Steel hypoeutectoid

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. 

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. 

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. 11.4. Microstructures during the slow cooling of a hypoeutectoid steel from the hot working temperature. A3 is the standard labelling for the temperature at which or first appears, and A, is standard for the eutectoid temperature. Hypoeutectoid means that the carbon content is below that of a eutectoid steel (in the same sense that hypodermic means "under the skin" ).

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.

Hypoeutectoid steels, 23 275 Hypohalous acid, 73 100 Hypomagnesia, 75 414 Hypomanganate, 75 592 Hypoparathyroidism, 25 791 Hypophosphites, 19 55 Hypophosphorous acid, 79 20, 54 Hypotension, antianginal agents for,... 

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. 

The phases of austenite, pearlite, and ferrite are relatively soft hence, the observed high hardness of steels is obtained through processing of these materials. For instance, hypoeutectoid steel is first heated to form austenite and then slowly cooled so the pearlite/ferrite phases may be worked into desired shapes. If the material is... 

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.

Bow] Bowman, F.E., Partition of Molybdenum in Hypoeutectoid Iron-Carbon-Molybdenum Alloys , Trans. Am. Soc. Met., 36, 61-80 (1946) (Experimental, Phase Relations, 10) [1950Kra] Krainer, H., (in German), X-Ray Study of Carbides in W-, Mo-, and V-Steels , Arch. 

The phases of austenite, pearlite, and ferrite are relatively soft hence, the observed high hardness of steels is obtained through processing of these materials. For instance, hypoeutectoid steel may be heated to form austenite and then slowly cooled so the cementite/ferrite phases may be worked into desired shapes. If the material is re-austenized and quickly quenched to room temperature, a very hard phase known as martensite is formed. Some of the remaining pearlite and ferrite phases (if present) would still remain in the matrix. Hence, only when the steel has been heated to temperatures sufficiently high to convert all of the ferrite into austenite, that quenching will result in pure martensitic steel. It should be noted that the martensite phase does not appear in the above Fe-C phase diagram since it is a non-equilibrium phase. 

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. 

Relative amounts of both pearlite and proeutectoid FcjC microconstituents may be computed for hypereutectoid steel alloys in a manner analogous to that for hypoeutectoid materials the appropriate tie line extends between 0.76 and 6.70... 

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