Supercooled Austenite Research Articles

Structural changes after high-speed impact of tungsten powder with a steel target

Shock-wave high-velocity impact of tungsten powder with a steel target was studied in the work. It is determined that shock wave treatment of the samples leads to a refinement, flattening and stretching of ferrite and perlite grains in the surface layer. The structural transition from plate perlite in the near-surface layer of the target to coarse-plate perlite in the bulk of the target, the microstructure of which does not differ from the microstructure of the initial sample, was detected in the structure of U8 steel after a particle flux. It is found that cellular supercooled austenite with tungsten carbide mesh along grain boundaries is formed in the structure of steel. The microhardness of the target in depth after exposure to tungsten particles is analyzed. It was shown that the microhardness distribution along the width of the sample is wave-like, and the microhardness decreases monotonically along the depth of the sample. The microhardness maximizes by 24% at a distance of 4 mm from the surface compared to the original microhardness.

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.

A Study of the Effect of Nickel on Structural and Phase Transformations and Properties of High-Strength Medium-Carbon Complexly Alloyed Steel

The effect of nickel content in a high-strength medium-carbon complexly alloyed steel on the structural and phase transformations, location of critical points, quantitative parameters of microstructure and critical quenching rate is studied. Thermokinetic diagrams of decomposition of supercooled austenite in the steel with original composition are plotted and the characteristic types of microstructure forming under cooling at different rates are described. The effect of nickel on the combination of mechanical properties of thick rolled sheets after quenching and low-temperature tempering is determined. The results obtained may be used for development of high-strength cold-resistant steels and modes of their heat hardening.

Numerical modeling of heat transfer at cooling a high-temperature steel cylinder by the flow of a water-air medium with account taken of the gamma - alpha transformation

Based on the mathematical model of the conjugate heat transfer from a steel cylinder to a water-air medium, the results of the calculation of the cooling rate and the mass fraction of the α-phase in supercooled austenite during the γ → α transformation are obtained. To find the mass fraction of the α-phase in austenite during supercooling, a modified Kolmogorov – Johnson – Mel – Avrami equation is used. The calculation results have been obtained during the cooling of the high-temperature metal cylinder by the laminar gas-liquid flow in an annular channel with account taken of vaporization in the liquid. The regulation of the heat and mass transfer balance of the system during the vaporization is based on the energy model of the heat balance and is performed using the effective thermophysical parameters of the cooling medium. The mathematical model is presented in a two-dimensional non-stationary formulation taking into account the symmetry of the cooling medium flow relative to the longitudinal vertical axis of the cylinder. To solve the system of differential equations, the control volume method is used. The parameters of the cooling medium flow are calculated using an algorithm SIMPLE. For the iterative solution of the system of linear algebraic equations, the methods of Gauss-Seidel and under-relaxation are used. The calculations, are carried out using a grid with a condensing profile at the «metal cylinder – liquid» and «liquid - outer metal ring» boundaries on the liquid side and the metal side.

Transformations, Structure and Properties of Steel 22Kh2G2S2MF Under Continuous Cooling

Transformations of structure in structural steel 22Kh2G2S2MF under continuous cooling are studied by the methods of dilatometric analysis and optical microscopy. Athermokinetic diagram of decomposition of supercooled austenite is plotted. It is shown that steel 22Kh2G2S2MF may undergo simultaneous martensitic and bainitic transformations under continuous cooling in the range – 200°C. As a result of continuous cooling at a rate vcool = 1.5 – 0.05 K/sec the ultimate strength of steel 22Kh2G2S2MF attains 1500 – 1700 MPa, which makes it possible to classify it as a high-strength one.

Kinetics of Phase Transformations in Roll Steel 8Kh3SMF

The structure and the hardness of steel 8Kh3SMF after isothermal quenching at 200 – 700°C with a step of 50°C and holds for 20 – 5000 sec are studied. An experimental isothermal diagram of decomposition of supercooled austenite is plotted. The kinetics of phase transformations in steel 8Kh3SMF is described with the help of the Austin–Rickett equation.

The Chemical Composition Effect of Medium-Carbon Chromomanganesemolybdenum Steel on the Structure of Drill Pipes

Dilatometric studies on medium-carbon Cr–Mn–Mo 40KhGMA steel with various chemical compositions are performed. Thermokinetic diagrams for the transformation of supercooled austenite are plotted, and the temperature-time ranges for the formation of structural components are determined. The metallographic study of the steel microstructure have been carried out after cooling at different rates. Steel hardness depending on the tempering temperature and mechanical properties in the heat-treated state are studied. By using a numerical simulation, cooling paths under heat treatment have been determined for a drill pipe with a wall thickness of 30 mm.

