Chromium Ferritic-martensitic Steel Research Articles

Structural-phase transformations of 12% chromium ferritic-martensitic steel EP-823

The features of phase transformations of 12 % chromium ferritic-martensitic steel EP-823 under heating and cooling conditions in the temperature range from 30 to 1100 ℃ were studied by the methods of high-temperature X-ray diffraction analysis (XRD) in situ and differential scanning calorimetry (DSC). According to XRD in situ data, upon heating, the temperatures of the beginning and end of the (α → γ) transformation of ferrite (martensite – austenite) are Ac1 ≈ 880 °C, Ac3 ≈ 1000 °C, respectively. Upon cooling, a diffusion (γ → α) transformation occurs with critical points – Аr1 ≈ 860°С (beginning temperature) and Аr3 ≈ 840 °С (end temperature). According to DSC data, during heating, the critical points of the (α → γ) transformation are Ac1 ≈ 840 °C and Ac3 ≈ 900 °C. During cooling, a martensitic (γ → α) transformation is realized with critical points of the beginning of Ms = 344 ℃ and the end of Mf = 212 ℃ of this transformation. The XRD in situ analysis revealed no precipitation of carbide phases under heating and cooling conditions of steel EP-823. Position of the critical points of phase transformations depends on the research method (XRD in situ or DSC), which is determined by the difference in effective (taking into account the time for shooting in the XRD method) heating-cooling rate. The effect of elemental composition on the position of critical points of phase transformations and the formation of structural-phase states of ferritic-martensitic steels is discussed. It is shown that the increased content of ferrite-stabilizing elements (Cr, Mo, Nb) in composition of EP-823 steel, compared with other steels of the same class, expands the region of existence of the ferrite phase, which can contribute to an increase in the temperature of Ac1 .

Mechanisms of hardening of 12 % chromium ferritic-martensitic steel EP-823

Based on experimental data on microstructure parameters of the reactor high-strength high-chromium (12 % Cr) ferritic-martensitic steel EP-823, the authors identified the main factors responsible for its strength properties. The hardening mechanisms of this steel were analyzed after processing according to the modes that provide different level of steel strength properties. Traditional heat treatment (THT) and promising modifying high-temperature thermomechanical treatment (HTMT) are considered. The main mechanisms of steel hardening, regardless of the processing mode, are: dispersed hardening by nanoscale particles of the MeX type (Me = V, Nb, Mo; X = C, N) by the Orovana mechanism; grain-boundary hardening by high-angle boundaries of martensitic blocks and ferrite grains; substructural hardening by small-angle boundaries of martensitic lamellae; dislocation hardening by increased dislocation density. HTMT mode, which includes hot deformation in the austenitic area, leads to a significant modification of the structural-phase state of steel relative to THT: a decrease in the average size of blocks and lamellae of martensite, as well as ferrite grains, an increase in the density of dislocations and the volume fraction of nanoscale particles of the MeX type. At the same time, the corresponding contributions to value of the steel yield strength from grain boundary, substructural and dispersed hardening increase by 1.2, 1.3 and 1.8 times in comparison with THT. The relative contributions of the considered hardening mechanisms to the yield strength of ferritic-martensitic steel EP-823 were discussed. The values closest to the experimental yield strength after two treatment modes studied are obtained when the Langford-Cohen model is used to estimate the magnitude of substructural hardening.

Hardening Mechanisms of 12% Chromium Ferritic-Martensitic Steel EP-823

Hardening Mechanisms of 12% Chromium Ferritic-Martensitic Steel EP-823

Temperature Dependences of Elastic Young’s Modulus and Internal Friction of 12% Chromium Ferritic-Martensitic Steels EK-181 and EP-823 with Different Heat Treatment Modes

Temperature Dependences of Elastic Young’s Modulus and Internal Friction of 12% Chromium Ferritic-Martensitic Steels EK-181 and EP-823 with Different Heat Treatment Modes

Special features of the surface layer structure of ferritic-martensitic EP-823-Sh steel after prolonged exposure to the flowing lead at 630 °C under low oxygen concentration

The microstructure, the phase and elemental compositions of the surface and near-surface layers of the EP-823-Sh reactor 12 % chromium ferritic-martensitic steel after holding in an oxygen-containing (C[o] = (4–8) × 10−7 wt. %) molten lead flow at 630 °C for 2500 h are investigated. As a result of the liquid lead interaction with the steel samples, an oxide film from 250 nm to 18 µm is formed on their surfaces. It is a layer of Fe-Cr spinel enriched in Si and Mn. An internal oxidation zone of steel with a thickness of 4–10 µm is found under the scale layer. In the internal oxidation zone, both inside the matrix grains and along their boundaries, oxide particles (Cr, Si, Mn)2O3 and Fe3O4 with sizes up to 50 nm are observed. Below this zone, a matrix layer depleted in chromium (with a thickness of ≤ 15 µm) is located. The features of the oxidation zone structure are discussed, the role of alloying elements is determined (identified and validated), and a possible mechanism for the formation of such a zone is proposed.

