In pure iron, the A4 (1394 °C) and A3 (912 °C) transformations take place at constant temperatures. If an element enters into solid solution in iron — forming in that way a binary alloy — each of these transformations are required by the Phase Rule to occur over a range of temperature.
Some elements, such as chromium, lower the A4 and raise the A3 transformation temperatures, restricting the gamma loop (γ loop) in the iron-carbon phase diagram. As the binary iron-chromium phase diagram shows, the presence of chromium restricts the gamma loop (Figure 1).
Notice that above approximately 13 wt. % Cr, the binary Fe-Cr alloys are ferritic over the whole temperature range. A narrow (α + γ) range that exists between approximately 12 wt. % Cr and 13 wt. % Cr is also worth noting.
The addition of carbon to the Fe-Cr binary system widens the (α + γ) field and extends the gamma-loop to higher chromium contents (see, for example, the Fe-Cr-C ternary system at 1000 °C and 1100 °C).
The sigma (σ) phase, which is an intermetallic FeCr compound, can sometimes form in Fe-Cr alloys, such as AISI 316 or AISI 310 stainless steels. The harmful effects of the sigma phase on mechanical properties (e.g., ductility) and corrosion resistance are well documented.
Some elements, such as chromium, lower the A4 and raise the A3 transformation temperatures, restricting the gamma loop (γ loop) in the iron-carbon phase diagram. As the binary iron-chromium phase diagram shows, the presence of chromium restricts the gamma loop (Figure 1).
Notice that above approximately 13 wt. % Cr, the binary Fe-Cr alloys are ferritic over the whole temperature range. A narrow (α + γ) range that exists between approximately 12 wt. % Cr and 13 wt. % Cr is also worth noting.
The addition of carbon to the Fe-Cr binary system widens the (α + γ) field and extends the gamma-loop to higher chromium contents (see, for example, the Fe-Cr-C ternary system at 1000 °C and 1100 °C).
FigureFe-Cr phase diagram shows which phases are to be expected at equilibrium for different combinations of chromium content and temperature. The Fe-Cr phase diagram was calculated with Thermo-Calc, coupled with PBIN thermodynamic database. The melting point of iron and chromium at the pressure of 101325 Pa is 1538 °C and 1907 °C, respectively.
The sigma (σ) phase, which is an intermetallic FeCr compound, can sometimes form in Fe-Cr alloys, such as AISI 316 or AISI 310 stainless steels. The harmful effects of the sigma phase on mechanical properties (e.g., ductility) and corrosion resistance are well documented.
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