Eutectoid iron

Figore 10.14 Isothermal transformation diagram for a eutectoid iron-carbon alloy, with superimposed isothermal heat treatment curve (ABCD). Microstructures before, during, and after the austenite-to-pearlite transformation are shown. 

Superimposition of isothermal and continuous-cooling transformation diagrams for a eutectoid iron-carbon alloy. 

Figure 10.27 Continuous-cooling transformation diagram for a eutectoid iron-carbon alloy and superimposed cooUng curves, demonstrating the dependence of the final microstructure on the transformations that occur during coohng.

Determine the approximate tensile strengths O and ductilities (%RA) for specimens of a eutectoid iron-carbon alloy that have experienced the heat treatments described in parts (a) through... 

The important (3-stabilizing alloying elements are the bcc elements vanadium, molybdenum, tantalum, and niobium of the P-isomorphous type and manganese, iron, chromium, cobalt, nickel, copper, and siUcon of the P-eutectoid type. The P eutectoid elements, arranged in order of increasing tendency to form compounds, are shown in Table 7. The elements copper, siUcon, nickel, and cobalt are termed active eutectoid formers because of a rapid decomposition of P to a and a compound. The other elements in Table 7 are sluggish in their eutectoid reactions and thus it is possible to avoid compound formation by careful control of heat treatment and composition. The relative P-stabilizing effects of these elements can be expressed in the form of a molybdenum equivalency. Mo (29) ... 

The higher solubility of carbon in y-iron than in a-iroii is because the face-ceiiued 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 fenite together with cementite by eutectoid composition of austenite, leads to an increase in volume of the metal with accompanying compressive stresses at die interface between these two phases. 

To make martensite in pure iron it has to be cooled very fast at about 10 °C s h Metals can only be cooled at such large rates if they are in the form of thin foils. How, then, can martensite be made in sizeable pieces of 0.8% carbon steel As we saw in the "Teaching Yourself Phase Diagrams" course, a 0.8% carbon steel is a "eutectoid" steel when it is cooled relatively slowly it transforms by diffusion into pearlite (the eutectoid mixture of a + FejC). The eutectoid reaction can only start when the steel has been cooled below 723°C. The nose of the C-curve occurs at = 525°C (Fig. 8.11), about 175°C lower than the nose temperature of perhaps 700°C for pure iron (Fig. 8.5). Diffusion is much slower at 525°C than it is at 700°C. As a result, a cooling rate of 200°C s misses the nose of the 1% curve and produces martensite. 

Figures 11.2-11.6 show how the room temperature microstructure of carbon steels depends on the carbon content. The limiting case of pure iron (Fig. 11.2) is straightforward when yiron cools below 914°C a grains nucleate at y grain boundaries and the microstructure transforms to a. If we cool a steel of eutectoid composition (0.80 wt% C) below 723°C pearlite nodules nucleate at grain boundaries (Fig. 11.3) and the microstructure transforms to pearlite. If the steel contains less than 0.80% C (a hypoeutectoid steel) then the ystarts to transform as soon as the alloy enters the a+ yfield (Fig. 11.4). "Primary" a nucleates at y grain boundaries and grows as the steel is cooled from A3...

Fig. 11.3. Microstructures during the slow cooling of a eutectoid steel from the hot working temperature. As a point of detail, when peorlite is cooled to room temperature, the concentration of carbon in the a decreases slightly, following the a/a + FejC boundary. The excess carbon reacts with iron at the or-FejC interfaces to form more FejC. This "plates out" on the surfaces of the existing FejC plates which become very slightly thicker. The composition of Fe3C is independent of temperature, of course.

Eutectoid structures are like eutectic structures, but much finer in scale. The original solid decomposes into two others, both with compositions which differ from the original, and in the form (usually) of fine, parallel plates. To allow this, atoms of B must diffuse away from the A-rich plates and A atoms must diffuse in the opposite direction, as shown in Fig. A1.40. Taking the eutectoid decomposition of iron as an example, carbon must diffuse to the carbon-rich FejC plates, and away from the (carbon-poor) a-plates, just ahead of the interface. The colony of plates then grows to the right, consuming the austenite (y). The eutectoid structure in iron has a special name it is called pearlite (because it has a pearly look). The micrograph (Fig. A1.41) shows pearlite. 

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. 

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). 

Tempering According to the more common meaning of the term it consists in reheating (in heat treatments) hardened steel or cast iron to a temperature below the eutectoid in order to decrease hardness and increase toughness. 

The importance of metastable phases which persist at ambient pressure and temperature need not be emphasized. Control of the eutectoidal decomposition of the metastable BCC iron-carbon phase, austenite, below 996 K is essential to the steel... 

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 solubility of carbon in iron is reduced by the addition of phosphorus, but the temperature of formation of the eutectoid pearlite is not influenced by the presence of the phosphide. P. Goerens and W. Dobbelstein gave for the composition of the ternary eutectic E, Fig. 27, at 953°, l-96 per cent, of carbon, 6-89 per cent, of phosphorus, and 9145 per cent, of iron and J. E. Stead, respectively 1 92, 6 89, and 9149. In Fig. 26, A represents the iron-phosphorus eutectic, and B, the iron-carbon eutectic. They showed that when sat. solid soln. of iron phosphide in iron are heated or cooled they show no critical point at Ars, and the structure is not broken up even... 

The experimental procedure can be understood by reference to the Fe-C phase diagram, Figure 2. The specimen is reacted with methane at temperatures of 750-850°C. Carbon dissolves in the metal until the saturation solubility has been exceeded at the gamma-Fe/FejC phase boundary, or alternatively the specimen is cooled to a temperature below the iron-carbon eutectoid (721°C or 738°C). The solid and dotted lines in Figure 2 indicate the alternative iron-Fe3C and iron-graphite phase boundaries respectively. Either method produces similar reaction products but the isothermal experiment is prefereably carried out at higher pressures, e.g. 50 Torr, to minimise run times. 

Fig. 11.8 Mossbauer spectra of (a) a-Fe, (b) cast iron, (c) cementite separated from eutectoid carbon steel. [Ref. 85, Fig. 1 ]...

Perlite is an eutectoid phase mixture composed of approximately 87 % ferrite and 13 % cementite and occurs in iron materials with a carbon content between 0.02 % and 6.67 %. The eutectic point is at 723 °C and 0.83 % carbon. At carbon contents below 2.06 %, perlite appears as individual metallographic constituent, whereas above 2.06 % carbon, it occurs in a phase mixture with cementite and ledeburite. In perlite, cementite predominantly appears in lamellar form. 

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