An Fe-16%Ni-4%Si alloy having a large hardenability due to a maraging mechanism was used to study the effect of microstructures prior to aging upon the strength properties under a tensile test. In contrast to the conventional solution treatment for maraging steels in the γ field, the present alloy was cold rolled by 80% at a martensite state (marforming) and annealed at the temperatures in the α+γ two-phase range before aging, in an attempt to obtain microduplex structures showing a good combination of strength and toughness.
The marforming was found to increase both the tensile strength, σ
u, and the reduction of area, ψ, due to the formation of proper duplex structures. In case, annealing was made at the temperatures around 570°C, large uniform elongation, ε
u, was observed due to a transformation plasticity of austenite which was formed on the annealing and would be rich in Ni, but the strength, σ
u=120 kg/mm
2, was rather low. In case, annealing was carried out at the temperatures from 600 to 650°C (still in the two-phase field), the reverted austenite was transformed on cooling into martensite which in turn was strengthened by aging proportionally to the martensite fraction; the resultant duplex structure had a strength of σ
u=158 kg/mm
2, and a plasticity of ψ=27% and ε
u =1%, which were better than thd combination, σ
u=132 kg/mm
2, ψ=25% and ε
u=1% in the same alloy that was conventionally age-hardened without marforming and annealing.
The marforming had a dominant effect on the reduction of transgranular brittle fracture as well as intergranular fracture associated with large austenite grains which had been present prior to the annealing in the two-phase range.
View full abstract