8 lron and Steel 8.1.Phases and Microconstituents A deeper understanding of the diverse properties of iron and var- ious steels is gained by inspecting the iron-carbon phase dia- gram.Actually,for the present purposes only the portion up to 6.67%C is of interest;see Figure 8.1. The various phases are known by specific names,such as the hard and brittle intermetallic phase Fe3C (6.67%C),which is called iron carbide or cementite;the FCC,non(ferro)magnetic,y- phase named austenite,and the BCC a-phase known as ferrite. Further,a high-temperature BCC phase called 8ferrite and the eutectoid phase mixture (a+Fe3C)named pearlite.Not enough. Two more microconstituents known as bainite and martensite, respectively,exist which are formed by specific heat treatments. The latter will be discussed in Section 8.3.These names came into existence either because of their properties or appearance under the microscope (such as cementite and pearlite)or to com- memorate certain scientists who devoted their lives to the study of these microconstituents (such as Sir W.C.Roberts-Austen, English Metallurgist,1843-1902;A.Martens,German Engineer, 1850-1914;and E.C.Bain,American Metallurgist). Several three-phase reactions are evident from Figure 8.1.The eutectic reaction at Fe-4.3%C lowers the melting temperature of iron to 1148C,as mentioned in Chapter 7.Further,a eutec- toid reaction(y-a+Fe3C)at 727C and a peritectic reaction at 1495C need to be emphasized.Finally,two allotropic transfor- mations during cooling from 8-ferrite to austenite and from there to ferrite take place.The a-,y-,and 6-phases consist of solid so- lutions in which the carbon is interstitially dissolved in iron
8 A deeper understanding of the diverse properties of iron and various steels is gained by inspecting the iron–carbon phase diagram. Actually, for the present purposes only the portion up to 6.67% C is of interest; see Figure 8.1. The various phases are known by specific names, such as the hard and brittle intermetallic phase Fe3C (6.67% C), which is called iron carbide or cementite; the FCC, non(ferro)magnetic, - phase named austenite, and the BCC -phase known as ferrite. Further, a high-temperature BCC phase called -ferrite and the eutectoid phase mixture ( Fe3C) named pearlite. Not enough. Two more microconstituents known as bainite and martensite, respectively, exist which are formed by specific heat treatments. The latter will be discussed in Section 8.3. These names came into existence either because of their properties or appearance under the microscope (such as cementite and pearlite) or to commemorate certain scientists who devoted their lives to the study of these microconstituents (such as Sir W.C. Roberts–Austen, English Metallurgist, 1843–1902; A. Martens, German Engineer, 1850–1914; and E.C. Bain, American Metallurgist). Several three-phase reactions are evident from Figure 8.1. The eutectic reaction at Fe–4.3% C lowers the melting temperature of iron to 1148°C, as mentioned in Chapter 7. Further, a eutectoid reaction ( Fe3C) at 727°C and a peritectic reaction at 1495°C need to be emphasized. Finally, two allotropic transformations during cooling from -ferrite to austenite and from there to ferrite take place. The -, -, and -phases consist of solid solutions in which the carbon is interstitially dissolved in iron. Iron and Steel 8.1 • Phases and Microconstituents
142 8·Iron and Steel 1538 1500 1495° 8 L+Fe C T y+L (c) (Austenite) 2.11 4.3 1148° 1000 Ar Y+FegC (Ferrite) 0.77 727° 6.67 0.0218 500 FIGURE 8.1.Portion of the iron-car- a+FeC bon phase diagram.(Actually,this (Pearlite) section is known by the name Fe- Fe3C phase diagram.)Af is the highest temperature at which fer- rite can form.As before,the mass Fe 234 FeC percent of solute addition is used (Cementite) (formerly called weight percent). Composition (mass C) 8.2.Hardening Mechanisms Eutectoid Several hardening mechanisms take place.First,the a-,y-,and Steel 8-phases are solid-solution strengthened as discussed in Section 5.1.Second,pearlite involves dispersion strengthening caused by the interaction of hard and brittle cementite with the relatively soft and ductile ferrite(Section 5.4).More specifically,the a-and Fe3C phases grow in the form of thin plates or lamellae,simi- larly as in eutectic reactions and as schematically depicted in Fig- ure 8.2.However,the plates are much thinner for pearlite than in a eutectic structure,which is necessitated by the shorter dif- fusion lengths encountered at lower temperatures.In short,the primary reason why eutectoid steel (iron with 0.77 mass C) is harder than pure iron or ferrite is because of the dispersion of hard cementite in soft ferrite in the form of plate-shaped pearlite, as shown in Figure 8.2. Hypoeutectoid The above statements need some fine tuning.For hypoeutectoid Steel compositions(below 0.77%C;see Section 5.2.2)the ferrite is the primary and continuous phase which,upon cooling from the y field,nucleates and grows at the grain boundaries of austenite. In other words,the a-phase quasi-coats the grain boundaries of austenite.Below 727C,the pearlite finally precipitates in the re- maining y-phase by a eutectoid reaction.It is thus surrounded
