Identification of the Precipitates in Maraging Steels by Non-aqueous Electrolyte Extraction Method

Abstract

Many works with respect to precipitates in maraging steels have been presented. However, the precipitates at the peak-aging condition (the maximum tensile strength level) have hardly been identified due to their very fine shape, complicated multi-phase precipitation, and similar crystal structures.
Fine precipitates formed at peak-aging condition were extracted by a non-aqueous electrolyte extraction method. The chemical composition and the crystal structure of the extracted residue were analyzed by the ICP analysis and electron diffraction analysis, respectively. As the result, fine precipitates formed at the various aging processes (under aging, peak aging, and over aging conditions) of the maraging steels were identified. The change of the precipitates in the various alloy series of maraging steels with aging time at 500°C are as follows :
1) Ni-12.5Co-6Mo-1.2Ti steels :
The precipitation of Ni₃Ti and FeMo was confirmed. In 17.5%Ni steel, Ni₃Mo was identified at the peak-aging condition.
2) Ni-12.5Co-6Mo steels :
It was confirmed that FeMo precipitated at the start and then Ni₃Mo did. The precipitation of Ni₃Mo was accelerated by the increase of nickel concentration.
3) Ni-12.5Co-1.2Ti steels :
Only Ni₃Ti was confirmed. This precipitates was very fine and needle-like shaped.