Engineering and geological features of soils fundamentals buildings and engineering networks of the Dnipro city

The analysis of engineering and geological properties of soils both in the natural state and under the influence of human economic activity is performed in the work. The researches from the point of view of conditions of construction and operation of buildings and constructions of engineering infrastructure of the city of Dnipro are proved. As a result of the analysis of specific geomorphological zones, specific examples of violations of the operation of buildings on subsidence soils, a number of measures are proposed for maintenance and operation of buildings and structures located in areas composed of subsidence soils, on the example of specific areas exposed to past man-made events. An analytical assessment of the ecological catastrophe on the territory of the Topolya 1 railway in 1997 is provided. The paper considers a principled approach to the principles and conditions of operation of buildings and structures on specific subsidence soils of the city of Dnipro.В роботі виконано аналіз інженерно-геологічних властивостей грунтів як в природньому стані, так і під впливом господарської діяльності людини. Доведені дослідження з точки зору умов будівництва і експлуатації будівель та споруд інженерної інфрастуктури міста Дніпро. Внаслідок аналізу конкретних геоморфологічних зон, конкретних прикладів порушень умов експлуатації будівель на просідальних грунтах, пропонується ряд заходів по утриманню і експлуатації будівель і споруд, розташованих на територіях, складених просідальними грунтами, на прикладі конкретних районів, які піддавалися в минулому негативним подіям техногенного характреру. Надається аналітична оцінка екологічної катастрофи на території ж/м "Тополя -1" у 1997 році. В роботі розглядається принциповий підхід до принципів і умов експлуатації будівель і споруд на специфічних просідальних грунтах міста Дніпро

A method for the calculation of retained austenite evolution during heat-treatment of low-alloy TRIP-assisted steels

Despite the critical effect of heat-treatment, and in particular of the isothermal bainitic treatment stage, on the amount and stability of retained austenite in the microstructure of low-alloy TRIP-assisted steels, determination of optimum heat-treatment conditions is still largely empirical and experiment-dependent. This work proposes a method by which it is possible to calculate the vol. fraction of retained austenite in the microstructure as a function of intercritical annealing temperature and isothermal bainilic treatment temperature and holding time. The method assumes diffusionless lengthening of bainitic ferrite (αB) plates in austenite (γ), and subsequent thickness-wise C rejection from the αB plates to the adjacent γ layers. The relative thickness of αB plates and adjacent γ layers is determined by the To line of the transforming system at any given bainitic transformation temperature. The C-concentration profiles in γ are calculated with respect to a local time-scale, referring to any random section of any random αB plate. Determination of the variation of C-concentration profiles with local time in γ, together with the use of a simple austenite-retention criterion, allows the calculation of vol. fraction retained austenite (γR) as a function of transformation temperature and local time. Transition from local (calculation) time to actual heat treatment time is performed by introducing a time-scale factor, which depends on transformation temperature and initial C-content of αB. The calculated behaviour of vol. fraction γR vs. bainitic holding time conforms to the well established, experimentally observed one: vol. fraction γR initially increases with holding time, reaches a maximum and decreases at longer holding times. According to calculated results, the decrease is attributed to the gradual homogenization of C inside the γ layers, which leads to inadequate stabilization and transformation of γ to martensite on quenching. As regards quantitative comparison with available experimental data, calculations show reasonable agreement. Certain refinements of the method, which are underway, are reported, in order to further improve quantitative results. Nevertheless, the method in its present form provides a calculational tool, by which the effect of different heat-treatment conditions or steel compositions can be examined and compared to each other, which can be a useful step towards optimizing alloy compositions and heat-treating processes

A computational study of austenite formation kinetics in rapidly heated steels

Surface hardening of steels involves rapid austenitization and subsequent quenching of the surface. The resulting extent of hardening largely depends on the rate of austenitization of the surface under the applied high heating rates. In the present work the kinetics of austenite formation in Fe-C alloys during rapid, non-isothermal heating conditions, characterized by high heating rates and short austenitization periods, were studied by means of computational simulation. Austenitization of lamellar pearlite/proeutectoid ferrite microstructures was simulated by assuming two kinetically distinct stages: i) dissolution of lamellar pearlite followed by ii) dissolution of proeutectoid ferrite. The two stages were simulated by two corresponding I-D diffusion models employed in series. Numerical solution of the resultant moving-boundary diffusion problems provide calculated results regarding the dependency of vol. fraction austenite on thermal cycle parameters and on initial microstructural features of the steel. Analysis of calculated results showed that the vol. fraction of pearlite transforming to austenite during pearlite dissolution depended on maximum temperature, dwell time and pearlite interlamellar spacing. A functional relationship between these variables, consisting of a thermodynamic and a kinetic term, was established. On the other hand, the total vol. fraction of austenite forming in the steel, after both stages of austenitization, was found to follow a typical sigmoidal kinetic behaviour. (c) 2006 Elsevier B.V. All rights reserved