142 8 • Iron and Steel Several hardening mechanisms take place. First, the -, -, and -phases are solid-solution strengthened as discussed in Section 5.1. Second, pearlite involves dispersion strengthening caused by the interaction of hard and brittle cementite with the relatively soft and ductile ferrite (Section 5.4). More specifically, the - and Fe3C phases grow in the form of thin plates or lamellae, similarly as in eutectic reactions and as schematically depicted in Figure 8.2. However, the plates are much thinner for pearlite than in a eutectic structure, which is necessitated by the shorter diffusion lengths encountered at lower temperatures. In short, the primary reason why eutectoid steel (iron with 0.77 mass % C) is harder than pure iron or ferrite is because of the dispersion of hard cementite in soft ferrite in the form of plate-shaped pearlite, as shown in Figure 8.2. The above statements need some fine tuning. For hypoeutectoid compositions (below 0.77% C; see Section 5.2.2) the ferrite is the primary and continuous phase which, upon cooling from the - field, nucleates and grows at the grain boundaries of austenite. In other words, the -phase quasi-coats the grain boundaries of austenite. Below 727°C, the pearlite finally precipitates in the remaining -phase by a eutectoid reaction. It is thus surrounded Eutectoid Steel Hypoeutectoid Steel 8.2 • Hardening Mechanisms 1495 727 1148 1538 + L + Fe3C L + Fe3C + Fe3C (Pearlite) 1500 1000 500 (Ferrite) (Austenite) 0.77 Fe 1 2 3 4 5 Fe3C (Cementite) 0.0218 6.67 4.3 L 2.11 Af Composition (mass % C) T (C) FIGURE 8.1. Portion of the iron–carbon phase diagram. (Actually, this section is known by the name FeFe3C phase diagram.) Af is the highest temperature at which ferrite can form. As before, the mass percent of solute addition is used (formerly called weight percent)
8.2.Hardening Mechanisms 143 FIGURE 8.2.Schematic representation of a lamellar (plate-like)microstructure of steel called pearlite obtained by cooling a eutectoid iron-carbon alloy from austenite to below 727C.Pearlite is a mixture of a and Fe3C.Compare to Fig- ure 5.9. by primary a,as schematically depicted in Figure 8.3(a).The re- sulting steel is hard but still ductile due to the continuous and soft ferrite.The strength of hypoeutectoid steels initially in- creases with rising carbon content,but eventually levels off near the eutectoid composition. There are some more mechanisms that may further increase the hardness of hypoeutectoid steel.We learned in Section 5.3 that a large number of small particles pose an enhanced chance for blocking the moving dislocations.This causes an increase in strength compared to the action of only a few but large particles. The same is true for the number and size of pearlite domains or "colonies".The number of pearlite colonies can be increased by providing small austenitic grains to begin with on whose bound- aries the pearlite eventually nucleates.Specifically,the hardness a-coated grain boundaries pearlite cementite pearlite IP (a) (b) FiGURE 8.3.Schematic representation of(a)a hypoeutectoid microstruc- ture of steel at room temperature containing primary a and pearlite mi- croconstituents (the latter consisting of two phases,i.e.,a and Fe3C);(b) a hypereutectoid microstructure of steel.Note that the primary phases in both cases have "coated"the former grain boundaries of the austenite
8.2 • Hardening Mechanisms 143 by primary , as schematically depicted in Figure 8.3(a). The resulting steel is hard but still ductile due to the continuous and soft ferrite. The strength of hypoeutectoid steels initially increases with rising carbon content, but eventually levels off near the eutectoid composition. There are some more mechanisms that may further increase the hardness of hypoeutectoid steel. We learned in Section 5.3 that a large number of small particles pose an enhanced chance for blocking the moving dislocations. This causes an increase in strength compared to the action of only a few but large particles. The same is true for the number and size of pearlite domains or “colonies”. The number of pearlite colonies can be increased by providing small austenitic grains to begin with on whose boundaries the pearlite eventually nucleates. Specifically, the hardness Fe3C FIGURE 8.2. Schematic representation of a lamellar (plate-like) microstructure of steel called pearlite obtained by cooling a eutectoid iron–carbon alloy from austenite to below 727°C. Pearlite is a mixture of and Fe3C. Compare to Figure 5.9. cementite pearlite (b) -coated grain boundaries pearlite (a) FIGURE 8.3. Schematic representation of (a) a hypoeutectoid microstructure of steel at room temperature containing primary and pearlite microconstituents (the latter consisting of two phases, i.e., and Fe3C); (b) a hypereutectoid microstructure of steel. Note that the primary phases in both cases have “coated” the former grain boundaries of the austenite
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 begins
