Mechanical Characteristics Of Steel Research Articles

ANALYSIS OF MECHANICAL CHARACTERISTICS OF KNOT CONNECTIONS IN BUILDING STRUCTURES BASED ON ANSYS SIMULATION

The most common cross-section – the joining of two symmetrical I-beams – is investigated in this work. The transmission of force on the node connection in the form of a multi-bolt connection, welded connection, welded connection through the lining is modeled. The effect of node connections on the stress-strain state of the spatial structure made of thin-walled open section rods, torsional effects depending on the fastening of the elements is studied. The calculation of the structure is reduced to the determination of the stresses and displacements in the fastening nodes, by which the change in the mechanical characteristics of steel is then determined.

Optimization of cross-section dimensions of structural members made of cold-formed profiles using compromise search

The object of this study is a structural member made from a C-shaped cold-formed profile, investigated to search for optimal cross-sectional dimensions. The parametric optimization task is stated as the problem to find the optimal cross-sectional dimensions of the structural member under axial compression conditions, taking into account its post-buckling behavior (local buckling of the web and flanges, as well as a distortional buckling of the cross-section) and structural requirements. In this case, the material consumption and mechanical characteristics of steel, as well as the design lengths of the structural member, were considered constant and predefined. The considered criterion of optimality was the maximization of the load-carrying capacity of the structural member for the overall buckling under the axial compression. The stated optimization problem is solved using the method of exhaustive search while applying the developed software. Additionally, for fixed steel consumption, compromise solutions were searched that do not depend on the thickness of the profile and the design lengths of the structural member. The resulting cold-formed C-shaped profiles with optimal cross-sectional dimensions are characterized by a higher load-carrying capacity for the overall buckling under axial compression (to 24.45 % and 22.19 %) at the same steel consumption compared to the profiles offered by the manufacturer. Analysis of the reported results made it possible to devise recommendations for optimal ratios of dimensions and geometric characteristics of the structural members made from C-shaped profiles operating under axial compression. The ratios could be used both at the stage of selection of cross-sections of structural members from cold-formed profiles, and in the development of effective assortments of cold-formed profiles

Оперативная оценка температурного интервала синеломкости стали

A brief analysis of the blue brittleness temperature range of structural steels and the reasons for its occurrence has been carried out. It is noted that blue brittleness causes a change in all mechanical characteristics of steel. In particular, the strength characteristics increase, and the plasticity characteristics decrease in a narrow temperature zone of blue brittleness. It was established that hardness changed similarly to strength and can be used as a diagnostic parameter to identify the temperature range of blue brittleness. For three grades of structural steels, experiments were carried out to identify the temperature range of blue brittleness by tension of samples at various temperatures. A technique for express detection of the blue brittleness temperature range by changing the Brinell hardness determined by instrumented indentation is proposed. The technique eliminates the need to manufacture samples and saves material and labor costs.

Deoxidation and modification of steels with reduced silicon content

The object of research is the processes that affect the mechanical characteristics of steels after the treatment of melts with silicon-free complex master alloys according to existing technologies. One of the most problematic places is the negative effect of silicon on the weldability of low-alloy steel, sharply increasing the heterogeneity of welds in sulfur and phosphorus and increasing their susceptibility to hot cracks. The cyclic strength of welded joints decreases markedly with an increase in the silicon concentration of steel. Also in this case, there is a threat of the formation of silicon monoxide, which significantly increases the fragility of the finished steel products. Also, steels processed with alloys containing silicon are prone to decarburization, the formation of surface defects during hot working and graphite formation, which reduces their endurance limit. In the research it was possible to prove that the use of silicon-free complex ligatures (SFCL) in smelting made it possible to obtain an increase (2–3 times) in the entire complex of mechanical and operational properties, especially ductility, impact strength at normal and negative test temperatures (cold resistance) , fatigue strength. In some cases, the level of properties of cast metal reaches the deformed version of its manufacture, for example, rolled products and even metal obtained by electroslag remelting. Processing of the melt of various BKL steels when tapping from the furnace into a pouring ladle instead of aluminum master alloys, silicocalcium and ferrocerium (according to the current technology) provided the required level and high stability of the mechanical properties of 20GML steel. Also, thanks to the use of BCL, it was possible to reduce the consumption of ligatures and deoxidizers by 4.2 kg per ton of liquid metal and increase the yield of rejection of casting defects by 6–10 %. It has been established in the work that BCL treatment leads to stabilization of the chemical composition, refinement of the grain structure of steels, as well as an increase in its dispersion and the level of mechanical characteristics.