Revisiting Some Prospects for the Further Development of Ideas by Academician V.N. Antsiferov in the Field of Structural Powder Materials

Some developments by Academician V.N. Antsiferov and his scientific school are considered from the viewpoint of their further development when fabricating structural powder materials and products. The prospects of work for the fabrication of concentration-inhomogeneous and trip steels are noted. There is great potential in the possibility of controlling the thickness and volume of the zone of the deformation martensite transformation occurring during the fracture. It is advisable to continue works on the fabrication of fullerene-containing and nitrogen-containing powder compositions, as well as on studying the structural heredity of powder steels. The possibility of synthesizing fullerene-containing phases during the liquid-phase sintering of “iron–cast iron” and “iron–graphite” compositions and their subsequent redistribution in the material bulk during dynamic hot pressing deserves attention. It is reasonable to fabricate nitrogen-containing steels by the mechanical activation of powders with subsequent sintering in dissociated ammonia not only when forming wear-resistant and corrosion-resistant steels, but also heat-resistant materials. Works on studying the decomposition of supercooled austenite in powder steels of various alloying systems having different process prehistories (sintered, hot-deformed, infiltrated, etc.) are promising. Works on studying hot deformed concentration-inhomogeneous materials fabricated, in particular, based on powders of the Distaloy type, are promising. Original procedures developed by the school of Academician V.N. Antsiferov are valuable. This is, in particular, the procedure of determining the concentration variation coefficient, as well as the magnitometric complex and mathematical model providing the possibility to forecast the decomposition of supercooled austenite. Works by V.N. Antsiferov can be in-demand when solving the problem of lean alloyed powder steels with a lower bainite structure, which provide the optimal combination of strength and viscosity.

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.

Effect of Annealing on the Structure and the Hardness of Die Steel with a Controllable Austenitic Transformation during Operation

Medium-carbon (0.44% C) steel with a controllable austenitic transformation during operation is investigated. It differs from a commercial 4Kh2N5M3K5F (EP930) steel in a high content of manganese, the element that decreases the α → γ transformation temperature and increases the stability of supercooled austenite. Manganese partially substitutes for deficient nickel. The effect of heat treatment on the structure and the hardness of the steel is investigated. Heat treatment conditions are proposed to reduce the hardness of the initial hot-rolled steel for subsequent machining.

Effect of boron microaddition on hardenability of new-developed HSLA-type steels

Purpose: The paper presents the results of investigations on the effect of 0.003% boron microaddition on the hardenability of new-developed HSLA-type steels. In order to prevent the binding of the boron microaddition with nitrogen in BN nitrides, Ti microaddition at concentrations of 0.033% and 0.028% was also added into the tested steels. Design/methodology/approach: Evaluation of hardenability of the investigated steels was carried out on the basis of the Jominy test and the analytical method, according to the ASTM 255-89 standard, taking into account the effect of the boron microaddition. Additionally, developed of the CCT-diagram of investigated steel. A DIL 805A/D dilatometer with a LVDT-type measuring head was used to carry out the dilatometric test. Findings: Microaddition of boron, introduced into steel in a concentration of 0.003% along with Ti microaddition shielding (in concentration of 0.033% in steel A and 0.028% in steel B), advantageously improves hardenability. This is reflected in calculated ideal critical diameter DIB, which is equal 163 mm for steel A and 155 mm for steel B. The form of curves of phase transformations of supercooled austenite is typical for steels with microaddition of boron, with similar chemical composition. Research limitations/implications: Due to similar chemical composition of investigated steels, the kinetics of phase transformations of austenite, supercooled under continuous cooling, was determined for steel B containing 0.28% C, 1.4% Mn, 0.3% Si, 0.26% Cr, 0.22% Mo and Nb, Ti, V and B microadditions at 0.027%, 0.028%, 0.019% and 0.003% respectively. Practical implications: Tested steels have high hardenability and show the full usability for production of forged parts with the method of thermomechanical treatment, i.e., hot- deformed in the temperature range adjusted to the type of microadditions added to steel, with direct quenching of forgings from finishing forging temperature. The results of the tests may be useful for developing the parameters of heat treatment and thermomechanical treatment of investigated steels. Originality/value: The hardenability of new-developed HSLA-type steels was determined.

