Steels - An Introduction to Heat Treatment
Steels - An Introduction to Heat
Treatment
Apr 24 2001
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.
Figure 1. Part of the equilibrium phase diagram for the Fe-C system.
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Steels - An Introduction to Heat Treatment
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.
Primary author: Justin Furness
Source: Materials Information Service
For more information on this source please visit The Institute of Materials.
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