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CHAPTER 9 ENGINEERING ALLOYS 9.1 How is raw pig iron extracted from iron oxide ores? Raw pig iron is typically extracted from iron oxide ores in a blast furnace in which coke (carbon) acts as a reducing agent of the iron oxides. The pig iron produced contains approximately 4 percent carbon. 9.2 Write a typical chemical reaction for the reduction of iron oxide (Fe2O3) by carbon monoxide to produce iron. The typical chemical reaction is: Fe 2 O 3 + 3CO → 2Fe + 3CO 2 9.3 Describe the basic-ox
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  Smith Foundations of Materials Science and Engineering Solution Manual 189 CHAPTER 9 ENGINEERING ALLOYS 9.1 How is raw pig iron extracted from iron oxide ores?Raw pig iron is typically extracted from iron oxide ores in a blast furnace in which coke(carbon) acts as a reducing agent of the iron oxides. The pig iron produced containsapproximately 4 percent carbon.9.2 Write a typical chemical reaction for the reduction of iron oxide (Fe 2 O 3 ) by carbonmonoxide to produce iron.The typical chemical reaction is: 232 FeO3CO2Fe3CO + → +  9.3 Describe the basic-oxygen process for converting pig iron into steel.In the basic-oxygen process, pig iron is converted into steel through oxidation in arefractory-lined converter. An oxygen lance insert provides pure oxygen to the bath of molten pig iron and scrap steel. The iron oxide formed then reacts with the carbon in thesteel to form carbon monoxide: FeOCFeCO + → +  9.4 Why is the 3 Fe-FeCphase diagram a metastable phase diagram instead of a trueequilibrium phase diagram?The 3 Fe-FeCphase diagram is a metastable phase diagram rather than a true equilibrium phase diagram because the compound iron carbide ( 3 FeC) formed, cementite, is not atrue equilibrium phase. While cementite is typically very stable, it can decompose intoiron and carbon under certain conditions.9.5 Define the following phases that exist in the 3 Fe-FeCphase diagram:(a) austenite, (b) α   ferrite, (c) cementite, (d) δ ferrite.(a) Austenite, the interstitial solid solution of carbon in γ  iron, has an FCC crystalstructure and a relatively high maximum solid solubility of carbon (2.08 percent).(b) An interstitial solid solution of carbon in the BCC iron crystal lattice, α   ferrite has amaximum solid solubility of carbon of 0.02 percent.(c) Cementite, 3 FeC, is a hard, brittle intermetallic compound with a carbon content of 6.67 percent.(d) An interstitial solid solution in δ iron, δ ferrite has a BCC crystal structure and amaximum solid solubility of carbon of 0.09 percent.  Smith Foundations of Materials Science and Engineering Solution Manual 190 9.6 Write the reactions for the three invariant reactions that take place in the 3 Fe-FeCphasediagram.The three invariant reactions that take place in the 3 Fe-FeCphase diagram are a peritecticreaction, a eutectic reaction, and a eutectoid reaction.Peritectic: 1495C Liquid (0.53% C) + (0.09% C) (0.17% C) δ γ    → ¡  Eutectic: 1148C3 Liquid (4.3% C) austenite (2.08% C) + FeC (6.67% C) γ    → ¡  Eutectoid: 723C3 austenite (0.8% C) ferrite (0.02% C) + FeC (6.67% C) γ α   → ¡  9.7 What is the structure of pearlite?Pearlite has a lamellar (plate-like) structure consisting of alternating plates of  α  ferriteand cementite ( 3 FeC).9.8 Distinguish between the following three types of plain-carbon steels:(a) eutectoid, (b) hypoeutectoid, and (c) hypereutectoid.(a) Eutectoid plain-carbon steel contains 0.8 percent carbon.(b) Hypoeutectoid plain-carbon steel contains less than 0.8 percent carbon.