15Kh2NMFA Steel Research Articles

Characteristics of experimental hard alloy WC+(Co+Mo+Ti) in the process of friction against steels 15Kh2NMFA and 10GN2MFA without lubrication

Characteristics of experimental hard alloy WC+(Co+Mo+Ti) in the process of friction against steels 15Kh2NMFA and 10GN2MFA without lubrication

Influence of corrosion-active non-metallic inclusions on the accelerated destruction of the internal surface of the Oplot-M tank barrel

The metallographic features of operational damage to the inner surface of the barrel of the Oplot-M tank were analyzed. It was established that with the movement from the chamber to the cut of the barrel, the number and size of damage decreases, which is related to the distribution of pressures of working gases, temperature and aggressiveness of the environment. It is shown that the damage is placed on the inner surface chaotically and the nature of their destruction is corrosive and corrosive-mechanical in origin. Local analysis of their chemical composition revealed a high oxygen content, which indicates the formation of oxide-hydroxide compounds. The surface layers of the barrel were studied and it was established that a "white" layer with a thickness of ~50 μm and a microhardness of 930 kg/mm2 is formed on the surface, the structure of the inner layer of the steel of the barrel is troostite (HW 225 kg/mm2). 
 The effect of corrosion-active non-metallic inclusions (KANV) on the destruction of the surface of a barrel made of steel 38KHN3MFA was studied. It is shown that more sensitive to changes in the technical state of steel due to degradation are studies of corrosion resistance, microelectrochemical heterogeneity and impact toughness of steel. It was established that the presence of KANV in the steel structure leads to an increase in corrosion currents by 4 times and microelectrochemical heterogeneity, in particular, non-periodic potential peaks of 50-70 and 200-230 mV. Due to the influence of tap water on the surface of the steel, corrosion ulcers of a rounded shape and a size of 50-80 μm were recorded, which contribute to the local corrosion of the steel. The analysis of steel fractures revealed that inclusions of a round shape with a size of 3-5 μm contain impurities of harmful elements Al, Mg, F, which accelerate local corrosion on the surface of the steel of the tank barrel. It is shown that the study of fractures, and not of samples, provides an opportunity to estimate not only the size and density of inclusions, but also their chemical composition.

Hydrogen Cooling of Turbo Aggregates and the Problem of Rotor Shafts Materials Degradation Evaluation

Changes in the properties of 38KhN3MFA steel, from which the rotor shaft is made, were investigated by comparing the hardness of the shaft surface and hydrogen concentration in the chips and analyzing changes in the morphology of the chips under the influence of various factors. The microstructures obtained from the surface of the rotor shaft samples are presented, and histograms reflecting the parameters of the structural components are constructed. An abbreviated diagram of the “life cycle” of the turbine rotor shaft is given. It was found that, during long-term operation (up to 250 thousand hours), the hardness of the rotor shaft surface decreases from 290 HB to 250 HB. It was recorded that, in the microstructure of the shaft during 250 thousand hours of operation, the amount of cementite decreased from 87% to 62%, and the proportion of free ferrite increased from 5% to 20%. The average values of ferrite microhardness decreased from 1.9 GPa to 1.5 GPa. An increase in the content of alloying elements in carbides was recorded: Cr and V—by 1.15–1.6 times; and Mo—by 2.2–2.8 times. With the help of the developed program (using computer vision methods), changes in their microrelief were detected to study photos of chips.

Thermal resistance of steels with increased strength properties for pressure vessels of advanced VVER reactors of various designs

The paper considers the results of structural studies and mechanical tests after a long-term thermal exposure of laboratory heats of the metallurgically improved 15Kh2NMFA steel and steel with an increased content of nickel considered as materials for the pressure vessels of advanced VVER-type reactors of various designs. It has been shown that, both for the improved 15Kh2NMFA steel and the high-nickel steel, there are no signs of grain boundary embrittlement after an segregation provoking embrittlement heat treatment. This is explained by the extremely low grain boundary segregation of phosphorus in the initial state caused by a high degree of the structure dispersity as well as by rather a low content of impurities. Besides, no changes have been found in the yield strength value for the improved 15Kh2NMFA steel, which agrees with the structure investigation results. For the high-nickel steel, a tendency towards a minor yield strength decrease by 5 to 10% and a regular reduction of the critical brittleness temperature has been revealed. A decrease in the mechanical properties has been caused by a relatively low temperature of tempering for the high-nickel steel and, accordingly, by the potential occurrence of the structure recovery during long-term thermal exposure, as evidenced by the results of an X-ray diffraction analysis. Despite the structure recovery in the high-nickel steel under the long-term thermal exposure, the main strengthening carbide phases remain stable. Due to this, the yield strength value remains at a relatively high level that exceeds the values for the modern VVER-type vessel steels, even in the case of a thermal exposure much in excess of the expected operating conditions for advanced VVER reactors. The observed decrease of critical brittleness temperature during the long-term thermal exposure contributes to an increase in the steel resistance to brittle fracture.

