Dissertations / Theses on the topic 'Temperature of hardening'

In the automotive industry, press hardening is usually employed to produce safety orstructural components from advanced high–strength steels. This hot forming process, andthermomechanical forming processes in general, is highly dependent on friction betweentool and workpiece as friction affects and controls the deformation of the workpiece.However, friction is also directly associated with wear of the forming tools. Tool wear isa complex system response depending on contact conditions and is a serious issue whenit comes to process economy as it reduces the service life of the tool. Therefore, it isnecessary to enhance the durability of thermomechanical forming tools by studying theinfluence of parameters such as contact pressure, cyclic thermal loading, repetitive mech-anical loading and others on tool wear. Then, computational mechanics can be utilised tonumerically simulate and optimise the thermomechanical forming process by predictingwear of the tools.Dry sliding tests were carried out on a high temperature reciprocating friction andwear tester. The aim was to identify the occurring wear mechanisms and determine thetribological behaviour of prehardened hot work tool steel when sliding against 22MnB5boron steel. A normal load of 31 N, which corresponds to a contact pressure of 10 MPa, asliding speed of 0.2 ms −1 and temperatures ranging from 40◦Cto800◦ C were employed.It was found that the coefficient of friction and the specific wear rate decreased at elevatedtemperature because of the formation of compacted wear debris layers on the interactingsurfaces.Increasing material and energy expenses, rising demands for process flexibility andstability as well as requirements for minimal trial and error have led to a growing interestin numerical simulation of wear phenomena. Finite element simulations of a strip drawingtest were conducted to explore the possibility of predicting tool wear in press hardening.The focus laid on unveiling the contact conditions on the forming tools through numericalsimulation. The influence of high temperature on wear was studied and the results wereimplemented in Archard’s wear model to introduce temperature dependence. Further-more, another wear model used for warm forging was also considered. It was found thatthe extreme contact conditions occurred at tool radii and that the different wear modelsled to similar wear depth profiles on the radii but with different orders of magnitude.Standard high temperature tribometers allow fundamental tribological studies to becarried out in order to investigate the tribological behaviour of the materials in contact.However, the conditions prevalent during the interaction of the hot workpiece and toolsurfaces in thermomechanical forming are not adequately simulated in these tribometers.A novel high temperature tribometer has been employed in order to more closely simulatethe interaction between tool and workpiece at elevated temperatures during thermomech-anical forming. It was found that a higher load led to a lower and more stable coefficient
Godkänd; 2014; 20140919 (sermoz); Nedanstående person kommer att hålla licentiatseminarium för avläggande av teknologie licentiatexamen. Namn: Sergej Mozgovoy Ämne: Maskinelement/Machine Elements Uppsats: High Temperature Friction and Wear in Press Hardening Examinator: Professor Braham Prakash, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Diskutant: Dr Manel Rodriguez Ripoll, AC2T research GmbH, Österrike Tid: Fredag den 21 november 2014 kl 10:00 Plats: E231, Luleå tekniska universitet

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Aluminum-zirconium alloys exhibit superior strength at elevated temperature in comparison to traditional aluminum casting alloys. These alloys are heat-treatable and their strength depends to a large extent on the quenching and aging steps of the heat treatment process. However, measurements show that the critical cooling rate necessary to retain 0.6 wt. pct. zirconium(the minimum amount necessary for significant strengthening) in a super-saturated solid solution with aluminum is 90ºC/s, which is un-attainable with traditional casting processes. On the other hand, the critical cooling rate necessary to retain 0.4 wt. pct vanadium and 0.1 wt. pct. zirconium in a super- saturated solidsolution with aluminum is only 40ºC/s; which suggests that substituting vanadium for zirconium significantly decreases the critical cooling rate of the alloy. This is an important finding as it means that, unlike the Al-0.6Zr alloy, the Al-0.4V-0.1Zr alloy may be processed into useful components by traditional high pressure die-casting. Moreover, measurements show that the hardness of the Al-0.4V-0.1Zr alloy increases upon aging at 400ºC and does not degrade even after holding the alloy at 300ºC for 100 hours. Also, measurements of the tensile yield strength of the Al-0.4V-0.1Zr alloy at 300ºC show that it is about 3 times higher than that of pure aluminum. This increase in hardness and strength is attributed to precipitation of Al3(Zr,V) particles. Examination of these particles with high resolution transmission electron microscopy (HRTEM) and conventional TEM show that vanadium co-precipitates with zirconium and aluminum and forms spherical particles that have the L12 crystal structure. It also shows that the crystallographic misfit between the precipitate particles and the aluminum matrix is almost eliminated by introducing vanadium into the Al3Zr precipitate and thatthe mean radius of the Al3(Zr,V) particles is in the range from 1nm to 7nm depending on the alloy composition and aging practice. Finally, it is found that adding small amounts of silicon to the Al-0.4V-0.1Zr alloy effectively accelerates formation of the Al3(Zr,V) precipitate.

