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 thiourea (CH 4 N 2 S) at a current density j = 200 mA/cm 2 . Immediately after hydrogen saturation, the plates were rolled using two regimes: regime I – at room temperature (23 °C), regime II – with cooling of the plates down to liquid nitrogen temperature (–196 °C) before each rolling path. Plastic deformation per single pass through the rolls of the rolling mill was ≈3–4 %. The rolling reduction was calculated as e = (( h 0 – h 1 )/ h 0 )100 %, where h0 is the initial plate thickness (1 mm), h1 is the plate thickness after rolling. The total reduction was 25 and 50 %. The second portion of specimens was rolled using the identical regimes, but without preliminary saturation with hydrogen. Further in the text, the specimens after heat treatment (but non-deformed) are named as initial one. Themicrohardness of the specimenswas determinedby theVickersmethodon aDuramin5microhardness tester with a load on the indenter of 200 g. Uniaxial static tension was carried out on an Instron 3369 testing machine at room temperature with an initial strain rate of 4.2×10 –4 s –1 . Dumb-bell shaped flat tensile specimens were cut from the rolled plates. The tensile specimens had the dimensions of 9×2.6×h 1 mm in gauge section. X-ray phase and X-ray structural analyses (XRD) of phase composition and structural parameters of steel were carried out on a Rigaku Ultima IV diffractometer (CuKα radiation). The calculation of a crystal lattice microstrain (∆ d / d ) and the sizes of coherent scattering regions (CSR) were performed by the approximation method. The volume fraction of strain-induced αʹ-martensite ( V αʹ ) formed in specimens was determined by the measuring of a specific magnetization depending on magnetic field strength using a Magnetometer N-04 device (magnetic phase analysis, MFA) [26]. Electron microscopic studies of the specimens’ structure were carried out using a JEM-2100 transmission electronmicroscope (TEM) at an accelerating voltage of 200 kV. Foils for TEMresearchwere prepared by the standard method described in [27]. The dislocation density was calculated according to the procedure given in [27]. The grain size of the initial heat-treated billet was determined using the grain maps reconstructed from the electron backscattered diffraction (EBSD) data (a Quanta 200 3D scanning electron microscope, an accelerating voltage of 30 kV). Results and Discussion XRD results Figure 1 shows the X-ray diffraction patterns obtained for steel specimens in the initial state and after various regimes of the chemical-deformation processing. According to XRD results, the initial austenitic structure of Fe-17Cr-13Ni-3Mo-0.01С steel possesses a lattice parameter a = 0.3603 nm, microstrain of the crystal lattice – Δ d/d = 7.3×10 –4 and CSR >200 nm. X-ray phase analysis of steel specimens subjected to different regimes of chemical-deformation processing revealed the presence of γ-phase only. Thus, austenitic SS maintains a single-phase fcc crystal structure regardless of the processing regime (Fig. 1). XRD analysis indicates the deformation- induced refinement of the structure and the increase in internal stresses in the specimens. Regardless of the deformation temperature and hydrogen saturation, the microstrain of the crystal lattice increases after rolling up to 1.5–2.9×10 –3 . Rolling significantly reduces CSR values (Table 1). The lattice parameter of austenite changes insignificantly after all processing regimes. It should be noted that at the same reduction in rolling, two factors – a decrease in the rolling temperature and preliminary saturation with hydrogen– promote the increase in CSR values in comparison with specimens rolled at room temperature and without preliminary saturation with hydrogen. This result indicates, first, a hydrogen-assisted and temperature- assisted change in the fragmentation of the microstructure and, probably, the deformation mechanisms in specimens during rolling. Second, these data indicate similar effect of both aforementioned factors on the microstructure of the steel during rolling. A comparative analysis of the XRD patterns shows a decrease in the intensity and broadening of X-ray lines with an increase in the reduction (strain) (Fig. 1). During rolling, deformation texture of the {220} type is formed in the rolling plane, as it is illustrated by changing in the ratio of the X-ray lines intensi-