Understanding the Cooling Transformation of Steel

 

I Cooling Methods

 

Steel primarily undergoes two types of cooling methods: isothermal cooling and continuous cooling.

 

Isothermal Cooling

In this method, steel is heated to an austenitic state and then quickly cooled to a specific temperature, where it is held for a certain period, allowing the austenite to transform before further cooling to room temperature. This approach enables precise control over the transformation temperature and time, resulting in specific microstructures and properties.

 

▲ Isothermal Cooling and Temperature

 

Continuous Cooling

Here, the steel, initially in an austenitic state, is continuously cooled to room temperature at different rates (e.g., air cooling, furnace cooling, oil cooling, water cooling, etc.). The cooling rate in this method influences the transformation process of austenite and the final microstructure.

 

▲ Continuous Cooling and Temperature

 

 

II Isothermal Transformation Curves of Supercooled Austenite

 

▲ Supercooled austenite isothermal transformation curve of eutectoid steel

 

The C-curve (also known as the isothermal transformation curve of supercooled austenite or TTT curve) for eutectoid carbon steel illustrates the relationship between the transformation temperature, time, and transformation products when the steel is in a supercooled austenitic state.

 

Division of C-Curve Regions

Supercooled Austenite Zone: Located to the left of the transformation start line on the C-curve, this zone represents the area where supercooled austenite has not yet transformed.

 

Transformation Products Zone: Situated to the right of the transformation end line and above the Ms point, this zone indicates where supercooled austenite has transformed into its products.

 

Transformation Progress Zone: This zone lies between the transformation start and end lines, indicating the ongoing transformation process of supercooled austenite.

 

Lines of the C-Curve and Their Physical Significance

Transformation Start Line: A curve connecting the points where the supercooled austenite begins to transform, showing the time it takes for austenite to start transforming at different temperatures.

 

Transformation End Line: Indicates the time required for austenite to complete its transformation at different temperatures.

Ms Line: A horizontal line denoting the starting temperature for martensitic transformation, marking the point where austenite begins to transform into martensite.

 

Mf Line (sometimes called the Mf Point): A horizontal line representing the end temperature for martensitic transformation, where austenite completely transforms into martensite.

 

Significance of the C-Curve's Nose

At approximately 550°C, the C-curve of eutectoid carbon steel shows a bend known as the curve's nose. The corresponding temperature is referred to as the nose temperature, where the transformation rate of austenite is the fastest. Above this nose, austenite primarily undergoes pearlitic transformation; below this nose, bainitic transformation occurs; and below the Ms point, martensitic transformation takes place.

 

Main Factors Influencing the Shape and Position of the C-Curve

Chemical Composition of Steel: Carbon content and alloying elements affect the stability and transformation process of austenite. Generally, an increase in carbon content shifts the C-curve to the right, while alloying elements (except Co and Al) increase the stability of austenite and alter the C-curve's shape.

 

Microstructure of Austenite: Finer austenitic grains provide more grain boundaries per unit area, facilitating the nucleation and growth of transformation products, thus impacting the C-curve's position and shape.

 

Austenitization Temperature and Holding Time: Higher austenitization temperatures and longer holding times lead to coarser austenitic grains, shifting the C-curve further to the right.

 

 

III Continuous Cooling Transformation Curve of Supercooled Austenite

 

▲ Continuous Cooling Transformation Curve of Supercooled Austenite

 

▲ The parameters corresponding to each letter

 

The Continuous Cooling Transformation Curve (CCT curve) is an important tool used to describe the phase transformation process of supercooled austenite under continuous cooling conditions. It reflects the transformation patterns of supercooled austenite at different cooling rates and serves as a basis for analyzing the microstructure and performance of transformation products. It is also an essential reference for formulating heat treatment processes.

 

Definition and Significance of the CCT Curve

The CCT curve, or Continuous Cooling Transformation curve, records the start and end temperatures and times at which supercooled austenite transforms into different phases (such as pearlite, bainite, martensite, etc.) under different cooling rates. This curve is significant for understanding the phase transformation process of steel, optimizing heat treatment processes, and predicting the properties of steel components.

 

Method of Determining the CCT Curve

The method of determining the CCT curve usually involves the following steps:

 

Sample Preparation: Select representative steel samples and subject them to austenitizing treatment to ensure that all samples have the same initial microstructure before measurement.

 

Continuous Cooling: Continuously cool the austenitized samples at different constant rates while recording the temperature and time data during the cooling process.

 

Transformation Product Analysis: During or after cooling, determine the type and quantity of transformation products through metallographic analysis or other methods.

 

Curve Plotting: Plot the start and end temperature and time data for transformations at different cooling rates on a "temperature-time logarithmic" coordinate chart to form the CCT curve.

 

Characteristics of the CCT Curve

Transformation Regions: The CCT curve generally includes regions for pearlite transformation, bainite transformation (for certain steels), and martensite transformation. These regions correspond to the phase transformation processes occurring at different cooling rates.