(c) Hypereutectoid plain-carbon steel contains more than 0.8 percent carbon.9.9 Describe the structural changes that take place when a plain-carbon eutectoid steel isslowly cooled from the austenitic region just above the eutectoid temperature.When a plain-carbon eutectoid steel is slowly cooled from the austenitic region justabove the eutectoid temperature (723°C) to the eutectoid temperature or just below it, theaustenite structure transforms completely into pearlite through the eutectoid reaction.Further cooling to room temperature does not cause any discernable structural changessince the solid solubility of carbon in α  ferrite and cementite ( 3 FeC) changes very littlein this temperature interval.9.10 Describe the structural changes that take place when a 0.4 percent C plain-carbon steel isslowly cooled from the austenitic region just above the upper transformation temperature.When a 0.4 percent C plain-carbon steel is slowly cooled from the austenitic region justabove the upper transformation temperature, proeutectoid ferrite will nucleate and growcontinuously until the eutectoid temperature of 723°C is reached. At this point,approximately 50 percent of the austenite will have transformed into proeutectoid ferritewhile the remaining austenite will have an increased carbon content of 0.8 percent.Further slow cooling will cause the remaining austenite to transform into pearlite(eutectoid ferrite and cementite) through the eutectoid reaction at 723°C.  Smith Foundations of Materials Science and Engineering Solution Manual 191 9.11 Distinguish between proeutectoid ferrite and eutectoid ferrite.Proeutectoid ferrite is the ferrite formed during cooling from the upper transformationtemperature to just above the eutectoid temperature, where as, eutectoid ferrite is produced during the eutectoid reaction at the eutectoid temperature of 723°C.9.12 A 0.65 percent C hypoeutectoid plain-carbon steel is slowly cooled from about 950ºC to atemperature just slightly above 723ºC. Calculate the weight percent austenite and weight percent proeutectoid ferrite in this steel.The weight percent austenite is calculated from the ratio of the segment of the tie line tothe left of the 0.65 percent C to the entire length of the tie line.0.650.020.63Wt % austenite 100%100%0.800.020.78 −= × = × =− 80.8%  The weight percent proeutectoid ferrite is calculated from the ratio of the segment of thetie line to the right of the 0.65 percent C to the entire length of the tie line.0.800.650.15Wt % proeutectoid ferrite 100%100%0.800.020.78 −= × = × =− 19.2%  9.13 A 0.25 percent C hypoeutectoid plain-carbon steel is slowly cooled from about 950ºC to atemperature just slightly below 723ºC.(a) Calculate the weight percent proeutectoid ferrite in the steel.(b) Calculate the weight percent eutectoid ferrite and weight percent eutectoid cementitein the steel.(a) The weight percent proeutectoid ferrite just below 723ºC will be the same as that justabove 723ºC. It is therefore calculated based upon a tie line similar to that shown inP9.12.0.800.250.55Wt % proeutectoid ferrite 100%100%0.800.020.78 −= × = × =− 70.5%  (b) The weight percent total cementite andtotal ferrite are calculated based on thetie line shown to the right. 0.80%C0.65% C0.02% C α    α γ   + α    γ     γ     6.67% C0.25% C0.02% C α    3 FeC α  3 FeC  Smith Foundations of Materials Science and Engineering Solution Manual 192 0.250.020.23Wt % total cementite 100%100%3.46%6.670.026.656.670.256.42Wt % total ferrite100%100%96.5%6.670.026.65 −= × = × =−−= × = × =−  The eutectoid ferrite is equal to the difference between the total ferrite and the proeutectoid ferrite:Wt % eutectoid ferrite = 96.5% - 70.5% = 26.0% Since the steel contains less than 0.8 percent carbon, no proeutectoid cementite wasformed during cooling. Thus,Wt % eutectoid cementite = Wt % total cementite = 3.46%  9.14 A plain-carbon steel contains 93 wt % ferrite and 7 wt % 3 FeC. What is its averagecarbon content in weight percent?The average weight percent carbon is calculated based upon the tie line shown below.6.670.936.670.026.670.93(6.670.02)  x x x −=−= − −= 0.49% C  9.15 A plain-carbon steel contains 45 wt % proeutectoid ferrite. What is its average carboncontent in weight percent?0.800.450.800.020.800.45(0.800.02)  x x x −=−= − −= 0.45% C  9.16 A plain-carbon steel contains 5.9 wt % eutectoid ferrite. What is its average carboncontent in weight percent?The carbon content is related to the eutectoid ferrite by considering the proeutectoidferrite and the total ferrite present after the eutectoid reaction.Wt % eutectoid ferriteWt % total ferrite Wt % proeutectoid ferrite = −   6.67%C  x % C0.02% C α    3 FeC α  3 FeC 0.80% C  x % C0.02% C α    α γ   + α γ     γ    
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