Structural Transformations and Mechanical Properties of Metastable Austenitic Steel under High Temperature Thermomechanical Treatment

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.

Effect of Multistage High Temperature Thermomechanical Treatment on the Microstructure and Mechanical Properties of Austenitic Reactor Steel

“…In the present study, the values of the yield strength achieved after HTMT-3 are close to those indicated in [13] and are significantly higher than that in [15]. The latter is associated with a lower degree of deformation and, as a consequence, less fragmented substructure.…”

“…It should be noted that the achieved mechanical properties of EK-164 steel after multistage HTMT are comparable with the properties presented in [13,15] on other austenitic steels. It is shown [13] that the yield strength reaches 875 MPa (at 20 • C), 580 MPa (at 600 • C), and 430 MPa (at 700 • C) after HTMT with plastic deformation at 700 • C with a total strain degree of e = 2.…”

“…In this case, the elongation to failure decreases (from 10 to 5%) with a change in the tensile temperature from 20 to 700 • C. The authors note that DSA is not observed in this case. In [15], HTMT with a sequence of hot and warm deformation (total deformation e ≈ 1.5) leads to an increase in the yield strength up to 677 MPa, while maintaining the elongation to fracture ≈ 12% at 20 • C.…”

“…Previously, the possibility of improving mechanical properties and modifying the microstructure under conditions of HTMTs was shown on such austenitic steels as AISI 304 [10,11], 310 [12], 316 [13] and 321 [11,14,15]. Most of these works, focus on the formation of dynamic recrystallization (DRX) grains; the main methods for their formation are HTMT with hot rolling [13,16] or compression [12,17,18] with degrees of deformation e = 0.7-3 in the temperature range of 800-1200 • C. These treatments promote an intensive formation of DRX grains at high temperatures of 900-1200 • C. In this case, a higher deformation temperature more strongly promotes the growth of DRX grains and an increase in the volume fraction of DRX [17].…”

“…In [15], the effect of hot deformation at 1100 • C (e = 1.6), and a sequence of hot deformation at 1100 • C (e = 0.8) and warm deformation at 600 • C (e = 0.7) on the microstructure and mechanical properties of 321 type austenitic steel is demonstrated. It is shown that HTMT, with decreasing temperature in the sequence of treatments, causes a high-intensity fragmentation of the steel structure and has a positive effect on the strength properties (yield strength of 677 MPa) while maintaining acceptable plasticity (elongation to fracture 12%).…”