The experimental researches aimed to find a way how to increase the percentage of γ’-Ni3(Al, Ti) ... more The experimental researches aimed to find a way how to increase the percentage of γ’-Ni3(Al, Ti) phase in the superficial layers of the nickel base super-alloys, particularly of the INCONEL 738 super-alloy and also to determine the thermodynamic stability conditions of this strengthening phase by ensuring an over-unity Al wt%/Ti wt% ratio in these layers. The chosen solution was to realize a simultaneous Al-Ti cementation (TitanoAluminizing) using solid powdery media containing TiO2 and Al powder as active suppliers of Ti and Al. Besides these components, in the cementation media there are, or not, other components which participate actively in the reduction of TiO2 such as fluoride and magnesium powder. These components have as role the dispersing/blocking of the sintering tendency of the mixture components. In the mixture, there are also components which have as role cleaning/activation of the surfaces subject to thermochemical processing. The experimental results led to the concl...
Simultaneous saturation of the metallic surface with sulphur, nitrogen and carbon – named sulphon... more Simultaneous saturation of the metallic surface with sulphur, nitrogen and carbon – named sulphonitrocarburizing (SNC) process may be carried out in a multitude of variants, differ in media state of aggregation which provide the elements in native state, or differ in phase composition. The most commonly used media in SNC process are liquid or gaseous and from standpoint of phase composition, there are media made of compounds generator of the cyanides, representing important source of carbon and nitrogen, and as a sulphur source, may be used iron sulphide, sodium sulphide, sodium thiocyanides or ammonium. Carbamide/urea (CON2H4) it is frequently used in the media composition met in nitro-carburizing or carbo-nitriding process, alongside being added carbonates (sodium or potassium carbonate), thus resulting in cyanides (primary sources of the elements helping superficial saturation of the metallic products), as a result of the reactions which take place at the operating temperature of...
The paper deals with the effects of the structural state modification of the main pulverulent com... more The paper deals with the effects of the structural state modification of the main pulverulent component used for the alloying of metallic products made of steels, cast iron or nonferrous alloys, on the layers kinetics formation. The aliting process is most often done in a powdery mixtures, composed of three components: the active component providing aluminum, a neutral one with the role of dispersing the others, also having the role of blocking the sintering tendency and a halide as an activator, by cleaning the metal surfaces to be saturated. The aim of the paper is to present the ways to ensure the kinetics of the aliting layer formation, while reducing the heat treatment temperature. These could be accomplished by replacing the aluminum or ferroaluminum powder with equimassic amounts mixture of thermitic powders, consisting of ferrous oxides and aluminum powders, mechanically alloyed in high energy ball mills. Thus, it is possible to produce the aluminothermic reduction reaction ...
In this paper was followed the processing flow of aluminum-alumina compositions (10÷20% alumina) ... more In this paper was followed the processing flow of aluminum-alumina compositions (10÷20% alumina) in powder state, aiming to obtain aluminum matrix composites reinforced with alumina particles, starting from selecting and mixing the grading fraction of both components reaching up to sintering; it was analyzed the way in which reflects the variation of grading fraction ratio (expressed through average particle diameter in the analyzed fractions limits) on the level of technological interest features: apparent density, tapped density, flowability, presability and on densification after sintering (in various environments). By transmission electron microscopy was observed that aluminum particles showed on the surface a nanoscale oxide film, so the sintering occurs between congeneric areas – by solid phase sintering mechanisms [1, 2, 3]. The analysis of thermophysical properties revealed a decrease of thermal diffusivity at an increase of alumina, simultaneous with the decrease of the densification level.
The main aim of this research was to study the sintering behavior in hydrogen atmosphere of mixed... more The main aim of this research was to study the sintering behavior in hydrogen atmosphere of mixed powders of triuranium octoxide (U3O8) powder in dioxide uranium (UO2) powders. By cold pressing the green pellets of mixed powders of triuranium octoxide in hyper stoichiometric uranium dioxide has density over 40 % UO2 theoretical density (TD). The sintered pellets had a height diameter ratio (H / D) over one and oxygen uranium ratio (O/U) near stoichiometry of uranium dioxide (UO2). The densities of sintered pellets were up to 97% of uranium dioxide theoretical density. The proposed treatment of sintering are function of green pellets density. Increasing the proportion of triuranium octoxide leads to a decrease in the density of the sinter pellets. The pellets made by 100 % of triuranium octoxide not resist to proposed sintering treatment. The metallographic analysis show uniform grains for sinter pellets made by the 30 % mixture of triuranium octoxide in uranium dioxide. This UO2 sinter pellets are without surface defects.