This work deals with the assessment of the influence of low-temperature treatment on the structure and properties of casehardened surface layer of parts. The objective was to assess whether low-temperature treatment is sufficient, insufficient, or unnecessary for the given purpose. Gears which form a part of Zetor tractors gear boxes were used as samples. Thirteen pairs of frozen and non-frozen samples were used; they were taken from production batches throughout 2007, their hardness was assessed and furthermore, the experiment itself, freezing casehardened and hardened samples to different temperatures reaching as low as -196°C, was carried out. A moderate increase in hardness was registered with the majority of the frozen samples, which proved the effectiveness of the low-temperature treatment.

Among the potential high temperature shape memory alloys, due to its low cost, medium ductility and high work output NiTiHf seems to be the most promising HTSMA for a wide range of applications in the 100-250ºC. A detailed investigation into the shape memory properties and transformation behavior for the Ni-rich HTSMA with the compositions of Ni45.3Cu5Ti29.7Hf20, Ni50.3Ti29.7Hf20 and Ni45.3Pd5Ti29.7Hf20 was carried out. It is possible to form Ni-rich precipitates in Ni-rich NiTiHf alloys and tailor the TTs by heat treatments that results in increased strength and stable response at high temperatures. The coherent Ni-rich precipitates deplete the Ni content from the matrix increasing the transformation temperatures and strengthen the material by hindering the dislocation motion. The effect of aging on the microstructure, shape memory and mechanical properties are revealed. Optimum aging conditions have been found determined to get the most favorable combination of high transformation temperatures with stable and good shape memory properties. The Ni50.3Ti29.7Hf20 and Ni45.3Pd5Ti29.7Hf20 aged at 500ºC-600 ºC were found to be formidable candidates for high temperature applications.

Cement hydration is the result of a series of simultaneous chemical reactions occurring during the production of concrete. An excessive amount of heat is generated, which consequently may give rise to thermal stresses and cause early age cracks in concrete that may affect its structural integrity, and load bearing capacity. Incorporating fly ash into the concrete mixture has shown to be an efficient method to reduce the temperatures developed during early age hydration, especially for massive concrete structures. Fly ash does additionally affect the concrete's development of compressive strength, tensile strength and Young's modulus. The MahanaKhon tower's mat foundation is divided into 14 layers, with fly ash incorporated in the concrete mix. A finite element model was developed of the mat foundation with COMSOL Multiphysics to simulate the developed temperatures and thermal stresses during curing. The simulations were carried out as parametric studies with different strain reference temperatures. The simulated temperatures were compared with existing temperature measurements that were conducted in three different elevations in each concrete layer. The result of the temperature analyses showed that the measured temperatures were generally larger than the simulated ones, which may have been the result of the numerical model's heat conductivity and convective heat transfer coeffcient not reflecting the actual case. Furthermore, the numerical model did not take into account the effects of solar radiation, which would most likely have increased the temperature of the concrete. The maximum simulated temperatures were mostly found in the center level of the concrete, followed by the lower level, and the lowest at the top. It was also observed that the maximum temperatures in some of the mat foundation layers could exceed 70 °C, which is generally considered high since the risk of delayed ettringite formation may arise. The large temperature is partially a result of not using cooling methods, such as cooling pipes, but also due to the high initial and ambient temperatures. The result of the thermal stress analyses showed that no tensile stresses arose when the strain reference temperature, Tref, was specified to 30 °C, corresponding to the mean ambient temperature. This is due to the concrete temperature not falling below Tref, and the concrete will therefore be in expansion and only be subject to compressive stresses. Increasing Tref to 50 °C, which was considered a reasonable estimation, resulted in developed tensile stresses in all mat foundation layers, where the majority of the mat foundation layers showed a risk of superficial surface cracks. The maximum tensile stresses were found at the final time of the simulations, which was expected, since the temperatures were at their lowest as a result of removing the curing insulation. Finally, setting Tref to 70 °C, corresponding to the maximum temperature during hardening, increased the induced tensile stresses considerably, due to the large temperature gradient between Tref and the concrete temperature. The maximum stresses were, as expected, located at the top level and caused by internal restraint. The second largest tensile stresses were found in the center level, also subject to internal restraint. The lowest tensile stresses were located in the lower level, subject to external restraint.
Cementhydratation är resultatet av en serie kemiska reaktioner som sker under tillverkningen av betong. Stora mängder värme genereras, vilket följaktligen kan ge upphov till termiska spänningar och orsaka tidig sprickbildning som påverkar betongens hållfasthet, och bärförmåga. Inkludering av flygaska i betongblandningen har visat sig vara en effektiv metod avsedd att minska temperaturerna som utvecklas under hydratationen i ung betong, särskilt i massiva betongkonstruktioner. Flygaska påverkar också betongens utveckling av tryckhållfasthet, draghållfasthet och elasticitetsmodul. MahanaKhon towers bottenplatta är uppdelad i 14 lager, där flygaska inkluderades i bottenplattans betong. En finit elementmodell av bottenplattan skapades i COMSOL Multiphysics, där de utvecklade temperaturerna och termiska spänningarna i den unga betongen simulerades under bottenplattans härdningsfas. Simuleringarna genomfördes som parameterstudier med olika referenstemperaturer. De simulerade temperaturerna jämfördes vidare med befintliga temperaturmätningar som utfördes i tre olika elevationer i varje gjutetapp. Resultaten av temperaturerna visade att de uppmätta temperaturerna var generellt högre än de simulerade, vilket bland annat kan bero på att betongens värmeledningsförmåga, samt konvektiva värmeöverföringskoefficient inte återspeglade det aktuella fallet. Den numeriska modellen tog inte heller hänsyn till effekten av solinstrålning, som sannolikt skulle ökat betongens temperatur. De maximala temperaturerna hittades mestadels i betongens mittnivå, följt av den lägre nivån och slutligen lägsta nivåerna vid toppen. Det observerades även att de maximala temperaturerna i bottenplattan kunde överstiga 70 °C, vilket generellt anses vara högt då risken för fördröjd ettringitbildning kan uppstå. De höga temperaturerna beror delvis på avsaknad av kylmetoder, såsom kylrör, men även på den höga initialtemperaturen och omgivningstemperaturen. Resultaten av spänningsanalysen påvisade att inga dragspänningar uppstod när referenstemperaturen Tref denierades till 30 °C, som motsvarar den genomsnittliga omgivningstemperaturen. Detta förklaras av att betongen kommer att vara i expansion och följaktligen endast utsättas för tryckspänningar. Efter att Tref ökats till 50 °C, vilken ansågs vara en rimlig estimering i denna studie, uppstod dragspänningar i alla lager i bottenplattan, där vissa utsattes för risk för ytsprickor. De maximala dragspänningarna uppstod vid simuleringarnas slut, vilket var förväntat då temperaturerna var som lägst vid den tidpunkten till följd av att isoleringen avlägsnades. Slutligen höjdes Tref till 70 °C, vilket motsvarar den maximala temperaturen i bottenplattan under härdning. De inducerade dragspänningarna ökade avsevärt på grund av den stora temperaturgradienten mellan Tref och betongtemperaturen. Samtliga lager utsattes i detta fall för risk för genomgående sprickor. De maximala dragspänningarna påträffades på toppnivån och orsakades av inre tvång. De näst största dragspänningarna fanns i mitten av plattan och var också resultatet av inre tvång. De lägsta dragspänningarna påträffades vid plattans lägre nivå, som utsattes för yttre tvång.

