Abstract
The effect of high-temperature thermomechanical treatment on the structural transformations and mechanical properties of metastable austenitic steel of the AISI 321 type is investigated. The features of the grain and defect microstructure of steel were studied by scanning electron microscopy with electron back-scatter diffraction (SEM EBSD) and transmission electron microscopy (TEM). It is shown that in the initial state after solution treatment the average grain size is 18 μm. A high (≈50%) fraction of twin boundaries (annealing twins) was found. In the course of hot (with heating up to 1100 °C) plastic deformation by rolling to moderate strain (e = 1.6, where e is true strain) the grain structure undergoes fragmentation, which gives rise to grain refining (the average grain size is 8 μm). Partial recovery and recrystallization also occur. The fraction of low-angle misorientation boundaries increases up to ≈46%, and that of twin boundaries decreases to ≈25%, compared to the initial state. The yield strength after this treatment reaches up to 477 MPa with elongation-to-failure of 26%. The combination of plastic deformation with heating up to 1100 °C (e = 0.8) and subsequent deformation with heating up to 600 °C (e = 0.7) reduces the average grain size to 1.4 μm and forms submicrocrystalline fragments. The fraction of low-angle misorientation boundaries is ≈60%, and that of twin boundaries is ≈3%. The structural states formed after this treatment provide an increase in the strength properties of steel (yield strength reaches up to 677 MPa) with ductility values of 12%. The mechanisms of plastic deformation and strengthening of metastable austenitic steel under the above high-temperature thermomechanical treatments are discussed.
Highlights
We focus on the microstructure features and their impact on the strength and plastic properties of metastable AISI 321 austenitic steel after hot deformation and a combination of hot and warm deformations at moderate deformation degrees
The results of electron back-scattered diffraction (EBSD) studies and their statistical analysis are presented in Figures 1–3 and in Supplementary Materials Figure S2
For the first time, the microstructure and mechanical properties of 321type steel have been studied in detail after a combination of the deformation above the recrystallization start temperature and significantly below this temperature
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Austenitic steels are widely used in many industries, but their application is still limited by relatively low strength properties (yield strength of 200–340 MPa) in the initial (quenched or solution treated) state [1,2,3]. The mechanical properties of these steels are successfully improved relative to the initial values by plastic deformation and (or) by thermomechanical treatments [4,5,6]. Under conditions of cold plastic deformation in metastable austenitic steels, dislocation gliding is not the only deformation mechanism of plastic deformation; there are strain-induced γ → ε (face-center-cubic (fcc) →
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