The Microstructure and Mechanical Properties of Ferritic-Martensitic Steel EP-823 after High-Temperature Thermomechanical Treatment

The effect of high-temperature thermomechanical treatment (HTMT) with plastic deformation by rolling in austenitic region on the microstructure and mechanical properties of 12% chromium ferritic-martensitic steel EP-823 is investigated. The features of the grain and defect microstructure of steel are studied by Scanning Electron Microscopy with Electron Back-Scatter Diffraction (SEM EBSD) and Transmission Electron Microscopy (TEM). It is shown that HTMT leads to the formation of pancake structure with grains extended in the rolling direction and flattened in the rolling plane. The average sizes of martensitic packets and ferrite grains are approximately 1.5–2 times smaller compared to the corresponding values after traditional heat treatment (THT, which consists of normalization and tempering). The maximum grain size in the section parallel to the rolling plane increases up to more than 80 µm. HTMT leads to the formation of new sub-boundaries and a higher dislocation density. The fraction of low-angle misorientation boundaries reaches up to ≈68%, which exceeds the corresponding value after HTMT (55%). HTMT does not practically affect the carbide subsystem of steel. The mechanical properties are investigated by tensile tests in the temperature range 20–700 °C. It is shown that the values of the yield strength in this temperature range after HTMT increase relative to the corresponding values after THT. As a result of HTMT, the elongation decreases. A significant decrease is observed in the area of dynamic strain aging (DSA). The mechanisms of plastic deformation and strengthening of ferritic-martensitic steel under the high-temperature thermomechanical treatments are also discussed.

Микроструктура, структурно-фазовые превращения и механические свойства малоактивируемой 12 %-й хромистой ферритно-мартенситной стали ЭК-181 в зависимости от условий обработки

The results of investigations of the treatment conditions influence on the regularities of structural-phase transformations, microstructure and mechanical properties of heat-resistant low-activation 12 % chromium ferritic-martensitic steel EK-181 are summarized. The physical factors responsible for increasing the yield strength of steel, the relationship of microstructure features with strength and plastic properties are revealed. The regularities of plastic deformation of steel are established. The modes of step-by-step heat treatment (SHT) and high-temperature thermomechanical treatment (HTMT) are presented, which lead to the formation of structural states with high values of dispersion and volume fraction of nanoscale particles V(C, N), dislocation density and internal stresses. These modes provide a significant (by ≈ 20 %) increase in the strength properties of steel in a wide (from –196 to 800 °C) temperature range, while maintaining a sufficient reserve of plasticity and increasing its thermal stability, compared with the traditional heat treatment mode (THT).

Fracture Mechanisms of Low Activation 12% Chromium Ferritic-Martensitic Steel EK-181 in the Temperature Range from –196 to 800°C

Fracture Mechanisms of Low Activation 12% Chromium Ferritic-Martensitic Steel EK-181 in the Temperature Range from –196 to 800°C

The Features of Martensitic Transformation in 12% Chromium Ferritic-Martensitic Steels.

An anomaly in martensitic transformation (the effect of martensitic two-peak splitting in the temperature-dependent thermal expansion coefficient) in complex alloyed 12% chromium steels Fe-12%Cr-Ni-Mo-W-Nb-V-B (ChS-139), Fe-12%Cr-Mo-W-Si-Nb-V (EP-823) and Fe-12%Cr-2%W-V-Ta-B (EK-181) was investigated in this study. This effect is manifested in steels with a higher degree of alloying (ChS-139). During varying temperature regimes in dilatometric analysis, it was found that the splitting of the martensitic peak was associated with the superposition of two martensitic transformations of austenite depleted and enriched with alloying elements. The anomaly was subsequently eliminated by homogenization of the steel composition due to high-temperature aging in the γ-region. It was shown that if steel is heated to 900 °C, which lies in the (α + γ) phase region or slightly higher during cooling, then the decomposition of austenite proceeds in two stages: during the first stage, austenite is diffused into ferrite with carbides; during the second stage, shear transformation of austenite to martensite occurs.