The automotive industry currently demands materials with improved formability and crash performance. Austenitic stainless steels have been singled out for potential development of high strength steels to achieve exceptional combinations of strength and ductility due to their high strain hardening abilities. Austenitic 301LN stainless steel is the least alloyed and most metastable among the 300-series austenitic stainless steels. Plastic deformation at a temperature below Md transforms the metastable austenite phase to a thermodynamically more stable martensite phase accompanied with enhanced strain hardening. There are two different deformation mechanisms of austenitic stainless steels which are TRIP and TWIP effects. In this work, both mechanisms were observed at different deformation temperatures with both phase transformations and twin formations contributing towards the strain hardening. This research work concentrated largely on the derivation of constitutive equations of both volume fraction of martensitic transformation and mechanical response of a metastable austenitic stainless steel alloy as a function of applied strain and temperature. The alloy investigated was a lean version of AISI 301LN. A calibration evaluation of the Ferritescope values was performed with the use of magnetizing tests (VSM), X-ray and neutron diffraction analyses to arrive at a reliable methodology for the determination of the martensite content during the tests. A calibration factor of 1.70 was obtained when tensile deformed samples were used (with analyses done using Vibrating Sample Magnetometer measurements, X-ray and Neutron diffraction techniques) and a calibration factor of 1.62 was obtained when cold rolled samples were used with analysis done using the neutron diffraction technique only. A series of interrupted uniaxial tensile tests at temperatures ranging between -60 and 180 °C at a constant strain rate of 6.67 x 10-4 s-1 were performed. A low strain rate and a small interruption interval were chosen to minimize the heating effect due to adiabatic heating. The strain hardening behavior of AISI 301LN metastable austenitic stainless steels was observed to be a complex process which is related not only to the generation of a dislocation structure but transformation and twinning hardening as well depending on the deformation temperature. Strain hardening curves were derived at different temperatures and were found to be following the same basic mathematical equation for the formation of the strain-induced martensitic transformation product as a function of true strain. Prior cold rolling was also done to different gauges ranging from 5% and 70% at ambient temperature, with small reduction passes applied to minimize adiabatic heating. A series of interrupted uniaxial tensile tests were done on the prior cold rolled samples at a low strain rate of 6.67 x 10-4 s-1. All the derived strain hardening curves were extended up to the true strain levels of 1.0, to arrive at estimates of the strength coefficient, K. The strength coefficient, K was found to be in the range of 1500 MPa ~ 1780 MPa, as calculated from the convergence of sigmoidal hardening curves at a log stress of ~ 3.25 (at log true strain of 0).This was found to be in accordance with the tensile strength of 1715 MPa after a cold rolling of 63.2% (which is equivalent to the compressive true strain of 1.0). The calculated values of strain hardening and martensite formed, using the developed constitutive equations, agree well with the experimental results for a wide range of deformation temperatures and prior cold rolling percentages. A linear variation of stress as a function of strain-induced martensite, observed at moderate martensite fraction levels, was explained as being due to the dispersion hardening effect. An abrupt change from a linear variation that occurred on exceeding a threshold value of martensite formed, was believed to be due to “percolation effect of martensite,” where clusters of martensite forms a continuous network linking up in 3D, adding more blockage to dislocation movement in the austenite phase. A percolation threshold of martensite was found to be in the range of 30 ~ 45%. This was found to be the percentage of martensite present when the rate of martensitic transformation reaches a maximum. At the percolation threshold of martensite, there is an interchange of roles of martensite and austenite, where martensite behaves as a matrix phase and austenite as dispersions embedded in the martensite phase. This results in higher strength as more stress is required to move the dislocations past the percolated martensite barriers which also reduces the plasticity of the austenite.
Thesis (PhD)--University of Pretoria, 2020.
1. Columbus Stainless (Pty) Ltd (No grant number) 2. Department of Science and Technology, S.A. Government, through their FMDN (Ferrous Metals Development Network) programme as administered by Mintek
Materials Science and Metallurgical Engineering
PhD
Unrestricted