Surface hardening of low-alloy 15CrNi6 steel by CO2 laser beam

The study investigates laser transformation hardening in the low-alloy 15CrNi6 case-hardening steel. The effect of process parameters such as beam power, beam diameter and travel speed on microstructure, case depth and hardness was examined. In addition, the effect of surface preparation was also investigated. For this purpose, all specimens were initially sandblasted, and some were additionally coated with graphite in order to enhance surface absorptivity. In most cases, a heat affected zone (HAZ) formed below the surface. This consisted of two discrete areas, a surface layer and a transition area between this surface layer and the base metal. The microstructure of the surface layer was found to consist of lath martensite, while carburization was found for certain process conditions in the graphite-coated specimens. The transition area consisted of a dispersion of fine carbides in a ferrite matrix. A substantial increase in surface hardness was achieved, by a factor of 2.5 times the base metal hardness. Depths of the HAZ up to approximately 0.6 mm were obtained, without surface melting. The additional use of graphite coating enhanced, in most cases, the coupling of laser-beam to the surface resulting in greater HAZ depths. (C) 1999 Elsevier Science S.A. All rights reserved

A Semi-Empirical Model for the evolution of Retained Austenite via Bainitic Transformation in Multiphase TRIP Steels

Multiphase TRIP steels exhibit unique combinations of strength and cold formability, characteristics especially desirable in automotive applications. This behaviour is largely determined by the amount and stability of the retained austenite dispersion in the microstructure, produced by a two-stage heat-treatment, consisting of intercritical annealing followed by an isothermal bainitic treatment. The amount and stability of retained austenite is mainly determined by the proper selection of the temperature and temporal duration of the bainitic treatment. In the present work an approach is presented which allows for the calculation of the amount of retained austenite in the microstructure, as a function of bainitic treatment conditions. The approach is based on the physical characteristics of the bainitic transformation and on the stabilizing effects of the formation of bainitic ferrite in austenite. Each bainitic ferrite platelet is considered to chemically stabilize a part of the surrounding austenite due to carbon rejection. The spatial and temporal extent of this stabilization is determined by solving the corresponding carbon-diffusion problem, and thus the amount of retained austenite contributed by any individual platelet is determined. Subsequently, the evolution of the population of the platelets in the entire microstructure is determined and, thus, the volume fraction of retained austenite as a function of transformation time is calculated. Application and comparison of calculations with experimental results, obtained from four different multiphase TRIP steel compositions, exhibited very good qualitative and quantitative agreement

Laser-beam carburizing of low-alloy steels

The present study investigated the use of a laser-beam, in order to carburize the surface of DIN 15CrNi6 low-alloy, case-hardening steel. The surface of the material was coated with graphite prior to laser irradiation. Two different kinds of coatings were used: (i) a dilute commercial graphite spray; and (ii) a slurry of graphite powder in ethanol. A CO, laser-beam was used as the heat source, in order to activate the introduction of carbon in the steel surface. Carburizing was achieved by two distinct mechanisms: (a) the surface alloying mechanism, which incorporates melting of the substrate and dissolution of the graphite in the liquid phase; and (b) the solid-state diffusion mechanism, which incorporates austenitization of the substrate and carbon diffusion in austenite. A variety of microstructures and microhardness profiles were produced, depending mainly on the type of graphite coating used, as well as on processing parameters. In general, the carburized layer was accompanied by a heat-affected zone, which was also significantly hardened, mainly due to secondary hardening. Cracking and porosity was observed in some specimens carburized by the surface alloying mechanism, as a result of carbon enrichment and high solidification rates. Finally, an effort was made to investigate the possibility for solid-state diffusion of carbon in austenite, under the short heating times imposed by laser treatment, with the use of computational kinetics simulation. (C) 2001 Elsevier Science B.V. All rights reserved