Investigations on carburized layers phase composition are usually made by optical microscopy, e... more Investigations on carburized layers phase composition are usually made by optical microscopy, electron microscopy and X-ray diffraction. A microstructure consisting in different proportions of superior bainite, inferior bainite, martensite and retained austenite was found after carburizing in paste of the 21NiCrMo2 steel which has appropriate bainitic hardenability. The investigations carried out on 21NiCrMo2 steel samples, carburized in paste with additions of about 10% Ce, highlighted a significant change of phase proportions ratio in the carburized layer resulting in an important increasing of the lower bainite ratio. It was also observed, that the carburized layer case depth obtained in the carburizing paste with Ce additions does not significantly differ in relation to that attained in the carburizing paste without Ce additions but the microhardness increases considerably. The reason is connected also to the presence of Ce carbide identified by X-ray diffraction in the carburized layer. The Ce hard affinity towards oxygen and its influence on bainitic hardenability increasing is well known and adopted, however neither Ce transfer mechanism from carburizing paste to the carburizing layer and nor the effects of its high affinity to oxygen (for example) are not addressed in this particular case. The present study proposes a possible transfer mechanism of Ce in the carburized layer and an explanation of the consequences of Ce hard affinity towards oxygen on the phase composition of the carburized layer. Nanoscale iron oxides such as wüstite, magnetite and maghemite were identified in the surface of the carburized layer by Mössbauer spectrometry.
Besides other possible applications, the thermitic compositions based on iron oxides are meant to... more Besides other possible applications, the thermitic compositions based on iron oxides are meant to the achieving of the permanent joints of the railroad tracks. In addition to the iron oxides, the thermitic kit include also the reducing agent – aluminum powder, a carbon source to adjust the percent of this element in the obtained steel and a flux with a view to allow the steel composition and temperature control. The metallothermic process efficiency is given by strictness in the selection of ratio of the thermitic components and mainly in that of the iron oxides types. Their correct dosage makes possible the estimation and rigorous control of the thermal effect of reactions. In this work, using thermodynamic calculations, is analyzed and quantified the development of the thermal effect of the metallothermic reduction reactions of iron oxides depending on the iron oxides proportions in the thermitic composition.
The influence of rare-earth metals adsorbed in the surface of the metallic material subject to th... more The influence of rare-earth metals adsorbed in the surface of the metallic material subject to thermochemical processing as well as of those pre-added in the material matrix on the kinetics of layers growth is presented in the technical literature. It is generally concluded that the presence of rare earths is accelerating the kinetics of layer growth.
Metallurgical and Materials Transactions, Mar 18, 2014
ABSTRACT We report on the changes on the microstructural, hardness, and corrosion properties indu... more ABSTRACT We report on the changes on the microstructural, hardness, and corrosion properties induced by carbo-chromization of 316L stainless steel prepared by Spark Plasma Sintering technique. The thermo-chemical treatments have been performed using pack cementation. The carburizing and chromization were carried out between 1153 K (880 °C)/4 h to 1253 K (980 °C)/12 h and 1223 K (950 °C)/6 h to 1273 K (1000 °C)/12 h in a solid powder mixture of charcoal/BaCO3 and ferrochromium/alumina/NH4Cl, respectively. The obtained layers were investigated using X-ray and electron diffraction, optical and scanning electron microscopies, Vickers micro-hardness, and potentiodynamic measurements. The thickness of the carbo-chromized layer ranges between 300 and 500 μm. Besides the host γ-phase, the layers are mainly constituted of carbides (Fe7C3, Cr23C6, Cr7C3, and Fe3C) and traces of α′-martensite. The average hardness values decrease smoothly from 650 HV at the sample surface down to 200 HV at the center of the sample. The potentiodynamic tests revealed that the carbo-chromized samples have smaller corrosion resistance with respect to the untreated material. For strong chromization regimes, the corrosion rate is increased by a factor of four with respect to that of the untreated material, while the micro-hardness of the layer is three times larger. Such materials are suited to be used in environments where good corrosion resistance and wear properties are required.
