Wever categorises iron binary equilibrium diagrams into four main classes:
1. Phase Diagram with Open γ-Field:
Alloying elements like Ni and Mn raise Ae4 and lower Ae3 as well as Ae1 temperatures, narrowing the range of α-phase and extending the range of γ-phase, and when present in sufficiently high concentrations, point 0 in Fig 1.42 (1) and more, make γ-phase to be present even at room temperature.
Thus, iron having such elements in concentrations higher than (point 0) shown in Fig. 1.42 (1) undergo no phase transformations (α Û γ) and have, at all temperatures, a solid solution of the alloying element in γ-iron. Such alloys are called austenitic alloys.
Alloys in range 0 to P undergoing partly α Û γ transformation, are called semi-austenitic alloys. Even in moderate concentrations, alloys quenched from γ-phase region are able to retain metastable austenite at room temperature.
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The two elements, Ni and Mn are very useful in obtaining austenitic steels. Austenitic stainless steels (18/8) and Hadfield Manganese steel (1% C, 12% Mn) are very common examples. Fe-Mn phase diagram is illustrated in Fig. 1.43. Some other elements like cobalt and inert elements like rhodium, palladium, ruthenium, osmium, iridium, and platinum too have similar effects.
In 18/8 steel, γ/α transformation temperature is 650°C- The diffusion of alloying elements to form ferrite is slow and does not occur even during air cooling. Nickel depresses the MS temperature to retain metastable austenite at room temperature.
2. Phase Diagram with Expanded γ-Field:
Elements like carbon and nitrogen expand γ-region but due to their limited solubility in iron and by the formation of compound, the range of γ-field is shortened, though, the two phase region containing γ-phase remains. Heat treatment of steels is based on this expansion of γ-field by carbon and reduction of α-field. Elements like copper, zinc and gold have similar influence.
3. Phase Diagram with Closed γ-Field:
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Elements like Cr, W, Mo, V, Ti (strong carbide forming), Si, Al, Be and P lower Ae4 and raise Ae3 to contract the γ-region to a small area called gamma loop. See Fig. 1.42, where α region and δ-region have become continuous, that means these elements encourage ferrite formation.
Alloys with more than X % element [Fig. 1.42′ (3)] have at all temperature the solid solution of the alloying element in α-iron and are called ferritic alloys and are not amenable to the normal heat treatments involving α Û γ transformation except recrystallisation annealing after cold working.
Alloys with only a partial γ → α transformation are called semi-ferritic alloys.
Fig. 1.44 illustrates equilibrium diagram of Fe-Cr. Table 1.10 gives compositions of alloying elements in iron at which γ-phase disappears:
4. Phase Diagram with Contracted γ-Field:
B, Ta, Nb, Zr strongly contract the γ-region, but due to their low solubility in iron, two phase alloys are formed before the γ-phase region is completely enclosed. Zener and Andrews have given thermodynamic reasons for alloying behaviour to cause open-gamma field and closed-gamma field in phase diagrams.
According to them, the following relationship holds good:
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Where,
Cα is the fractional concentration of an alloying element in α-iron,
Cγ is the fractional concentration of the alloying element in γ-iron,
ΔH is the change in enthalpy (heat absorbed per unit of solute dissolving in γ-iron minus heat absorbed per unit of solute dissolving in α-iron), i.e. = Hγ – Hα,
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β is a constant. Thus, solute forms closed-gamma loop, or, is a ferrite stabliser if,
Hα < Hγ, or Δ H is positive,
and the solute shows open gamma field, or is a austenite stabliser, if
Hα > Hγ, or Δ H is negative.