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
146 8·Iron and Steel (d)Lower or fine bainite is even harder (but less ductile)than coarse bainite due to its larger number of small cementite par- ticles.This microconstituent forms by reducing the quenching temperature even further,as indicated by "d"in Figure 8.4.The long times for heat treatment to complete the transformation are, however,often prohibitive for proceeding on this avenue,par- ticularly since other treatments can be applied to achieve simi- lar results;see below. (e)If austenitic,eutectoid steel is very rapidly quenched to room temperature(to prevent the formation of pearlite or bai- nite),a very hard and brittle,body-centered tetragonal (BCT) structure,called martensite,is instantly formed.No diffusion of atoms is involved.Instead,a slight shift of the location of atoms takes place.This allows the transformation from FCC to BCT to occur with nearly the velocity of sound.Indeed,needle-shaped microconstituents can be observed in the electron microscope to shoot out from the matrix.The reason for the increased hard- ness and the greatly reduced ductility is that BCT has no close- packed planes on which dislocations can easily move.Another cause is the large c/a ratio,which distorts the lattice and leads to substantial twinning.The hardness of steel martensite in- creases with rising carbon content,leveling off near 0.6%C. When austenitic steel is quenched to temperatures between the Ms and Mr temperatures (see Figure 8.4)only a portion of the austenite is transformed into martensite.Specifically,the amount of martensite,and thus the hardness,increase with decreasing temperature.Prolonging the annealing time at a given tempera- ture does not change the amount of martensite,as can be de- duced from Figure 8.4. The quenching medium has an influence on the martensitic transformation.It affects the rate at which a work piece is cooled from austenite to below the Mf temperature without allowing pearlite or bainite microconstituents to form.As an example,the cooling rate in brine is five times faster than in oil and two times faster than in plain water.The quench rate can be even doubled by stirring the medium.(The severity of a quench is determined by the H-coefficient of the medium.) Further,the shape and size of a piece to be heat-treated influ- ences the rate of transformation and thus its hardness.For ex- ample,if a thick part is quenched from austenite,the surface is affected more severely than the interior.This may cause a more complete martensitic transformation on the outside compared to the interior,and may thus result in quench cracks due to resid- ual stresses.Moreover,a large mass as a whole may not be ef- fectively quenched because of a lack of efficient heat removal. Martensitic steel is essentially too brittle to be used for most
146 8 • Iron and Steel (d) Lower or fine bainite is even harder (but less ductile) than coarse bainite due to its larger number of small cementite particles. This microconstituent forms by reducing the quenching temperature even further, as indicated by “d” in Figure 8.4. The long times for heat treatment to complete the transformation are, however, often prohibitive for proceeding on this avenue, particularly since other treatments can be applied to achieve similar results; see below. (e) If austenitic, eutectoid steel is very rapidly quenched to room temperature (to prevent the formation of pearlite or bainite), a very hard and brittle, body-centered tetragonal (BCT) structure, called martensite, is instantly formed. No diffusion of atoms is involved. Instead, a slight shift of the location of atoms takes place. This allows the transformation from FCC to BCT to occur with nearly the velocity of sound. Indeed, needle-shaped microconstituents can be observed in the electron microscope to shoot out from the matrix. The reason for the increased hardness and the greatly reduced ductility is that BCT has no closepacked planes on which dislocations can easily move. Another cause is the large c/a ratio, which distorts the lattice and leads to substantial twinning. The hardness of steel martensite increases with rising carbon content, leveling off near 0.6% C. When austenitic steel is quenched to temperatures between the Ms and Mf temperatures (see Figure 8.4) only a portion of the austenite is transformed into martensite. Specifically, the amount of martensite, and thus the hardness, increase with decreasing temperature. Prolonging the annealing time at a given temperature does not change the amount of martensite, as can be deduced from Figure 8.4. The quenching medium has an influence on the martensitic