Residual and Internal Stress States in Duplex Steel with TWIP Effect
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abstract
A new variety of duplex steels with superior mechanical properties has been studied.
They exhibit a very interesting combination of strength (tensile strength of 680 MPa) and ductility
values (more than 45% total elongation) due to the competition between different plasticity
mechanisms. These steels contain two phases: austenite and ferrite and are characterized by low
stacking fault energy at room temperature. In this work, four duplex steels with different chemical
composition and phase volume fraction are studied. Residual and internal stresses in each phase
were determined using the classical X-ray diffraction sin²ψ method. In the as-received state, both
longitudinal and transverse residual stresses are in compression (until -350 MPa) for the ferrite and
in tension (until +410 MPa) for the austenite. However, residual stresses in the austenitic phase
decrease when its volume fraction increases. Moreover, internal stress distribution in one alloy was
determined by X-ray diffraction during an in situ tensile test. The austenitic phase stress along the
loading direction is higher than the macroscopic applied one, which is higher than the ferritic stress
state, verifying a mixture rule and consistent with the initial residual stresses. For an applied
macroscopic strain of about 1%, the austenite phase is subjected to a stress of about 600 MPa
whereas the stress in the ferritic phase is about 300 MPa. It was also observed that as macroscopic
strain increases, stress difference between the austenite and the ferrite decreases.
