Conditioning of steel by thermal or heat treatment relies on the different mechanical properties which are exhibited by the various structures present in plain carbon steels. Figure 1 illustrates the equilibrium structures present at different temperatures with changing carbon content for the iron-carbon system. Figure 2 demonstrates the effect of chill rate upon final structure, and is called a time-temperature-transformation or TTT diagram. Essentially, when cooling from the melt or high temperature phases, there is an incubation period below the equilibrium melting point or transformation temperature (723°C in the case of the steel shown) before the transformation occurs. This undercooling provides the driving force for the transformation. During a furnace cool (i.e. slow cooling rate) the austenite will start to transform to ferrite and cementite after sufficient undercooling, resulting in a microstructure of coarse pearlite. With a high cooling rate such as that experienced with a water quench, it is possible to miss the 'nose' of the TTT curve altogether. Martensite is produced starting at about 220°C for the composition shown. The finish temperature of the martensite reaction for certain alloys can be below room temperature, so that at room temperature some unstable austenite is present.
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| Figure 1. Part of the equilibrium phase diagram for the Fe-C system. |
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| Figure 2. The TTT diagram for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised at 900°C. |
The design of steels and cooling conditions to produce required amounts of martensite has its own branch of technology, 'hardenability'. In plain carbon steels, the nose of the TTT curve occurs at very short times, hence fast cooling rates are required to produce martensite. In thin sections of steel, a rapid quench can produce distortion and cracking. In thick plain carbon steels, it is not possible to produce an all martensitic structure. All common alloying elements shift the nose of the TTT diagram to longer times, thus allowing the development of martensite in thick sections at slower cooling rates.