Исследование влияния длительной высокотемпературной эксплуатации на структуру и свойства аустенитной хромоникельмолибденовой стали

The effect of long-term operation at a temperature of 515°C on the structure and properties of steel 08Cr16Ni11Mo3 (grade 316) is studied. Data on the structure and phase composition of the steel were obtained using optical and scanning electron microscopy. The phase composition of steel in the equilibrium state was determined by thermodynamic modeling in the FactSage software package. As a result of the study, it was found that there are the structure changes with the formation of secondary phases initiated by the precipitation of elements with limited solubility from the supersaturated solid solution during operation at a temperature of 515°C for 195 000 hours in steel 08Cr16Ni11Mo3 (grade 316). The presence of the following secondary precipitates in the structure of the solid solution of austenite was revealed: carbide of the М23С6 type (M — Cr, Mo), ferrite (α), intermetallic phases. Based on a comparison of the thermodynamic modeling data and the experimental determination of the phase composition, it was found that the structure of steel is close to equilibrium. The results of static tensile tests have shown that a change in the phase composition during heat aging leads to embrittlement of steel, i. e. a decrease in ductility. In general, the mechanical properties of the material in the state after operation are close to the original ones. The tensile strength at room temperature is 5 % higher than the initial one, and in the range of elevated temperatures of 350 – 600°С it is 5 –10 % lower than the initial level. The yield strength at temperatures of 20, 500 and 600°С after operation increases by 5 –10 % of the initial level, and at temperatures of 350 and 550°С it practically does not change. In the temperature range 350 – 600°C, the ductility values of steel 08Cr16Ni11Mo3 (grade 316) decrease by 5 –17 % from the initial level. The main contribution to the change in the mechanical characteristics of steel is made by secondary precipitates of carbide and intermetallic phases.

Dispersion of the Steel Structure and its Effect on Magnetic and Mechanical Properties of Steel

The mechanical properties of steel often determine to a greater extent its performance characteristics, including service life of the equipment. Often, for operational control of mechanical properties, magnetic parameters are used such as, for example, the coercive force. Both mechanical and magnetic properties of steel are associated precisely with the structure of the material. The influence of the structure on these properties became known a long time ago, however, in similar works, the influences exerted by the structure with different degrees of dispersion are not considered. In this work, the goal was set to consider the effect of grain size difference on the magnetic and mechanical characteristics of steel.

Moment at elastic-plastic bending of steel sheet. Part 1. Parabolic approximation of steel’s hardening zone

To calculate the power parameters of metallurgical machines in the production of pipes from steel sheet, it is extremely necessary to know the analytical dependence of the elastic-plastic hardening curve of steel and the numerical value of bending moment of steel sheet depending on the curvature’s radius of its neutral plane during bending. Otherwise, the power characteristics of links of metallurgical machines (driving mechanisms of punches, torsion shafts, power frames and so on) may exceed the permissible values, which will lead to their breakage. To obtain the mechanical characteristics of steel, a round or flat steel sample of standard dimensions is usually stretched and the stretching diagram is obtained in the coordinates “normal stress σ – relative elongation ε”, which is given as a graph. In the experimental stretching diagram, the yield strength σy (or σ0.5), the ultimate strength σu, the relative elongation at break δ and the relative narrowing ψ (not always determined experimentally) of the neck of the round sample at break are usually found (determined). The stretching curve has the maximum at the moment of the beginning of the neck formation in the round steel sample (at the relative deformation εu). For a number of steels (for example, for the low-carbon steels), the deformation curve may have the yield area, where the normal stress σ is practically unchanged when the relative longitudinal deformations ε change. At this moment, the internal restructuring of the steel microstructure occurs at the level of the steel grains — sliding at the grains’ boundary and changing the orientation of the grains relative to each other. The size of the yield area is significantly smaller than the steel hardening zone. Therefore, the yield area is usually neglected in the analytical and numerical calculations.

