# Solution: ΔG = a 3 ΔG v. + 6a 2 γ. Differentiation of this expression with respect to a is as. d ΔG da = d(a3 ΔG v. + d(6a2 γ) = 3a 2 ΔG v.

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1 14:440:407 ch10 Question 10.2: (a) Rewrite the expression for the total free energy change for nucleation (Equation 10.1) for the case of a cubic nucleus of edge length a (instead of a sphere of radius r). Now differentiate this expression with respect to a (per Equation 10.2) and solve for both the critical cube edge length, a*, and also ΔG*. (b) Is ΔG* greater for a cube or a sphere? Why? (a) This problem first asks that we rewrite the expression for the total free energy change for nucleation (analogous to Equation 10.1) for the case of a cubic nucleus of edge length a. The volume of such a cubic radius is a 3, whereas the total surface area is 6a 2 (since there are six faces each of which has an area of a 2 ). Thus, the expression for ΔG is as follows: ΔG = a 3 ΔG v + 6a 2 γ Differentiation of this expression with respect to a is as d ΔG da = d(a3 ΔG v ) + d(6a2 γ) da da = 3a 2 ΔG v + 12a γ If we set this expression equal to zero as 3a 2 ΔG v + 12a γ=0 and then solve for a (= a*), we have a *= 4 γ ΔG v Substitution of this expression for a in the above expression for ΔG yields an equation for ΔG* as ΔG *=(a*) 3 ΔG v + 6(a*) 2 γ

2 = 4 γ 3 ΔG ΔG v + 6 γ v 4 γ 2 ΔG v = 32 γ3 (ΔG v ) 2 γ (b) ΔG v for a cube i.e., 3 (32) (ΔG v ) 2 16 π γ is greater that for a sphere i.e., 3 3 (ΔG v ) 2 γ 3 (16.8) (ΔG v ) 2. The reason for this is that surface-to-volume ratio of a cube is greater than for a sphere. = Question 10.3: If copper (which has a melting point of 1085 C) homogeneously nucleates at 849 C, calculate the critical radius given values of J/m 3 and J/m 2, respectively, for the latent heat of fusion and the surface free energy. This problem states that copper homogeneously nucleates at 849 C, and that we are to calculate the critical radius given the latent heat of fusion ( J/m 3 ) and the surface free energy (0.200 J/m 2 ). Solution to this problem requires the utilization of Equation 10.6 as r * = 2 γt m 1 ΔH f T m T = (2)(0.200 J / m2 )( K) J/m C 849 C = m = 1.30 nm

3 Question 10.4: (a) For the solidification of iron, calculate the critical radius r* and the activation free energy ΔG* if nucleation is homogeneous. Values for the latent heat of fusion and surface free energy are J/m 3 and J/m 2, respectively. Use the supercooling value found in Table (b) Now calculate the number of atoms found in a nucleus of critical size. Assume a lattice parameter of nm for solid iron at its melting temperature. (a) This portion of the problem asks that we compute r* and ΔG* for the homogeneous nucleation of the solidification of Fe. First of all, Equation 10.6 is used to compute the critical radius. The melting temperature for iron, found inside the front cover is 1538 C; also values of ΔH f ( J/m 3 ) and γ (0.204 J/m 2 ) are given in the problem statement, and the supercooling value found in Table 10.1 is 295 C (or 295 K). Thus, from Equation 10.6 we have r * = 2γT m 1 ΔH f T m T = (2)(0.204 J / m2 )( K) J/m K = m = 1.35 nm For computation of the activation free energy, Equation 10.7 is employed. Thus ΔG * = 16 πγ3 T 2 m 1 3ΔH 2 f (T m T) 2 = (16)(π)(0.204 J /m2 ) 3 ( K) 2 1 (3)( J/m 3 ) 2 (295 K) 2 = J (b) In order to compute the number of atoms in a nucleus of critical size (assuming a spherical nucleus of radius r*), it is first necessary to determine the number of unit cells, which we then multiply by the number of atoms per unit cell. The number of unit cells found in this critical nucleus is just the ratio of critical nucleus and unit cell

4 volumes. Inasmuch as iron has the BCC crystal structure, its unit cell volume is just a 3 where a is the unit cell length (i.e., the lattice parameter); this value is nm, as cited in the problem statement. Therefore, the number of unit cells found in a radius of critical size is just # unit cells /particle = 4 πr *3 3 a 3 = 4 (π)(1.35 nm) 3 3 (0.292 nm) 3 = 414 unit cells Inasmuch as 2 atoms are associated with each BCC unit cell, the total number of atoms per critical nucleus is just (414 unit cells /critical nucleus)(2 atoms /unit cell) = 828 atoms /critical nucleus

