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Phase Diagrams

Martensitic Stainless Steel for Knife Applications – Part 2: Fe-0.8Mo-Cr-C

Some stainless steel mills add approximately 0.8 wt. pct. molybdenum to AISI 440C. Figures 3 and 4 show the isothermal sections of Fe-0.8Mo-Cr-C quaternary phase diagram at 1000°C (1832°F) and 1100°C (2012°F), respectively, calculated with Thermo-Calc, coupled with TCFE2000 thermodynamic database.

Fe-0.8Mo-Cr-C quarternary phase diagram at 1000 degree C

Figure 3. Isothermal section of Fe-0.8Mo-Cr-C quaternary phase diagram at 1000°C (1832°F) calculated with Thermo-Calc coupled with TCFE2000 thermodynamic database.

According to thermodynamic calculations, the austenitic matrix of Fe-0.8Mo-17Cr-1.075C alloy at 1000°C (1832°F) has the following chemical composition (in weight percent):

Cr = 10.9
C = 0.32
Mo = 0.37

The amount of chromium-rich M23C6 primary carbides in Fe-0.8Mo-17Cr-1.075C alloy at 1000°C (1832°F) is calculated to be 17.2 mol. percent. It is worth noting that even a small addition of molybdenum to the Fe-Cr-C system (in this case 0.8 wt. %) expands the γ + M23C6 field at the expense of the γ + M7C3 field (compare Figure 1 — Isothermal Section of Fe-Cr-C Ternary Phase Diagram at 1000°C — with Figure 3).

Fe-0.8Mo-Cr-C quarternary phase diagram at 1100 degree C

Figure 4. Isothermal section of Fe-0.8Mo-Cr-C quaternary phase diagram at 1100°C (2012°F) calculated with Thermo-Calc coupled with TCFE2000 thermodynamic database.

According to thermodynamic calculations, the austenitic matrix of Fe-0.8Mo-17Cr-1.075C alloy at 1100°C (2012°F) has the following chemical composition (in weight percent):

Cr = 12.8
C = 0.52
Mo = 0.60

The amount of chromium-rich M23C6 and M7C3 primary carbides in Fe-0.8Mo-17Cr-1.075C alloy at 1100°C (2012°F) is calculated to be 11.6 and 1.0 mol. percent, respectively.

Part 1: Fe-Cr-C Ternary Phase Diagrams
Part 3: Fe-4.0Mo-Cr-C Quaternary Phase Diagrams

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