10000 7270 Temperature( C) The TTT diagram for a eutectoid Fe-C alloy Y (stable) TN γ' (metastable) Ms -0% ME 50%' 10-1 25% 75% a+Fe3C 0% Bainite 50% 100% Eutectoid temperature 100%- a' + + + 1 101 102 103 104 time (s)
By using the time-temperature-transformation (TTT) diagram for a steel of eutectoid
composition (Fe-C alloy with 0.77 wt% carbon) given below, answer the following questions:
Sketch the cooling path(s) on the TTT diagram provided below in answering each question and
label them clearly.
a) What is meant by the so-called critical cooling rate (CCR)? What happens if this
eutectoid steel is quenched from 1000 oC at a cooling rate that is faster than the critical cooling
rate? Trace the cooling path on the diagram. What is the crystal structure before and after the
quenching?
b) What are the effects of alloying elements on the position of the boundaries on a
TTT diagram? What are the engineering implications of such effects on the boundaries on a TTT
diagram? Concisely elaborate on your answer.
c) A steel that is comprised of 25% Pearlite, 37.5% Bainite and 37.5% Martensite is
needed for a special application. Design a heat treatment by which such a steel microstructure and
be created. Sketch your heat treatment protocol on the TTT diagram provided herein.
d) What do the temperatures Ms and Mf designate on the TTT diagram? What
happens when a eutectoid steel is first quenched to Ms > T > Mf at a rate faster than the CCR, and
then isothermally held for a very long time (t>104 sec), followed by quenching to room
temperature? What phases are present at room temperature?
e) What is the resultant phase or phases present in a eutectoid steel after the
following heat treatment cycle: First quenched to Ms>T>MF at a cooling rate faster than the critical
cooling rate and held for t > 104 sec, followed by reheating to T > 727 oC and held for a long time,
and then quenched to Ms < T < TN at a rate faster than the CCR, and held for t > 104 seconds and
then quench to room temperature.
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