Question

By referring to the iron-iron carbide phase diagram given below, answer the questions that follow. Competens 1000 10 -14970 20 1.400 1.200 1307C Top Die 1500 7270 0.76 0.020 le Care 1000 400 0 Dompet Iron-Iron Carbide Phase Diagram (from Textbook) (a) Give the temperature and composition (carbon wel) corresponding to: (i) the cutectic point, and (ii) the cutectoid point (2 points) (b) Descnbe the structural changes that take place when a 1.0 wtf carbon steel is slowly cooled from 1000 °C. (after it is fully austenite to room temperature, (4 Points) (c) Consider 5.0 kg of austenite containing 0.48 % which is slowly cooked to below 727 °C. () What is the protectoid phase! How many kilograms cach of totalferrite and cementite for? i) How many kilograms cach of pearlite and the procutectoid phase form! (iv) Sketch and label the resulting microstructure. (10 Points) (d) Explain why known steel structures such as bainite and martensite are not shown on the iron-iron carbide phase diagram above. (2 Points)

Answer 1

(a)

Comptes 10 25 1200 -140 1400 Eutectic Point 1200 1107 9 2.14 2000 1000 012 ytre Eutectoid Point 1500 Cente 1000 Desiones Iron-Iron Carbide Phase Diagram (from Textbook)

For Eutectic Point ; Temperature - 1143 C ; Composition - 4.3 %

For Eutectoid Point ; Temperature - 727 C ; Composition - 0.76 %

(b)

Comptes 10 20 TOO 143 1200 1167C 2000 1000 AT 012 B 1500 08 2.024 c Cente 1000 D Dagens Iron-Iron Carbide Phase Diagram (from Textbook) (b) Describe the structural changes that take place when a 1.0 wts carbon steel is slowly cooled from 1000 °C. (after it is fully austenite to room temperature.

When austenite with 1 % carbon is cooled from 1000 C i.e. point A following structural changes takes place:

As soon as the temperature drops to point B, some Austenite will start to solidify into Cementite, this cementite that is formed before the eutectoid temperature is known as Pro-Cementite phase.

Just before point C i.e. eutectoid temperature there will be Austenite and Pro-Cementite phase.

As soon as the temperature reaches eutectoid temperature, the remaining austenite will convert into alpha ferrite and cementite.

Further cooling to room temperature will not result in any structural changes.

(c)
430 1000 Temperatur 012 y+ 1500 0.48 0.022 lec Cente 1000 Carpetont Iron-Iron Carbide Phase Diagram (from Textbook)

pro-euse toid ferrure (0) (1) The proente toid phone is pro-a Applying Lever nule just below 17²6, mars frouion of total ferrite- m 6.70 -0.48 feite 6.70 – 0.022 Mfavite = 0.93 14 or 93.14 % i cementite (100 - 93.14% - 6.86 % As Total mass is 5kg, 5x0.9314 Mass of total ferrite = = 4.657 kg total cementite = 0.343 kg. Mors of 9 () For moss fraction of pro- eutectoid ferrite or p310-d , applying lever rule just above 727°C ie. blue line. mpro-da = 0.3794 0.76-0:48 0.76 -0.022 37,94% in pro-d = Hence procwe toid phase = (0.3794 x 5) kg 1.897 kg 3.103 kg. Rest will be pearlite (iv) fezc -prod 2

(d) Steel microstructures such bainite , martensite are drawn on non equilibrium diagrams or TTT diagram (Time-Temperature-Transformation diagram) . They are not shown in the equilibrium phase diagram of the iron-carbon system because they are not equilibrium phase. Equilibrium phases form by slow cooling rates that allow sufficient time for diffusion, whereas martensite , bainite are usually formed by very high cooling rates.