Multiscaling effects in low alloy TRIP steels

Low-alloy TRIP steels are a new class of steels with excellent combinations of strength and formability, which offer a unique field for the study of multiscale effects in materials, in the sense that experimental observations and models referring to different scale levels have to be combined, for the understanding and the design of these steels. In the present work, models involving multiscale physical quantities are reported, which regard prediction of the stability of retained austenite and of the kinetics of its mechanically-induced transformation to martensite, optimization of the heat-treatment stages necessary for austenite stabilization in the microstructure, as well as prediction of the mechanical behaviour of these steels under deformation. Austenite stability depends on chemical composition, austenite particle size, strength of the matrix and stress state, i.e. on factors ranging from the nano- to the macro-scale. The stability of retained austenite against mechanically-induced transformation to martensite is characterized by the s M temperature, which can be derived as a function of the aforementioned multiscale factors by an appropriate model presented in this work. The kinetics of the mechanically-induced transformation of retained austenite to martensite are also dependent on multiscale factors, such as the population density of martensitic nucleation sites, the retained austenite particle size and the macroscopic level of plastic deformation. In the present work, a model describing the kinetics of this mechanically-induced transformation as a function of these factors is presented. Furthermore, the mechanical behaviour of TRIP steels also depends on the amount of retained austenite present in the microstructure, which is determined by the combinations of temperature and temporal duration of the heat-treatment stages undergone by the steel. Optimum amounts of retained austenite require optimization of the heat-treatment conditions. A physical model is presented in this work, which is based on the interactions between bainite and austenite during the heat-treatment of TRIP steels, which allows for the selection of treatment conditions leading to the maximization of retained austenite in the final microstructure. Finally, a constitutive micromechanical model is presented, which describes the mechanical behaviour of TRIP steels under deformation, taking into account the evolution of the microstructure during plastic deformation. This model is then used for the calculation of forming limit diagrams (FLD) for these complex steels, thus allowing for the optimization of stretch-forming and deep-drawing operations. © 2007 Springer

Stress-corrosion cracking of a Monel 400 tube

A bent Ni-Cu Monel 400 alloy tube, which operated as part of a pipeline in a petrochemical distillery installation, failed by through-thickness cracking. The pipeline was used to carry a stream of gaseous hydrocarbons containing hydrochloric acid (HCl) into a reaction tower. The tower provided a caustic solution (NaOH) to remove HCl from the stream, before the latter was directed to a burner. Metallographic examination showed that the cracks were intergranular and were frequently branched. Although nominal chemical composition of the component was found within the specified range, electron dispersive analysis by X-ray (EDXA) indicated significant segregation of sulfur and chlorine on grain boundaries. Failure was attributed to hypochlorous-acid (HClO)-induced stress-corrosion cracking (SCC). The HClO was formed by the reaction of HCl with atmospheric O2, and the oxygen entered the tube during shutdowns/ startups of the installation. Residual stresses, originating from the in situ bend forming of the tube during assembly of the line, provided a driving force for crack growth, and the segregation of sulfur on grain boundaries enhanced the susceptibility of the material to cracking. © ASM International

Modelling of transformations in TRIP steels

Industrial processing of low-alloy Transformation Induced Plasticity (TRIP) steels involves various stages of heat-treating, such as Intercritical Annealing (IA) and Bainitic Isothermal Treatment (BIT), in order to produce a dispersion of retained austenite (gamma(R)) particles and bainite (alpha(B)) in a ferritic matrix (alpha). Retained austenite then transforms to martensite (alpha') during forming processes undergone by the steel. In the present work an effort was made to model these stages of processing, i.e. IA, BIT and the gamma(R)-->alpha' strain-induced transformation. Simulation of heat-treatment stages was implemented using computational kinetics methods. Investigation of the strain-induced alpha(R)-->alpha' transformation kinetics was performed by means of a simple analytical model. Simulation of IA and comparison with available experimental data showed that the amount of austenite (gamma) forming during IA reaches the values predicted by thermodynamic equilibrium only at high annealing temperatures (>825degreesC). It was also observed that kinetic and thermodynamic predictions set a lower and an upper limit, respectively, within which the actual amount of austenite experimentally observed is contained. Results from the simulation of the BIT indicated considerable carbon enrichment, and thus stabilization of gamma(R), in agreement with recent experimental observations. As regards the strain-induced gamma(R)-->alpha' transformation, the analytical model employed in the present work was fitted to available experimental results, showing reasonably good adaptation to the kinetic behaviour of the microstructure during plastic deformation

Simulation of intercritical annealing in low-alloy TRIP steels

A coupled thermodynamic/kinetic calculation of austenite formation during intercritical annealing of low-alloy TRIP steels is presented. The simulation was performed with the use of Dictra computational kinetics software, which employs a procedure for the numerical solution of the coupled diffusion equations involved, as well as mobility databases for the retrieval of the appropriate kinetic data. Calculated results are compared with available experimental data, in order to evaluate the model. Simulation results, regarding the amount and composition of austenite, the rate of transformation and the effect of annealing temperature and heating conditions, are presented and discussed. It is concluded that the simulation can assist the design of the intercritical annealing in these steels