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 After low temperature deformation of hydrogen-free specimens (cooling of the plates before each rolling cycle, regime II), the details of its microstructure are quite similar to those deformed at room temperature after preliminary saturation with hydrogen (Figs. 2 and 3). A decrease in SFE of steel due to decrease in the deformation temperature is a well-known fact, and here it causes effect similar to alloying of the specimens with hydrogen before rolling. Figure 3, a shows bright-field TEM image of microstructure and corresponding microdiffraction pattern (inserts) after rolling according to regime II without hydrogen saturation (ε = 25 %). A decrease in the de - formation temperature promotes the formation of the larger number of twins in the microstructure, thinner twin plates and shorter distances between it relative to the room-temperature deformation. This contributes to the increase in the linear density of twin boundaries (Table 2). Analysis of the diffraction pattern (Fig. 3, a , inserts show variants of interpretation of reflections corresponding to twins and the ε-phase) indicate that, in addition to twins in the austenite grains, plates of ε-martensite are observed. It ought to be noted that in deformation regime I, ε-martensite plates have been observed only after rolling with hydrogen saturation (Fig. 2, b). Figure 3, c shows bright-field TEM image of misoriented grain/subgrain structure corresponded to the rolling of the steel in regime II with 50 % reduction without preliminary saturation with hydrogen. Compar- ison of TEM images in Figures 2, c and 3, c indicates that, in contrast to deformation at room temperature, the cooling of the specimens is accompanied with the formation of a more homogeneous misoriented struc- ture with deformation microbands, twins, and high density of dislocations (Table 2). Diffraction analysis of these specimens indicates the formation of αʹ-phase in the structure. Selected area electron diffraction pattern has a quasi-ring character. It shows numerous austenitic reflections with high azimuthal diffusions, as well as reflections of ε- and α’-deformation martensite (Fig. 3, c , insert). However, the fraction of ε- and αʹ-phases is not high, since it is not identified by XRD (Fig. 1, c ). According to magnetic phase analysis, the volume fraction of α’-martensite in these specimens is V αʹ = 4.5 %. In specimens after hydrogen saturation and rolling in regime II (ε = 25 %), a homogeneous dense grid of twin boundaries is formed (Fig. 3, b ). It consists of twin lamellae of 30–60 nm thick. Inside of this grid, there are even thinner twins of 10–15 nm thick and the density of twin boundaries reaches ρ tω = 30×10 6  m –1 (Table 2). Selected area diffraction patterns show point reflections corresponding to α’-phase, as well as reflections for γ-phase with strong azimuthal diffusions (Fig. 3, d , insert). In this case, the austenitic reflec - tions do not form a quasi-ring pattern in contrast to that for the specimens rolled without hydrogen satura- tion (Fig. 3, c , insert). Magnetic phase analysis indicates that a deformation α’-phase is also present in the specimens processed according to this regime, but its volume fraction is lower than that in specimens rolled without hydrogen saturation ( V αʹ = 2.7 %). This experimental fact requires further detailed research and analysis in a separate publication. The realization of phase transformations, deformation microscopical localization (shear bands), high linear density of twin boundaries and dislocations after chemical-deformation processing by regime II all contribute to intensive grain refinement and formation of more uniform grain/subgrain structure in comparison with that in specimens subjected to processing in regime I. Preliminary saturation with hydrogen of austenitic SS and decrease in the deformation temperature both facilitate high density of twin boundaries in the microstructure of the specimens in this case. Tensile test results Fig. 4 shows “stress-elongation” diagrams obtained by uniaxial static tension of the specimens pro- cessed according to different chemical-deformation regimes. The mechanical properties determined from these diagrams (elongation (δ), yield strength (σ 0.2 ) and ultimate tensile strength (σ в )) are summarized in Table 3. In Fig. 4, curve 1 demonstrates the tensile diagram of the initial coarse-grained specimen (without any deformation pre-treatments). At the initial state steel possesses high ductility (≈ 63 %) and low values of the yield strength (σ 0.2 =370 MPa) and ultimate tensile strength (σ в = 660 MPa). Chemical-deformation pro- cessing leads to an increase in microhardness, a significant increase in strength properties and a decrease in