5 Question 10.5: (a) Assume for the solidification of iron (Problem 10.4) that nucleation is homogeneous, and the number of stable nuclei is 10 6 nuclei per cubic meter. Calculate the critical radius and the number of stable nuclei that exist at the following degrees of supercooling: 200 K and 300 K. (b) What is significant about the magnitudes of these critical radii and the numbers of stable nuclei? (a) For this part of the problem we are asked to calculate the critical radius for the solidification of iron (per Problem 10.4), for 200 K and 300 K degrees of supercooling, and assuming that the there are 10 6 nuclei per meter cubed for homogeneous nucleation. In order to calculate the critical radii, we replace the T m T term in Equation 10.6 by the degree of supercooling (denoted as ΔT) cited in the problem. For 200 K supercooling, r* 2 γt 200 = m 1 ΔH f ΔT = (2)(0.204 J / m2 )( K) J/m K = m = 2.00 nm And, for 300 K supercooling, r* (2)(0.204 J / m 2 )( K) = J/m K = m = 1.33 nm In order to compute the number of stable nuclei that exist at 200 K and 300 K degrees of supercooling, it is necessary to use Equation However, we must first determine the value of K 1 in Equation 10.8, which in turn requires that we calculate ΔG* at the homogeneous nucleation temperature using Equation 10.7; this was done in Problem 10.4, and yielded a value of ΔG* = J. Now for the computation of K 1, using the value of n* for at the homogenous nucleation temperature (10 6 nuclei/m 3 ): K 1 = n * exp ΔG * kt

6 = exp 10 6 nuclei / m J ( J / atom K)(1538 C 295 C) = nuclei/m 3 Now for 200 K supercooling, it is first necessary to recalculate the value ΔG* of using Equation 10.7, where, again, the T m T term is replaced by the number of degrees of supercooling, denoted as ΔT, which in this case is 200 K. Thus ΔG* 200 = 16 πγ3 T 2 m 1 3ΔH 2 f (ΔT) 2 = (16)(π)(0.204 J / m2 ) 3 ( K) 2 1 (3)( J/m 3 ) 2 (200 K) 2 = J And, from Equation 10.8, the value of n* is n* 200 = K 1 exp ΔG * 200 kt = ( nuclei / m )exp 18 J ( J /atom K) (1538 K 200 K) = stable nuclei Now, for 300 K supercooling the value of ΔG* is equal to ΔG* (16)(π)(0.204 J /m = ) 3 ( K) 2 1 (3)( J/m 3 ) 2 (300 K) 2 = J

7 from which we compute the number of stable nuclei at 300 K of supercooling as * = K 1 exp ΔG * 300 kt n 300 n* = ( nuclei / m )exp 18 J ( J / atom K) (1538 K 300 K) = stable nuclei (b) Relative to critical radius, r* for 300 K supercooling is slightly smaller that for 200 K (1.33 nm versus 2.00 nm). [From Problem 10.4, the value of r* at the homogeneous nucleation temperature (295 K) was 1.35 nm.] More significant, however, are the values of n* at these two degrees of supercooling, which are dramatically different stable nuclei at ΔT = 200 K, versus stable nuclei at ΔT = 300 K!

8 Question 10.6: For some transformation having kinetics that obey the Avrami equation (Equation 10.17), the parameter n is known to have a value of 1.7. If, after 100 s, the reaction is 50% complete, how long (total time) will it take the transformation to go to 99% completion? This problem calls for us to compute the length of time required for a reaction to go to 99% completion. It first becomes necessary to solve for the parameter k in Equation In order to do this it is best manipulate the equation such that k is the dependent variable. We first rearrange Equation as exp( kt n ) = 1 y and then take natural logarithms of both sides: kt n = ln (1 y) Now solving for k gives k = ln (1 y) t n And, from the problem statement, for y = 0.50 when t = 100 s and given that n = 1.7, the value of k is equal to k = ln (1 0.5) (100 s) 1.7 = We now want to manipulate Equation such that t is the dependent variable. The above equation may be written in the form: t n = ln (1 y) k And solving this expression for t leads to t = ln (1 y) k 1/n

9 Now, using this equation and the value of k determined above, the time to 99% transformation completion is equal to t = ln (1 0.99) /1.7 = 305 s