Regularities of the deformed microstructure of ferritic-martensitic steel EP-823 after high-temperature thermomechanical treatment

The features of the microstructure of 12% chromium ferritic-martensitic steel EP-823 near the neck region of samples deformed by tension at T=-70, -40 °C, 20 °C and in the temperature range close to the reactor core operating temperatures of T = 650, 720 °C after high-temperature thermomechanical treatment (HTMT) are investigated. It is shown that at negative and room temperatures plastic deformation leads to curvature and fragmentation of martensitic lamellae, formation of new low-angle boundaries and a significant increase in the dislocations density. Deformation at high temperature contributes to the dynamic recovery, dynamic polygonization and increased sizes of carbide and carbonitride particles (M23C6 and MX type).

Behavior of 12% Cr low-activation ferritic-martensitic steel EK-181 after holding in a static lead melt at 600 °С for 3000 hours

The behavior of low-activation 12% chromium ferritic-martensitic steel EK-181 (RUSFER-EK181) is investigated during holding in an oxygen-containing lead melt at 600 °C for 3000 h. Following this exposure, a 5 to 45 µm bi-layer oxide scale is formed on steel surface. Its outer and inner layers represent oxide films mainly enriched with iron and chromium, respectively. The X-ray diffraction and electron microscopy analyses show that the oxide scale is primarily represented by particles of Fe3O4 and Fe2O3 iron oxides, and to a lesser extent by Cr2O3 chromium oxides. A chromium-depleted region of the matrix several micrometers thick is found under the scale layer. It is shown that the bulk microstructure of steel retains its stability during long-term exposure in liquid lead. No qualitative changes are observed after such exposure.

Correction to: Features of Phase Transformations of Low-Activation 12%-Chromium Ferritic-Martensitic Steel EK-181

The last paragraph of the article should read “The work was performed according to the Government research assignment for ISPMS SB RAS, project No. III.23.2.6.”

Features of Phase Transformations of Low-activation 12%-Chromium Ferritic-Martensitic Steel Ek-181

An investigation of the features of structural-phase transformations during heating and cooling of the lowactivation 12%-chromium ferritic-martensitic steel EK-181 (Fe–12Cr–2W–V–Ta–B) is performed in the range of temperatures from 30 to 1100°C by the methods of high-temperature X-ray diffraction analysis (XRD) in situ. The critical points of an (α →γ) - transformation (Ас1 ≈ 850°С, Ас3 ≈ 950°С) in the course of steel heating and the temperature interval (910–890°C) of the diffusion-induced transformation of the austenite →α-ferrite type under cooling are determined. It is shown that the position of these points changes depending on the method (XRD or differential scanning calorimetry) of their identification, which is attributed to the differences in their heating-cooling rates. During heating in the range of temperatures of 830–875°C, a significant increase in the volume fraction of М23С6 carbides is observed. Further temperature increase up to 1000–1100°C leads to their complete dissolution. In the course of cooling of the steel specimens from 1100 to 30°C, there is no carbide particle precipitation.

Structural-Phase Evolution of Quenched Low-Activation Ferritic–Martensitic Steel in Dependence of the Temperature of Isothermal Holding

This paper is devoted to the investigation of the effect of nitrogen and carbon content, as well as different temperatures of isothermal holding,—on the structural-phase state evolution of the quenched low-activation 12% chromium ferritic–martensitic steel. Basic temperature ranges within which the structural-phase state of the quenched steel changes upon heating have been found using the differential scanning calorimetry technique. Correlations between changes in the fine atomic structure of steel and structural-phase transformations during isothermal heating have been established by Mossbauer spectroscopy.

Закономерности фазовых превращений малоактивируемой 12 %-й хромистой ферритно-мартенситной стали ЭК-181

Закономерности фазовых превращений малоактивируемой 12 %-й хромистой ферритно-мартенситной стали ЭК-181

Correlation of 12% Chromium Ferritic-Martensitic Steel Heat Resistance with Supercooled Austenite Stability Indices

Development of new heat-resistant ferritic-martensitic steels with 12% chromium designed for operation at 700–720 °С is a complex multifactorial task. The main characteristics of such steels that define the possibility of their application at elevated temperature are high values of strength within the temperature range of 20–720 °С, and high values of creep resistance at temperatures of 650 °C and above with a satisfactory ductility level (at least 6–12% at 20–720 °С). A significant amount of time is required in the case of large range of steels investigated to determine the abovementioned properties. An important task is finding readily and quickly obtained properties by which it is possible to estimate the level of steel heat resistance properties. Diagrams for isothermal decomposition of supercooled austenite are analyzed and equations are found in the present work for their connection with yield strength and creep strength of the materials studied.

