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Phase Diagrams

Martensitic Stainless Steel for Knife Applications – Part 3: Fe-4Mo-Cr-C

Martensitic stainless steel such as 154CM contains about 4 wt. percent molybdenum (in addition to 1.05 wt. % C and 14.0 wt. % Cr). To determine the effect of 4 wt. % Mo on the Fe-Cr-C ternary system, consider Figures 5 and 6, which show the isothermal sections of Fe-4Mo-Cr-C quaternary phase diagram at 1000°C (1832°F) and 1100°C (2012°F), respectively.

Fe-4Mo-Cr-C quarternary phase diagram at 1000 degree C

Figure 5. Isothermal section of Fe-4Mo-Cr-C quaternary phase diagram at 1000°C (1832°F) calculated with Thermo-Calc coupled with TCFE2000 thermodynamic database.

According to Thermo-Calc calculations, the austenitic matrix of Fe-4Mo-14Cr-1.05C alloy at 1000°C (1832°F) has the following chemical composition (in weight percent):

Cr = 8.6
C = 0.33
Mo = 2.6

The amount of chromium-rich M23C6 primary carbides in Fe-4Mo-14Cr-1.05C alloy at 1000°C (1832°F) is calculated to be 16.8 mol. percent. It is worth noting that the addition of 4 wt. % Mo to the Fe-Cr-C system expands significantly the presence of M23C6 phase at the expense of the M7C3 phase (compare Figure 1 — Isothermal Section of Fe-Cr-C Ternary Phase Diagram at 1000°C — and Figure 3 — Isothermal Section of Fe-0.8Mo-Cr-C Quaternary Phase Diagram at 1000°C — with Figure 5).

Fe-4Mo-Cr-C quarternary phase diagram at 1100 degree C

Figure 6. Isothermal section of Fe-4Mo-Cr-C quaternary phase diagram at 1100°C (2012°F) calculated with Thermo-Calc coupled with TCFE2000 thermodynamic database.

According to Thermo-Calc calculations, the austenitic matrix of Fe-4Mo-14Cr-1.05C alloy at 1100°C (2012°F) has the following chemical composition (in weight percent):

Cr = 10.6
C = 0.58
Mo = 3.4

The amount of chromium-rich M23C6 primary carbides in Fe-4Mo-14Cr-1.05C alloy at 1100°C (2012°F) is calculated to be 11.6 mol. percent.

The amount of chromium and molybdenum in the matrix is also an indicator of the secondary-hardening response — in general, the higher the amount of chromium and molybdenum in the matrix, the stronger the secondary-hardening response during tempering (especially at higher tempering temperatures.)

Part 1: Fe-Cr-C Ternary Phase Diagrams
Part 2: Fe-0.8Mo-Cr-C Quaternary Phase Diagrams

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