3. b. Answer the following questions by referring to the phase diagram for iron-carbon at 1...

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3. b. Answer the following questions by referring to the phase diagram for iron-carbon at 1 bar shown below. The solid a and y phases are commonly referred to as ferrite and austenite respectively. The solid 8 phase is called delta ferrite, and Fe3C is called cementite. (a) Sketch cooling curves at the following %wt values of carbon: (i) 0.25 (ii) 0.83 (i.e. passing through point S) (iii)2.05 (i.e. passing through point E) (iv)4.3 (i.e. passing through point C) Liquid having 3.5 wt% C is cooled to 1200C. What are the compositions of the liquid and the austenite? Calculate the quantities (percentages) of each. C. How many degrees of freedom are there at: d. (i) (i) At Point S (ii) Along the line between points E and C (iii) At 1.0% C and 600C (iv) At 1700C Austenite is much harder than ferrite, and so is more useful in many applications. According to the phase diagram, can austenite exist at room temperature? (ii) Heat treating steel is done to increase its hardness. It consists of heating ferrite (say, 0.5 wt% C) to 1000C and rapidly cooling the steel, by plunging it into water for instance. The result is austenite. Why does this happen? Temperature (C) 1800 1700 1600- A 1534 1500 H 8. 1400 1300 1200 1100 1000- 900 G Atomic % 2.5 5 7.5 B N Liquid+y 800 7+a 700 P S 600 - a+Fe3C 10 12-5 15 17.5 20 Liquid E 1147 7+ Fe3 C 723 K 500 1 1 0 0.5 1.0 1-5 2.0 2.5 3.0 3.5 4.0 4-5 5.0 Weight % Carbon 3. b. Answer the following questions by referring to the phase diagram for iron-carbon at 1 bar shown below. The solid a and y phases are commonly referred to as ferrite and austenite respectively. The solid 8 phase is called delta ferrite, and Fe3C is called cementite. (a) Sketch cooling curves at the following %wt values of carbon: (i) 0.25 (ii) 0.83 (i.e. passing through point S) (iii)2.05 (i.e. passing through point E) (iv)4.3 (i.e. passing through point C) Liquid having 3.5 wt% C is cooled to 1200C. What are the compositions of the liquid and the austenite? Calculate the quantities (percentages) of each. C. How many degrees of freedom are there at: d. (i) (i) At Point S (ii) Along the line between points E and C (iii) At 1.0% C and 600C (iv) At 1700C Austenite is much harder than ferrite, and so is more useful in many applications. According to the phase diagram, can austenite exist at room temperature? (ii) Heat treating steel is done to increase its hardness. It consists of heating ferrite (say, 0.5 wt% C) to 1000C and rapidly cooling the steel, by plunging it into water for instance. The result is austenite. Why does this happen? Temperature (C) 1800 1700 1600- A 1534 1500 H 8. 1400 1300 1200 1100 1000- 900 G Atomic % 2.5 5 7.5 B N Liquid+y 800 7+a 700 P S 600 - a+Fe3C 10 12-5 15 17.5 20 Liquid E 1147 7+ Fe3 C 723 K 500 1 1 0 0.5 1.0 1-5 2.0 2.5 3.0 3.5 4.0 4-5 5.0 Weight % Carbon