Abstract
Nickel is one of the alloying elements promoting the formation of acicular ferrite at the expense of proeutectoid ferrite. The Ni addition reduces the steady-state nucleation rates of allotriomorphic ferrite in Fe-C-Ni alloys. Due to such reasons, Ni was added to modify the microstructure of two sintered steels with different compositions-with the aim of improvement in mechanical properties. Two pre-alloyed Fe-3.0Cr-0.5Mo and Fe-1.5Cr-0.2Mo powders were employed as starting powders for experimental works. The sintered steels were produced under the same cooling rate of 0.1 °C/s. The pre-alloyed powder compositions, differentiated by alloying element contents of Cr and Mo, showed clear influence on the final microstructures of the sintered steels. For the sintered Fe-3.0Cr-0.5Mo-0.3C-xNi steels, additions of ≥ 1 wt.% Ni could change the microstructure from dual-phase type to bainitic structure. Ni additions in the sintered Fe-1.5Cr-0.2Mo-0.3C-xNi steels could also promote bainitic transformation. However, bainitic grain portion in the sintered Fe-1.5Cr-0.2Mo -0.3C-xNi steels was lower than that of the sintered Fe-3.0Cr-0.5Mo-0.3C-xNi steels. The microstructures with dual-phase nature still existed in the sintered Fe-1.5Cr-0.2Mo-0.3C-xNi steels, even Ni additions of up to 3.0 wt.% were conducted. The Ni addition not only altered the sintered steel microstructure but also improved the hardness of sintered steels.
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