Abstract
The article presents the influence of structure formation on the properties of 321 metastable austenitic stainless steel in the process of cold radial forging (CRF). The steel under study after austenitization was subjected to CRF at room temperature with degrees of true strain (e) 0.26, 0.56, 1.00, 1.71 and 2.14. It has been shown that structure formation of the studied steel during CRF consists of three stages: formation of the lamellar structure of austenite, formation of the trapezoidal structure, and formation of the equiaxial grain structure. The kinetics of the strain-induced α′-martensitic transformation is related to the stages of structure evolution. Hardness, ultimate tensile strength and yield strength uniformly increase in all stages of structure formation with a significant decrease of elongation to fracture during the first stage of structure formation while the value of elongation to fracture remains constant in the subsequent stages of deformation. Impact strength of fatigue cracked specimens (KCT) decreases sharply at the first stage of structure formation and smoothly increases at the second and third stages. However, the impact strength of V-notch specimens (KCV) continuously decreases when deformation degree increases in the overall investigated deformation range.
Highlights
Metastable austenitic stainless steels (MASS), along with high corrosion resistance, impact strength and plasticity, have a rather low yield strength, which significantly limits the scope of their application.One of the most common ways to increase the yield strength of MASS is cold plastic deformation.It should be noted that significant structural changes occur as a part of this process
The total structure refinement leads to the stabilization of austenite relative to γ→α0 -transformation including due to the decreasing of strain-induced martensitic transformation (SIMT) interval by the temperature scale [38], which significantly reduces steel plasticity during tensile tests due to no transformation-induced plasticity (TRIP) effect being present during the tensile tests [44]
When compared with the results of mechanical properties tests on 321 MASS steel obtained by other authors after treatment using other methods of structure formation, such as cryogenic and repetitive cold rolling, equal-channel angular pressing, high-pressure torsion, fast neutron irradiation, as well as post-deformation annealing in some cases, we have shown that patterns of change in the yield strength and elongation to fracture coincide with the data presented by other authors (Figure 10)
Summary
Metastable austenitic stainless steels (MASS), along with high corrosion resistance, impact strength and plasticity, have a rather low yield strength, which significantly limits the scope of their application. Equal-channel angular pressing [14,15,16] have been the most commonly studied processes These methods of strain hardening differ in the strain pattern and rate, which, as is well known, significantly affect the processes of structure formation during deformation processing, and the properties of the processed steel [3,13,17,18,19]. It is known that during radial forging, the plastic flow develops inhomogeneously, and at the same time a fairly soft plastic deformation pattern is formed, which is close to uniform triaxial compression [20,21] This allows us to form high degrees of non-destructive deformation, which is combined with a fairly high productivity of the processing. The paper is dedicated to the research of peculiarities of the formation of the structure and properties of MASS during CRF in order to study how the staging of the structure formation influence the behavior of mechanical properties
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