10 Question 10.8: It is known that the kinetics of recrystallization for some alloy obey the Avrami equation and that the value of n in the exponential is 2.5. If, at some temperature, the fraction recrystallized is 0.40 after 200 min, determine the rate of recrystallization at this temperature. This problem gives us the value of y (0.40) at some time t (200 min), and also the value of n (2.5) for the recrystallization of an alloy at some temperature, and then asks that we determine the rate of recrystallization at this same temperature. It is first necessary to calculate the value of k. We first rearrange Equation as exp( kt n ) = 1 y and then take natural logarithms of both sides: kt n = ln (1 y) Now solving for k gives k = ln (1 y) t n which, using the values cited above for y, n, and t yields ln (1 0.40) k = = (200 min) 2.5 At this point we want to compute t 0.5, the value of t for y = 0.5, which means that it is necessary to establish a form of Equation in which t is the dependent variable. From one of the above equations t n = ln (1 y) k And solving this expression for t leads to

11 1/n ln (1 y) t = k For t 0.5, this equation takes the form t 0.5 = ln (1 0.5) k 1/n and incorporation of the value of k determined above, as well as the value of n cited in the problem statement (2.5), then t 0.5 is equal to t 0.5 = ln (1 0.5) /2.5 = min Therefore, from Equation 10.18, the rate is just rate = 1 t 0.5 = min = (min) -1

12 Question 10.9: The kinetics of the austenite-to-pearlite transformation obey the Avrami relationship. Using the fraction transformed time data given here, determine the total time required for 95% of the austenite to transform to pearlite: Fraction Transformed Time (s) The first thing necessary is to set up two expressions of the form of Equation 10.17, and then to solve simultaneously for the values of n and k. In order to expedite this process, we will rearrange and do some algebraic manipulation of Equation First of all, we rearrange as follows: 1 y = exp ( kt n ) Now taking natural logarithms ln (1 y) = kt n Or ln (1 y) = kt n which may also be expressed as 1 ln = kt n 1 y Now taking natural logarithms again, leads to 1 ln ln = ln k + n ln t 1 y

13 which is the form of the equation that we will now use. Using values cited in the problem statement, the two equations are thus 1 ln ln = ln k + n ln (12.6 s) ln ln = ln k + n ln (28.2 s) Solving these two expressions simultaneously for n and k yields n = and k = Now it becomes necessary to solve for the value of t at which y = One of the above equations viz ln (1 y) = kt n may be rewritten as t n = ln (1 y) k And solving for t leads to t = ln (1 y) k 1/n Now incorporating into this expression values for n and k determined above, the time required for 95% austenite transformation is equal to 1/2.453 ln (1 0.95) t = = 35.7 s

14 Question 10.11: (a) From the curves shown in Figure and using Equation 10.18, determine the rate of recrystallization for pure copper at the several temperatures. (b) Make a plot of ln(rate) versus the reciprocal of temperature (in K 1 ), and determine the activation energy for this recrystallization process. (See Section 5.5.) (c) By extrapolation, estimate the length of time required for 50% recrystallization at room temperature, 20 C (293 K). This problem asks us to consider the percent recrystallized versus logarithm of time curves for copper shown in Figure (a) The rates at the different temperatures are determined using Equation 10.18, which rates are tabulated below: Temperature ( C) Rate (min) (b) These data are plotted below.

15 The activation energy, Q, is related to the slope of the line drawn through the data points as Q = Slope (R) where R is the gas constant. The slope of this line is equal to Slope = Δ ln rate Δ 1 T = ln rate 1 ln rate 2 1 T 1 1 T 2 Let us take 1/T 1 = K -1 and 1/T 2 = K -1 ; the corresponding ln rate values are ln rate 1 = -2.6 and ln rate 2 = Thus, using these values, the slope is equal to Slope = 2.6 ( 9.4) K K -1 = K And, finally the activation energy is Q = (Slope)(R) = ( K -1 )(8.31 J/mol- K) = 94,150 J/mol

16 (c) At room temperature (20 C), 1/T = 1/( K) = K -1. Extrapolation of the data in the plot to this 1/T value gives ln (rate) 12.8 which leads to rate exp ( 12.8) = (min) -1 But since rate = 1 t 0.5 t 0.5 = 1 rate = (min) 1 = min = 250 days