The experimental researches aimed to find a way how to increase the percentage of γ’-Ni3(Al, Ti) ... more The experimental researches aimed to find a way how to increase the percentage of γ’-Ni3(Al, Ti) phase in the superficial layers of the nickel base super-alloys, particularly of the INCONEL 738 super-alloy and also to determine the thermodynamic stability conditions of this strengthening phase by ensuring an over-unity Al wt%/Ti wt% ratio in these layers. The chosen solution was to realize a simultaneous Al-Ti cementation (TitanoAluminizing) using solid powdery media containing TiO2 and Al powder as active suppliers of Ti and Al. Besides these components, in the cementation media there are, or not, other components which participate actively in the reduction of TiO2 such as fluoride and magnesium powder. These components have as role the dispersing/blocking of the sintering tendency of the mixture components. In the mixture, there are also components which have as role cleaning/activation of the surfaces subject to thermochemical processing. The experimental results led to the concl...
Simultaneous saturation of the metallic surface with sulphur, nitrogen and carbon – named sulphon... more Simultaneous saturation of the metallic surface with sulphur, nitrogen and carbon – named sulphonitrocarburizing (SNC) process may be carried out in a multitude of variants, differ in media state of aggregation which provide the elements in native state, or differ in phase composition. The most commonly used media in SNC process are liquid or gaseous and from standpoint of phase composition, there are media made of compounds generator of the cyanides, representing important source of carbon and nitrogen, and as a sulphur source, may be used iron sulphide, sodium sulphide, sodium thiocyanides or ammonium. Carbamide/urea (CON2H4) it is frequently used in the media composition met in nitro-carburizing or carbo-nitriding process, alongside being added carbonates (sodium or potassium carbonate), thus resulting in cyanides (primary sources of the elements helping superficial saturation of the metallic products), as a result of the reactions which take place at the operating temperature of...
The paper deals with the effects of the structural state modification of the main pulverulent com... more The paper deals with the effects of the structural state modification of the main pulverulent component used for the alloying of metallic products made of steels, cast iron or nonferrous alloys, on the layers kinetics formation. The aliting process is most often done in a powdery mixtures, composed of three components: the active component providing aluminum, a neutral one with the role of dispersing the others, also having the role of blocking the sintering tendency and a halide as an activator, by cleaning the metal surfaces to be saturated. The aim of the paper is to present the ways to ensure the kinetics of the aliting layer formation, while reducing the heat treatment temperature. These could be accomplished by replacing the aluminum or ferroaluminum powder with equimassic amounts mixture of thermitic powders, consisting of ferrous oxides and aluminum powders, mechanically alloyed in high energy ball mills. Thus, it is possible to produce the aluminothermic reduction reaction ...
In this paper was followed the processing flow of aluminum-alumina compositions (10÷20% alumina) ... more In this paper was followed the processing flow of aluminum-alumina compositions (10÷20% alumina) in powder state, aiming to obtain aluminum matrix composites reinforced with alumina particles, starting from selecting and mixing the grading fraction of both components reaching up to sintering; it was analyzed the way in which reflects the variation of grading fraction ratio (expressed through average particle diameter in the analyzed fractions limits) on the level of technological interest features: apparent density, tapped density, flowability, presability and on densification after sintering (in various environments). By transmission electron microscopy was observed that aluminum particles showed on the surface a nanoscale oxide film, so the sintering occurs between congeneric areas – by solid phase sintering mechanisms [1, 2, 3]. The analysis of thermophysical properties revealed a decrease of thermal diffusivity at an increase of alumina, simultaneous with the decrease of the densification level.
The main aim of this research was to study the sintering behavior in hydrogen atmosphere of mixed... more The main aim of this research was to study the sintering behavior in hydrogen atmosphere of mixed powders of triuranium octoxide (U3O8) powder in dioxide uranium (UO2) powders. By cold pressing the green pellets of mixed powders of triuranium octoxide in hyper stoichiometric uranium dioxide has density over 40 % UO2 theoretical density (TD). The sintered pellets had a height diameter ratio (H / D) over one and oxygen uranium ratio (O/U) near stoichiometry of uranium dioxide (UO2). The densities of sintered pellets were up to 97% of uranium dioxide theoretical density. The proposed treatment of sintering are function of green pellets density. Increasing the proportion of triuranium octoxide leads to a decrease in the density of the sinter pellets. The pellets made by 100 % of triuranium octoxide not resist to proposed sintering treatment. The metallographic analysis show uniform grains for sinter pellets made by the 30 % mixture of triuranium octoxide in uranium dioxide. This UO2 sinter pellets are without surface defects.
