BARC-4: Why is the TTT Curve in C-Shape ? And why is the lower portion of CCT curve not represented?

# BARC-4: Why is the TTT Curve in C-Shape ? And why is the lower portion of CCT curve not represented?

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Asked: 2020-04-16T23:31:38+05:30
2020-04-16T23:31:38+05:30In: Physical Metallurgy & Heat treatment

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At the eutectoid temperature, the rate of under cooling is very less that the driving force for transformation is small. As the undercooling increases, the rate of transformation takes place at a much faster rate with maximum at the nose of the curve, which is the incubation period. Thereafter, as tRead more

At the eutectoid temperature, the rate of under cooling is very less that the driving force for transformation is small. As the undercooling increases, the rate of transformation takes place at a much faster rate with maximum at the nose of the curve, which is the incubation period. Thereafter, as the degree of undercooling increases, the transformation takes place slowly since it is impeded by slow diffusion. Hence, TTT curve has a characteristic C shape.

See lessThis answer was edited.TTT diagram appearing C shape: Driving force for phase transformation undercooling. If increase undercooling transformation occur rapidly. But transformation of atoms occurs through diffusion. If temperature increases, diffusion rate increase. Upto noise of TTT diagram, undercooling dominate the traRead more

TTT diagram appearing C shape:

Driving force for phase transformation undercooling. If increase undercooling transformation occur rapidly. But transformation of atoms occurs through diffusion. If temperature increases, diffusion rate increase.

Upto noise of TTT diagram, undercooling dominate the transformation, below diffusion dominate.

2. Bottom of CCT diagram not appearing due diffusionless transformation

See lessThis answer was edited.TTT curve stands for the Time-Temperature-Transformation curve. This is usually experimental. When we observe the fraction transformation vs time curve we get an S-Shape curve. This can be related with the Temp. Vs time(log scale) curve at different temperatures and we can get at what time how muchRead more

TTT curve stands for the Time-Temperature-Transformation curve. This is usually experimental. When we observe the fraction transformation vs time curve we get an S-Shape curve. This can be related with the Temp. Vs time(log scale) curve at different temperatures and we can get at what time how much fraction has converted.This gives us an idea of start time and finish time which on plotting takes a C-Shape. This can also be interpreted by the fact that at lower temperatures (or higher undercooling) it takes lower time for transformation and vice versa.(See by temp. Vs rate curve). The C curve for TTT is usually represented by superimposing both Pearlite and Bainite region.

Now in the CCT curve which is a Continuous Cooling Transformation curve. The lower curve is not formed because of the insufficient time for upper range of transformation and hence it Directly forms Martensite.

Talking about the Bainite transformation rate which decreases with decreasing temperature (NOTE: bainitic transformation is not a full diffusion process) and due to small time no significant Bainite is formed which is visible on the CCT curve.

See lessAt close to Ae1 temperature, coarse pearlite forms at close to Ae1 temperature due to low driving force or nucleation rate. At higher under coolings or lower temperature finer pearlite forms. At the nose of TTT diagram very fine pearlite forms Close to the eutectoid temperature, the undercooling isRead more

Ae1 temperature due to low driving force or nucleation rate.

forms.

Close to the eutectoid temperature, the undercooling is low so

that the driving force for the transformation is small. However,

as the undercooling increases transformation accelerates until

the maximum rate is obtained at the “nose” of the curve.

continues to increase but the reaction is now impeded by slow

diffusion.

This is why TTT curve takes on a “C” shapewithmost rapid overall transformation at some intermediate

temperature.

Bottom of CCT diagram not appearing due diffusionless transformation.During Heat Treatment of various steel smple Heating about Austenitising temperature and hold(Isothermally) at different temperature till the transformation complete from Austenite to pearlite,Bainite and Martensite. we found the curve of transformation start and transformation complete. This curveRead more

CCT curve in heat treatment means continuous cooling curves. In this sample is taken to austenite temperature then cooled down to room temperature with a single cooling rate. We are maintaining a cooling rate so there is no chance of incubation so the CCT curve doesn't represent after nose. And in CRead more

CCT curve in heat treatment means continuous cooling curves. In this sample is taken to austenite temperature then cooled down to room temperature with a single cooling rate. We are maintaining a cooling rate so there is no chance of incubation so the CCT curve doesn’t represent after nose.

And in CCT there is no formation of banite.