The carburizing atmosphere during the case hardening process contains a large proportion of hydrogen. Due to the rapid diffusion of hydrogen a high amount of hydrogen can be absorbed by the carburizing component. The amount of absorbed hydrogen is dependent on some factors such as for example the carburizing time and component dimensions. Hydrogen diffused in material can then cause hydrogen embrittlement and in some cases cause cracking under a static load. This hydrogen must therefore be removed. High amounts of hydrogen diffuse out spontaneously at room temperature. Tempering accelerates the process. The aim of this study was to experimentally measure the amount of absorbed hydrogen after case hardening and hydrogen content after storage at room temperature and also after tempering. The effect of the enriching gas in carburizing furnace on hydrogen absorption was investigated in this study. Three steel grades with different content of alloying elements were used in this investigation. Steel samples were case hardened by gas carburizing and tempering. The hydrogen content analyses included the measurement of hydrogen content before case hardening, after case hardening and after tempering using Leco-RHEN602. Based on the results in this study it was concluded that all steel grades used in this investigation absorb hydrogen during case hardening by gas carburizing. A major part of the absorbed hydrogen is then released by effusion after being stored at room temperature and during tempering. Around 50% of the absorbed hydrogen content during gas carburizing is due to the presence of the enriching gas in the carburizing atmosphere. Around 50 % of hydrogen diffuses out of the steel specimens after one day. It is likely that all of free diffusible) hydrogen has diffused out of the specimens of two steel grades after one week at room temperature or after tempering.
Vid sätthärdning består den uppkolande atmosfären till stor del av vätgas och p.g.a. vätets snabba diffusion kan stora mängder av väte absorberas i komponenten. Halten av absorberade väte beror bl.a. på sätthärdningstid och komponentens dimensioner. Väte i materialet kan sedan leda till sprickbildning vid statisk belastning. Detta väte måste därför avlägsnas. En stor del av väte diffunderar ut spontant vid rumstemperatur. Vid anlöpning går processen fortare. Syftet med denna studie var att experimentellt mäta halten av väte som absorberas under sätthärdning, samt efter att metallen har lagrats i luft vid rumstemperatur. Dessutom mättes vätehalten efter anlöpning. Dessutom undersöktes effekten av ugnsatmosfärens tillsatsgas på mängden absorberad väte efter uppkolning. Tre olika höghållfasta och låg legerade stål sorter sätthärdades genom gas uppkolning. Mängden väte analyserades innan sätthärdning, efter sätthärdning, efter lagring i rumstemperatur och efter anlöpning med hjälp av Leco-RHEN602. Enligt resultaten i denna studie, absorberar alla av de tre undersökta stålsorterna väte under sätthärdning. En stor del av det absorberade vätet diffunderar ut efter att stålet har lagrats i luft vid rumstemperatur och under anlöpning. Omkring 50 % av den absorberade vätehalten under uppkolningen är på grund av reaktionen med tillsatsgasen i ugnsatmosfären. Omkring 50 % av vätet diffunderar ut ur proverna efter en dag. Möjligen all fritt (diffunderbart) väte har diffunderat ut ur proverna i två av stålsorterna efter en vecka i rumstemperatur eller efter anlöpning.

