Eutectoid ferrite

The newly formed cementite nucleates at many sites simultaneously. The resulting solid, which is a mixture of ferrite and cementite, is called pearlite because it has a lustrous appearance in an optical microscope. Pearlite is not a compound or single phase but is a microstmeture, made up of thin lamellae of cementite and a-ferrite side by side (Figure 8.7c). In this context, these phases are called eutectoid ferrite and eutectoid cementite. 

As noted above, most steels are, in practice, hypo-eutectoid. Consider, by way of example, a low-alloy steel containing 0 3%C. The Fe-C phase diagram (Fig. 20.44) indicates that as a steel of this composition is cooled below about 1 080K the equilibrium state is a two-phase structure containing primary, pro-eutectoid ferrite and austenite. On further cooling to below 996K the equilibrium structure consists of pro-eutectoid ferrite and pearlite. These differences are reflected in the rather more complicated T-T-T diagram used for this steel, shown in Fig. 20.48b. 

The microstructure of the welding metal is based on acicular and eutectoid ferrite. The acicular ferrite has blunter rods due to the annealing of the welding splice. 

The mass fraction of eutectoid ferrite in an iron-Q carbon alloy is 0.71. On the basis of this information, is it possible to determine the composition of the alloy If so, what is its composition If this is... 

Precipitation Hardening. With the exception of ferritic steels, which can be hardened either by the martensitic transformation or by eutectoid decomposition, most heat-treatable alloys are of the precipitation-hardening type. During heat treatment of these alloys, a controlled dispersion of submicroscopic particles is formed in the microstmeture. The final properties depend on the manner in which particles are dispersed, and on particle size and stabiUty. Because precipitation-hardening alloys can retain strength at temperatures above those at which martensitic steels become unstable, these alloys become an important, in fact pre-eminent, class of high temperature materials. 

When a eutectoid steel is slowly cooled from the austenite range, the ferrite and cementite form in alternate layers of microscopic thickness. Under the microscope at low magnification, the diffraction effects from this mixture of ferrite and cementite give an appearance similar to that of a pearl, hence the material is called peadite. 

Changes on Heating and Cooling Hypoeutectoid Steel. Hypoeutectoid steels are those that contain less carbon than the eutectoid steels. If the steel contains more than 0.02% carbon, the constituents present at and below 727°C are usually ferrite and peadite. The relative amounts depend on the carbon content. As the carbon content increases, the amount of ferrite decreases and the amount of peadite increases. 

On slow cooling the reverse changes occur. Ferrite precipitates, generally at the grain boundaries of the austenite, which becomes progressively richer in carbon. Just above A, the austenite is substantially of eutectoid composition, 0.76% carbon. 

The peritectic transformation generally has little effect on the structure, properties or corrosion resistance of steels at room temperature an exception to this occurs in the welding of certain steels, when 6-ferrite can be retained at room temperature and can affect corrosion resistance. Furthermore, since most steels contain less than about 1 -0 oC (and by far the greatest tonnage contains less than about 0-3%C) the eutectic reaction is of relevance only in relation to the structure and properties of cast irons, which generally contain 2-4%C. This discussion, therefore, will be limited to the eutectoid reaction that occurs when homogeneous austenite is cooled. 

Fig. 20.49 Schematic illustration of some of the ferritic/pearlitic microstructures observed in hypo-eutectoid steels after various heat treatments...

The higher the carbon content of a hypo-eutectoid steel, the more pearlite there will be in a ferritic/pearlitic structure and the greater will be the strength of the steel, other factors (grain-size, pearlite spacing, etc.) being equal. 

The austenite phase which can contain up to 1.7 wt% of carbon decomposes on cooling to yield a much more dilute solution of carbon in a-iron (b.c.c), Ferrite , together with cementite, again rather than the stable carbon phase, at temperatures below a solid state eutectoid at 1013 K (Figure 6.3). 

The higher solubility of carbon in y-iron than in a-iron is because the face-centred lattice can accommodate carbon atoms in slightly expanded octahedral holes, but the body-centred lattice can only accommodate a much smaller carbon concentration in specially located, distorted tetrahedral holes. It follows that the formation of ferrite together with cementite by eutectoid composition of austenite, leads to an increase in volume of the metal with accompanying compressive stresses at the interface between these two phases. 

In the most frequently used steels, having less than the eutectoid content of carbon (about 0.8 wt%), the various forms in which the cementite phase can be produced in dispersion in ferrite depend upon the rate of cooling to... 

Figure 5.29. Fe-rich region of the Fe C phase diagram. Stable Fe-C (graphite) diagram solid lines metastable Fe-Fe3C diagram dashed lines. The following current names are used ferrite (solid solution in aFe), austenite (solid solution in 7Fe) and cementite (Fe3C compound). Pearlite is the name given to the two-phase microstructure which originates from the eutectoid reaction ...

The best-known eutectoid reaction is that which occurs in steel where the austenite phase, stable at high temperatures, transforms into (he eutectoid structure known as pcarlitc In this transformation, the austenite phase, containing 0.8% carbon in solid solution, transforms to a mixture of ferrite (nearly pure body-centered cubic irom anti iron-carbide (Fe-.Ct. Al atmospheric pressure, the equilibrium temperature for this reaction is 723 C. This temperature is the eutectoid temperature... 

In binary alloy systems, a eutectoid alloy is a mechanical mixture of two phases which form simultaneously from a solid solution when it cools through Ihe eutectoid temperature. Alloys leaner or richer in one of the metals undergo transformation from the solid solution phase over a range of temperatures beginning above and ending al the eutectoid temperature. The structure of such alloys will consist of primary particles of one of the stable phases in addition to ihe eutectoid. lor example ferrite and pearlite in low-carbon steel. See also Iron Metals, Alloys, and Steels. 

The effect of this extrapolation can be seen in isothermal diagrams. Figure 7.8 is the isothermal transformation diagram for a 1050 steel. Note that proeutectoid ferrite must form before cementite for transformation temperatures above about 600 °C in accordance with Figure 7.8. The agreement is not perfect because in addition to 0.50% C, the 1050 steel contains 0.91 % Mn, which lowers the eutectoid temperature and composition. 

Lithium-Structure Compatibility. One of the critical chemistry problems of HYLIFE is the compatibility of structural alloys with the molten liquid of the jet array. Two candidate liquid metals are lithium and Pbg3Lij 7. High-Z metal (such as lead from target debris) will enter the liquid metal and may affect the compatibility. The structural alloy selected in the HYLIFE study is Cr-1 Mo, a ferritic steel. The carbides usually present in this steel are M3C (cementite) and M2C, where M is primarily Fe. Both of these carbides are unstable in lithium. M3C is usually present as platelets within pearlite, the eutectoid structure in pearlitic steel. The most common microstructure for the 2 4 Cr-1 Mo steel is large grains of ferrite with small islands of pearlite. M2C is present as a fine spray of precipitate within large ferrite grains. Lithium... 

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