time temperature-transformation diagram

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MM: 304 Heat treatmentTopic: Transformation diagrams

Compiled for T.E, MetallurgybyEngr. Muhammad Ali SiddiquiLecturerDepartment of Metallurgical EngineeringNED university of Engineering and technologyEmail: muetali@yahoo.com

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Isothermal Transformation (I.T) / Time Temperature Transformation (TTT)

Diagram

Today we will discuss about its Construction

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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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