transformation. It affects the rate at which a work piece is cooled from austenite to below the Mf temperature without allowing pearlite or bainite microconstituents to form. As an example, the cooling rate in brine is five times faster than in oil and two times faster than in plain water. The quench rate can be even doubled by stirring the medium. (The severity of a quench is determined by the H-coefficient of the medium.) Further, the shape and size of a piece to be heat-treated influences the rate of transformation and thus its hardness. For example, if a thick part is quenched from austenite, the surface is affected more severely than the interior. This may cause a more complete martensitic transformation on the outside compared to the interior, and may thus result in quench cracks due to residual stresses. Moreover, a large mass as a whole may not be effectively quenched because of a lack of efficient heat removal. Martensitic steel is essentially too brittle to be used for most
8.3·Heat Treatments 147 engineering applications.Thus,a subsequent heat treatment, called tempering,needs to be applied.This causes the precipita- tion of equilibrium ferrite in which very fine cementite particles are dispersed.The result is an increase in ductility at the expense of hardness.Tempering between 450 and 600C is typical.Con- siderable skill and experience are involved when performing quenching and tempering.Because of the importance of these heat treatments,many metal shops have wall charts that provide guidance for the proper procedures which allow one to obtain specific mechanical properties. It should be noted in passing that diffusionless phase transfor- mations (i.e.,martensitic transformations)are also observed in other alloys or substances.Among them are martensitic transfor- mations in certain copper-zinc alloys,in cobalt,or many poly- morphic ceramic materials.Some alloys (such as NiTi,Cu-Al-Ni, Au-Cd,Fe-Mn-Si,Mn-Cu,Ag-Cd,or Cu-Zn-Al)which have under- gone a thermo-mechanical treatment that yields a martensitic structure possess a shape memory effect.After deformation of these alloys,the original shape can be restored by a proper heat treat- ment which returns the stress-induced martensite into the origi- nal austenite.Some materials also change their shape upon re- cooling.They are called two-way shape memory alloys in contrast to one-way alloys which change only when heated.Only those ma- terials that exert a significant force upon shape change are of commercial interest,such as Ni-Ti and the copper-based alloys. (An Italian entrepreneur exploited this effect to create a smart shirt that automatically rolls up its sleeves at elevated temperatures and that can be smoothed out by activating a hair dryer. The TTT diagrams for noneutectoid steels need to be modi- fied somewhat to allow for the austenite-containing two-phase regions (i.e.,y+a or y+Fe3C);see Figure 8.1.Let us consider, for example,a hypoeutectoid steel.To accommodate for the transformation from y to (a+y)and from there to a pearlite, etc.,an additional line has to be inserted beginning at the nose of the TTT diagram and reaching to higher temperatures.It rep- resents the ferrite start temperature F;see Figure 8.5.Let us con- sider again a few specific cases. (a)Quenching a hypoeutectoid steel from above Af (i.e.,the highest temperature at which ferrite can form)to a temperature between Ar and the eutectoid temperature results in a mixture of y and primary a;see Figures 8.1 and 8.5.Once formed,the amount of ferrite does not change any further when extending the annealing time;see "a"in Figure 8.5. (b)Austenitizing and quenching a hypoeutectoid steel to a tem- perature slightly above the nose in a TTT diagram yields rela- tively quickly a mixture of y and primary a.The remaining
8.3 • Heat Treatments 147 engineering applications. Thus, a subsequent heat treatment, called tempering, needs to be applied. This causes the precipitation of equilibrium ferrite in which very fine cementite particles are dispersed. The result is an increase in ductility at the expense of hardness. Tempering between 450 and 600°C is typical. Considerable skill and experience are involved when performing quenching and tempering. Because of the importance of these heat treatments, many metal shops have wall charts that provide guidance for the proper procedures which allow one to obtain specific mechanical properties. It should be noted in passing that diffusionless phase transformations (i.e., martensitic transformations) are also observed in other alloys or substances. Among them are martensitic transformations in certain copper–zinc alloys, in cobalt, or many