What is the difference between the austenite isothermal transformation curve and the continuous cool

The austenite isothermal transformation curve (TTT curve) and the continuous cooling transformation curve (CCT curve) are the core tools for describing the cooling transformation laws of austenite. However, there are significant differences between the two in terms of definitions, experimental methods, curve characteristics, transformation products, and application scenarios. The following provides a detailed comparison from multiple dimensions: 

1. Definition and Nature Difference

TTT Curve (Isothermal Transformation Curve)

The full name is "Time-Temperature-Transformation Curve", which describes the transformation law of austenite under "constant temperature": quickly cool the austenitized steel to a certain temperature below A₁ (such as 300℃, 500℃, etc.) and maintain a constant temperature, then record the start time of transformation, the end time of transformation, and the relationship between the transformation and the transformation products. The horizontal coordinate of the curve is time (usually taken as logarithmic), and the vertical coordinate is temperature, reflecting "the influence of time under constant temperature on the transformation".

CCT Curve (Continuous Cooling Transformation Curve)

The full name is "Continuous Cooling Transformation Curve", which describes the transformation law of austenite during "continuous cooling process": cool the austenitized steel at different speeds (such as air cooling, oil cooling, water cooling, etc.), and record the start temperature of transformation, the end temperature of transformation, and the relationship between time and temperature. The horizontal coordinate of the curve is time (logarithmic), and the vertical coordinate is temperature, reflecting "the combined influence of temperature and time on the transformation during continuous cooling". 

II. Different Experimental Methods

Measurement of TTT Curve

The multi-sample method should be adopted:

1. Take multiple samples of the same composition, all heated to complete austenitization;

2. Cool each sample rapidly to a preset isothermal temperature (such as 350℃, 500℃, etc., all lower than A₁), and hold it at this temperature for a certain period;

3. Through metallographic observation or expansion method, record "the time when transformation begins (maturation period) and ends at this temperature";

4. Connect the transformation time points at different temperatures to form the TTT curve.

Measurement of CCT Curve

The single-sample continuous cooling method should be adopted:

1. After the single sample is heated to complete austenitization, continuously cool it at a certain constant speed (such as 10℃/s, 50℃/s, etc.);

2. Monitor the volume change during the transformation process in real time (reflecting the beginning and end of the transformation) using a thermal expansion instrument or high-temperature metallographic microscope;

3. Change the cooling speed, repeat the experiment, and record the "relationship between the transformation starting temperature, transformation ending temperature and time" under different cooling speeds;

4. After organizing the data, draw the CCT curve. 

III. Different Curve Shapes and Characteristics

Both have the abscissa as time (usually in logarithmic form due to the wide time span), and the ordinate as temperature. However, the shapes of the curves and their key characteristics are significantly different: Picture One

IV. Differences in Transformation Products

Due to the different transformation conditions (isothermal vs. continuous cooling), the corresponding transformation products of the two are significantly different:

TTT Curve: At the same temperature, the transformation products are single and uniform.

For example: At 600°C isothermal, only pearlite is generated; at 300°C isothermal, only lower bainite is generated; at isothermal below the Ms point, only martensite is generated.

CCT Curve: During continuous cooling, austenite may undergo multiple stages of transformation at different temperature intervals, and the products may be a mixed structure.

For example: If the cooling rate is between the critical rate of pearlite and bainite, it may first generate some pearlite in the high-temperature zone, and the remaining austenite continues to cool to the medium-temperature zone to generate bainite, and the final product is "pearlite + bainite"; if the cooling rate is slightly lower than the critical rate of martensite, it may generate "a small amount of pearlite + martensite". 

V. Different Application Scenarios

The two have completely different applications corresponding to different heat treatment processes:

TTT Curve: Primarily used to guide isothermal heat treatment processes, which are "rapidly cooling to a certain temperature and then maintaining at a constant temperature".

Typical applications:

Isothermal annealing (annealing at the pearlite transformation zone to obtain uniform pearlite and reduce hardness);

Isothermal quenching (annealing at the bainite transformation zone to obtain lower bainite and enhance strength and toughness).

CCT Curve: Primarily used to guide continuous cooling heat treatment processes, which are "oxygenation followed by continuous cooling" (more common in actual production).

Typical applications:

Normalizing (air cooling, controlling the cooling speed according to the CCT curve to obtain uniform subgrain structure);

Quenching (water cooling / oil cooling, determining the critical cooling speed according to the CCT curve to ensure the formation of martensite);

Spheroidizing annealing (slow continuous cooling to obtain spherical pearlite). 

VI. Summary of Core Differences

Image 2 

Summary

The TTT curve and the CCT curve are "twin tools" for studying austenite transformation. However, the TTT curve focuses on "the time effect at constant temperature", while the CCT curve focuses on "the temperature-time synergy during continuous cooling". In actual production, since continuous cooling processes (such as quenching and normalizing) are more common, the CCT curve is more widely applied; while the TTT curve provides a foundation for understanding the transformation mechanism and optimizing isothermal processes. The combination of the two can comprehensively grasp the transformation laws of austenite, thereby precisely controlling the microstructure and properties of steel materials.