Role of Nonmetallic Inclusions in Premature Stress-Corrosion Fractures of Drill Pipes

We analyze the causes of the in-service loss of integrity of drill pipes made of G105 steel connected with their premature stress-corrosion fractures. In this case, the mechanical characteristics of steel are satisfactory, except a somewhat lowered elongation. At the same time, both in the structure and on the fracture surfaces of impact test specimens, we reveal a large number of corrosion-active nonmetallic inclusions of the calcium-aluminate type, which may increase the rate of local corrosion by an order of magnitude, promote the formation of deep pits on the inner surface of the pipe, and play the role of sites of initiation of corrosion-fatigue cracks. This is why we assume that the stage of crack initiation, which mainly determines the total durability of drill pipes after premature failures is controlled by the presence of corrosion-active nonmetallic inclusions and their density in the structure of steel.

Mechanical Characteristics of Steels with a Gas-Phase Nickel Coating

A promising method of corrosion protection of steel parts in chemical engineering is a gas-phase coating, which has a high density and chemical stability to the influence of fluorine and good adhesion to substrate over entire coating area even for parts of complex shapes. The characteristics of the static and fatigue fracture of BSt.3sp, 40, and E10 steels are considered after coating with gas-phase nickel of different thicknesses (5, 10, 20, 50, and 100 μm) at various substrate temperatures (130, 160, 180, and 200°C). The gas-phase nickel coating 30 μm thick of steel parts can be considered useful due to its high chemical stability and the simplicity of the technological process in spite of some decrease in the mechanical characteristics.

Effect of Thermocyclic Treatment with Different Cooling Rates on the Mechanical Characteristics of 42CD4 Low-Alloy Steel

The aim of this study was to investigate the influence of thermo-cyclic treatments on the mechanical characteristics (Hardness and Resilience) of low-alloy 42CD4 steel. Thermocyclic treatment on 42CD4 steel was operated for four cycles at 850 °C for 30 min. After each cycle, the steel sample was cooled in different medium (open air and water) in order to check the effect of the cooling rate on the microstructure characteristics. It was found that the cooling rate can affect the mechanical characteristics of the steel. The hardness values of steel cooled in water were higher than those of steel cooled in air. Additionally, there was an increase in the resilience of steel sample with the increase of thermocyclic number.

DEVELOPMENT AND VERIFICATION OF COLD BENDING OFFSET NUMERICAL MODEL

For profiling of the pipeline route at the crossed land relief the elastic bend of pipes and curve inserts (branches) which in most cases are made cold flexible pipes are used. When calculating on durability and stability of the section of the pipeline with cold bending offsets (CBO), it is necessary to take into consideration the level of residual stresses and deformations in a branch wall. For the analysis of the residual stress-strain state (SSS) of CBO on stream, in this article the CBO generalized mathematical (numerical) model is developed and also verification on compliance to empirical formulas and the pilot studies is carried out. The developed CBO numerical model allows to define characteristics of the SSS depending on many parameters, such as diameter, pipe wall thick- ness, mechanical characteristics of steel, tap curvature value and a bending angle. Analytical dependences which allow to carry out primary assess- ment of the residual SSS of CBO without application of the program environment of model operation are also presented.