 

Critical Cooling Rates: Two important critical cooling rates exist within the CCT curve: the upper critical cooling rate (Vk) and the lower critical cooling rate (Vk'). The upper critical cooling rate is the minimum rate required to ensure that austenite does not decompose during continuous cooling and is fully supercooled into the martensite region. The lower critical cooling rate is the maximum rate that ensures austenite fully decomposes without undergoing martensitic transformation during continuous cooling.

 

Transformation Complexity: Continuous cooling transformation is more complex than isothermal transformation. Since the continuous cooling process passes through various transformation temperature regions sequentially, multiple transformations may occur in sequence, and different cooling rates may lead to different transformation products and relative quantities.

 

Applications of the CCT Curve

Heat Treatment Process Formulation: The CCT curve can provide insights into the transformation products and performance changes of steel at different cooling rates, allowing for the formulation of reasonable heat treatment parameters such as heating temperature, holding time, and cooling rate.

 

Performance Prediction: The CCT curve can be used to predict the properties of steel components under specific heat treatment conditions, such as hardness, strength, and toughness.

 

Material Selection: During material selection, the CCT curves of different materials can be compared to evaluate their heat treatment performance and potential applications.

 

 

IV Types of Cooling Transformation

 

▲ Different transformations below the A temperature

 

▲ Different transformations below the A temperature

 

The cooling transformations of steel mainly include pearlite transformation, bainite transformation, and martensite transformation.

 

Pearlite Transformation: This high-temperature diffusion transformation is completed through nucleation and growth processes. The morphology of pearlite changes with decreasing formation temperature; the interlamellar spacing decreases, and the strength and hardness increase while maintaining good ductility and toughness.

 

Bainite Transformation: Occurring in the medium temperature range, bainite transformation is a semi-diffusion transformation. Bainite exists in various forms, such as upper bainite and lower bainite, and its properties lie between those of pearlite and martensite.

 

Martensite Transformation: This low-temperature, non-diffusion transformation results in martensite characterized by high hardness and strength but lower ductility and toughness. Martensite can be lath-like or plate-like, corresponding to low-carbon and high-carbon steels, respectively.

 

 

V Relationship Between Continuous Cooling Transformation and Isothermal Transformation

 

▲ Comparison of Isothermal Cooling Transformation Curve of Eutectoid Steel and Transformation Structure

 

Relationship

Both continuous cooling transformation and isothermal transformation are important methods of austenite phase transformation in heat treatment. They are crucial for understanding the phase transformation behavior of materials, formulating heat treatment processes, and predicting material properties. In certain cases, the continuous cooling transformation process can be approximately analyzed using the isothermal transformation diagram (C curve) due to the relative difficulty in determining the continuous cooling transformation diagram.

 

Differences

Transformation Conditions: Continuous cooling transformation occurs under continuously changing temperature conditions, while isothermal transformation takes place at a specific constant temperature.

 

Transformation Process: During continuous cooling, the supercooled austenite completes its phase transformation within a temperature range, potentially resulting in uneven transformation. The initially transformed microstructure may be coarser, while the later-transformed microstructure may be finer, often leading to a mixture of various microstructures. Isothermal transformation, on the other hand, occurs under constant temperature conditions, leading to relatively uniform phase transformation.

 

Transformation Products: Due to the differing transformation conditions, the types and proportions of transformation products obtained from the two methods can vary. For example, in eutectoid steel, continuous cooling might result only in pearlitic transformation without bainite, while isothermal transformation conditions might yield a richer variety of phase transformation products.

 

Applications and Selection

In practical production, the choice between continuous cooling transformation and isothermal transformation depends on the specific material's chemical composition, microstructure, and the desired heat treatment effects and performance requirements. Continuous cooling transformation is typically used in large-scale production and continuous processing due to its simplicity and lower cost. In contrast, isothermal transformation is more suitable for scenarios requiring precise control over the phase transformation process and product types, such as in the preparation of high-end materials and the production of parts with special performance requirements.

 

 

VI. Factors Influencing Cooling Transformation

 

Austenite Composition

The carbon content and alloying elements affect the stability and transformation process of austenite. For instance, increasing carbon content shifts the C curve to the right, and alloying elements (except Co and Al) dissolved in austenite enhance its stability and alter the shape of the C curve.

 

Austenite Microstructure

Finer austenite grains, with more grain boundaries per unit area, facilitate the nucleation and growth of transformation products.

 

Stress and Plastic Deformation

Supercooled austenite under tensile stress accelerates transformation, whereas compressive stress has the opposite effect. Plastic deformation also accelerates the transformation of austenite.

 

 

VII. Applications of Cooling Transformation

 

Understanding the cooling transformation of steel is crucial for formulating heat treatment processes. By controlling the cooling method and rate, steel with different microstructures and properties can be produced to meet various requirements. For example, the quenching process rapidly cools steel to form a martensitic structure, thereby increasing hardness and strength; the tempering process involves heating and holding after quenching to relieve internal stresses and improve toughness.

 

Cooling transformation in steel is a critical aspect of the heat treatment process, influenced by numerous and complex factors. In practical applications, selecting the appropriate cooling method and rate based on specific conditions is necessary to achieve the desired microstructure and properties.