causing the lattice parameter to increase progressively with the carbon content. Cementite. Cementite is an iron carbide with the chemical formula Fe3 C. At room temperature,   cementite is a hard, brittle, and ferromagnetic material with a Curie temperature of   210°C. It is formed by chemical reaction between iron and excess carbon. Three distinct origins must, however, be distinguished:

(i) primary cementite resulting from the separation during solidification of liquid iron with carbon content ranging between 4.3 wt.% and 6.69 wt.% C;

(ii) secondary cementite formed after demixion of carbon as a result of a decrease in miscibility during the cooling of ferrite;

(iii) tertiary cementite resulting from demixion during the cooling of austenite. Actually, at

room temperature under conditions of equilibrium, any carbon present in excess of that small amount must exist in a form other than that of a solute in a solid solution.

Perlite. A biphasic eutectoidic constituent that consists of an interlamellar growth of ferrite and cementite. Perlite is formed during the transformation of austenite with a eutectoid composition (i.e., 0.77 wt.% C).

 

Ledeburite. A biphasic eutectic constituent resulting from the solidification of a molten metal having a eutectic composition. Hence it consists of an austenite containing 1.7 wt.% C in solid solution and cementite.

 

In the phase diagrams in Figures 2.1 and 2.2, the transition temperatures or critical points   previously identified for the four iron allotropes must now be replaced by two temperature  limits or points. Actually, due to hysteresis phenomena occurring upon heating and cooling, the equilibrium curves are greatly influenced by the rate of cooling and heating, and they form distinct plots. The various temperatures at which pauses occur in the rise or fall of  temperatures when iron or steel is heated from room temperature or cooled from the molten  state are called arrest points, denoted by uppercase letter A. Due to the previously mentioned hysteresis behavior during heating and cooling, the arrest points obtained on heating   are denoted Ac and those obtained on cooling are denoted Ar,  while arrest points at equilibrium are denoted Ae. Historically, the subscripts c , r, and e were derived from the first letters  of the French words chauffage (heating), refroidissement (cooling), and équilibre (equilibrium),  respectively. These arrest points are described in detail in Table 2.

 

 

From the iron-carbon phase diagram several important characteristics regarding the classification   of iron and iron alloys can be seen. Iron alloys are classified according to their total content of carbon. Steel are particular iron alloys having a carbon content below 2.1 wt.% C.  Above this limit, we have cast irons up to a practical limit of 3.75 wt.% C. A steel containing  0.77 wt.% C is called a eutectoid steel.  Eutectoid steel consists of an intimate mixture of alpha- ferrite and cementite forming an intergrowth of thin plates or lamellae known as perlite.  Therefore, a steel having a carbon content below 0.77 wt.% C is called a hypotectoid steel. Its  structure consists of a small amount of pearlite with an excess of alpha-ferrite, which collectsat the grain boundaries. Hypotectoid steels are hence softer and more ductile than eutectoid steels. On the other hand, a steel with more than 0.77 wt.% C is called a hypertectoid steel. Its   structure consists of pearlite with an excess of cementite. Hypertectoid steels are harder,  more brittle, and les ductile than eutectoid steels. Above 2.11 wt.% C, molten iron solidifies   always below 1350°C and the resulting low liquidus temperature iron alloys are called cast irons due to their ease of melting. The eutectic point in the Fe-C diagram is located at  4.3 wt.% C. At this composition, when the alloy solidifies, it forms a mixture of austenite  with 1.7 wt.% C in solid solution and cementite; this eutectic structure is called ledeburite. In  practice, cast irons exhibit a carbon content ranging between 2.11 and 3.75 wt.%. Upon cooling, cast irons exhibit a mixture of pearlite and cementite.

 

 

 

The iron-carbon phase diagram only applies to alloys that contain only iron and carbon. But because other desired or undesired alloying elements are usually originally present from the ironmaking process (e.g., O, C, Si, P, Mn, V) or are added intentionally (e.g., Ni, Cr, Mo), during steelmaking the iron-carbon phase diagram cannot show accurately the conditions that apply to actual steels. Hence it has to be modified appropriately to take into account the effect of the elements. These additional elements impact both the arrest points and equilibrium lines, and they also determine the existence or not of certain phases. Alloying elements with their related impact on iron-carbon phases are listed in Table 3.