144 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 austenitizing 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 (Figure 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 annealing, 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 cementite 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 ductility 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 annealing, 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–temperature–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 (indicated by the arrow “a” in Figure 8.4), only little undercooling of the austenite takes place. The driving force for ferrite and cementite nucleation is therefore small. As a consequence, the time span is relatively long until ferrite and cementite nuclei start to form at the grain boundaries of austenite. The time at which the pearlite begins Hypereutectoid Steel TTT Diagrams 8.3 • Heat Treatments
8.3·Heat Treatments 145 T Hardness 727C Austenite --A Pearlite B coarse FIGURE 8.4.Schematic repre- Bainite sentation of a TTT diagram for eutectoid steel.The an- 220C fine nealing temperatures (a) through (e)refer to specific 下M cases as described in the Martensite text.Note that the hardness 1 sec 1 hr 1 day scale on the right points log time downward. to nucleate is called the pearlite start time,or abbreviated Ps.Upon holding the work piece further at the same temperature,the nuclei grow in size until all austenite has been eventually transformed into ferrite and cementite platelets,that is,into pearlite.This has oc- curred at the pearlite finish time,Pf.Since the transformation tem- perature is quite high,the diffusion is fast and the diffusion dis- tances may be long.For this reason,and because the density of the nuclei was small,the pearlite is coarse and the hardness of the work piece is relatively low;see Figure 8.4.In summary:A small tem- perature difference during quenching causes little undercooling which yields only a small number of nuclei.As a consequence the pearlite is coarse and the hardness is relatively small. (b)The situation is somewhat different if austenitic steel is quenched to a lower temperature,as indicated by"b"in Figure 8.4.The undercooling is now larger,which causes a shorter nu- cleation time.Moreover,one encounters shorter diffusion dis- tances and a larger number of nuclei due to the lower tempera- ture.As a consequence,the pearlite is finer and thus harder.The time until the entire transformation is completed is relatively short,as can be deduced from Figure 8.4. (c)If the temperature to which austenitic,eutectoid steel is quenched is reduced even further,the interplay between an en- hanced tendency toward nucleation and a reduced drive for dif- fusion causes the cementite to precipitate in microscopically small, elongated particles(needles)that are imbedded in a ferrite matrix. This new microconstituent has been named bainite,and the re- spective times for the start and finish of the transformation have been designated as Bs and Bf.Heat treatments just below the "nose" of the TTT curves (e.g.,"c"in Figure 8.4)produce upper or coarse bainite.Bainite is harder than pearlite,and the presence of a fer- rite matrix causes the steel to be ductile and tough
8.3 • Heat Treatments 145 to nucleate is called the pearlite start time, or abbreviated Ps. Upon holding the work piece further at the same temperature, the nuclei grow in size until all austenite has been eventually transformed into ferrite and cementite platelets, that is, into pearlite. This has occurred at the pearlite finish time, Pf. Since the transformation temperature is quite high, the diffusion is fast and the diffusion distances may be long. For this reason, and because the density of the nuclei was small, the pearlite is coarse and the hardness of the work piece is relatively low; see Figure 8.4. In summary: A small temperature difference during quenching causes little undercooling which yields only a small number of nuclei. As a consequence the pearlite is coarse and the hardness is relatively small. (b) The situation is somewhat different if austenitic steel is quenched to a lower temperature, as indicated by “b” in Figure 8.4. The undercooling is now larger, which causes a shorter nucleation time. Moreover, one encounters shorter diffusion distances and a larger number of nuclei due to the lower temperature. As a consequence, the pearlite is finer and thus harder. The time until the entire transformation is completed is relatively short, as can be deduced from Figure 8.4. (c) If the temperature to which austenitic, eutectoid steel is quenched is reduced even further, the interplay between an enhanced tendency toward nucleation and a reduced drive for diffusion causes the cementite to precipitate in microscopically small, elongated particles (needles) that are imbedded in a ferrite matrix. This new microconstituent has been named bainite, and the respective times for the start and finish of the transformation have been designated as Bs and Bf. Heat treatments just below the “nose” of the TTT curves (e.g., “c” in Figure 8.4) produce upper or coarse bainite. Bainite is harder than pearlite, and the presence of a ferrite matrix causes the steel to be ductile and tough. T 727C 220C Austenite a b c d e Ps Af Pf Bf Bs Ms Mf Pearlite coarse Bainite fine Martensite 1 sec 1 hr 1 day log time Hardness FIGURE 8.4. Schematic representation of a TTT diagram for eutectoid steel. The annealing temperatures (a) through (e) refer to specific cases as described in the text. Note that the hardness scale on the right points downward