Eutectoid point

But the diagram shows another feature which looks like a eutectic it is the V at the bottom of the austenite field. The transformation which occurs there is very like the eutectic transformation, but this time it is a solid, austenite, which transforms on cooling to two other solids. The point at the base of the V is called a eutectoid point. 

The compositions of the two new phases are given by the ends of the tie line through the eutectoid point. 

The copper-zinc system (which includes brasses) has one eutectoid reaction. Mark the eutectoid point on the phase diagram (Fig. A 1.38). 

Fig. 14. Phase diagrams, in which the superplastic alloys are found, show the alloys texclusively located at compositions corresponding to eutectic or eutectoid points.These compositions are marked by a vertical line...

By shifting the eutectoid point to lower carbon concentrations, tungsten increases the amount of undissolved and excess carbide in the hardened steel. Tungsten is a carbideforming element and binds the carbon to form straight tungsten carbide or combines with... 

Even below the solidification point, changes can take place. As the saturated austenite is cooled below 1125°, separation of cementite takes place along GH, the percentage of carbon in the saturated austenite diminishing with fall of temperature, until at about 700 , the limit of saturation is reached at 0-89 per cent, of carbon (point H). Below this eutectoid point H, separation into a-ferrite and cementite takes place (Al transformation). This eutectoid mixture is known as pearhte. 

A similar feature, but involving only solid phases, occurs at the lowest temperature of the austenite phase field, 727 °C, and at 0.76 wt% carbon. This is also an invariant point, called a eutectoid point. At a eutectoid point, a solid transforms to two solids on cooling ... 

On cooling a homogeneous solid phase through a eutectoid point it forms ... 

When austenite is cooled slowly through the eutectoid point, the material that forms is called ... 

At lower temperatures the austenitic phase decomposes into ferrite and cementite. The lowest point of stability of austenitic crystals is at a concentration of 0.8% carbon and a temperature of 723 °C. At this point the austenite transforms to a eutectoid mixture of ferrite with 0.02% carbon and cementite with 6.67% carbon. This eutectoid point of the solid solution is adequate to the eutectic point of a melt where the melt decomposes into a eutectic mixture. The eutectoid mixture of ferrite and cementite is called pearlite. 

Eutectic and Eutectoid Point in the Fe-C System (Polish), Kovove Mater., 20(4), 417-425 (1982) (Calculation, Thermodyn., 12)... 

An isothermal reversible reaction in which a solid solution is converted into two or more intimately mixed solids on cooling, the number of solids formed being the same as the number of components in the system. (2) An alloy having the composition indicated by the eutectoid point on an equilibrium diagram. (3) An alloy structure of intermixed solid constituents formed by a eutectoid reaction. 

Fe-FesC phase diagram. The eutectoid point at 727°C and 0.76 wt% FeiC is the focal point for steel making. (From Massalski, T.B., Senior Editor, Handbook of Binary Alloy Phase Diagrams, Vols. 1-3, American Society for Metals, 1990. Reprinted with permission of ASM International. All rights reserved.)... 

Of particular importance to steel making is the eutectoid point at 727°C 0.76 wt% C. When austenite of this composition is cooled to this temperature, it undergoes euctectoid reaction y a + FeaC. Further cooling produces lamellas of alternating a-Fe and cementite called pearlite because its appearance resembles mother-of-pearl. The spacing of the lamella can be controlled by the cooling rate as discussed in Chapter 12. 

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.

Time-temperature-transformation (T-T-T) diagrams are used to present the structure of steels after isothermal transformation at different temperatures for varying times. The T-T-T diagram for a commercial eutectoid steel is shown in Fig. 20.48a. Also shown on the curves are the points at which the microstructures illustrated in Figs. 20.46 and 20.47 are observed, and the thermal treatments producing these structures. When a steel partially transformed to, say, pearlite, is quenched from point a in Fig. 20.48a to below nif, the untransformed austenite transforms to martensite. 

Characteristics and implementation of the treatments depend on the expected results and on the properties of the material considered a variety of processes are employed. In ferrous alloys, in steels, a eutectoid transformation plays a prominent role, and aspects described by time-temperature-transformation diagrams and martensite formation are of relevant interest. See a short presentation of these points in 5.10.4.5. Titanium alloys are an example of the formation of structures in which two phases may be present in comparable quantities. A few remarks about a and (3 Ti alloys and the relevant heat treatments have been made in 5.6.4.1.1. More generally, for the various metals, the existence of different crystal forms, their transformation temperatures, and the extension of solid-solution ranges with other metals are preliminary points in the definition of convenient heat treatments and of their effects. In the evaluation and planning of the treatments, due consideration must be given to the heating and/or cooling rate and to the diffusion processes (in pure metals and in alloys). 

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... 

At this point it would be worthwhile to discuss in little more detail the unique nature of eutectic and eutectoid alloy composition as follows ... 

If a substance is allotropic this will affect the shape of phase diagrams for systems involving the substance. Consider a system which involves two allotropic substances, A and B. The following figure shows one of the possible diagrams which involve allotropic substances. The point e in the diagram is called the eutectoidpoint, and the eutectoid reaction is... 

In pure titanium, the crystal structure is dose-packed hexagonal (a) up to 882°C and body-centered cubic (p) to the melting point. The addition of alloying dements alters the a—p transformation temperature. Elements that raise the transformation temperature are called a-stabilizers those that depress the transformation temperature, p-stabilizers the latter are divided into p-isomorphous and p-eutectoid types. The p-isomorphous elements have limited a-solubility and increasing additions of these dements progressively depresses the transformation temperature. The p-eutectoid elements have restricted p-solubility and form intermetallic compounds by eutectoid decomposition of the p-phase. The binary phase diagram illustrating these three types of alloy... 

The absorption-isotherm method provides a very useful approach to the study of a gas-solid system. Inflection points in the isotherms locate phase boundaries, and a plot of log P vs. 1/T (Arrhenius plot) permits calculation of heats of solution and estimation of the eutectoid temperature. The shape of the isotherms depends only on those phases which affect the hydrogen absorption, rather than all phases present. The actual phases present were identified by high-temperature x-ray diffraction. The designation of the phases follows terminology commonly used in U. S. Atomic Energy Commission studies ... 

Melting point peritectoid decomposition ° peritecdc decomposition eutectoid formation ... 

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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