Influence of hydrogen saturation on the structure and mechanical properties of Fe-17Cr-13Ni-3Mo-0.01С austenitic steel during rolling at different temperatures

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 2 2021 Fig. 3. TEM images of the microstructure in steel after processing by regime II: a , c – rolling of hydrogen-free specimens; b , d – rolling of hydrogen saturated specimens. Reduction: 25 % ( a , b ) and 50 % ( c , d ). Selected area electron diffraction patterns are obtained from an area of 1.4 μm 2 a b с d Figure 2 shows bright-field TEM images of the microstructure and the corresponding selected area electron diffraction patterns (inserts) for specimens rolled at room temperature (regime I). After rolling of specimens without preliminary saturation with hydrogen with ε = 25 %, the initial large austenitic grains are seen. The diffraction contrast inside of these grains testifies the accumulation of high dislocation density (Fig. 2, a, Table 2). Microdiffraction patterns for such a microstructure have a point character, with weak azimuthal diffusion of reflections. The dislocation arrangement indicates that the steel is characterized by “wavy” slip, typical for deformation of steels with medium-to-high SFE [31]. Both grains, in which deformation is realized due to slip only, and those with active slip and twinning are observed in TEM images. This is due to the orientation dependence of the twinning in austenitic steels [32, 33]. Under conditions of constrained deformation, the twinning stresses in Fe-17Cr-13Ni-3Mo-0.01С steel, which does not contain interstitial atoms, are not achieved in all grains, but in some grains this mechanism is realized. Twins with a thickness (lamella width) t = 50–100 nm and with a distance between lamellas е = 60–100 nm are frequently observed (Table 2). The linear density of twin boundaries in such grains is ρ t ω = 2×10 6  m –1 (Table 2). Rarely, individual grains are observed, in which the density of twin boundaries is higher than the average value (ρ t ω = 12×10 6  m –1 ). Obviously, these grains were most favorably oriented for the development of mechanical twinning (these grains possess the maximum Schmid factors for twinning). The inhomogeneous grain/subgrain structure is formed after rolling with 50 % reduction (Fig. 2, c ). Microdiffraction patterns contain numerous reflections of the austenite phase, distributed over the ring