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 Ta b l e 2 Microstructure characteristics (ρ – dislocation density, t – twin thickness, e – distance between twins, ρ tω – the linear density of twin boundaries) of steel microstructure after chemical-deformation processing CDT ρ, m –2 t , nm e , nm ρ tω , m –1 Rolling without hydrogen saturation regime I, ε = 25 % 0.4×10 15 50–100 (15–25 * ) 60–100 (15–40 * ) 2×10 6 (12×10 6* ) regime I, ε = 50 % 0.8×10 15 60–150 (15–30 * ) 40–130 (20–40 * ) 6×10 6 (16×10 6* ) regime II, ε = 25 % 0.7×10 15 20–100 50–150 7×10 6 regime II, ε = 50 % 1.0×10 15 30–60 30–60 10×10 6 Rolling after hydrogen saturation regime I, ε = 25 % 0.8×10 15 50–200 (20– 50)* 70–250 (25–50 * ) 8×10 6 (29×10 6* ) regime I, ε = 50 % 1.2×10 15 50–100 (15–45 * ) 50–150 (30–50 * ) 13×10 6 (34×10 6* ) regime II, ε = 25 % 0.8×10 15 10–60 40–150 30×10 6 regime II, ε = 50 % 1.5×10 15 10–40 20–60 40×10 6 * in individual grains that are favorably oriented for twinning. (Fig. 2, c , insert). This indicates the formation of high-angle misorientations in the steel structure as a result of plastic deformation, while high azimuthal diffusions of the reflections confirm the presence of low-angle misorientations as well. Localized deformation bands (shear bands) of various scales are formed in rolling. Broken and separated twin boundaries are observed inside and between such bands (Fig. 2, c ). Deformation twins are observed in most grains after 50 % reduction. The analysis of TEM images indicates the increase in the linear density of twin boundaries and the density of dislocations in comparison with the specimens rolled to 25 % reduction (Table 2). Preliminary saturation with hydrogen before rolling promotes the development of deformation twin- ning, which leads to a significant increase in the linear density of twin boundaries in comparison with the structure after rolling in regime I without hydrogen saturation (Table 2, Fig. 2). Twinning is observed in almost all grains at 50 % reduction (Fig. 2, d ). Mechanical twinning as a deformation mechanism is facili- tated primarily due to a decrease in SFE of steel alloyed with hydrogen atoms. The formation of twins can be observed even in grains unfavorably oriented for this deformation mechanism [34, 35]. TEM studies also show the presence of thin ε-martensite lamellae inside of austenitic grains, but its amount is not high (Fig. 2, b , d ). Since the ε-phase is not determined by XRD, its volume fraction is not above 5 %. The formation of the ε-phase also confirms the decrease in SFE of hydrogen-alloyed steel. After rolling up to 50 % reduction, diffraction patterns are predominantly point-like, although azimuthal diffusions of the reflections are pres - ent (Fig. 2, d , inserts). TEM studies have shown that preliminary saturation with hydrogen of the specimens before rolling contributes to the formation of a less misoriented structure relative to those deformed without preliminary saturation with hydrogen. This is obviously associated with the formation of a high density of special boundaries (twins and ε-phase). Along with the twinning, an increase in dislocation density is ob - served in comparison with specimens rolled to the similar reduction (strain) but without preliminary satu- ration with hydrogen (Table 2). This may be due to the hindered transfer of shear across twin boundaries and the accumulation of slip dislocations in the regions between special boundaries. Microstructural TEM studies confirm X-ray structural analysis data on the texture formation during rolling of the specimens, presented and discussed above. In terms of the special boundaries network formation, the microstructure of hydrogen saturated and rolled specimens is more uniform in comparison with one produced without preliminary saturation with hydrogen.