\A\ The Iron-Carbon System

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Critical Temperatures and Transformation Diagrams

The critical temperatures of steel define either the onset or completion of a phase transformation. Three important critical temperatures in steel heat treatment are:

  • A1, Critical temperature for the start of the transformation to austenite, and the so-called eutectoid temperature, which is the minimum temperature required for austenite to form. A1 is also referred to as the lower critical temperature.

  • A3, Critical temperature for complete transformation of the + two-phase mixture into 100% austenite (). Referred to as the upper critical temperature, A3 is the high temperature boundary of the + solid-solution phase region with carbon contents below the eutectoid composition of 0.77 %C.

  • Acm, Critical temperatures for complete transformation of the  + Fe3 two-phase mixture into 100% austenite (). The Acm critical temperature is the high temperature boundary of the cementite in solid solution with austenite in hypereutectoid steels (C > 0.77 %C).

Between A1 and A3, is the Curie temperature (A2) for transition between magnetic and non-magnetic ferrite. The A2 temperature is 770 C (1418 F).

One important factor is whether the steel is being heated or cooled and at what rate. Because diffusion is sluggish in solids, the rate of heating and cooling can be important factors in determining the critical temperatures for both the onset and completion of equilibrium transformations. Therefore, sometimes the subscripts c, r, and e are included to denote heating, cooling, or equilibrium conditions (Table 3). For example, critical temperatures for the start and completion of the transformation to austenite under conditions of thermal equilibrium are denoted, respectively, by Ae1 and Ae3.

For a given steel, the critical temperatures depend on whether the steel is being heated or cooled. In practice, critical temperatures occur at lower temperatures during cooling than during heating and depend on the rate of change of temperature. Table 4 provides approximate critical temperatures for selected steels, measured at heating and cooling rates of 28 °C/h (50 °F/h). The equilibrium critical temperatures generally lie about midway between those for heating and cooling at equal rates. Because annealing treatments may involve various ranges of heating and cooling rates in combination with isothermal treatments, the less specific terms A1, A3, and Acm are used when discussing the basic concepts.

It is possible to calculate upper (A3) and lower (A1) critical temperatures using the actual chemical composition of the steel. The following equations will give an approximate critical temperature for a hypoeutectoid steel (J. Iron Steel Inst., Vol 203, 1965, p 721):

Ac1 (°C) = 723 - 20.7(% Mn) - 16.9(%Ni) + 29.1(%Si) - 16.9(%Cr)

[with Standard deviation of ± 11.5 °C]

Ac3 (°C) = 910 - 203 - 15.2(% Ni) + 44.7(% Si) + 104(% V) + 31.5(% Mo)

[with Standard deviation = ± 16.7 °C]

The presence of other alloying elements will also have marked effects on these critical temperatures.

Transformation Diagrams.

The kinetic aspects of phase transformations are as important as the equilibrium diagrams for the heat treatment of steels. Any practical heat treatment requires heating and/or cooling. Therefore, another important tool in steel heat treatment is the use of transformation diagrams that plot phase transformation as a function of temperature and either time or the rate of temperature change.

In general, there are four different types of transformation diagrams that can be distinguished. These include:

  • Isothermal transformation diagrams describing the formation of austenite during heating (sometimes as ITh diagrams)

  • Isothermal transformation (IT) diagrams, also referred to as time-temperature-transformation (TTT) diagrams, describing the decomposition of austenite

  • Continuous heating transformation (CHT) diagrams

  • Continuous cooling transformation (CCT) diagrams

Steel heat treatment methods often rely on the use of TTT diagrams, which were developed by Bain and were the basis of understanding the conditions of bainite formation in steels (see the section Bainite). However, continuous cooling diagrams are very practical tools in heat treatment. Continuous cooling diagrams plot phase formation as a function of cooling rate (Fig.22). This type of diagram is very useful, because cooling rates are a key process factor in quenching or cooling operations. In fact, CCT diagrams may also plot cooling rates in terms of section size and cooling medium (Fig. 22). This enhances the practical use of CCT diagrams as a tool based on part thickness and cooling medium. Computer simulations are also becoming more effective in modeling phase transformations.
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