Time-Temperature-Transformation (TTT) Diagrams

A Time-Temperature-Transformation (TTT) diagram represents the isothermal phase transformations of a material. It is crucial in heat treatment and metallurgical processes to predict microstructural changes, such as the formation of pearlite, bainite, and martensite in steels.

1. Purpose of TTT Diagrams

✅ Understand phase transformations under isothermal conditions
✅ Determine the cooling rate required for desired microstructure
✅ Optimize heat treatment processes (annealing, quenching, tempering)
✅ Prevent undesirable phases in materials

2. Features of a TTT Diagram

📌 X-Axis: Time (log scale, seconds to hours)
📌 Y-Axis: Temperature (°C)
📌 Curves: Represent different transformation start and finish lines
📌 Critical Points:

• Austenitizing Temperature (Above A₃/A₁ Line) – Starting point before cooling

• Nose of the Curve (~500–600°C) – Fastest transformation to pearlite

• Ms (Martensite Start) and Mf (Martensite Finish) Lines – Indicate martensitic transformation

• 3. Key Transformations in a TTT Diagram

• (a) Pearlite Formation (~600–700°C)

• 🔹 Reaction: γ(Austenite)→α(Ferrite)+Fe3C(Cementite)
  🔹 Slow cooling → Coarse Pearlite (Soft, ductile)
  🔹 Faster cooling → Fine Pearlite (Stronger, harder)

• (b) Bainite Formation (~250–500°C)

• 🔹 Reaction: γ(Austenite)→Bainite
  🔹 Upper Bainite (~400–500°C): Coarse structure, moderate hardness
  🔹 Lower Bainite (~250–400°C): Finer structure, higher strength and toughness

• (c) Martensite Formation (<Ms Temperature)

• 🔹 Reaction: γ(Austenite)→Martensite
  🔹 Occurs at high cooling rates (quenching)
  🔹 Very hard and brittle phase, requires tempering for toughness

• Industrial Applications of TTT Diagrams

• ✅ Heat Treatment Processes (quenching, tempering, annealing)
  ✅ Tool and Die Manufacturing (hard martensitic steels)
  ✅ Automotive & Aerospace (control of strength and toughness)
  ✅ Welding (avoid unwanted transformations causing brittleness)