Abstrait

Influence of the chromium content on the as-cast microstructures of ternary alloys very rich in carbon: Part II: Ni-based and Fe-based alloys with 3 to 5 wt.%C

Patrice Berthod, Elise Souaillat, Oph�������©lie Hestin, Thierry Schweitzer

High fractions of carbides are favourable for high hardness but the possible presence of graphite for very high carbon contents may have the opposite effect. Limited modifications of the chemical composition may correct the microstructures to achieve the demanded properties. This was studied for three ternary alloys based on nickel and three other ones based on iron, all rich in carbon, by modifying their chromium contents (from 30wt.% Cr to new values comprised between 20 and 37wt.%). Preliminary thermodynamic calculations were first performed to explore the possible microstructures, and to select several chromium contents potentially allowing either to promote graphite, or on the contrary to avoid it, or to enhance the carbide fraction at constant carbon content. Second, alloys with the chromium contents selected above were really elaborated by foundry. Metallography samples were prepared and the as-cast microstructures observed. As in the first part of this work concerning cobalt alloys, the calculated and metallography results were compared to the ones previously obtained for the analogous nickel and iron containing 30wt.%Cr. Adding 4.5 and 7% of chromium to the graphite-rich nickel alloysNi-30Cr-4.5C andNi-30Cr-5.0C respectively, induces a significant decrease in graphite fraction while subtracting 4.5 %Cr to the graphite-free Ni-30Cr-3.0C is not sufficient to make graphite appear but it decreases the carbideÂ’s quantity. Removing 10.3% Cr and 3.9%Cr to the Fe- 30Cr-4.0C ant the Fe-30Cr-5.0C respectively, decreases the mass fraction of carbides but does not promote the appearance of graphite. In contrast adding 2.6%Cr to the Fe-3Cr-5.0C induces only a small increase in carbide quantity.