Formation of Structure in Metastable Austenitic Steel during Cold Plastic Deformation by the Radial Forging Method

Metastable Austenitic Steel Structure and Mechanical Properties Evolution in the Process of Cold Radial Forging

“…The full quantitative evaluation of the dislocation and twins density, as well as the size of the substructure elements was carried out in the previous work [24]. The TEM micrographs were used to estimate the size of the substructure elements of the steel being studied, by which the size of relatively dislocation-free regions bounded by dislocation, twinning, intergranular or interphase boundaries was meant.…”

“…The shear bands are not thinned during the deformation and only increase in their number. In other words, the average width of the lamella in the process of implemented deformation degrees is at the same level and corresponds to the diameter of the grain obtained at the deformation degree of e = 2.14 in cross section of a rod of 240 ± 10 nm [24]. It should be noted that the formation of a band structure in metastable austenitic steels in the deformation direction is recorded by other deformation methods [3,33], however, in our study the globular structure in the cross section indicates the columnar form of the formed grains.…”

“…The total structure refinement leads to the stabilization of austenite relative to γ→α′-transformation including due to the decreasing of SIMT interval by the temperature scale [38], which significantly reduces steel plasticity during tensile tests due to no TRIP effect being present during the tensile tests [44]. Thus, at the first stage of structure formation, there has been a significant drop in elongation to fraction from 67% in its original state to 12% after deformation with a degree of e = 0.56 (Figure 6b), which is due to the occurrence of shear bands in the austenite structure, a significant increase of the density of dislocations and deformation twins [24]. As a result, there is an increase in the number of boundaries impervious to dislocations, which significantly complicates their movement and redistribution and leads to a drop in the relative elongation [8].…”

“…Thus, the increase in the degree of deformation is accompanied by the decrease in the origination of cracks, as the difference between the KCV and KCT. Within the interval of deformation from e = 0 to e = 0.56, the relative work of crack growth drastically reduces, which is caused by accumulation of the number of deformation twins [24]; while during the deformation in the interval from e = 0.56 to e = 2.14, the relative work of crack growth slightly increases and the relative work for crack initiation is sensibly reduced (see Figure 7a). This is caused by the formation of the band structure in the cross-section, which complicates the growth of cracks in the transverse direction due to the increasing in number of inter-grains and inter-phase borderlines on the way of the crack growth, which are local ductility barriers.…”

Mechanisms of the Reverse Martensite-to-Austenite Transformation in a Metastable Austenitic Stainless Steel

“…In the transversal section of the bar in the as-processed state, regions of austenite (Figure 2b and c) and DIM (Figure 2d) possessed lamellar packet-like and nearly equiaxed shape. The transition from lamellar to the equiaxed structure occurred at the latter stage (at the strain above 60%) of cold swaging [10,37], but the transformation was not complete at the strain of 90% (Figure 2e). Additional lattice defects introduced in the DIM-containing alloy during cold deformation caused the transformation [26].…”