Effect of long-term thermal aging in heat exchange equipment of fast neutron switchgears on the structure and properties of austenitic chromium-nickel steel

The influence of long-term operation at 515 °C on structure and properties of 09Cr18Ni9 steel was investigated. Structure and phase composition were obtained using optical and scanning electron microscopy. The phase composition of the steel in equilibrium state was determined by thermodynamic modeling in the software package Fact-Sage. As a result of the study, it was found that during the operation at 515 °C with a duration of 195,000 h, the structure changes occurred in the 09Cr18Ni9 steel with the formation of secondary phases, initiated by the release of elements with limited solubility from the supersaturated solid solution. The following secondary precipitates in structure of the solid solution of austenite presented: Cr23C6 chromium carbide, ferrite (a), G-phase. Based on comparison of the thermodynamic modeling results and on experimental determination of the phase composition, it was established that the steel structure is in a state close to equilibrium. The mechanism of structural transformations course and sequence of the secondary phases’ formation were revealed and described. At the initial stage, chromium carbide is formed, then a-ferrite is formed near the carbides, and then G-phase is formed. Results of the tests for impact strength and static elongation have shown that the change in phase composition in process of thermal aging leads to embrittlement of the steel - a reduction in ductility and impact energy. Fractografic studies of fracture surfaces of the samples have shown that the decrease in plasticity during long-term high-temperature operation is associated with softening of the grain body and strengthening of the boundaries due to secondary precipitations of the carbide phase. As a result of this process, plastic deformation is localized in the weakened volume of the body of grain surrounded by strong boundaries. The structure evolution during prolonged heat aging has the greatest effect on impact strength. At the same time, the change in ultimate and yield stress is insignificant. The main contribution to the change in mechanical characteristics of steel is made by the secondary precipitates of the carbide phase.

Complex method of defects diagnostics in underground structures

The article presents a comprehensive method for diagnosing underground structures using the example of an underground pedestrian crossing located in Rostov-on-Don. The problem of assessing the condition of buildings and structures is very relevant at all stages of the life cycle. There is a special need for continuous monitoring of bearing structures for many buildings with critical applications especially if reliability of which determines the life and health of people. This work considered complex method included geodetic research, geological study of the state of sub-soil, analysis of state of steel and reinforced concrete structures, vibro diagnostics of load-bearing structures under dynamic technogenic impacts and an assessment of the mechanical characteristics of steel and reinforced concrete structures in order to develop a conclusion on the overall state of the transition and the possibility of its further operation. Analysis of the response from arbitrary non-stationary effects on structural elements is carried out on the base of the calculated spectrum of reference impacts.

Dynamic response of a plate laying on elastic base during the impact of a conical indenter

The article presents an analytical and numerical study of a dynamic conical indentation during impact process. The circular plate laying on the elastic base is subjected to impact load by the conical indenter. Local deformations in imprint zone are considered as elastic-plastic and general plate deformations are only elastic. According to Timoshenko method, the general plate displacement is considered as the sum of the elastic-plastic displacement of the indenter and the elastic displacement of the plate at the impact point. The solution is constructed with Fourier transforms and the Green function of the dynamic process is obtained. The numerical study of impact indentation is received by FEM method. The dependences of displacements, velocities, and accelerations of the plate on time is obtained during impact. The experimental data obtained with the special device is compared with the results of the numerical analysis. The method of an assessment of the mechanical characteristics of steel is proposed on the basis of the research. Analysis of the dynamic response and non-stationary effects on structural elements is carried out on the base of the calculated impact process.

Determination of a Real Strain-stress State of the Steel-wire Rope Elements

Today steel-wire ropes are used as a pre-stressed reinforcement for the large-span reinforced concrete structures that are considered as solid isotropic bars. Meanwhile, it's not taken into account that the rope represents a composite structure with the axial spiral anisotropy of properties, where a combined strain-stress state (SSS) occurs. On the basis of Kirchhoff differential equations for thin curved bars, the author develops a generic mathematical model of the steel twisted rope with a linear contact of wires. This model enables one to determine a combined strain-stress state of the rope elements taking into consideration its geometrical characteristics (wire size, twist angle of a twisted rope) and mechanical characteristics of steel (elasticity and shear moduli). The paper presents calculation of irregularity of stress distribution between a core and wires of the steel rope К-7 (1+6) in case of different rates of wire twisting. Knowing the real strain-stress state in the steel rope elements, one can specify an effective pre-stressing of the rope.

