Question

Austenite Pearlite Question 3 (24 pts): We are interested in performing different heat treatments to an iron-carbon alloy of eutectoid compostion. Use the provided Temperature-Time Transformation (TTT) and Continuous Cooling Transformation (CCT) diagrams to specify the nature of the final microstructure in terms of microconstituents present and approximate percentages) of a small specimen that has been subjected to the following heat treatments. In each case, assume that the specimen begins at 760°C and that it has been at this temperature long enough to have achieved complete and homogeneous austenitic structure. 800 800 A 1400 Eutectoid temperature 1400 Eutectoid temperature 700 700 A 1200 1200 P 600 600 1000 1000 500 500 B Temperature (°C) 800 Temperature (°F) 800 400 Temperature (°C) Temperature (°F) 400 A 300 600 600 300 Critical cooling rate 1 Mistart) M (start) 200 50% M + A 400 200 1 400 1 M(50%) M(90%) 100 200 100 Martensite 1 200 Martensite Pearlite Pearlite 0 10-2 1 10 103 101 102 Time (s) 105 0 10-1 1 10 103 104 105 102 Time (s) TTT Diagram a) (4) The specimen is cooled to room temperature at a rate of 20 °C/s. CCT Diagram b) (4) The specimen is rapidly cooled to 600 °F, held for 10s, and quenched to room temperature c) (4) The specimen is rapidly cooled to 175 °C and maintained at this temperature until stability of the microstructure. d) (4) The specimen is rapidly cooled to 350 °C, held for 100s, and quenched to room temperature e) (4) The specimen is rapidly cooled to 600 °C, held for 10s, and quenched to room temperature f) (4) The material obtained in part e) was heated again to 600 °C for a long time until stability of the microstructure.

Answer 1

a.

100% pearlite

From figure 2 cooling rate 35/°C is very slow hence pearlite will form.

B.

100% Martensite

At 600F for 10 seconds No transformation take place hence after quenching all Austenite gets converted to Martensite.

C.

50% Martensite+ 50% Austenite

At 175°C half of austenite gets converted to Martensite and remaining half gets stablizes.

D.

50% Bainite+ 50 % Martensite

At 350°C holding for 100 seconds half of austenite gets converted to bainite and after quenching remaining half gets converted to Martensite.

E.

100% Pearlite

At 600°C holding for 10 seconds all Austenite gets converted to Pearlite.

F.

100% Spherodite

Reheating will improve properties of steel and spherodite steel formation will take place.

800 A Eutectoid temperature 1400 700 1200 600 P 1000 500 B Temperature (°C) 800 Temperature (°F) 400 300 600 Mistart) 200 50% 400 M(90%) 100 78 C 200 0 10-1 1 103 101 105 10 102 Time (s) TTT Diagram