Investigations on carburized layers phase composition are usually made by optical microscopy, e... more Investigations on carburized layers phase composition are usually made by optical microscopy, electron microscopy and X-ray diffraction. A microstructure consisting in different proportions of superior bainite, inferior bainite, martensite and retained austenite was found after carburizing in paste of the 21NiCrMo2 steel which has appropriate bainitic hardenability. The investigations carried out on 21NiCrMo2 steel samples, carburized in paste with additions of about 10% Ce, highlighted a significant change of phase proportions ratio in the carburized layer resulting in an important increasing of the lower bainite ratio. It was also observed, that the carburized layer case depth obtained in the carburizing paste with Ce additions does not significantly differ in relation to that attained in the carburizing paste without Ce additions but the microhardness increases considerably. The reason is connected also to the presence of Ce carbide identified by X-ray diffraction in the carburized layer. The Ce hard affinity towards oxygen and its influence on bainitic hardenability increasing is well known and adopted, however neither Ce transfer mechanism from carburizing paste to the carburizing layer and nor the effects of its high affinity to oxygen (for example) are not addressed in this particular case. The present study proposes a possible transfer mechanism of Ce in the carburized layer and an explanation of the consequences of Ce hard affinity towards oxygen on the phase composition of the carburized layer. Nanoscale iron oxides such as wüstite, magnetite and maghemite were identified in the surface of the carburized layer by Mössbauer spectrometry.
Besides other possible applications, the thermitic compositions based on iron oxides are meant to... more Besides other possible applications, the thermitic compositions based on iron oxides are meant to the achieving of the permanent joints of the railroad tracks. In addition to the iron oxides, the thermitic kit include also the reducing agent – aluminum powder, a carbon source to adjust the percent of this element in the obtained steel and a flux with a view to allow the steel composition and temperature control. The metallothermic process efficiency is given by strictness in the selection of ratio of the thermitic components and mainly in that of the iron oxides types. Their correct dosage makes possible the estimation and rigorous control of the thermal effect of reactions. In this work, using thermodynamic calculations, is analyzed and quantified the development of the thermal effect of the metallothermic reduction reactions of iron oxides depending on the iron oxides proportions in the thermitic composition.
The influence of rare-earth metals adsorbed in the surface of the metallic material subject to th... more The influence of rare-earth metals adsorbed in the surface of the metallic material subject to thermochemical processing as well as of those pre-added in the material matrix on the kinetics of layers growth is presented in the technical literature. It is generally concluded that the presence of rare earths is accelerating the kinetics of layer growth.
Metallurgical and Materials Transactions, Mar 18, 2014
ABSTRACT We report on the changes on the microstructural, hardness, and corrosion properties indu... more ABSTRACT We report on the changes on the microstructural, hardness, and corrosion properties induced by carbo-chromization of 316L stainless steel prepared by Spark Plasma Sintering technique. The thermo-chemical treatments have been performed using pack cementation. The carburizing and chromization were carried out between 1153 K (880 °C)/4 h to 1253 K (980 °C)/12 h and 1223 K (950 °C)/6 h to 1273 K (1000 °C)/12 h in a solid powder mixture of charcoal/BaCO3 and ferrochromium/alumina/NH4Cl, respectively. The obtained layers were investigated using X-ray and electron diffraction, optical and scanning electron microscopies, Vickers micro-hardness, and potentiodynamic measurements. The thickness of the carbo-chromized layer ranges between 300 and 500 μm. Besides the host γ-phase, the layers are mainly constituted of carbides (Fe7C3, Cr23C6, Cr7C3, and Fe3C) and traces of α′-martensite. The average hardness values decrease smoothly from 650 HV at the sample surface down to 200 HV at the center of the sample. The potentiodynamic tests revealed that the carbo-chromized samples have smaller corrosion resistance with respect to the untreated material. For strong chromization regimes, the corrosion rate is increased by a factor of four with respect to that of the untreated material, while the micro-hardness of the layer is three times larger. Such materials are suited to be used in environments where good corrosion resistance and wear properties are required.
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