Hypereutectoid

Microstmcture and Grain Size. The carbon steels having relatively low hardenabihty do not contaia martensite or bainite ia the cast, roUed, or forged state. The constituents of the hypoeutectoid steels are therefore ferrite and peariite, and of the hypereutectoid steels, cementite and peariite. 

Fig. 11.5. Microstructures during the slow cooling of a hypereutectoid steel. is the standard labelling for the temperature at which FejC first appears. Hypereutectoid means that the carbon content is above that of a eutectoid steel (in the sense that a hyperactive child has an above-normal activity ).

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). 

Fig. 11.8. TTT diagrams for (a) eutectoid, (b) hypoeutectoid and ( ) hypereutectoid steels, (b) and ( ) show (dashed lines) the C-curves for the formation of primary a and FejC respectively. Note that, os the carbon content increases, both A s and Mf decrease.

The entectoid transformation is an important one not only for this specific carbon composition, bnt for classifying all types of steels. Carbon steels have carbon contents between 0.1 and 1.5 wt%. Those with carbon contents less than 0.8% are termed hypoeutectoid steels, and those with greater than 0.8% C are called hypereutectoid steels. Further classifications of steels are given in Table 2.4. 

Figure 3.11. Comparison of the microstmctural changes upon very slow cooling of hypoeutectoid (upper) and hypereutectoid (lower) steel. Reproduced with permission from Machine Tools and Machining Practices, White, W. Wiley New Jersey, 1977. Copyright John Wiley Sons Limited.

Equation 9 quantifies the effects of dopant concentrations on the temperature required for the onset of martensite formation, Mg. The greatest effects are seen for the austenite-forming elements of C, Mn, and Ni where even small concentrations result in a sharp decrease in Mg. Whereas pure y-iron may be converted to martensite at temperatures in excess of 500°C, hypereutectoid steel is not transformed to martensite until a temperature of ca. 160°C is reached during quenching. At carbon concentrations above 0.7%, martensite is still being formed at temperatures well below 0°C. Hence, high-C steels must be quenched in low-temperature media e.g., dry ice/acetone, liquid nitrogen) to ensure full conversion of austenite to martensite. 

In very hard tool steels, for example, cold work steels tempered at low temperature, the wear resistance is originated mainly by hypereutectoid carbide which is not affected by hardening. These carbides are very small and evenly distributed and very hard (heavy-duty... 

Slow cooling of a hypereutectoid steel composition produces a microstructure composed of ... 

Interphase Precipitation in Hypereutectoid Fe-C-Cu Alloys , Acta Metall. Mat., 43(7), 2589-2604 (1995) (Phase Relations, Experimental, 46)... 

Schl] Schissler, J.-M., Metauer, G., Dilatometric Study of the Second Stage of the Isothermal Bai-nitic Transformation at 420°C of Hypereutectoid Fe-C-Si Alloys with 3.9% Siheon. Observation of a New Carbide (in French), Compt. Rend. Hebd. Acad. Sci., Ser. C, 277(21), 1081-1083 (1973) (Experimental, Crys. Stmcture, 6)... 

A, 3 Temperature at which austenite and cementite coexist for steel with hypereutectoid composition and above which only austenite exists and below which only cementite exists. 

Iroii alloys with caibon content of less than 2% are known as steels and those with more than 2% are known as cast iron. Steels are further divided into those with carbon content of more than 0.8%, called hypereutectoid steels, and those with caihon content of less than 0.8%, known as hypoeutectoid steels. Most steels used in pressure vessel applications have a carbon content of less than 0.4%. Steels with carbon content of over 0.4% are very brittle and hard to weld. 

The temperature at which transformation of ferrite to austenite is completed during heating The temperature at which austenite transforms to S ferrite during heating The temperatures of phase changes at equilibrium In hypereutectoid steel... 

The fee y-phase of Fe (austenite) is stable above 727°C but there is a euctectoid reaction at 0.76 wt% C, where the austenite transforms into bcc a-ferrite and FesC cementite. The lamellar structure of alternating layers of ferrite and cementite is called pearlite. Hypoeu-tectoid steels (steels with lower carbon content than the eutectoid composition) are called low carbon steels and are weaker but more ductile than the high carbon hypereutectoid steels. The strength and ductility of steels ako depends on the lamellar spacing of the pearlite, which can be controlled by time and temperature profiles during cooling. 

Figore 9.32 Schematic representations of the microstructures for an iron-carbon alloy of hypereutectoid composition Q (containing between 0.76 and 2.14 wt% C) as it is cooled from within the austenite-phase region to below the eutectoid temperature. 

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