polymorphic ceramic materials. Some alloys (such as NiTi, Cu-Al-Ni, Au-Cd, Fe-Mn-Si, Mn-Cu, Ag-Cd, or Cu-Zn-Al) which have undergone a thermo-mechanical treatment that yields a martensitic structure possess a shape memory effect. After deformation of these alloys, the original shape can be restored by a proper heat treatment which returns the stress-induced martensite into the original austenite. Some materials also change their shape upon recooling. They are called two-way shape memory alloys in contrast to one-way alloys which change only when heated. Only those materials that exert a significant force upon shape change are of commercial interest, such as Ni-Ti and the copper-based alloys. (An Italian entrepreneur exploited this effect to create a smart shirt that automatically rolls up its sleeves at elevated temperatures and that can be smoothed out by activating a hair dryer.) The TTT diagrams for noneutectoid steels need to be modified somewhat to allow for the austenite-containing two-phase regions (i.e., or Fe3C); see Figure 8.1. Let us consider, for example, a hypoeutectoid steel. To accommodate for the transformation from to ( ) and from there to pearlite, etc., an additional line has to be inserted beginning at the nose of the TTT diagram and reaching to higher temperatures. It represents the ferrite start temperature Fs; see Figure 8.5. Let us consider again a few specific cases. (a) Quenching a hypoeutectoid steel from above Af (i.e., the highest temperature at which ferrite can form) to a temperature between Af and the eutectoid temperature results in a mixture of and primary ; see Figures 8.1 and 8.5. Once formed, the amount of ferrite does not change any further when extending the annealing time; see “a” in Figure 8.5. (b) Austenitizing and quenching a hypoeutectoid steel to a temperature slightly above the nose in a TTT diagram yields relatively quickly a mixture of and primary . The remaining
148 8·Iron and Steel Hardness a+y -P---Eut.Temp. b a+y+Pearlite 一P o+Pearlite FIGURE 8.5.Schematic repre- y+Bainite B sentation of a TTT diagram C- 令* Bainite for a hypoeutectoid plain M carbon steel.Ar is the highest y+Martensite temperature at which ferrite can form;see Figure 8.1.Fs M Martensite 1 is the ferrite start tempera- 1sec 1 hr 1 day ture. log time austenite eventually transforms into pearlite upon some further isothermal annealing.The transformation into pearlite is com- pleted at the pearlite finish temperature,Pr;see "b"in Figure 8.5. (c)Finally,quenching and holding the same steel to a tem- perature just below the nose yields,after crossing the Brline,only bainite,which has,in contrast to pearlite,no fixed composition. Similar TTT diagrams as in Figure 8.5 are found for hyper- eutectoid plain carbon steels.The differences are an Fe3C +y field (instead of the a +y field)and a cementite start curve,Cs (instead of the ferrite start line,Fs). The martensitic transformations for hypo-and hypereutectoid steels behave quite similar as outlined above.However,the M and Mr temperatures depend on the carbon content,as shown in Figure 8.6.Unfortunately,the M temperature cannot be clearly determined by visual inspection only.Other techniques,such as resistivity or X-ray diffraction measurements,need to be applied to obtain a reliable value.Further,the martensitic transforma- 100 T 500 (C) 400 Retained M 300 Retainedy 50 FIGURE 8.6.Schematic representa- 200 tion of the influence of carbon con- M austenite(Vol. 100 centration on the Ms and Mf tem- 8 peratures in steel and on the --一了1 amount of retained austenite(given 0.20.40.6 0.81.01.2 1.4 in volume percent). Mass carbon
148 8 • Iron and Steel austenite eventually transforms into pearlite upon some further isothermal annealing. The transformation into pearlite is completed at the pearlite finish temperature, Pf; see “b” in Figure 8.5. (c) Finally, quenching and holding the same steel to a temperature just below the nose yields, after crossing the Bf line, only bainite, which has, in contrast to pearlite, no fixed composition. Similar TTT diagrams as in Figure 8.5 are found for hypereutectoid plain carbon steels. The differences are an Fe3C field (instead of the field) and a cementite start curve, Cs (instead of the ferrite start line, Fs). The martensitic transformations for hypo- and hypereutectoid steels behave quite similar as outlined above. However, the Ms and Mf temperatures depend on the carbon content, as shown in Figure 8.6. Unfortunately, the Mf temperature cannot be clearly determined by visual inspection only. Other techniques, such as resistivity or X-ray diffraction measurements, need to be applied to obtain a reliable value. Further, the martensitic transformaT a b c Fs Af Ps Pf Bf Bs Ms Mf + Pearlite + Martensite + Bainite Bainite Martensite 1 sec 1 hr 1 day log time Hardness Eut. Temp. + + + Pearlite FIGURE 8.5. Schematic representation of a TTT diagram for a hypoeutectoid plain carbon steel. Af is the highest temperature at which ferrite can form; see Figure 8.1. Fs is the ferrite start temperature. Ms Mf 500 400 300 200 100 0.2 0.4 0.6 0.8 1.0 1.2 1.4 100 50 Retained T (C) Mass % carbon Retained austenite (Vol. %) FIGURE 8.6. Schematic representation of the influence of carbon concentration on the Ms and Mf temperatures in steel and on the amount of retained austenite (given in volume percent)