On the Determination of the Mechanical Characteristics of Steels from Hardness Test Results

A comparative analysis is performed for Brinell hardness values obtained with the help of different testing equipment and procedures for determining the mechanical characteristics of steels from the results of hardness testing used in the nuclear power engineering, including the testing unit and procedure developed at the Pisarenko Institute of Problems of Strength of the National Academy of Sciences of Ukraine.

ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ ШАРНИРНОГО УЗЛА СТАЛЬНЫХ ПРОСТРАНСТВЕННЫХ СТЕРЖНЕВЫХ КОНСТРУКЦИЙ

The authors carried out a research of the stress - strain state of the hinge unit of light spatial grid framings of regular structure. The solid model of the hinge unit was created previously and numerical simulation was performed. Calculation identified, that the weak point of the hinge unit is connecting disk. Plastic deformation can arise in the disk during loading. The experimental research was required for verification of results of a pilot study. The test of disk was executed with application of a specially designed installation on universal tensile testing machine in the laboratory of the University. Mechanical characteristics of steel, from which the disk was made, were identified by a certified laboratory destructive control methods in accordance with the GOST. The experimental results are compared with the simulation results. The authors give recommendations for optimizing the design of the hinge unit.

Effect of Pile-Up on the Mechanical Characteristics of Steel by Depth Sensing Indentation

Mechanical properties by depth sensing indentation are derived from the indentation load-displacement data used a micromechanical model developed by Oliver & Pharr (O&P). However, O&P analysis on the indentation unloading curve is developed from a purely elastic contact mechanics (sink-in). The applicability of O&P analysis is limited by the materials pile-up. However, when it does, the contact area is larger than that predicted by elastic contact theory (material sinks-in during purely elastic contact), and both hardness H and Youngs modulus E are overestimated, because their evaluation depends on the contact area deduced from the load-displacement data. H can be overestimated by up to 60 % and E by up to 30 % depending on the extent of pile-up [1,2]. It is therefore important to determine the effect of pile-up on obtained mechanical characteristics of the material by depth sensing indentation. The work experimentally analyses the effect of pile-up height on mechanical characteristics H and E, which are determined by O&P analysis. Pile-up height was measured by atomic force microscopy (AFM).

Modeling of the in-service degradation of steels of cargo seaport structures under the laboratory conditions

We study the influence of the duration of high-cycle loading on the complex of mechanical characteristics of low-strength steels specifying their structural strength. We distinguish three stages of cycling characterized by well-pronounced regularities in the behavior of the properties of these steels. It is shown that, in the first stage, the fatigue-sensitive characteristics of steels vary almost identically: their hardness somewhat increases, whereas the relative elongation and impact toughness decrease. The microcracks appearing and propagating in the second stage affect only the impact toughness of the metal: it becomes almost twice lower. After the formation of a macrocrack in the third stage, the subsequent cycling almost does not affect the mechanical characteristics of steels. It is shown that high-cycle fatigue is responsible for changes in the state of the metal similar to those observed for the steels of cargo port structures in the course of their long-term operation. Thus, it can be used for the simulation of in-service degradation of the mechanical properties of steels under laboratory conditions.

Changes of Mechanical Properties of Steel 12Cr18Ni10Тi after Electrolytic-Plasma Cementation

In the present work the results of electrolytic-plasma treatment influence on the mechanical properties and structural-phase condition of steel’s 12Cr18Ni10Ti surface layers. In the work, the mechanical characteristics of steel’s surface layers are investigated, cemented in electrolytic plasma with the composition of 10 % Nа2CO3 and 10 % С3Н8О3. The operational parameters for the processing are determined. The optimal content of components in saturating mixtures of plasma by cementation is defined. According to the study it is found that, after electrolyte plasma processing, high wear resistance and hardness increased in 2-2.5 times more than in the original condition. The hardened layer with thickness 30-65 µm is formed. It is shown that after the electrolyte-plasma processing of steel 12Cr18Ni10Ti, microstructure contains particles of carbides and has a fine-grained martensitic structure.