Analyse des phenomenes de coalescence orientee de la microstructure au cours du fluage dans differents superalliages a base de nickel durcis par precipitation de phase gamma '. Correlation entre la microstructure et la resistance au fluage. Determination, a partir de l'interpretation des resulats, un critere simple de resistance au fluage

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.18.06 – технологія жирів, ефірних масел і парфумерно-косметичних продуктів. – Національний технічний університет "Харківський політехнічний інститут", Харків, 2018. Дисертаційну роботу присвячено науковому обґрунтуванню перспективної технології одержання жирових продуктів з наперед заданими властивостями. Визначено триацилгліцерольний склад найбільш поширеної жирової сировини. Показано, що при загальній кількості триацилгліцеролів від 11 до 20 у кожному компоненті визначальними (за вмістом) є 7 – 10 триацилгліцеролів, від яких і залежать фізико-хімічні властивості жирових сумішей. На діаграмах диференційної скануючої калориметрії проаналізовані фазові перетворення для процесів плавлення та кристалізації в широкому діапазоні температур для одинадцяти сумішей. Виконано порівняльний аналіз температури в характерних точках з температурами плавлення і застигання. В результаті проведених обчислень знайдені коефіцієнти апроксимації для обчислення вмісту твердих триацилгліцеролів в характерних точках на діаграмі диференційної скануючої калориметрії і отриманих за допомогою ядерного магнітного резонансу, що дозволяє зіставляти і перераховувати результати обох аналізів, а це, в свою чергу, дозволяє досягати більш достовірних результатів. За допомогою плану Шеффе отримані рівняння регресії на базі яких розраховано рівняння залежності фізико-хімічних властивостей від триацилгліцерольного складу, що підтверджує можливість створення жирових основ жировмісних продуктів за допомогою відомого триацилгліцерольного складу. Розроблена методологія дозволяє скорегувати рецептуру жирових сумішей згідно із заданими фізико-хімічними властивостями або відтворювати вже існуючі рецептури за допомогою іншої сировини, на стадії лабораторних досліджень, ще до початку технологічного процесу і мінімізувати втрати енергоресурсів та час роботи лабораторії.
Thesis for a candidate Degree in Technical sciences, specialty 05.18.06 – technology of fats, essential oils and perfumery and cosmetic products. – National Technical University "Kharkiv Polytechnic Institute", of Ministry of Education and Science of Ukraine Kharkiv, 2018. The thiacylglycerol composition of the most common fatty material is determined. It is shown that in the total number of triacylglycerols from 11 to 20 in each component, the determinant (in content) is 7-10 triacylglycerols, on which the physical and chemical properties of fatty mixtures depend. In the diagrams of differential scanning calorimetry, phase transformations for melting and crystallization processes in a wide temperature range for eleven mixtures are analyzed. A comparative analysis of temperature at characteristic points with melting and freezing temperatures has been performed. As a result of the calculations, the coefficients of approximation were calculated for the calculation of the content of solid triacylglycerols in the characteristic points on the differential scanning calorimetry diagram and obtained with the help of a nuclear magnetic resonance, which allows to compose and recalculate the results of both analyzes, which in turn allows us to achieve more reliable results. Using the Scheffe plan, regression equations were derived on the basis of which the equation of the dependence of physical and chemical properties on the triatsilglycerol composition was calculated, which confirms the possibility of creating fatty bases of fat-containing products with the help of knowledge of the triacylglycerol composition. The developed methodology allows to adjust the formula of fat mixtures according to the given physical and chemical properties or to reproduce already existing recipes with the help of other raw materials, at the stage of laboratory research, before the beginning of the technological process and to minimize energy losses and the time of the laboratory.

Дисертація на здобуття наукового ступеня кандидата технічних наук за спеціальністю 05.18.06 – технологія жирів, ефірних масел і парфумерно-косметичних продуктів. – Національний технічний університет "Харківський політехнічний інститут", Харків, 2018. Дисертаційну роботу присвячено науковому обґрунтуванню перспективної технології одержання жирових продуктів з наперед заданими властивостями. Визначено триацилгліцерольний склад найбільш поширеної жирової сировини. Показано, що при загальній кількості триацилгліцеролів від 11 до 20 у кожному компоненті визначальними (за вмістом) є 7 – 10 триацилгліцеролів, від яких і залежать фізико-хімічні властивості жирових сумішей. На діаграмах диференційної скануючої калориметрії проаналізовані фазові перетворення для процесів плавлення та кристалізації в широкому діапазоні температур для одинадцяти сумішей. Виконано порівняльний аналіз температури в характерних точках з температурами плавлення і застигання. В результаті проведених обчислень знайдені коефіцієнти апроксимації для обчислення вмісту твердих триацилгліцеролів в характерних точках на діаграмі диференційної скануючої калориметрії і отриманих за допомогою ядерного магнітного резонансу, що дозволяє зіставляти і перераховувати результати обох аналізів, а це, в свою чергу, дозволяє досягати більш достовірних результатів. За допомогою плану Шеффе отримані рівняння регресії на базі яких розраховано рівняння залежності фізико-хімічних властивостей від триацилгліцерольного складу, що підтверджує можливість створення жирових основ жировмісних продуктів за допомогою відомого триацилгліцерольного складу. Розроблена методологія дозволяє скорегувати рецептуру жирових сумішей згідно із заданими фізико-хімічними властивостями або відтворювати вже існуючі рецептури за допомогою іншої сировини, на стадії лабораторних досліджень, ще до початку технологічного процесу і мінімізувати втрати енергоресурсів та час роботи лабораторії.
Thesis for a candidate Degree in Technical sciences, specialty 05.18.06 – technology of fats, essential oils and perfumery and cosmetic products. – National Technical University "Kharkiv Polytechnic Institute", of Ministry of Education and Science of Ukraine Kharkiv, 2018. The thiacylglycerol composition of the most common fatty material is determined. It is shown that in the total number of triacylglycerols from 11 to 20 in each component, the determinant (in content) is 7-10 triacylglycerols, on which the physical and chemical properties of fatty mixtures depend. In the diagrams of differential scanning calorimetry, phase transformations for melting and crystallization processes in a wide temperature range for eleven mixtures are analyzed. A comparative analysis of temperature at characteristic points with melting and freezing temperatures has been performed. As a result of the calculations, the coefficients of approximation were calculated for the calculation of the content of solid triacylglycerols in the characteristic points on the differential scanning calorimetry diagram and obtained with the help of a nuclear magnetic resonance, which allows to compose and recalculate the results of both analyzes, which in turn allows us to achieve more reliable results. Using the Scheffe plan, regression equations were derived on the basis of which the equation of the dependence of physical and chemical properties on the triatsilglycerol composition was calculated, which confirms the possibility of creating fatty bases of fat-containing products with the help of knowledge of the triacylglycerol composition. The developed methodology allows to adjust the formula of fat mixtures according to the given physical and chemical properties or to reproduce already existing recipes with the help of other raw materials, at the stage of laboratory research, before the beginning of the technological process and to minimize energy losses and the time of the laboratory.

