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Abstract:
AISI 304 steel experiences plastic flow instability during tension at room temperature if appropriate conditions are applied: a low strain rate and a sufficiently long gauge section of the sample. Then, propagation of the strain-localised band is activated. The electron backscattered diffraction (EBSD) research revealed that the reason is not only the difference in the content of the secondary phase – martensite α’ across the front face, but also the change in the volume fraction of austenite grains with Copper (Cu) and Goss-Brass (GB) orientation. Consequently, there is a division between two areas of high and limited deformation capacity. The tendency to maintain the continuity of deformation fields induces a massive rotation of austenite grains to Cu and GB orientations, which then undergo shearing and phase transformation. As a result, momentary strain accumulation leaves behind a stiffer zone. It is shown that the trapping of austenite grains prone to large deformations, inside the matrix with Cu and GB orientations, makes the formation of a plastic strain front possible. These features improve the ductility and strength of the 304 steel over 316L and 316LN at room temperature. The in-situ EBSD tension studies for the considered grades reveal three developing textures, with their comparison showing a gradual decrease in the preferences of the Cu and GB components. Thus, the appearing bands of the accumulated strains in 316L are limited by the Cu and GB areas, while such blockages do not occur in 316LN. The presented strengthening mechanism is confirmed by the digital image correlation (DIC) measurements. The root-mean-square (RMS) function of strains along the tensile direction, characterising the linear surroundings of the considered point, is introduced as a tool for linking the micro and macro scales. The experimental results provide a basis for explaining discontinuous front propagation at a temperature near 0 K.

Keywords:
Plastic flow instability, Martensitic transformation, Austenitic stainless steels, RMS strain amplitude

Abstract:
A remarkable plastic flow instability is observed during tensile deformation of the commercial 304 stainless-steel sheet at room temperature. It has been found that the occurrence of plastic flow instability in 304 is dependent on the strain rate and specimen gage length. Moreover, it is essentially the same as the necking caused by plastic instability in 316L. However, the enhanced strain hardening resulting from deformation-induced martensitic transformation facilitates the orderly propagation of the strain-localized band.