Increasing Turbine Hall Safety by Using Fire-Resistant, Hydrogen-Containing Lubricant Cooling Liquid for Rotor Steel Mechanical Treatment

This paper is devoted to the development of hydrogen-containing, environmentally safe, fire-resistant, and corrosion-protected lubricant cooling liquids (LCLs) from vegetable oils with improved sanitary and hygienic parameters for the machining of parts and equipment made from high-strength steels for application during the interoperation period in turbine halls. The use of plant raw materials as ecologically and fire-safe LCLs increased the efficiency of LCLs when evaluating drilling steel in terms of the dependence of the stability of the cutting tool on the drilling speed. Chips formed from LCLs during turning had a compact, cylindrical appearance, and the addition of both water and coolant during turning significantly changed the morphology of the cutting particles. Using water and LCL intensified the physical and chemical destruction processes. After the use of water and LCL, the concentration of hydrogen in the cutting products of 38KHN3MFA steel increased, which indicated its participation in facilitating the destruction during machining. In the chips formed when using LCL, the amount of hydrogen increased by 2.25 times compared to the chips obtained with the dry treatment, while with coolants, it increased by 2.6 times, indicating the intense flow of decomposition products of LCL through diffusion processes in the cutting zone. Hydrogen reduces the energy costs for the destruction of structural and phase components and promotes their dispersion. The creation of 2D and 3D images allowed for a more detailed approach to the study of the influence of LCL on surface treatments.

MICROPLASTICITY OF 15Kh2NMFA VESSEL STEEL IN A STRESS-STRAIN STATE DURING MAGNETIC TREATMENT

Earlier it was found that the treatment of pre-strained ferrite-pearlite vessel steel 15Kh2NMFA, welded joints of this steel by alternating magnetic field of industrial frequency leads to a decrease in the level and alignment of the stress field’s relief and significantly improves the complex of physical and mechanical properties. When non-stationary magnetic fields are applied to a structurally inhomogeneous ferromagnetic material, several different mechanisms of rearrangement of the domain and defect structure can act. In this work, we propose a model of dislocation system response of ferromagnetic in a stress-strain state to a magnetic effect, based on the microscopic properties of dislocations in a mechanically unstable state.

Evaluation of the structural stability of welded joints in welding

Brittle (intergranular) destruction of the heat-affected zone metal (local failure) of welded joints of power equipment during welding and heat treatment (tempering) was an unexpected and serious problem. Its solution based on the development of progressive technological solutions is an urgent task of modern power engineering. The article considers an applied method of fluctuation analysis for assessing the stability of the structure and the probability of local destruction (LD) of the metal of the heat-affected zone (HAZ) of thick-walled welded structures of power equipment made of low-alloy steel 15Kh2NMFA. A computational - experimental method for calculating the range of optimal values of heat input is stated. The LD mechanism is stated and its connection with the technology and thermal conditions of welding and the structural stability of the HAZ metal is established. It was found that the growth of austenite grains of the HAZ metal stimulates the tendency to LD, if a dendrid-type carbide phase is precipitated at grain boundaries during welding. By optimizing the heat input during welding, it is possible to control the structural and chemical homogeneity of the HAZ metal and the associated tendency to form LD.

Determination of Fracture Energy Characteristics and Their Anisotropy for 15Kh2NMFA Steel in Impact Bending Tests

Determination of Fracture Energy Characteristics and Their Anisotropy for 15Kh2NMFA Steel in Impact Bending Tests

Influence of Modification of 38khN3MFa Steel on the Structural-Phase State and Cutting Products under Variable Technological Conditions

We study the influence of modification on the structural-phase state of 38KhN3MFA steel and the morphology of chips formed in the course of turning of billets at a rotation speed of 315 rpm under dry conditions, in the presence of water, and with lubricoolants. It is shown that, in the case of application of lubricoolants, the sizes of chips decrease, their shape becomes more compact and folded, and the roughness of the treated surface becomes ten times lower than after dry turning.

