Relationships between microstructure and mechanical properties of microduplex α + (γ + ɛ) 6 and 9%Mn steels for cryogenic applications

Room temperature tensile properties and low temperature toughness of quenched and doubly tempered 6 and 9%Mn steels were studied for a variety of tempering treatments, and the influence of the various phases present (α tempered martensite, α′ and ɛ martensites, and austenite) on the mechanical properties has been established. The intergranular brittleness of the 9%Mn steels is plain temper embrittlement due to P and Mn cosegregation to the former austenite grain boundaries, which can be suppressed by small Mo additions. ɛ martensite is not intrinsically embrittling and the mechanical properties are controlled by the total volume fraction of (γ + ɛ) islands dispersed in the tempered α lath martensitic matrix: the impact transition temperature is lowered and the strain hardening capacity σ_R–α_Y increases as (γ + ɛ) increases. These relationships are explained in terms of the morphology of the dispersed phases and of the strain induced phase transformations. The influence of Mo on the microstructures and phase transformations is shown to be beneficial to toughness.