3xxx aluminiumslegeringer har Mangan som hovedlegeringselement, og får sin styrke fra plastisk bearbeidelse. 3xxx legeringer har tidligere blitt ansett for å oppnå neglisjerbare endringer i hardhet og styrke som følge av presipitering av dispersoider.Denne masteroppgaven tar for seg hardhetsutvikling ved lave homogeniseringstemperaturer i fire 3xxx Al-Mn-Fe-Si legeringer som følge av presipitering av dispersoider. Legeringene ble homogenisert ved forskjellige temperaturer og holdetider. Utvikling av antallstetthet, gjennomsnittlig ekvivalent diameter og volumfraksjon av dispersoider er koblet til blant annet elektrisk ledningsevne, hardhetsmålinger og strekktesting. Oppgaven belyser teoretiske utregninger av volumfraksjoner, i tillegg til eksperimentelle målinger av disse gjort ved hjelp av transmisjons- elektronmikroskopi (TEM). Scanning elektronmikroskopi (SEM) og NanoSight partikkelanalyse har blitt brukt for å undersøke pulver ekstrahert fra aluminiumsmatrix. Den gjennomsnittlige diameteren av dispersoidene ble sammenlignet med resultater fra TEM-statistikk, disse ble ansett for å være konsistente med hverandre. Identifisering av en konstituent partikkelfase og en silisiumfase ble utført ved hjelp av diffraksjon, disse ble sammenlignet med faser som allerede var kjente legeringen. Det konkludert at økende volumfraksjon av dispersoider førte til en økning i styrke for alle legeringene. En homogen distribusjon av dispersoider i fast løsning, der dispersoidene var relativt små med kort gjennomsnittlig avstand mellom hver partikkel, ga størst effekt.Det ble klart etter flere undersøkelser at en økning av Mn i fast løsning resulterte i en klart minkende elektrisk ledningsevne, samtidig var legeringene med høyt Mn og Si innhold de ledende i presipitering av dispersoider ved økende holdetid.

This doctoral thesis consists of a literature review, presented in two papers, and another six papers describing experimental studies of the influence of different kinds of wax and polyphosporic acid on bitumen and asphalt concrete mixture properties. The literature review should give an extensive description of the field of knowledge concerning wax in bitumen. Effects of wax in crude oil, bitumen and asphalt concrete as well as test methods for studying these effects are described. Theories behind possible mechanisms are also discussed, and commercial wax as additive to bitumen for different purposes included. The experimental parts comprise laboratory studies involving totally five 160/220 penetration base bitumens from different sources, two isolated bitumen waxes, five commercial waxes and one polyphosphoric acid. Asphalt concrete slabs, containing base or modified bitumen were prepared and tested. Binder properties were evaluated using different types of laboratory equipment, such as dynamic shear rheometer (DSR), bending beam rheometer (BBR), differential scanning calorimeter (DSC), force ductilometer, as well as equipment for determining conventional parameters like penetration, softening point, viscosity, and Fraass breaking point. Fourier Transform Infrared (FTIR) spectroscopy and Thin Layer Chromatography (TLC-FID) were used for chemical characterization. The binders were aged by means of the rolling thin film oven test (RTFOT) and pressure ageing vessel (PAV) in combination. Asphalt concrete properties were evaluated at low temperatures using the tensile strain restrained specimen test (TSRST) and creep test at -25°C. Dynamic creep testing was performed at 40°C, as well as complex modulus tests between 0 and 20°C. Binder test results indicated that the magnitude and type of effect on bitumen rheology depend on the bitumen itself, type of crystallizing fraction in the bitumen and/or type and amount of additive used. Bitumen composition was found to be of decisive importance. Adding polyethylene wax or polyphosphoric acid, especially to a non-waxy 160/220 penetration grade bitumen, showed no or positive effects on the rheological behaviour at low temperatures (decrease in stiffness) as well as medium and high temperatures (increase in complex modulus and decrease in phase angle). However, the corresponding positive effects could not be shown in dynamic creep testing (at 40°C) of asphalt concrete mixtures containing these modified binders. Adding FT-paraffin decreased the physical hardening index for all bitumens. Also polyethylene wax and montan wax showed this effect for some bitumens. Slack wax showed a large increasing effect on physical hardening, and polyphosphoric acid none or a minor negative effect. No correlation between physical hardening index (PHI) and wax content by DSC was found in this study, involving both natural bitumen wax and commercial wax. Addition of the commercial waxes used showed no or marginally positive influence on bitumen ageing properties for the bitumens and test conditions used. Comparing asphalt mixture test results to the corresponding binder test results, the effects on asphalt mixtures from adding commercial wax or polyphosphoric acid were less evident. Significant binder physical hardening by BBR could not be confirmed by TSRST.
QC 20101006

