The other main group is the heat treatable aluminium alloys Steels can be heat

The other main group is the heat treatable aluminium

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The other main group is the heat-treatable aluminium alloys. Steels can be heat treated because of the structural changes that can take place within solid iron- carbon alloys. The various heat-treatment processes appropriate to plain carbon steels are: o Annealing; o Normalising; o Hardening; and o Tempering. In all the above processes the steel is heated slowly to the appropriate temperature for its carbon content, soaked and then cooled. It is the rate of cooling which determines the ultimate structure and properties that the steel will have, providing that the initial heating has been slow enough for the steel to have reached phase equilibrium at its process temperature.
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~ Page 19 of 73 ~ The Iron-Carbon Equilibrium Diagram A study of the constitution and structure of all steels and irons must first start with the iron- carbon equilibrium diagram. Many of the basic features of this system (Fig. 1) influence the behaviour of even the most complex alloy steels. For example, the phases found in the simple binary Fe-C system persist in complex steels, but it is necessary to examine the effects alloying elements have on the formation and properties of these phases. The iron-carbon diagram provides a valuable foundation on which to build knowledge of both plain carbon and alloy steels in their immense variety. Fig. 1 The iron-carbon diagram. It should first be pointed out that the normal equilibrium diagram really represents the metastable equilibrium between iron and iron carbide (cementite). Cementite is metastable, and the true equilibrium should be between iron and graphite. Although graphite occurs extensively in cast irons (2-4 wt % C), it is usually difficult to obtain this equilibrium phase in steels (0.03-1.5 wt %C). Therefore, the metastable equilibrium between iron and iron carbide should be considered, because it is relevant to the behavior of most steels in practice.
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~ Page 20 of 73 ~ The much larger phase field of γ-iron (austenite) compared with that of α-iron (ferrite) reflects the much greater solubility of carbon in γ-iron, with a maximum value of just over 2 wt % at 1147 °C (E, Fig.1). This high solubility of carbon in γ-iron is of extreme importance in heat treatment, when solution treatment in the γ-region followed by rapid quenching to room temperature allows a supersaturated solid solution of carbon in iron to be formed. The α-iron phase field is severely restricted, with a maximum carbon solubility of 0.02 wt% at 723 °C (P), so over the carbon range encountered in steels from 0.05 to 1.5 wt%, α-iron is normally associated with iron carbide in one form or another. Similarly, the δ-phase field is very restricted between 1390 and 1534 °C and disappears completely when the carbon content reaches 0.5 wt% (B). There are several temperatures or critical points in the diagram, which are important, both from the basic and from the practical point of view.
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