This document discusses isothermal transformation (TTT) diagrams. It explains that a TTT diagram plots temperature versus the logarithm of time for a steel of definite composition. It shows the transformations that begin and end during isothermal heat treatment of an austenitized alloy. The document provides steps for constructing a TTT diagram, including austenitizing a sample, cooling it to different subcritical temperatures and checking its hardness/microstructure over time. A TTT curve is generated by repeating these steps at different temperatures. An example TTT curve is shown for carbon steel AISI 1050. Cooling curves can also be superimposed on a TTT diagram.

1. MM: 304 Heat treatment
Topic: Transformation diagrams
Compiled for T.E, Metallurgy
by
Engr. Muhammad Ali Siddiqui
Lecturer
Department of Metallurgical Engineering
NED university of Engineering and technology
Email: muetali@yahoo.com
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2. Isothermal Transformation (I.T) /
Time Temperature Transformation (TTT)
Diagram
Today we will discuss about its Construction
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What is TTT?
 T (Time) T (Temperature) T (Transformation) diagram is a plot
of temperature versus the logarithm of time for a steel of
definite composition.
 It is used to determine when transformations begin and end for
an isothermal (constant temperature) heat treatment of a
previously austenitized alloy.

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4. Step: 4 After Cooling, check sample hardness
and microstructure.
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• Step 2,3 and 4 are shown in the following Figure
• If we draw a curve b/w
%age of Pearlite Transformed and Time

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Step: 5 all steps are repeated at different sub-critical
temperature below Ac1 in order to get sufficient points for
plotting the curve.

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Composition of
AISI 1080 Steel
C= 0.75-0.88,
Mn= 0.60-0.90 ,
P= 0.040,
S= 0.050
Fe= balance

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(TTT) curve of carbon steel AISI 1050 (Hypoeutectoid steel)

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Example: Effect of Alloying Elements

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 A cooling curve is
determined experimentally
by placing a thermocouple
at a definite location in a
steel sample and then
measuring the variation of
temperature with time.
 Since the coordinates of
the I-T diagram are the
same as those for a
cooling curve,
 it is possible to
superimpose various
cooling curves on the I-T
diagram. This is shown in
the figure given below.
Cooling Curves and the I-T Diagram