Abstract
Precipitation behaviour of intermetallic compounds is one of the important factors which control the strength and toughness of maraging steels. However, systematic investigations on the precipitation behaviour have hardly carried out.
In the present paper, the effect of alloy series (Ni-Co-Ti, Ni-Co-Mo and Ni-Co-Mo-Ti steels) with the variation of nickel content on the strength and toughness was studied. Precipitation behaviour of intermetallic compounds was identified by non-aqueous electrolyte extraction method and transmission electron microscopy. Further, the relation between fracture pattern and the precipitation behaviour was investigated in the alloy series with various toughness levels. Results are summerized as follows:
1) Titanium containing steels showed the marked decrease in toughness due to the grain boundary and lath boundary precipitates of η-Ni3Ti.
2) Molybdenum containing steels showed the highest toughness level with wide range of nickel contents due to the finely dispersed precipitates of σ-FeMo in the grain.
3) Molybdenum and titanium containing steels including 18% nickel commercial steels showed the highest tensile strength level due to the multi-precipitations of η-Ni3Ti, σ-FeMo and Ni3Mo. It was ascertained that the addition of molybdenum and the increase in nickel content suppressed the grain boundary and lath boundary precipitates of η-Ni3Ti and resulted in the highest toughness level.
