144 8·Iron and Steel is increased by annealing the steel in the y-field slightly (e.g., 25C)above Ar to prevent grain growth.This is called an austen- itizing treatment.Alternatively,grain refiners can be used.Still another technique for increasing the strength is to produce a finer pearlite,that is,by reducing the size of the individual plates(Fig- ure 8.2).This can be accomplished by increasing the cooling rate (called normalizing),for example,by air-cooling the work piece. (On the other hand,slow cooling in a furnace,called full an- nealing,yields coarse pearlite,that is,steel with less strength.) Hypereutec- The situation is somewhat different for hypereutectoid steels toid Steel (iron with carbon concentrations between 0.77 and 2.11%C).In this case,the primary constituent is the hard and brittle cemen- tite which nucleates on the grain boundaries of austenite upon cooling.These cementite nuclei grow and eventually join each other,thus forming a continuous Fe3C microconstituent.Upon further cooling below 727C,the pearlite precipitates out of the remaining y microconstituent.This results in pearlite particles (colonies)that are dispersed in a continuous cementite;Figure 8.3 (b).The resulting steel is therefore brittle.To improve the ductil- ity one would have to anneal the steel for an extended time just slightly above or below the eutectoid temperature.This produces rounded discontinuous cementite due to the tendency of elongated constituents to reduce their surface energy(i.e.,their boundary area),thus eventually forming spherical particles.In other words, the extended heat treatment near the eutectoid temperature yields spherical Fe3C particles in a ferrite matrix.This process,called spheroidizing,improves the machinability of hypereutectoid steel. 8.3●Heat Treatments TTT Diagrams We learned in Chapter 7 that earlier civilizations had an intuitive knowledge of the fact that certain heat treatments such as an- nealing,quenching,and tempering would alter and improve the mechanical properties of steel.We shall now provide the scientific basis for understanding these treatments.For this a time-tem- perature-transformation (TTT)diagram needs to be presented as depicted in Figure 8.4.Let us consider a few specific cases. (a)By quenching a eutectoid steel from above 727C,that is,from the austenite region,to a temperature slightly below 727C (indi- cated by the arrow "a"in Figure 8.4),only little undercooling of the austenite takes place.The driving force for ferrite and cementite nu- cleation is therefore small.As a consequence,the time span is rel- atively long until ferrite and cementite nuclei start to form at the grain boundaries of austenite.The time at which the pearlite begins144 8 • Iron and Steel is increased by annealing the steel in the -field slightly (e.g., 25°C) above Af to prevent grain growth. This is called an austen￾itizing treatment. Alternatively, grain refiners can be used. Still another technique for increasing the strength is to produce a finer pearlite, that is, by reducing the size of the individual plates (Fig￾ure 8.2). This can be accomplished by increasing the cooling rate (called normalizing), for example, by air-cooling the work piece. (On the other hand, slow cooling in a furnace, called full an￾nealing, yields coarse pearlite, that is, steel with less strength.) The situation is somewhat different for hypereutectoid steels (iron with carbon concentrations between 0.77 and 2.11% C). In this case, the primary constituent is the hard and brittle cemen￾tite which nucleates on the grain boundaries of austenite upon cooling. These cementite nuclei grow and eventually join each other, thus forming a continuous Fe3C microconstituent. Upon further cooling below 727°C, the pearlite precipitates out of the remaining  microconstituent. This results in pearlite particles (colonies) that are dispersed in a continuous cementite; Figure 8.3 (b). The resulting steel is therefore brittle. To improve the ductil￾ity one would have to anneal the steel for an extended time just slightly above or below the eutectoid temperature. This produces rounded discontinuous cementite due to the tendency of elongated constituents to reduce their surface energy (i.e., their boundary area), thus eventually forming spherical particles. In other words, the extended heat treatment near the eutectoid temperature yields spherical Fe3C particles in a ferrite matrix. This process, called spheroidizing, improves the machinability of hypereutectoid steel. We learned in Chapter 7 that earlier civilizations had an intuitive knowledge of the fact that certain heat treatments such as an￾nealing, quenching, and tempering would alter and improve the mechanical properties of steel. We shall now provide the scientific basis for understanding these treatments. For this a time–tem￾perature–transformation (TTT) diagram needs to be presented as depicted in Figure 8.4. Let us consider a few specific cases. (a) By quenching a eutectoid steel from above 727°C, that is, from the austenite region, to a temperature slightly below 727°C (indi￾cated by the arrow “a” in Figure 8.4), only little undercooling of the austenite takes place. The driving force for ferrite and cementite nu￾cleation is therefore small. As a consequence, the time span is rel￾atively long until ferrite and cementite nuclei start to form at the grain boundaries of austenite. The time at which the pearlite begins Hypereutec￾toid Steel TTT Diagrams 8.3 • Heat Treatments