8.4·Alloyed Steels 149 tion is seldom entirely completed even at very low temperatures. This results in some retained austenite,as indicated in Figure 8.6.Retained austenite can be of concern after tempering in that it may lead to some brittleness due to its transformation to martensite upon tempering and cooling to room temperature. 8.4·Alloyed Steels Alloying elements,such as Mn,Si,Ni,Cu,Mo,and V,are often added to steel in various quantities (often well under 1%)in or- der to favorably alter its properties.These additional constituents generally shift the nose in a TTT diagram to longer times.As a consequence,no pearlite or bainite is inadvertently formed upon quenching,and the martensitic transformation can be brought to completion even in large work pieces despite the fact that the cooling rate might have been relatively slow.This feature is re- ferred to as hardenability and expresses the ease with which martensite is formed upon quenching. A second effect that alloying elements provide is a shift of the eutectoid composition to lower carbon concentrations.One mass of molybdenum,for example,reduces the eutectoid composi- tion of plain carbon steel from 0.77 to 0.4%C.This has some in- fluence on the primary microconstituent which is formed upon cooling.Specifically,a reduction of the eutectoid composition might lead to primary cementite instead of primary ferrite in a given plain-carbon-steel. Third,some constituents(such as Mn and Ni)considerably de- crease the eutectoid temperature,TE,and the temperatures at which ferrite and cementite are first formed (Figure 8.1).For ex- ample,5%Ni decreases Te of plain carbon steel by about 70C. Other elements,such as Cr,W,and Mo,increase the eutectoid temperature instead.These changes have to be considered when austenitizing treatments are conducted.Fourth,the martensitic start and finish temperatures are reduced by alloying elements. Moreover,the entire TTT diagram might undergo some varia- tions.Fifth,the time needed for tempering is generally dimin- ished by alloying. Stainless Steel Sixth,and nearly most importantly,appreciable additions of chromium to iron (at least 12%)yield corrosion-resistant steels called stainless steels.They derive this property from a protective layer of chromium oxide which forms on the free surface.How- ever,with rising Cr concentrations the amount of austenite de- creases in either binary Fe-Cr or some chromium-containing iron-carbon steels which cause the ferrite to be the dominant mi-
8.4 • Alloyed Steels 149 tion is seldom entirely completed even at very low temperatures. This results in some retained austenite, as indicated in Figure 8.6. Retained austenite can be of concern after tempering in that it may lead to some brittleness due to its transformation to martensite upon tempering and cooling to room temperature. Alloying elements, such as Mn, Si, Ni, Cu, Mo, and V, are often added to steel in various quantities (often well under 1%) in order to favorably alter its properties. These additional constituents generally shift the nose in a TTT diagram to longer times. As a consequence, no pearlite or bainite is inadvertently formed upon quenching, and the martensitic transformation can be brought to completion even in large work pieces despite the fact that the cooling rate might have been relatively slow. This feature is referred to as hardenability and expresses the ease with which martensite is formed upon quenching. A second effect that alloying elements provide is a shift of the eutectoid composition to lower carbon concentrations. One mass % of molybdenum, for example, reduces the eutectoid composition of plain carbon steel from 0.77 to 0.4% C. This has some influence on the primary microconstituent which is formed upon cooling. Specifically, a reduction of the eutectoid composition might lead to primary cementite instead of primary ferrite in a given plain-carbon–steel. Third, some constituents (such as Mn and Ni) considerably decrease the eutectoid