Eutectoid transformation

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 volume fraction of reinforced phase in eutectics is 7.7 % and 31-wol.% for systems Ti-B and Ti-Si, respectively. The typical structures of eutectic alloys for Ti-Si system is shown in Fig. 2. According to binary diagrams of phase equlibria, an essential solubility of silicon in a- and 13-phases is observed, which is dependent on temperature there is an eutectoid transformation (in this respect diagram Ti-Si is similar to Fe-C diagram), but in system Ti-B essential solubility of boron in a- and 3- phases does not occur. For this reason the structure of composites of Ti-B system is more stable at temperature variation. 

The eutectoid transformation, which takes place as austenite is cooled below the eutectoid temperature, is of great importance in steelmaking. (See Section 8.2.4.)... 

Zhu] Zhu, Y.T., Devletian, J.H., Precise Determination of Isomorphous and Eutectoid Transformation Temperatures in Binary and Ternary Zr Alloys , J. Mater. Sci., 26(22), 6128-6122 (1991) (Experimental, Phase Relations, Thermodyn., 13)... 

Nauk Ukr. RSR, A, (6), 566-569 (1978) (Experimental, Crys. Structure, Phase Diagram, 10) [1978Sha2] Shapovalov, V.I., Influence of Hydrogen on the Eutectoid Transformation in Iron-Carbon Alloys , Phys. Met. Metallogr, 46(3), 183-185 (1978), translated fiomEVz. Metal Metalloved, 46(3), 664-666 (1978) (ExperimentaL Phase Relations, Morphology, 8)... 

Fri] Fridberg, J., Hillert, M., On the Eutectoid Transformation of 6-Ferrite in Fe-Mo-C Alloys , Acta MetalL, 25(1), 19-24 (1977) (Phase Relations, Experimental, Transport Phenomena, Thermodyn., Calculation, 17)... 

Various authors have reported different temperatures of eutectoid decomposition, ranging from 1000 to 1140 °C [183-185]. The eutectoid transformation is a rather slow process, and is therefore not seen in conventionally aged samples. However, a large cubic phase field also exists which, together vfith slow transformation, facilitates the existence of a fully cubic structure, providing the basis for calcia-stabilized zirconia solid electrolytes. 

The phase diagram for this system, as reported by Naumkin et al. (1970) and Savitskii et al. (1970) is shown in fig. 15. Savitskii et al. did not reveal the purity of their alloying materials but Naumkin et al. reported a 99.7 wt% purity for their lanthanum (impurities, reported in wt%, were 0.04 Ce, 0.07 each Pr and Nd and 0.012 Fe). Their scandium was reported to be 99.7 wt% pure (impurities included 0.11 wt% O). Their alloys were formed by arc-melting the metals under an atmosphere of purified helium. Below the solidus, the body-centered cubic yLa and jSSc form a solid solution. Two eutectoid transformations at about 750°C and 45 at% Sc and 233°C and 14at% Sc result in regions of two-phase immiscibility. 

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. 

A nonaHoyed carbon steel having 0.76% carbon, the eutectoid composition, consists of austenite above its lowest stable temperature, 727°C (the eutectoid temperature). On reasonably slow cooling from above 727°C, transformation of the austenite occurs above about 550°C to a series of parallel plates of a plus cementite known as peadite. The spacing of these plates depends on the temperature of transformation, from 1000 to 2000 nm at about 700°C and below 100 nm at 550°C. The corresponding BrineU hardnesses (BHN), which correspond approximately to tensile strengths, are about BHN... 

Fig. 17. Isothermal transformation (IT) diagram for a plain carbon eutectoid steel (1). Ae is A temperature at equiUbnum BHN, BrineU hardness number ...

Fig. 6.7. How pearlite grows from undercooled y during the eutectoid reaction. The transformation is limited by diffusion of carbon in the y, and driving force must be shared between all the diffusionol energy barriers. Note that AH is in units of J kgn2 is the number of carbon atoms that diffuse from or to Fe3C when 1 kg of y is transformed. (AH/njKfT - 7]/TJ is therefore the free work done when a single carbon atom goes from or to Fe,C.

Fig. 8.11. The TTT diagram for a 0.8% carbon (eutectoid) steel. We will miss the nose of the 1% curve if w quench the steel at = 200°C s. Note that if the steel is quenched into cold water not all the i/will transform to martensite. The steel will contain some "retained" / which can only be turned into martensite if the steel is cooled below the Mf temperature of -50°C.

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...

We saw in Chapter 8 that, if we cool eutectoid y to 500°C at about 200°C s , we will miss the nose of the C-curve. If we continue to cool below 280°C the unstable y will begin to transform to martensite. At 220°C half the y will have transformed to martensite. And at -50°C the steel will have become completely martensitic. Flypoeutectoid and hypereutectoid steels can be quenched to give martensite in exactly the same way (although, as Fig. 11.8 shows, their C-curves are slightly different). 

The figure below shows the isothermal transformation diagram for a coarse-grained, plain-carbon steel of eutectoid composition. Samples of the steel are austenitised at 850°C and then subjected to the quenching treatments shown on the diagram. Describe the microstructure produced by each heat treatment. 

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 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. 20.48 Isothermal time temperature transformation curves for (a) a eutectoid steel and...

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. 

Isothermal transformation (IT) diagrams, 17 16 23 277-280 for eutectoid steel, 23 279 Isothiazolone marine antifoulant,... 

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