17 Question 10.14: (a) Briefly describe the phenomena of superheating and supercooling. (b) Why do these phenomena occur? (a) Superheating and supercooling correspond, respectively, to heating or cooling above or below a phase transition temperature without the occurrence of the transformation. (b) These phenomena occur because right at the phase transition temperature, the driving force is not sufficient to cause the transformation to occur. The driving force is enhanced during superheating or supercooling. Question 10.15: Suppose that a steel of eutectoid composition is cooled to 550 C (1020 F) from 760 C (1400 F) in less than 0.5 s and held at this temperature. (a) How long will it take for the austenite-to-pearlite reaction to go to 50% completion? To 100% completion? (b) Estimate the hardness of the alloy that has completely transformed to pearlite. We are called upon to consider the isothermal transformation of an iron-carbon alloy of eutectoid composition. (a) From Figure 10.22, a horizontal line at 550 C intersects the 50% and reaction completion curves at about 2.5 and 6 seconds, respectively; these are the times asked for in the problem statement. (b) The pearlite formed will be fine pearlite. From Figure 10.30a, the hardness of an alloy of composition 0.76 wt% C that consists of fine pearlite is about 265 HB (27 HRC). Question 10.16: Briefly cite the differences between pearlite, bainite, and spheroidite relative to microstructure and mechanical properties. The microstructures of pearlite, bainite, and spheroidite all consist of α-ferrite and cementite phases. For pearlite, the two phases exist as layers which alternate with one another. Bainite consists of very fine and parallel needle-shaped particles of cementite that are surrounded an α-ferrite matrix. For spheroidite, the matrix is ferrite, and the cementite phase is in the shape of sphere-shaped particles. Bainite is harder and stronger than pearlite, which, in turn, is harder and stronger than spheroidite.

18 Question 10.17: What is the driving force for the formation of spheroidite? The driving force for the formation of spheroidite is the net reduction in ferrite-cementite phase boundary area. Question 10.18: Using the isothermal transformation diagram for an iron carbon alloy of eutectoid composition (Figure 10.22), specify the nature of the final microstructure (in terms of microconstituents present and approximate percentages of each) of a small specimen that has been subjected to the following time temperature treatments. In each case assume that the specimen begins at 760 C (1400 F) and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure. (a) Cool rapidly to 700 C (1290 F), hold for 10 4 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment.

19 After cooling and holding at 700 C for 10 4 s, approximately 50% of the specimen has transformed to coarse pearlite. Upon cooling to room temperature, the remaining 50% transforms to martensite. Hence, the final microstructure consists of about 50% coarse pearlite and 50% martensite. (b) Reheat the specimen in part (a) to 700 C (1290 F) for 20 h. spheroidite. Heating to 700 C for 20 h the specimen in part (a) will transform the coarse pearlite and martensite to (c) Rapidly cool to 600 C (1110 F), hold for 4 s, rapidly cool to 450 C (840 F), hold for 10 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment.

20 After cooling to and holding at 600 C for 4 s, approximately 50% of the specimen has transformed to pearlite (medium). During the rapid cooling to 450 C no transformations occur. At 450 C we start timing again at zero time; while holding at 450 C for 10 s, approximately 50 percent of the remaining unreacted 50% (or 25% of the original specimen) will transform to bainite. And upon cooling to room temperature, the remaining 25% of the original specimen transforms to martensite. Hence, the final microstructure consists of about 50% pearlite (medium), 25% bainite, and 25% martensite. (d) Cool rapidly to 400 C (750 F), hold for 2 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment. After cooling to and holding at 400 C for 2 s, no of the transformation begin lines have been crossed, and therefore, the specimen is 100% austenite. Upon cooling rapidly to room temperature, all of the specimen transforms to martensite, such that the final microstructure is 100% martensite.

21 (e) Cool rapidly to 400 C (750 F), hold for 20 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment. After cooling and holding at 400 C for 20 s, approximately 40% of the specimen has transformed to bainite. Upon cooling to room temperature, the remaining 60% transforms to martensite. Hence, the final microstructure consists of about 40% bainite and 60% martensite. (f) Cool rapidly to 400 C (750 F), hold for 200 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment.

22 After cooling and holding at 400 C for 200 s, the entire specimen has transformed to bainite. Therefore, during the cooling to room temperature no additional transformations will occur. Hence, the final microstructure consists of 100% bainite. (g) Rapidly cool to 575 C (1065 F), hold for 20 s, rapidly cool to 350 C (660 F), hold for 100 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment.