FORECASTING OF STRUCTURAL TRANSFORMATIONS IN HEAT AFFECTED ZONE STEEL OF 15Kh2NMFA AT ANTI-CORROSION CLADDING

Using mathematical modeling based on existing parametric regression equations the prediction of microstructure-phase transformations in heat affected zone( HAZ) metal of base material (steel 15Kh2NMFA) of WWER-1000 reactor vessel in arc cladding of protective anti-corrosion layer was carried out as well as performed comparative analysis of modeling results with obtained experimental data of dilatometric and metallographic analysis. The comparison of results ensures formation of bainite-martensite structure in HAZ metal of WWER-1000 reactor vessel, however a value of content of martensite in calculation and experimental determination is different on ~ 40…50%. To clarify the calculated results, a thermokinetic diagram of the decomposition of supercooled austenite was constructed for characteristic cooling rates (3; 4, and 5 °C/s) during the welding/surfacing thermal cycle. As a result, adequate parameters of the microstructure-phase composition in the HAZ of steel 15Kh2NMFA at corrosion-resistant cladding were obtained.

High-Rate Deformation and Fracture of 15Kh2NMFA Steel under Impact Loading at Normal and Elevated Temperatures

In this study, we present the results of the recording of shock-wave phenomena in 15Kh2NMFA steel at room temperature and 504 ± 5°C. Based on measurements of the free surface velocity histories, the dependences of the material resistance to deformation and fracture on the strain rate are determined. 15Kh2NMFA steel was found to be characterized by a twice higher dynamic tensile strength than iron, with almost equal values of the Hugoniot elastic limit. At increasing temperature, the Hugoniot elastic limit of steel decreases faster than that of iron while the dynamic tensile strength varies slightly.

New Type of Corrosion-Active Nonmetallic Inclusions and their Influence on the Corrosion of 38KHN3МFА Steel

We study the influence of corrosion-active nonmetallic inclusions on the corrosion of 38KhN3MFA steel. A new type of fluorine-containing inclusions of this kind with a size 2–30 μm is discovered. These inclusions are responsible for anomalously rapid damage of drill pipes made of the indicated type of steel. It is difficult to identify them with the help of metallographic analysis. They do not affect the general chemical composition of steel, the characteristics of strength and impact toughness at room and lowered (– 50°С) temperatures. We propose a new approach to the detection of corrosion-active nonmetallic inclusions in steels reduced to the investigation of the fracture surfaces of specimens instead of their microsections. Unlike the existing methods, this approach enables one to determine the chemical composition and correctly reproduces the shape of inclusions.

Study of the character and causes of destruction of the cardan shaft of the propeller engine

The results of studying operational destruction of a high-loaded cardan shaft of the propeller engine made of steel 38KhN3MFA are presented to elucidate the cause of damage and develop a set of recommendations and measures aimed at elimination of adverse factors. Methods of scanning electron and optical microscopy, as well as X-ray spectral microanalysis are used to determine the mechanical properties, chemical composition, microstructure, and fracture pattern of cardan shaft fragments. It is shown that the mechanical properties and chemical composition of the material correspond to the requirements of the regulatory documentation, defects of metallurgical origin both in the shaft metal and in the fractures are absent. The microstructure of the studied shaft fragments is tempered martensite. Fractographic analysis revealed that the destruction of cardan shaft occurred by a static mechanism. The fracture surface is coated with corrosion products. The revealed cracks developed by the mechanism of corrosion cracking due to violation of the protective coating on the shaft. The results of the study showed that the destruction of the cardan shaft of a propeller engine made of steel 38Kh3MFA occurred due to formation and development of spiral cracks by the mechanism of stress corrosion cracking under loads below the yield point of steel. The reason for «neck» formation upon destruction of the shaft fragment is attributed to the yield point of steel attained during operation. Regular preventive inspections are recommended to assess the safety of the protective coating on the shaft surface to exclude formation and development of corrosion cracks.

Влияние сварочного цикла охлаждения на структурно-фазовый состав стали 15Х2НМФА

Влияние сварочного цикла охлаждения на структурно-фазовый состав стали 15Х2НМФА

Effect of cooling cycle of welding on structure-phase composition of 15Kh2NMFA steel

Effect of cooling cycle of welding on structure-phase composition of 15Kh2NMFA steel

Inspection of Reactor Steel Degradation by Magnetic Adaptive Testing.

