K. V. Aksenova

Physical nature of rail strengthening in long term operation

Regularities of changes in structure-phase states and the defect substructure of rail surface layers up to 10 mm along the fillet in long-term operation (the gross tonnage 1000 mln tons) were determined by methods of transmission electron diffraction microscopy and by measuring microhardness. The possible reasons of the observed regularities were discussed. It was noticed that two competitive processes may proceed in rail operation: (1) cementite segregation followed by their carrying to the volume of ferrite grains or plates (in the pearlite structure) and (2) cutting, subsequent dissolution of cementite particles, transition of carbon atoms at dislocations (Cottrell atmospheres), and carbon atom transfer by dislocations into the volume of ferrite grains (or plates) followed by the formation of cementite nanoparticles. A qualitative analysis of rail hardening mechanisms at different distance from the tread surface along the fillet after long-term operation was done. It was shown that hardening had a mult...

Degradation of structure and properties of rail surface layer at long-term operation

ABSTRACT The microstructure evolution and properties variation of the surface layer of rail steel after passed 500 and 1000 million tons of gross weight (MTGW) have been investigated. The wear rate increases to 3 and 3.4 times after passed 500 and 1000 MTGW, respectively. The corresponding friction coefficient decreases by 1.4 and 1.1 times. The cementite plates were destroyed and formed the cementite particles of around 10–50nm in size after passed 500 MTGW. The early stage dynamical recrystallisation was observed after passed 1000 MTGW. The mechanisms for these have been suggested. The large number of bend extinction contours is revealed in the surface layer. The internal stress field is evaluated. This paper is part of a themed issue on Materials in External Fields.

Comparative Analysis of the Structure and Phase States and Defect Substructure of Bulk and Differentially Quenched Rails

Optical and transmission electron microscopy (TEM) methods are used for layer by layer comparative analysis of low-temperature reliability rails with increased wear resistance and contact-fatigue strength of the highest quality after bulk quenching and differential hardening by different regimes. Quantitative relationships are established for changes in structural parameters, phase composition, and dislocation substructure over the central axis and fillet at different distances from the running surface. The degree of structure and phase composition inhomogeneity and defective substructure is revealed. It is shown that with respect to structural component content and interlamellar distance the structure after bulk hardening compared with differential hardening is more uniform in a layer 2 mm thick and less uniform at a distance of 10 mm from the running surface. With respect to stress concentration density, the rail structure after bulk hardening (compared with differential hardening) is less uniform in a layer 2 mm thick and more uniform in a layer at a distance of 10 mm from the running surface.

The Influence of Electron Beam Treatment on Al-Si Alloy Structure Destroyed at High-Cycle Fatigue

By scanning and transmission electron diffraction microscopy method the analysis of structure-phase states and defect substructure of silumin subjected to high-intensity electron beam irradiation in various regimes and subsequent fatigue loading up to failure was carried out. It is revealed that the sources of fatigue microcracks are silicon plates of micron and submicron sizes that are not soluble in electron beam processing. The possible reasons of the silumin fatigue life increase under electron-beam treatment are discussed.

FORMATION AND EVOLUTION OF STRUCTURE-PHASE STATES IN RAILS AFTER DRAWN RESOURCE

By optic and transmission electron microscopy, the regularities of the transformation of structural-phase states, the defective substructure of the rail surface layer to the depth of 10 mm under long-term operation (passed gross tonnage 500 and 1000 mln ton) are determined. In the initial state the structure is presented by perlite grains with predominantly lamellar morphology, grains of a ferrite-carbide mixture and structurally free ferrite grains. It is shown that operation of rail steel is accompanied by a complete failure of lamellar pearlite grains in the 15 μm thick surface layer and the formation of a ferrite-carbide mixture with nano-size particles. The strain-induced transformation of steel leads to the increase in the scalar and excessive density of dislocations, the curvature-torsion value of the crystal lattice and the amplitude of internal stress fields.

Redistribution of carbon in the deformation of steel with bainite and martensite structures

Recent years have been marked by considerable increase in the use of high-strength steel—especially martensitic and bainitic steel—for the manufacture of key industrial components and structures. The high strength depends on strain hardening of the steel. It is important to understand the strain hardening of steel of different structural classes with active plastic deformation, in order to ensure specified structural and phase states and mechanical properties. In the present work, by transmission electron-diffraction microscopy, the evolution of the structure, the phase composition, and the state of the defect substructure is compared for steel samples with martensite and bainite structure in active plastic deformation to failure. After austenitization at 950°C (1.5 h) and subsequent quenching in oil (for 38KhN3MFA steel) and cooling in air (for 30Kh2N2MFA steel), multiphase structure (α phase, γ phase, and cementite) based on packet martensite (38KhN3MFA steel) and lower bainite (30Kh2N2MFA steel) is formed. Quantitative results for the structural parameters of steel in plastic deformation permit analysis of the distribution of the carbon atoms in the structure of the deformed steel. The points of localization of carbon atoms in the martensite (quenched 38KhN3MFA steel) and bainite (air-cooled 30Kh2N2MFA steel) are identified. Deformation of the steel is found to be accompanied by the destruction of cementite particles. For quenched martensitic steel, the total quantity of carbon atoms in the solid solution based on α and γ iron is reduced, while their content at structural defects is increased. The redistribution of carbon atoms in the bainitic steel with increase in the strain involves the increase in the quantity of carbon atoms in the α iron, defects in the crystalline structure, and cementite at the intraphase boundaries; and the decrease in the content of carbon atoms in the cementite particles within the bainite plates and the γ iron.

Nanoscale localization of plastic deformation in steel with a bainitic structure

Transmission electron microscopy is used to reveal the formation of localized deformation channels on a nanoscale in 30Kh2N2MFA steel subjected to compressive deformation of 40% or more. The channels are found to be located mainly along the interfaces of neighboring bainite lamellae or along grain boundaries. The structure of the deformation channels, their sizes, and the change in their volume fraction with the strain are analyzed.

Evolution of the structure and the phase composition of a bainitic structural steel during plastic deformation

The evolution of the phase composition and the imperfect substructure of the 30Kh2N2MFA bainitic structural steel subjected to compressive deformation by 36% is quantitatively analyzed. It is shown that deformation is accompanied by an increase in the scalar dislocation density, a decrease in the longitudinal fragment sizes, an increase in the number of stress concentrators, the dissolution of cementite particles, and the transformation of retained austenite.

Analysis of Strain Hardening Mechanisms for Steel with a Bainitic Structure

Transmission electron diffraction microscopy is used to analyze strain hardening of steel with a bainitic structure. Contributions to strengthening are considered for lattice friction, interphase boundaries, dislocation substructure, carbide phases, alloying element atoms, and remote stress fields. It is established that substructural and solid solution strengthening make the greatest contributions. The reason for steel loss of strength with large degrees of deformation is connected with activation of deformation microtwinning.

Structure and properties of strengthening layer on Hardox 450 steel

ABSTRACT The microstructure and microhardness distribution in the surface of low-carbon Hardox 450 steel coated with alloyed powder wires of different chemical compositions are studied. It is shown that the microhardness of 6–8 mm-thick surfaced layer exceeds that of base metal by more than two times. The increased mechanical properties of surfaced layer are caused by the submicro and nanoscale dispersed martensite, containing the niobium carbides Nb2C, NbC and iron borides Fe2B. In the bulk plates, a dislocation substructure of the net-like type with scalar dislocation density of 1011 cm−2 is observed. The layer surfaced with the wire containing B possesses highest hardness. The possible mechanisms and temperature regimes of niobium and boron carbides in surfacing are discussed.