The TTT curve (1080/1084)

Summary

The video script delves into the intricate process of heat treating steel, focusing on the Time-Temperature-Transformation (TTT) diagram. It explains how the diagram, which includes time on a logarithmic scale, predicts phase transformations in steel at various temperatures and cooling rates. The key to hardening steel is highlighted as avoiding pearlite formation by rapid cooling, while the diagram also illustrates the formation of martensite, a phase that occurs when austenite cools below the Ms temperature, creating a hard, body-centered tetragonal structure.

Takeaways

• ⏱️ The TTT (Time-Temperature-Transformation) diagram is a critical tool for understanding the phase transformations in steel during heat treatment.
• 📊 The TTT diagram incorporates time as a variable to show how different phases of steel form at various temperatures and cooling rates.
• 🔍 The time scale on the TTT diagram is logarithmic, ranging from fractions of a second to days, to accommodate the wide range of transformation rates.
• 🌡️ The diagram indicates that austenite, a phase of steel, can exist below the normal A1 temperature due to the time required for diffusion.
• 🔑 The key to hardening steel is to cool it at a rate fast enough to prevent pearlite formation, which is a softer phase.
• 📉 The TTT diagram shows a critical zone where pearlite forms the fastest, and avoiding this zone is essential for achieving a hard steel.
• 🛠️ When intentionally cooled slowly through the critical zone, complete transformation to pearlite occurs, resulting in soft steel.
• 🔬 The M sub s line on the TTT diagram represents the Martensite Start temperature, indicating the onset of martensite formation.
• 💎 Martensite is a hard phase formed when the cooling rate is fast enough to prevent carbon diffusion, leading to a body-centered tetragonal structure.
• 🔄 The formation of martensite is not time-dependent but rather controlled by the rate of cooling and the strain energy in the steel.
• 🛑 The TTT diagram illustrates that the key to achieving different properties in steel, such as hardness, is through controlling the cooling rate and avoiding phase transformations like pearlite.

Q & A

What is the purpose of the TTT diagram in the context of steel heat treating?

The TTT (Time-Temperature-Transformation) diagram is used to determine the phases that will form in steel at any given temperature when cooled from the austenitizing temperature. It provides a clear picture of the transformation rates and the time required for these transformations to occur.

Why is the time scale on the TTT diagram logarithmic rather than linear?

The time scale is logarithmic to accommodate the wide range of transformation rates, from fractions of a second to days. This allows the diagram to fit all the necessary information on a single chart.

What does the term 'austenitic' refer to in the context of steel?

Austenitic refers to the state of steel when it is in the austenite phase, which is a face-centered cubic structure. This phase is typically present at high temperatures and is characterized by its ability to dissolve carbon.

Why is it important to understand the transformation from austenite to other phases in steel?

Understanding the transformation from austenite to other phases is crucial because it determines the mechanical properties of the steel, such as hardness, toughness, and ductility. Different phases like pearlite and martensite have distinct properties that are desirable for different applications.

What is the significance of the pearlite formation zone on the TTT diagram?

The pearlite formation zone indicates the temperature and time at which pearlite, a mixture of ferrite and cementite, forms rapidly. Pearlite is generally softer and less hard than other phases like martensite, so controlling this zone is essential for achieving the desired hardness in steel.

How does the rate of cooling affect the formation of pearlite in steel?

If the steel is cooled slowly through the pearlite formation zone, it will transform into pearlite, resulting in a softer steel. To prevent pearlite formation and achieve a harder steel, the steel must be cooled quickly enough to outpace the diffusion process.

What is the role of diffusion in the transformation of steel phases?

Diffusion is the process by which atoms move and rearrange themselves to form new phases. In the context of steel, diffusion controls the rate at which austenite transforms into other phases like pearlite or martensite, and it is influenced by temperature.

What is meant by the term 'lamellar annealing' in the script?

Lamellar annealing is a process where steel is cooled slowly through the pearlite formation zone, allowing complete transformation to pearlite. This results in a softer steel, which is desirable for certain applications.

What is the significance of the M sub s line on the TTT diagram?

The M sub s line, or martensite start line, indicates the temperature below which austenite begins to transform into martensite. This transformation is rapid and does not involve diffusion, making it crucial for achieving a hard steel.

How does the formation of martensite differ from other phase transformations in steel?

Martensite formation is unique because it is a diffusionless transformation that occurs rapidly at lower temperatures. It involves a rearrangement of the crystal lattice from face-centered cubic to body-centered tetragonal, trapping carbon atoms and resulting in a very hard steel.

What is the key to hardening steel according to the script?

The key to hardening steel is to cool it at a rate fast enough to keep any pearlite from forming. This traps the carbon in the austenite solution, allowing the formation of martensite, which is harder than pearlite.