temperature, TE, and the temperatures at which ferrite and cementite are first formed (Figure 8.1). For example, 5% Ni decreases TE of plain carbon steel by about 70°C. Other elements, such as Cr, W, and Mo, increase the eutectoid temperature instead. These changes have to be considered when austenitizing treatments are conducted. Fourth, the martensitic start and finish temperatures are reduced by alloying elements. Moreover, the entire TTT diagram might undergo some variations. Fifth, the time needed for tempering is generally diminished by alloying. Sixth, and nearly most importantly, appreciable additions of chromium to iron (at least 12%) yield corrosion-resistant steels called stainless steels. They derive this property from a protective layer of chromium oxide which forms on the free surface. However, with rising Cr concentrations the amount of austenite decreases in either binary Fe–Cr or some chromium-containing iron–carbon steels which cause the ferrite to be the dominant miStainless Steel 8.4 • Alloyed Steels
150 8·Iron and Steel croconstituent.The strengthening mechanism in ferrite stainless steels (up to 30%Cr and less than 0.12%C)is then by solid-solu- tion hardening.Still,martensitic stainless steels (for knives or ball bearings)and austenitic stainless steels (containing Ni and Cr)are also frequently manufactured and used if the high price can justify this.For details we refer again to the above-mentioned handbooks. 8.5●Cast Irons Cast iron is the principal material which the old Chinese manu- factured 3000 years ago as a result of increasing the carbon con- tent during the smelting of iron ore.This enabled them to reduce the melting temperature possibly to as low as 1150C,thus gain- ing liquid metal that could be effectively cast(see Chapter 7).Fur- ther,raw cast iron(today called pig iron)is the material that flows out of a blast furnace,as likewise explained in Chapter 7.Specif- ically,iron having a carbon content above 2.11 mass percent is generally referred to as cast iron even though carbon concentra- tions between 2.5 and 4.5%are more typical for practical appli- cations.(See in this context the carbon concentrations of common iron-containing materials shown on the bottom of Figure 8.7.) 1600 T (C) 1400 L+ Graphite 1200 Y+L 1154° (Austenite) 2.08 4.26 1000 Y+Graphite 800 738° 0.68 600 a+Graphite FIGURE 8.7.The equilibrium iron-car- (Ferrite) bon phase diagram showing graphite 400 as the stable phase.Compare to Figure Fe 1 LL人o0 234 100 8.1,in which the metastable inter- (Graphite) metallic phase Fe3C is prominently in- Composition (mass C) cluded.The dashed line marks the sol- ubility of graphite in liquid Fe.Note Tool steels Cast iron the carbon contents of common iron- Alloy steels containing materials,which are shown Construction steels below the phase diagram. Soft ferromagnets
150 8 • Iron and Steel croconstituent. The strengthening mechanism in ferrite stainless steels (up to 30% Cr and less than 0.12% C) is then by solid-solution hardening. Still, martensitic stainless steels (for knives or ball bearings) and austenitic stainless steels (containing Ni and Cr) are also frequently manufactured and used if the high price can justify this. For details we refer again to the above-mentioned handbooks. Cast iron is the principal material which the old Chinese manufactured 3000 years ago as a result of increasing the carbon content during the smelting of iron ore. This enabled them to reduce the melting temperature possibly to as low as 1150°C, thus gaining liquid metal that could be effectively cast (see Chapter 7). Further, raw cast iron (today called pig iron) is the material that flows out of a blast furnace, as likewise explained in Chapter 7. Specifically, iron having a carbon content above 2.11 mass percent is generally referred to as cast iron even though carbon concentrations between 2.5 and 4.5% are more typical for practical applications. (See in this context the carbon concentrations of common iron-containing materials shown on the bottom of Figure 8.7.) 8.5 • Cast Irons 738 1154 + L + Graphite L + Graphite + Graphite 1600 1400 1200 1000 800 600 400 (Ferrite) (Austenite) 0.68 Fe 1 2 3 4 90 100 (Graphite) L 2.08 4.26 Composition (mass % C) Tool steels Cast iron Alloy steels Construction steels Soft ferromagnets T (C) FIGURE 8.7. The equilibrium iron–carbon phase diagram showing graphite as the stable phase. Compare to Figure 8.1, in which the metastable intermetallic phase Fe3C is prominently included. The dashed line marks the solubility of graphite in liquid Fe. Note the carbon contents of common ironcontaining materials, which are shown below the phase diagram