23 After cooling and holding at 575 C for 20 s, the entire specimen has transformed to fine pearlite. Therefore, during the second heat treatment at 350 C no additional transformations will occur. Hence, the final microstructure consists of 100% fine pearlite. (h) Rapidly cool to 250 C (480 F), hold for 100 s, then quench to room temperature in water. Reheat to 315 C (600 F) for 1 h and slowly cool to room temperature. Below is Figure upon which is superimposed the above heat treatment.

24 After cooling and holding at 250 C for 100 s, no transformations will have occurred at this point, the entire specimen is still austenite. Upon rapidly cooling to room temperature in water, the specimen will completely transform to martensite. The second heat treatment (at 315 C for 1 h) not shown on the above plot will transform the material to tempered martensite. Hence, the final microstructure is 100% tempered martensite.

25 Question 10.20: Using the isothermal transformation diagram for a 0.45 wt% C steel alloy (Figure 10.39), determine the final microstructure (in terms of just the microconstituents present) of a small specimen that has been subjected to the following time-temperature treatments. In each case assume that the specimen begins at 845 C (1550 F), and that it has been held at this temperature long enough to have achieved a complete and homogeneous austenitic structure. (a) Rapidly cool to 250 C (480 F), hold for 10 3 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment. While rapidly cooling to 250 C about 80% of the specimen transforms to martensite; during the 1000 s isothermal treatment at 250 C no additional transformations occur. During the final cooling to room temperature, the untransformed austenite also transforms to martensite. Hence, the final microstructure consists of 100% martensite. (b) Rapidly cool to 700 C (1290 F), hold for 30 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment.

26 After cooling to and holding at 700 C for 30 s, a portion of specimen has transformed to proeutectoid ferrite. While cooling to room temperature, the remainder of the specimen transforms to martensite. Hence, the final microstructure consists proeutectoid ferrite and martensite. (c) Rapidly cool to 400 C (750 F), hold for 500 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment.

27 After cooling to and holding at 400 C for 500 s, all of the specimen has transformed to bainite. Hence, the final microstructure consists of 100% bainite. (d) Rapidly cool to 700 C (1290 F), hold at this temperature for 10 5 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment.

28 After cooling to and while holding at 700 C the specimen first transforms to proeutectoid ferrite and coarse pearlite. Continued heat treating at 700 C for 10 5 s results in a further transformation into spheroidite. Hence, the final microstructure consists of 100% spheroidite. (e) Rapidly cool to 650 C (1200 F), hold at this temperature for 3 s, rapidly cool to 400 C (750 F), hold for 10 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment.

29 After cooling to and holding at 650 C for 3 s, some of the specimen first transformers to proeutectoid ferrite and then to pearlite (medium). During the second stage of the heat treatment at 400 C, some (but not all) of the remaining unreacted austenite transforms to bainite. As a result of the final quenching, all of the remaining austenite transforms to martensite. Hence, the final microstructure consists of ferrite, pearlite (medium), bainite, and martensite. (f) Rapidly cool to 450 C (840 F), hold for 10 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment.

30 After cooling to and holding at 450 C for 10 s, a portion of the specimen first transformers to bainite. During the quenching to room temperature, the remainder of the specimen transforms to martensite. Hence, the final microstructure consists of bainite and martensite. (g) Rapidly cool to 625 C (1155 F), hold for 1 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment.

31 After cooling to and holding at 625 C for 1 s, a portion of the specimen first transformers to proeutectoid ferrite and pearlite. During the quenching to room temperature, the remainder of the specimen transforms to martensite. Hence, the final microstructure consists of ferrite, pearlite, and martensite. (h) Rapidly cool to 625 C (1155 F), hold at this temperature for 10 s, rapidly cool to 400 C (750 F), hold at this temperature for 5 s, then quench to room temperature. Below is Figure upon which is superimposed the above heat treatment.

32 After cooling to and holding at 625 C for 10 s, all of the specimen transformers to proeutectoid ferrite and pearlite. During the second part of the heat treatment at 400 C no additional transformation will occur. Hence, the final microstructure consists of ferrite and pearlite.

