Heat treatment is a controlled way to change the properties of steel and other alloys by changing temperature, holding time, and cooling rate. Engineers use it to adjust hardness, toughness, ductility, wear resistance, and internal stress. A Time-Temperature-Transformation diagram, or TTT diagram, helps predict what microstructures form when a hot alloy is held or cooled through different temperature ranges.
This matters because the microscopic arrangement of atoms and phases strongly controls how a part performs in service.
A TTT diagram plots temperature on the vertical axis and time on a logarithmic horizontal axis, showing when austenite begins and finishes transforming into phases such as pearlite, bainite, or martensite. Slow cooling usually allows diffusion and forms softer structures such as coarse pearlite, while faster cooling can bypass diffusion-controlled transformations and produce hard martensite. Annealing, normalizing, quenching, and tempering are heat treatment routes that use these transformations in different ways.
By comparing cooling paths with the TTT curves, engineers choose processing conditions that create the desired balance of hardness and toughness.
Key Facts
- A TTT diagram shows transformation behavior at constant temperature after austenitizing.
- The horizontal axis is usually logarithmic time, often in seconds, because transformations can range from fractions of a second to hours.
- Pearlite and bainite form by diffusion-controlled transformations, so time and temperature strongly affect their formation.
- Martensite forms by a diffusionless transformation when cooling reaches the martensite start temperature, Ms.
- Cooling rate = ΔT/Δt, and faster cooling is more likely to avoid the pearlite and bainite regions.
- Tempering reduces brittleness in quenched martensite by reheating below the eutectoid temperature, improving toughness while lowering hardness.
Vocabulary
- Austenite
- A high-temperature phase of steel with a face-centered cubic crystal structure that can dissolve more carbon than ferrite.
- Pearlite
- A layered mixture of ferrite and cementite that forms when austenite transforms slowly at moderate temperatures.
- Bainite
- A fine microstructure of ferrite and carbide that forms at temperatures below pearlite formation but above martensite formation.
- Martensite
- A very hard, brittle phase formed by rapid cooling that traps carbon in a distorted crystal structure.
- Quenching
- A heat treatment step in which hot metal is cooled rapidly, often in water, oil, or air, to change its microstructure.
Common Mistakes to Avoid
- Reading the time axis as linear instead of logarithmic, which leads to large errors when estimating transformation times from a TTT diagram.
- Assuming quenching always gives the best material, which is wrong because untempered martensite can be too brittle for many engineering parts.
- Confusing TTT diagrams with continuous cooling transformation diagrams, which is wrong because TTT diagrams are based on holding at constant temperature.
- Ignoring austenitizing before heat treatment, which is wrong because the starting austenite condition affects the final microstructure and properties.
Practice Questions
- 1 A steel sample is austenitized at 800 °C and quenched to 500 °C, then held for 100 s. If the TTT diagram shows pearlite begins at 20 s and finishes at 200 s at 500 °C, has transformation started, finished, or partially occurred?
- 2 A part cools from 800 °C to 200 °C in 6 s during quenching. Calculate the average cooling rate in °C/s using cooling rate = ΔT/Δt.
- 3 Two identical steel parts are austenitized, then one is furnace cooled and the other is water quenched and tempered. Explain which one is likely harder and which one is likely tougher, using microstructure in your reasoning.