Etude du comportement en fatigue plastique de l'acier inoxydable austenitique 316l fatigue sous vide a 20, 300 et 600c dans une gamme de deformations plastiques imposees. Analyse par microscopie electronique en transmission des sous-structures a20 et 600c, les microstructures sont constituees de bandes de glissement persistantes, de murs-canaux des labyrinthes et des structures en echelle evoluant vers la structure cellulaire. A300c, une structure de contraste en cotes de velours est observee. Cette structure est fonction de la deformation plastique et provoque un durcissement cyclique secondaire. Presentation de modeles de formation et d'evolution de microstructures de fatigue

Os aços inoxidáveis endurecidos por precipitação vêm sendo largamente empregados na indústria aeronáutica, por combinar resistência mecânica, tenacidade à fratura e resistência à corrosão. E deste modo, são materiais que possibilitam a substituição dos aços carbonos utilizados atualmente, que necessitam de tratamento superficial adicional, como o cádmio, para melhorar a resistência à corrosão. A utilização desses revestimentos traz desvantagens como o custo, a fabricação, a susceptibilidade à fragilização por hidrogênio além dos aspectos ambientais. Neste contexto, o aço endurecido por precipitação UNS S46500, designado como Custom 465® foi avaliado considerando o efeito da temperatura de envelhecimento sobre as propriedades mecânicas e a resistência à corrosão por pite. Amostras tratadas nas condições solubilizada e envelhecida a 510ºC (H950) e 538ºC (H1000) foram submetidas ao ensaio de tração, caracterização microestrutural e ensaios de polarização potenciodinâmica (PP) para determinar a resistência à corrosão por pite. Os exames microestruturais foram realizados com auxílio de microscopia óptica (MO), microscopia eletrônica de varredura (MEV), espectroscopia de energia dispersiva (EDS) e difração de raios X. Também foram realizadas análises utilizando o software Thermo-Calc. A resistência à corrosão por pite foi avaliada em solução 0,6M NaCl com adições crescentes de Na2SO4. Os resultados obtidos nos ensaios de PP nas duas condições de tratamento térmico foram comparados entre si e com resultados disponíveis na literatura (CALDERÓN-HERNANDEZ, 2012) para o aço inoxidável UNS S30403 (304L). Os exames e análises da microestrutura revelaram que o aço Custom 465® envelhecido apresenta uma matriz martensítica, precipitados de fase chi, austenita e precipitados Ni3Ti. O tratamento H950 apresentou maior resistência mecânica e menor alongamento do que o tratamento H1000. Tal comportamento foi devido à produção de maior porcentagem de fase chi e menor porcentagem de austenita nesse tratamento de envelhecimento. Os diferentes tratamentos térmicos, condição solubilizada, H950 e H1000 apresentaram praticamente a mesma resistência à corrosão por pite. Por outro lado, o aço Custom 465® apresentou ótima resposta à inibição da nucleação de pite com adições crescentes de sulfato em meio de 0,6M NaCl, sendo que a condição H1000 se sobressai sobre a H950 nessa questão. Além disso, através da adição de sulfato foi 7 possível obter maior resistência a corrosão por pite do aço Custom 465® comparativamente ao aço 304L. Tal comportamento foi discutido em termos da afinidade química entre níquel, cloreto e sulfato, levando a maior resistência à corrosão por pite quando o aço contém maior teor de níquel (que é o caso do aço Custom 465®). Este trabalho indicou que o critério na escolha do tratamento de envelhecimento do aço Custom 465® deve ser o das propriedades mecânicas almejadas, uma vez que a resistência à corrosão por pite mostrou-se praticamente independente do tratamento térmico.
The precipitation hardened stainless steels have been widely used in the aircraft industry to combine mechanical strength, fracture toughness and corrosion resistance. And therefore, are materials that enable replacement of the carbon steels used today, which require additional surface treatment, such as cadmium plating, to improve the corrosion resistance. The use of such coatings brings disadvantages such as cost, manufacturing, susceptibility to hydrogen embrittlement beyond environmental aspects. In this context, the precipitation hardened steel UNS S46500, known as Custom 465® were evaluated for the effect of aging temperature on the mechanical properties and the resistance to pitting corrosion .Treated samples in solubilized and aged condition at 510°C (H950) and 538ºC (H1000) were subjected to tensile strength test, microstructural characterization and potentiodynamic polarization (PP) tests to determine the pitting corrosion resistance. The microstructural studies were performed with the aid of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction. Analyzes were also performed using the Thermo -Calc software. The resistance to pitting corrosion was evaluated at 0.6M NaCl solution with increasing additions of Na2SO4. The results obtained from tests of PP in both heat treatment conditions were compared with each other and with results available in the literature (CALDERÓN HERNANDEZ, 2012) to stainless steel UNS S30403 (304L). The analysis of the microstructure revealed that the steel aged Custom 465 ® presents a martensitic matrix, chi precipitates, Ni3Ti precipitates and austenite phase. The H950 age treatment had higher mechanical strength and lower elongation than the treatment H1000. Such behavior was due to the production of higher percentage of chi and a lower percentage of austenite phase in aging treatment. The different heat treatments, solubilized condition, H950 and H1000 showed almost the same pitting corrosion resistance. On the other hand, the Custom 465® showed good response to pitting inhibition with increasing nucleating additions of sulfate in 0.6M NaCl, with enhanced result for H1000 condition. Furthermore, by adding sulfate was possible to obtain greater pitting corrosion resistance of Custom 465® compared to the 304L steel. This behavior has been discussed in terms of the chemical affinity between nickel, chloride and sulphate, leading to higher pitting corrosion resistance when the steel contains a higher nickel 9 content (which is the case of steel Custom 465®). This study indicates that the criterion in selecting the aging heat treatment of Custom 465® steel must be the desired mechanical properties, since the pitting corrosion resistance was found to be substantially independent of heat treatment.

