N. A. Polekhina

The features of microstructure and mechanical properties of austenitic steel after direct and reverse martensitic transformations

The features of structural states of metastable austenitic steel after thermomechanical treatments, including low-temperature deformation, warm deformation and subsequent annealing are investigated. It is shown that under these conditions the direct (γ → α′) and reverse (α′ → γ) martensitic transformations occur and submicrocrystalline structural states are formed. The proposed thermomechanical treatment allows varying the strength and plastic properties of austenitic steel in a wide range. The strength of steel in submicrocrystalline state is 4–6 times higher than its original value.

Effect of thermomechanical treatment modes on structural-phase states and mechanical properties of metastable austenitic steel

The features of the structural-phase states and mechanical properties of metastable austenitic steel after thermomechanical treatments have been investigated. It is shown that low-temperature and subsequent deformation in the temperature range 300–773 K contributes to the direct (γ → α′)-martensitic transformation. The combination of low-temperature, subsequent warm deformation at 873 K and annealing at 1073 K leads to the direct (γ → α′)- and reverse (α′ → γ)-martensitic transformations. As a result of thermomechanical treatments submicrocrystalline two-phase structural states with high strength properties (σ0.1 ≈ 1160–1350 MPa) are formed.

Features of deformation localization in stable austenitic steel under thermomechanical treatment

Features of structural states of Fe-18Cr-14Ni-Mo austenitic steel after thermomechanical treatment, including low-temperature and warm rolling deformation, were investigated by means of transmission electron microscopy. It is shown that mechanical twinning in multiple systems and strain localization bands contribute to grain fragmentation with the formation of the submicrocrystalline austenitic structure. These bands lie in the miсrotwin structure, have high-angle (≈60°–90°, 〈110〉) misorientations of the crystal lattice relative to the matrix and localize significant (up to ≈1) shear strain. In areas of the bands, structural states with high (tens of deg/μm) curvature of the crystal lattice and high local internal stresses are observed. The internal structure of the bands is presented by nanoscale fragments of austenite and α′-martensite. The presence of specific misorientations and fragments of martensite means that the formation mechanism of localized deformation bands are direct plus reverse (γ → α′ → ...

The microstructural stability of low-activation 12%-chromium ferritic-martensitic steel EK-181 during thermal aging

The results of structural investigations and mechanical tests of low-activation 12%-chromium ferritic-martensitic steel EK-181 after long-term (13500 h) aging at 450°C and 620°C are presented. It is shown that the high thermal stability of steel microstructure ensures that its original short-term mechanical properties are maintained at T ≤ 620°C.

Mechanisms for improving strength of metastable austenitic steel by thermomechanical treatments

Based on the data of structural investigations, an analysis of the strengthening mechanisms of SUS 321 Fe-18Cr-10Ni-Ti metastable austenitic steel under thermomechanical treatments combining low-temperature and subsequent warm rolling deformations is performed. Under these conditions, the direct (γ → α′) and reverse (α′ → γ) martensitic transformations occur, and submicrocrystalline structural states with different martensite/austenite phase ratios are formed. It is shown that a significant increase in the yield stress by Δσ ≈ 900 MPa can be achieved through the formation of a two-phase submicrocrystalline structure with the martensite/austenite phase ratio ≈30/70. It is primarily the austenite submicrocrystalline structure that can improve the yield strength by Δσ ≈ 890 MPa. It is shown that the main contribution to the steel strength increase under the above treatments comes from grain-boundary, substructural and dislocation strengthening. The possibilities of controlling the strength and plastic proper...

Strengthening mechanisms of heat-resistant 12% Cr ferritic-martensitic steels after different modes of heat treatment

The features of microstructure of 12% chromium ferritic-martensitic steels, EK-181 (Fe–12Cr–2W–V–Ta–B) and ChS-139 (Fe–12Cr–Ni–Mo–W–Nb–V–B), after heat treatments providing different levels of strength and plastic properties are investigated. A theoretical analysis of a number of strengthening mechanisms of these steels, depending on the conditions of heat treatment, is carried out. It is shown that dispersion hardening by MX carbonitride nanoparticles is one of the most effective ways of increasing the strength of ferritic-martensitic steels under study.

Microstructure and mechanical properties of heat-resistant 12% Cr ferritic-martensitic steel EK-181 after thermomechanical treatment

The effect of high-temperature thermomechanical treatment (TMT) with the deformation in the austenitic region on the features of microstructure, phase transformations and mechanical properties of low-activation 12% Cr ferritic-martensitic steel EK-181 is investigated. It is established, that directly after thermomechanical treatment (without tempering) the sizes and density of V(CN) particles are comparable with those after a traditional heat treatment (air quenching and tempering at 720°C, 3 h), where these particles are formed only during tempering. It causes the increasing of the yield strength of the steel up to ≈1450 MPa at room temperature and up to ≈430 MPa at the test temperature T = 650°C. The potential of microstructure modification by this treatment aimed at improving heat resistance of steel is discussed.

EFFECT OF TEMPERING TEMPERATURE ON THE PHASE TRANSFORMATIONS IN THE FERRITIC-MARTENSITIC 12% CHROMIUM STEEL EK-181

Институт физики прочности и материаловедения Сибирского отделения РАН, Томск, Россия Национальный исследовательский Томский государственный университет, Сибирский физико-технический институт им. В.Д. Кузнецова, Томск, Россия Высокотехнологический научно-исследовательский институт неорганических материалов имени академика А.А. Бочвара, Москва, Россия Национальный исследовательский ядерный университет «МИФИ», Москва, Россия