Temperature Dependences of Mechanical Properties and Fracture Features of Low-Activation Ferritic-Martensitic EK-181 Steel in a Temperature Range from – 196 to 720°C

The features of the microstructure, structural-phase transformations, and regularities of the change in the short-term mechanical properties of low-activation 12% chromium ferritic-martensitic steel EK-181 together with the features of its plastic deformation and fracture by the active stretching method in a temperature range from–196 to 720°C are investigated. The high efficiency of dispersive hardening by nanoscale particles V(C, N) provides a weak temperature dependence of the steel strength properties with increasing temperature from 20 to 450°C. A significant increase in the temperature dependence of the steel yield strength is observed in the ductile-to-brittle transition interval (below T ~ 20°C). In a temperature range from–196 to 720°С, the change in the plasticity regularities and the mode of steel fracture are closely related to the features of the temperature dependence of the yield stress and ultimate tensile strength. In the region of temperatures above 0°С (up to ~450°С), these features are determined by the weak temperature dependence of the value of dispersion hardening by nanoscale vanadium carbonitride particles; in the temperature range below 0°С, they are determined by a strong temperature dependence of the thermally activated mobility of dislocations in the crystalline relief (the Peierls barrier, solid solution of impurities).

Effect of high-temperature thermomechanical treatment in the austenite region on microstructure and mechanical properties of low-activated 12% chromium ferritic-martensitic steel EK-181

The effect of high-temperature thermomechanical treatment with deformation in the austenite region on the microstructure and mechanical properties in low-activated 12% chromium ferritic-martensitic steel EK-181 (Fe–12Cr–2W–V–Ta–B) has been investigated. This treatment leads to a significant increase (compared to traditional regime of treatment) in the density of dislocations, dispersity, and volume fraction of nanosized particles V(C,N) and, as a consequence, to an increase in the yield strength while maintaining a sufficient reserve of ductility.

Investigation of microstructure and thermal stability of pulsed plasma processed chromium ferritic-martensitic steels

This paper presents results of the microstructural evolution and thermal stability of the promising Russian ferritic-martensitic steels (EP 823, EP 900, EK 181 and ChS 139) for the nuclear and fusion application after surface modification by high temperature pulsed plasma flows (HTPPF) treatment. Investigations of microstructure, topography and elemental content changes associated with irradiation by nitrogen plasma with energy density 19-28 J/ cm2 and pulse duration 20 μs were carried out. Changes in microstructure and elemental content occurring in the modified surface layer were characterized by means of scanning electron microscopy (SEM) and X-ray microanalysis (EDS and WDS). It was shown that independently of initial microstructure and phase composition, HTPPF treatment of ferritic- martensitic steels leads to formation of ultrafine homogeneous structure in the near surface layers with typical grain size ∼100 nm. Results of microstructure investigations after annealing during 1 hour demonstrates significant thermal stability of nanostructure formed by HTPPF treatment.

Comparative Tensile Flow and Work-Hardening Behavior of 9 Pct Chromium Ferritic-Martensitic Steels in the Framework of the Estrin–Mecking Internal-Variable Approach

The comparative tensile flow and work-hardening behavior of P9 steel in two different product forms, normalized and tempered plate and thick section tube plate forging, and P91 steel were investigated in the framework of the dislocation dynamics based Estrin–Mecking (E–M) one-internal-variable approach. The analysis indicated that the flow behavior of P9 and P91 steels was adequately described by the E–M approach in a wide range of temperatures. It was suggested that dislocation dense martensite lath/cell boundaries and precipitates together act as effective barriers to dislocation motion in P9 and P91 steels. At room and intermediate temperatures, the evolution of the internal-state variable, i.e., the dislocation density with plastic strain, exhibited insignificant variations with respect to temperature. At high temperatures, a rapid evolution of dislocation density with plastic strain toward saturation with increasing temperature was observed. The softer P9 steel tube plate forging exhibited higher work hardening in terms of larger gains in the dislocation density and flow stress contribution from dislocations than the P9 steel plate and P91 steel at temperatures ranging from 300 K to 873 K (27 °C to 600 °C). The evaluation of activation energy suggests that the deformation is controlled by cross-slip of dislocations at room and intermediate temperatures, and climb of dislocations at high temperatures. The relative influence of initial microstructure on flow and work-hardening parameters associated with the E–M approach was discussed in the three temperature regimes displayed by P9 and P91 steels.