Abstract
The deformation microstructures formed by novel multistage high-temperature thermomechanical treatment (HTMT) and their effect on the mechanical properties of austenitic reactor steel are investigated. It is shown that HTMT with plastic deformation at the temperature decreasing in each stage (1100, 900, and 600 °C with a total strain degree of e = 2) is an effective method for refining the grain structure and increasing the strength of the reactor steel. The structural features of grains, grain boundaries and defective substructure of the steel are studied in two sections (in planes perpendicular to the transverse direction and perpendicular to the normal direction) by Scanning Electron Microscopy with Electron Back-Scatter Diffraction (SEM EBSD) and Transmission Electron Microscopy (TEM). After the multistage HTMT, a fragmented structure is formed with grains elongated along the rolling direction and flattened in the rolling plane. The average grain size decreases from 19.3 µm (for the state after solution treatment) to 1.8 µm. A high density of low-angle boundaries (up to ≈ 80%) is found inside deformed grains. An additional cold deformation (e = 0.3) after the multistage HTMT promotes mechanical twinning within fragmented grains and subgrains. The resulting structural states provide high strength properties of steel: the yield strength increases up to 910 MPa (at 20 °C) and up to 580 MPa (at 650 °C), which is 4.6 and 6.1 times higher than that in the state after solution treatment (ST), respectively. The formation of deformed substructure and the influence of dynamic strain aging at an elevated tensile temperature on the mechanical properties of the steel are discussed. Based on the results obtained, the multistage HTMT used in this study can be applied for increasing the strength of austenitic steels.
Highlights
These steels have a number of attractive qualities, such as high ductility, increased heat resistance, lack of tendency to low-temperature embrittlement compared to other structural materials
It should be noted that annealing twins are found in the grains with the widths of several microns (Figure 2a,b) and those narrower than 1 μm (Figure 2e,f)
Earlier [19], it was shown by Transmission Electron Microscopy (TEM) that fine MX particles based on vanadium were observed in the structure of steel EK-164
Summary
Austenitic stainless steels are used as structural materials in various industries, including the nuclear power engineering [1,2,3]. A common problem limiting the use of austenitic steels in the nuclear power industry is their radiation-stimulated swelling at high irradiation doses. Some authors [5] attribute this to the increased content of Ni (19% wt.) and the complex system of alloying—the presence of Ti, Nb, V, B, Si, etc It was shown [5] that austenitic steel EK-164 is a radiation-resistant material ensuring fuel burn-up of over 15%. At higher irradiation doses (more than 100 dpa) and higher fuel burnout, EK-164 and ChS-68 steels swell significantly, which limits their applications in nuclear power engineering
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