33 Question 10.23: Name the microstructural products of eutectoid iron carbon alloy (0.76 wt% C) specimens that are first completely transformed to austenite, then cooled to room temperature at the following rates: (a) 200 C/s, (b) 100 C/s, and (c) 20 C/s. We are called upon to name the microstructural products that form for specimens of an iron-carbon alloy of eutectoid composition that are continuously cooled to room temperature at a variety of rates. Figure is used in these determinations. (a) At a rate of 200 C/s, only martensite forms. (b) At a rate of 100 C/s, both martensite and pearlite form. (c) At a rate of 20 C/s, only fine pearlite forms. Question 10.24: Figure shows the continuous cooling transformation diagram for a 1.13 wt% C ironcarbon alloy. Make a copy of this figure and then sketch and label continuous cooling curves to yield the following microstructures: (a) Fine pearlite and proeutectoid cementite (b) Martensite (c) Martensite and proeutectoid cementite (d) Coarse pearlite and proeutectoid cementite (e) Martensite, fine pearlite, and proeutectoid cementite Below is shown a continuous cooling transformation diagram for a 1.13 wt% C iron-carbon alloy, with continuous cooling paths that will produce (a) fine pearlite and proeutectoid cementite; (b) martensite; (c) martensite and proeutectoid cementite; (d) coarse pearlite and proeutectoid cementite; and (e) martensite, fine pearlite, and proeutectoid cementite.

34 Question 10.28: Briefly describe the simplest continuous cooling heat treatment procedure that would be used in converting a 4340 steel from one microstructure to another. (a) (Martensite + bainite) to (ferrite + pearlite) (b) (Martensite + bainite) to spheroidite (c) (Martensite + bainite) to (martensite + bainite + ferrite) This problem asks that we briefly describe the simplest continuous cooling heat treatment procedure that would be used in converting a 4340 steel from one microstructure to another. Solutions to this problem require the use of Figure (a) In order to convert from (martensite + bainite) to (ferrite + pearlite) it is necessary to heat above about 720 C, allow complete austenitization, then cool to room temperature at a rate slower than C/s. (b) To convert from (martensite + bainite) to spheroidite the alloy must be heated to about 700 C for several hours. (c) In order to convert from (martensite + bainite) to (martensite + bainite + ferrite) it is necessary to heat to above about 720 C, allow complete austenitization, then cool to room temperature at a rate between 0.3 C/s and 0.02 C/s.

35 Question 10.32: Rank the following iron carbon alloys and associated microstructures from the highest to the lowest tensile strength: (a) 0.25 wt%c with spheroidite, (b) 0.25 wt%c with coarse pearlite, (c) 0.60 wt%c with fine pearlite, and (d) 0.60 wt%c with coarse pearlite. Justify this ranking. This problem asks us to rank four iron-carbon alloys of specified composition and microstructure according to hardness. This ranking is as follows: 0.60 wt% C, fine pearlite 0.60 wt% C, coarse pearlite 0.25 wt% C, coarse pearlite 0.25 wt% C, spheroidite The 0.25 wt% C, coarse pearlite is stronger than the 0.25 wt% C, spheroidite since coarse pearlite is stronger than spheroidite; the composition of the alloys is the same. The 0.60 wt% C, coarse pearlite is stronger than the 0.25 wt% C, coarse pearlite, since increasing the carbon content increases the strength. Finally, the 0.60 wt% C, fine pearlite is stronger than the 0.60 wt% C, coarse pearlite inasmuch as the strength of fine pearlite is greater than coarse pearlite because of the many more ferrite-cementite phase boundaries in fine pearlite. Question 10.33: Briefly explain why the hardness of tempered martensite diminishes with tempering time (at constant temperature) and with increasing temperature (at constant tempering time). This question asks for an explanation as to why the hardness of tempered martensite diminishes with tempering time (at constant temperature) and with increasing temperature (at constant tempering time). The hardness of tempered martensite depends on the ferrite-cementite phase boundary area; since these phase boundaries are barriers to dislocation motion, the greater the area the harder the alloy. The microstructure of tempered martensite consists of small sphere-like particles of cementite embedded within a ferrite matrix. As the size of the cementite particles increases, the phase boundary area diminishes, and the alloy becomes softer. Therefore, with increasing tempering time, the cementite particles grow, the phase boundary area decreases, and the

37 Question 10.35: (a) Briefly describe the microstructural difference between spheroidite and tempered martensite. (b) Explain why tempered martensite is much harder and stronger. (a) Both tempered martensite and spheroidite have sphere-like cementite particles within a ferrite matrix; however, these particles are much larger for spheroidite. (b) Tempered martensite is harder and stronger inasmuch as there is much more ferrite-cementite phase boundary area for the smaller particles; thus, there is greater reinforcement of the ferrite phase, and more phase boundary barriers to dislocation motion.

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