15) Answer the following questions using the phase diagram of the iron-carbon system. a) A plain carbon steel contains 70 wt % ferrite and 30 wt % pearlite. Estimate the carbon content of the steel. b) This steel is heat treated under a CO2-CO gas mixture at 900 C. Calculate the composition of the CO2-CO gas atmosphere that will keep the carbon content of the steel unchanged throughout the heat treatment. c) Will the steel carburize or decarburize if the composition of the CO2-CO gas mixture used for the heat treatment purpose consists of 80% CO-20% CO2 without changing the total pressure in the furnace? Briefly explain your reasoning. d) We would like to change the surface carbon content of the steel to 0.6 wt% carbon by heat treating it at 900 C in a CO2-CO gas atmosphere without changing the total pressure in the furnace. What should be the composition of the CO2-CO gas mixture? e) This steel is heat treated under a CO2-CO-20% N2 gas mixture at 900 C. The total pressure in the furnace is 0.9 atm. Calculate the composition of the CO2-CO-20% N2 gas atmosphere that will keep the carbon content of the steel unchanged throughout the heat treatment. f) Will the steel carburize or decarburize if the composition of the CO2-CO gas mixture used for the heat treatment purpose in part (b) consists of 80% CO-20% CO2 without changing the total pressure (0.9 atm) in the furnace. Briefly explain your reasoning. g) We would like to change the surface carbon content of the steel to 0.6 wt% carbon by heat treating it at 900 C in a CO2-CO gas atmosphere without changing the total pressure (0.9 atm) in the furnace. What should be the composition of the CO2-CO gas mixture? DATA: C+02 = CO2 C + 1/2 O2 = CO AG = -394184 0.84 T AG - = -111727 87.66 T joule/mole joule/mole R=8.314 joule/mol degree.