Microstructural effects on the magnitude of Lüders strains in a low alloy steel

Tensile testing of a 0.17% C–Ni–Cr–Mo pressure vessel steel showed the presence of Lüders banding. Lüders strains ranged from 0.6 to 0.7% for a ferrite–pearlite microstructure, 0.8–0.9% for an upper bainite microstructure to 3.2–4.1% for a tempered martensite microstructure. Lüders strains tended to slightly increase with an increase of strain rate in the range 10−4–10−2 s−1. Modelling of the deformation showed how the strain in the bands developed as a function of time and this was confirmed by the use of differential image correlation measurements of the straining of the specimen. Multiple banding was observed in some specimens but the magnitudes of the strains seen in each band were similar and comparable to the global Lüders strain. Confirmation of the decrease in magnitude of the Lüders strain with increasing carbon content was also seen in tests on lower carbon content steels, as was also the decrease in magnitude of Lüders strain associated with larger grain sizes.