Chart for one specific steel plotting the start and finish of austenite transformation against log time at constant temperatures, which gives the C-shaped curves for pearlite and bainite plus the Ms line. The nose of the curve marks the shortest time to transformation, and a quench must beat it to make martensite. Because real parts cool continuously, TTT charts guide isothermal processes like austempering more than ordinary quenches.
Pull up the TTT diagram for 1080, the nose is under a second, so you're not austempering a section that thick.
Also heard as
- isothermal transformation diagram
- IT diagram
- C-curve
- S-curve
Do not mix it up with
- CCT diagram: A TTT diagram assumes the steel is held at one temperature; a CCT diagram follows steady cooling, which matches real quenches better.
Related terms
CCT diagram
Continuous-cooling-transformation chart predicting which structures form as steel cools at various steady rates.
Austempering
Quenching steel into a salt bath above Ms and holding it until the austenite fully transforms to bainite.
Bainite
A tough mix of ferrite and fine carbides that forms between the pearlite and martensite temperature ranges.
Hardenability
A steel's ability to form martensite to a given depth, set by its chemistry and grain size, not its hardness.