Microstructure, Mechanical Properties and Fracture of EP-823 Ferritic/Martensitic Steel After High-Temperature Thermomec
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Russian Physics Journal, Vol. 63, No. 5, September, 2020 (Russian Original No. 5, May, 2020)
MICROSTRUCTURE, MECHANICAL PROPERTIES AND FRACTURE OF EP-823 FERRITIC/MARTENSITIC STEEL AFTER HIGH-TEMPERATURE THERMOMECHANICAL TREATMENT K. V. Almaeva,1,2 I. Yu. Litovchenko,1 and N. A. Polekhina2
UDC 669.018.25 539.219 539.25
The paper investigates the microstructure and mechanical properties of the EP-823 ferritic/martensitic steel containing 12 wt.% chromium after the high-temperature thermomechanical treatment with plastic deformation in the austenite region, at a temperature ranging from –70 to –40°С, 20°С and from 650 to 720°С. The hightemperature thermomechanical mode results in the formation of the hetero-phase microstructure with high dispersible nanoparticles of the МХ type (V, Nb, Mo) and (C, N) and the higher dislocation density. The mechanical properties of steel obtained in this mode are higher than that obtained in the mode of conventional thermal treatment in the indicated temperature range. At low temperatures from –70 to –40°С, the yield stress increases by approximately 100–200 MPa, whereas at higher temperatures from 650 to 720°С, it changes only by 14 MPa. At low temperatures, the steel fracture elongation varies between 12.7–14.3% in using the hightemperature thermomechanical mode. The steel fracture occurs with the neck formation. At high temperatures, a plastic cup fracture occurs, and at room temperature brittle failure and microcracks are observed along with the plastic cup fracture. The brittle fracture considerably increases at low temperatures, the size and the number of microcracks also grow. Keywords: ferritic/martensitic steel, thermal treatment, thermomechanical treatment, mechanical properties, ductile-brittle transition, transmission electron microscopy, fractography.
INTRODUCTION An interest in ferritic/martensitic (F/M) steels containing 9–12 wt.% chromium, is caused by the need to develop constructional materials applied in new generation of nuclear reactors. The study of their mechanical properties at elevated temperatures of 650–720 °C and the possibility of the durability increase at operating temperatures are among the most important tasks for this steel grade. The mechanical properties of constructional steels can be improved by using different thermal and thermomechanical treatment. Usually, the conventional thermal treatment (CTT) is used for F/M steels that includes normalizing and tempering. According to [1–4], the high-temperature thermomechanical treatment (HTMT) with plastic deformation in the austenite region is one of the ways to modify the hetero-phase microstructure and improve the mechanical properties of ferritic/martensitic steels. At negative temperatures, a ductile-brittle transition [5] is observed in F/M steels, which may shift to positive temperatures under irradiation while in operation. During the operation of nuclear reactors, the exhausted fuel cladding must be replaced without brittle fracture. It is therefore important to study the low-temperature mech
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