Degradation of nuclear pressure vessel steel materials, 15Kh2NMFA type and A508 Cl2 type (definition is given in the text) were investigated by a novel magnetic nondestructive testing method, so-called Magnetic Adaptive Testing (MAT), which is based on systematic measurement and evaluation of minor magnetic hysteresis loops. The measured samples were thermally treated by a special step cooling procedure, which generated structural changes in the material. It was found that this type of degradation can be easily followed by magnetic measurements. Charpy impact test were also performed and the results were compared with the magnetic parameters. In case of 15Kh2NMFA steel, a good, reliable and closely linear correlation was found between magnetic descriptors and transition temperature.

Influence of Lubricating Liquid on the Formation of the Products of Cutting of 38KhN3MFA Steel

We study the morphology of chips formed in the course of turning of a blank of 38KhN3MFA steel on a turning lathe at a rotational speed of 200 rpm under the conditions without lubrication and with an addition of water or lubricating liquids (LL) containing sunflower oil. It was discovered that the addition of water and LL in the process of cutting promotes a decrease in the size of chip and leads to the transformation of its shape from flat to barrel-like. The application of LL makes the surface roughness nine times smaller than in the case of dry turning.

Nonequilibrium kinetics of the electron–phonon subsystem can give rise to electric- and magnetic-plasticity effects in crystals in alternating electric and/or magnetic fields

Kinetic processes in magnetic crystals in an alternating magnetic field and/or a pulsed electric field are studied theoretically, experimentally, and numerically to establish the prime mechanisms by which they influence the structure and the mechanical, dissipative, and magnetic characteristics of crystals. The specific materials studied are severly strained ferritic pearlite steel 15Kh2NMFA and nickel. The paper presents a systematic kinetic analysis of the nonequilibrium dynamics of the electron–phonon subsystem of a magnetic crystal in an electric field. Our proposed method that underlies the analysis solves the system of Boltzmann equations for the electron and phonon distribution functions numerically without expanding the electron distribution function in a power series of the phonon energy. It is shown that an electronic subsystem excited by an electric field transfers energy to the phonon subsystem and thereby massively produces short-wave phonons which act strongly on lattice defects (such as point and linear ones and phase boundaries) and thus redistribute and decrease their density, as well as providing damage healing, decreasing local peak stresses, and reducing the degradation level of construction materials properties. It is found that, under the action of an induction electric field, the electron distribution function becomes nonequilibrium near the Fermi level energy and, as a result of electron–phonon collisions, transfers significant energy to the phonon subsystem, resulting in forming a nonequilibrium phonon distribution function. Based on modified Granato–Lücke and Landau–Gofman models, it is shown, using the calculated phonon distribution function, that the action of phonons on dislocations is much stronger than it would be in the case of thermodynamic equilibrium at the experimentally measured sample heating by 12 K.

Features of the estimation of the influence of various factors on the integrity of the RPV reactor under the action of a melt of metals during a severe accident

The results of experimental studies of material behaviour at different temperatures ranges and loading rates are presented in this paper. Using finite element method, an analysis of the influence of various factors on the integrity of the reactor vessel of the nuclear power plant under the action of the melted corium during a severe accident. The experimental data of special tests of the metal was used. In the part of experimental research, the characteristics of strength and high temperature creep for 15Kh2NMFA steel at temperatures typical of a severe accident were obtained. An estimation of the behaviour of high-temperature steel under conditions of exposure at temperature + 650°C is carried out. New experimental data of steel deformation were obtained. Numerical simulation of the stress-strain state of the reactor vessel WWER-1000 under different conditions of a severe accident is presented. The variation of the residual wall thickness in the melting zone and different temperature of cooling of the outer surface of the vessel’s wall were considered in the analysis.

Determination of mechanical properties of steel 15Kh2NMFA using instrumented indentation results

Розроблено алгоритм відтворення діаграми деформування зразків на розтяг за параметрами діаграми контактного деформування. Визначено механічні характеристики сталі 15Х2НМФА за діаграмою одновісного розтягу, отриманої методом інструментованого індентування. Показано, що відхилення між значеннями характеристик міцності, визначеними при індентуванні й випробуванні на розтяг, не перевищує 1%.