The combining effects of ausforming and below-Ms or above-Ms austempering on the transformation kinetics, microstructure and mechanical properties of low-carbon bainitic steel

The isothermal transformation kinetics, microstructure and mechanical properties of a low-carbon bainitic steel, subjected to below-Ms/above-Ms austempering with or without prior ausforming, have been investigated via dilatometric measurements, microstructural characterization and mechanical tests. The results show that for all austempered samples, prior ausforming largely refines the bainitic laths and enhances the mechanical stability of supercooled austenite, the latter leaving more austenite untransformed at the finish of isothermal treatment. Nevertheless, divergent consequences on the final microstructure arise: for the above-Ms austempered samples, prior ausforming increases the amount of large, unwanted brittle martensite/austenite blocks and decreases the volume fraction of retained austenite at room temperature; however, for the below-Ms austempered samples, prior ausforming decreases the size and amount of martensite/austenite blocks and increases the volume fraction of retained austenite. Accordingly, the below-Ms austempered samples with prior ausforming exhibit a product of strength and ductility of ∼ 43 GPa% and impact toughness of ∼180 J/cm2, in sharp contrast with those of the above-Ms austempered samples with prior ausforming, ∼33 GPa% and ∼80 J/cm2, respectively. The present results clearly demonstrate a favorable effect of the combined process of ausforming and below-Ms austempering against the adverse combining effect of ausforming and above-Ms austempering.

DECAY OF SUPERCOOLED AUSTENITE OF LOW-CARBON PIPE STEEL WITH THE USE OF GLEEBLE 3500 COMPLEX

The promising direction in improving the strength properties of low-carbon steels is the use of controlled rolling providing formation of structures with prevalence of a bainite component. Analysis of the references has shown that now there are no detailed researches which allow to approve what morphological type of bainite provides the most optimal properties. In this regard in the present work the influence of cooling rate on the structure, properties, and structuralphase transformations of low-carbon complex-alloyed pipe steel containing 0.062 % C; 1.80 % Mn; 0.12 % Mo; 0.032 % Cr, 0.90 % Ni and other elements (Al, Cu, V, Nb, Ti) was studied. The dilatometric method was used to construct the CCT diagram of the decay of supercooled austenite of low-carbon complex-alloyed pipe steel. The qualitative and quantitative analysis of microstructure was carried out and hardness after various speeds of cooling was determined. There were identified the cooling rates providing bainite structures and increase in the strength properties of steel with specified composition. At a cooling rate since 0.05 to 6 °C/s, along with ferrite, a globular bainite is formed in the microstructure, consisting of bainitic α-phase and “islands” of martensite-austenite component ranging in size from 1 – 6 μm. At a cooling rate of 6 °C/s, conversion to reed bainite is observed, along with the borders of which the carbides and residual austenite are located. At cooling rates of more than 16 °C/s, bainite becomes bag-rack. With an increase in cooling rate from 50 to 150 °C/s, the average width of the bainite α-phase rails decreases from 2.22 to 1.32 μm.

On some prospects for further development of Academician V.N. Antsiferov ideas in the field of structural powder materials

The paper discusses the further development of some ideas of V.N. Antsiferov and the scholar school created by him in obtaining structural powder materials and products. The prospects for obtaining concentration-inhomogeneous steels and trip-steels are noted. The great potential lies in the control of the thickness and volume of the zone of deformation martensitic transformation occurring during fracture. It is advisable to continue the preparation of fullerene- and nitrogen-containing powder compositions and to study the structural heredity of powder steels. The possibility of the synthesis of fullerene-containing phases during the liquidphase sintering of the iron–cast iron and iron–graphite compositions and their subsequent redistribution in the bulk of the material during dynamic hot pressing is worth noticing. Producing nitrogen-containing steels by mechanical activation of powders followed by sintering in dissociated ammonia is advisable to use for obtaining not only wear-and corrosion-resistant materials, but also heat-resistant ones. The studies on the decomposition of supercooled austenite in powder steels of various doping systems with different technological backgroung (sintered, hot-deformed, infiltrated, etc.) are promising. The potential for development is the research of hot-deformed concentration-inhomogeneous materials, obtained, in particular, on the basis of powders of the Distaloy type. The techniques developed by the Antsiferov’s school are significant. The most important one is the method for determining the concentration variation coefficient, as well as a magnetometric complex and a mathematical model, which makes it possible to predict the decomposition of supercooled austenite. Antsiferov’s works can be used for obtaining lean powder steels with the lower bainite structure, which provides the optimal combination of strength and toughness.

Decomposition of Supercooled Austenite in Low-Carbon Pipe Steel

Controlled rolling may facilitate the transition from ferrite–pearlite structure to bainite structure and hence improve the strength of low-carbon alloy steel. A literature review shows the lack of detailed research regarding the type of bainite structure that ensures optimal properties of low-carbon alloy steel. To determine the temperature, time, and other parameters of treatment to produce such structure, it is expedient to establish the influence of the cooling rate on the structure and properties of low-carbon alloy steel. In the present work, we investigate the decomposition of supercooled austenite in low-carbon alloy pipe steel with 0.062% C, 1.80% Mn, 0.120% Mo, 0.032% Cr, 0.90% Ni, and other elements (Al, Cu, V, Nb, Ti). The treatment conditions producing bainite structure with elevated steel are determined for such steel. At low cooling rates (no more than 6°C/s), not only ferrite is formed in the microstructure but also granular (or globular) bainite consisting of bainitic α phase and an insular martensite–austenite component (1–6 μm). At a rate of 6°C/s, transition to rack bainite is observed; around the boundaries of the racks, carbides and residual austenite are seen. At cooling rates above 16°C/s, the bainite has packet–rack structure, Between 50 and 150°C/s, the mean rack width of the bainitic α phase declines from 2.24 to 1.32 μm.

