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Using knowledge of the TTT diagrams, steels can be manufactured in different ways to produce a different properties for diverse applications.
Martempering
Martempering is a method of heat treatment of steels in order to produce martensite in a way to avoid cracking under internal stresses.
To achieve this the steel is quenched rapidly and held just above the martensite start temperature (typically in an salt/ oil bath) to allow thermal equilibrium in the sample. The steel is then cooled to below the martensite finish temperature before the transition time reaches the curve to prevent austenite forming bainite. By allowing the centre of the steel to reach thermal equilibrium before the martensite starting temperature, it results in martensite being formed at both the surface and centre at the same time as these regions cool with similar temperatures. This reduces internal stresses caused by the expansion when martensite is produced. After this stage tempering is typically carried out increasing the temperature to allow trapped carbon to diffuse forming tempered martensite.
[1] Figure 44
Austempering
Austempering is a method of converting all of austenite to bainite without any distortions or cracking by holding the steel just above the martensite starting temperature.
Austenite is cooled rapidly using a bath that is maintained at a temperature just above the martensite starting temperature and all cooling throughout the sample is done before the nose of the diagram. This allows thermal equilibrium throughout the steel to reduce internal stresses as bainite forms. This temperature is maintained until all of the austenite is converted to bainite where it is then left to cool at room temperature in air.
[1] Figure 45
Isothermal Annealing
Isothermal annealing is a process used to produce pearlite with uniform spacing of the alpha and cementite phases.
This is achieved by putting the steel directly into an annealing furnace which is maintained at a temperature just below the eutectic temperature allowing for thermal equilibrium throughout the steel. It is held at this temperature for a long period of time until all of the austenite has converted to pearlite. The nucleation of both alpha and cementite phases is equal resulting in uniform alternating phases. After this stage the steel can be cooled at room temperature as the rate will not impact the end product as all austenite has been converted.
[1] Figure 46
Patenting
Patenting is used to produce steels with both fine pearlite and upper bainite phases often used for ropes and springs.
The steel is rapidly cooled in a bath that is maintained at the temperature at the nose of the diagram (550°C). This allows the austenite to be converted into both pearlite and bainite equally (50% of each phase). Once enough time has elapsed for all the austenite to be converted the steel may then be cooled at room temperature as the rate does not impact the end product as all austenite has been converted.
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