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, σu, 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, εu, was observed due to a transformation plasticity of austenite which was formed on the annealing and would be rich in Ni, but the strength, σu=120 kg/mm2, 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 σu=158 kg/mm2, and a plasticity of ψ=27% and εu =1%, which were better than thd combination, σu=132 kg/mm2, ψ=25% and εu=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.