THE FORMATION OF THE FRICTION SURFACE OF MACHINE PARTS OF WEAR RESISTANT GRADIENT STRUCTURES BY MEANS OF COMBINED ELECTROMECHANICAL PROCESSING

The purpose of research in this work is to increase the wear resistance of friction surfaces of machine parts by implanting materials based on tungsten carbide, providing a modified surface layers with improved physical and mechanical properties in comparison with wear-resistant coatings obtained by traditional methods of processing. The technology of implanting a combined Electromechanical treatment is implemented on a special installation, which is a technological complex consisting of: a universal machine (used for machining workpieces) with the appropriate tools and devices for fixing the workpiece and the supply of electric current of high power and low voltage; power unit for the conversion of industrial electric current; control unit processing modes; means of switching and supply of lubricating-cooling technological medium; unit interface with PC. In the process of implanting tungsten carbide particles were introduced into the formed surface layer, reinforcing it. Then, on the same surface, Electromechanical treatment was carried out on the reinforcing modes. It is experimentally established that in fact the surface layer of carbon steel 45 is a gradient structure similar to the structure of tool steel P18, and with a higher hardness as a consequence of the composite hardening of the released carbide phases of different morphology. The gradient structure is a cellular supercooled austenite stabilized by tungsten and reinforced with a carbide mesh consisting of aggregated highly dispersed (less than 1 µm) filamentous and rounded tungsten carbide particles. The presence of a gradient structure with a smooth transition to the main metal matrix of steel 45 provides monolithic adhesion of hardened layers that do not peel off from each other during wear.

Effect of ausforming on the martensite microstructure of 20CrMnTiH steel

The hot compressive deformation behavior of 20CrMnTiH steel was studied using a Gleeble-3500 thermomechanical simulator at temperatures ranging from 600 °C to 650 °C at a strain rate of 5 s−1 and in a strain range of 30% to 70%. The effects of the ausforming temperature, strain rate and strain on the martensite microstructure were studied. The results show that in the absence of recrystallization, when the strain rate is large, the deformation temperature of the supercooled austenite is lowered, the deformation amount is increased, the martensite structure is refined, and the microhardness is increased. The deformation of supercooled austenite can create nanostructured lath martensite with high microhardness.

Structure and Properties of Steel 40Kh2N2MA After Thermomechanical Treatment with Martensitic-Bainitic Transformation of Austenite

Special features of structure formation in steel 40Kh2N2MA are studied after a high-temperature thermomechanical treatment with martensitic-bainitic transformation of austenite. It is shown that the treatment produces a mixed structure represented by alternating extended regions of tempered martensite and lower bainite. The treatment raises substantially the impact toughness and the crack resistance of the steel. The process is the most effective for hardening alloy steels with enhanced stability of supercooled austenite.

Austenite Decomposition and Precipitation Behavior of Plastically Deformed Low-Si Microalloyed Steel

The aim of the present study is to assess the effects of hot deformation and cooling paths on the phase transformation kinetics in a precipitation-strengthened automotive 0.2C–1.5Mn–0.5Si steel with Nb and Ti microadditions. The analysis of the precipitation processes was performed while taking into account equilibrium calculations and phase transitions resulting from calculated time–temperature–transformation (TTT) and continuous cooling transformation (CCT) diagrams. The austenite decomposition was monitored based on thermodynamic calculations of the volume fraction evolution of individual phases as a function of temperature. The calculations were compared to real CCT and DCCT (deformation continuous cooling transformation) diagrams produced using dilatometric tests. The research included the identification of the microstructure of the nondeformed and thermomechanically processed supercooled austenite products formed at various cooling rates. The complex interactions between the precipitation process, hot deformation, and cooling schedules are linked.

Transformations of Supercooled Austenite in Promising High-Hardenability Machine Steels

We study special features of the transformation of supercooled austenite under the continuous cooling of Si – Mn steels with reduced contents of nickel as compared with traditionally used machine steels. The temperature ranges of the phase and structural transformations running under the conditions of heating and cooling of steels are determined by the dilatometric method. We plot the thermokinetic diagrams of transformations of supercooled austenite. The microstructural components formed in the investigated steels are analyzed both qualitatively and quantitatively. We also propose the chemical compositions of promising steels characterized by a high stability of supercooled austenite and high hardenability.