Objetivo principal desta dissertação foi verificar os efeitos da temperatura de laminação na formação de martensita induzida por deformação e na sua posterior reversão da martensita para austenita no aço inoxidável austenítico AISI 304L. O estudo foi predominantemente microestrutural e para análise e caracterização foram utilizadas as técnicas de microscopia óptica, microscopia eletrônica de varredura, difração de raios X, medidas de dureza Vickers e medidas de fases ferromagnéticas por ferritoscopia. As amostras foram inicialmente solubilizadas a 1100 ºC por uma hora, visando a dissolução de uma pequena quantidade residual de ferrita encontrada nas amostras na condição como recebida, depois laminadas em diferentes temperaturas, determinando-se curvas de endurecimento por deformação e de formação de martensita induzida por deformação em função do grau de deformação. Em seguida, foram realizados pré-recozimentos a 600 ºC, favorecendo apenas a reversão da martensita para austenita, de maneira que não ocorresse a recristalização. A quantidade e a temperatura de deformação apresentaram forte influência na quantidade de martensita formada, no endurecimento por deformação e na cinética de amolecimento durante o recozimento. Os pré-tratamentos realizados a 600 ºC causaram acentuada reversão da martensita, algum amolecimento e pequeno efeito no tamanho de grão recristalizado durante o posterior recozimento a 600 ºC.
The main objective of the present dissertation was to verify the effects of the rolling temperature on the formation of strain induced martensite and in its subsequent martensite reversion to austenite in a AISI 304L stainless steel. The study was predominantly microstructural and, for the analysis and characterization, several techniques have been used, namely optical microscopy, scanning electron microscopy, X-ray diffraction, Vickers hardness measurements and magnetic phase measurements, using the ferritoscope. The samples were initially solution annealed at 1100 ºC for one hour, aiming at the dissolution of a small quantity of the residual -ferrite found in the samples in the as-received condition; then rolling was performed at different temperatures, evaluating strain hardening and the strain induced martensite as a function of strain. Following, pre-annealing treatments at 600 ºC have been performed, favoring only the martensite to austenite reversion, in a way that no recrystallization would occur. Strain and temperature had a strong influence on the amount of formed martensite, on the strain hardening and on the softening kinetics during annealing. The pre-annealing treatments at 600 ºC caused an accentuated effect on the martensite reversion, some softening and a small effect on the recrystallized grain size during the subsequent annealing at 600 ºC.

This work represents several years' research into the field of radiation hardening by design. The unique characteristics of a SiGe HBT, described in Chapter 1, make it ideally suitable for use in extreme environment applications. Chapter 2 describes the total ionizing dose effects experienced by a SiGe HBT, particularly those experienced on an Earth-orbital or lunar-surface mission. In addition, the effects of total dose are evaluated on passive devices. As opposed to the TID-hardness of SiGe transistors, a clear vulnerability to single-event effects does exist. This field is divided into three chapters. First, the very nature of single-event transients present in SiGe HBTs is explored in Chapter 3 using a heavy-ion microbeam with both bulk and SOI platforms [31]. Then, in Chapter 4, a new device-level SEU-hardening technique is presented along with circuit-design techniques necessarily for its implementation. In Chapter 5, the circuit-level radiation-hardening techniques necessarily to mitigate the effects shown in Chapter 3 are developed and tested [32]. Finally, in Chapter 6, the performance of the SiGe HBT in a cryogenic testing environment is characterized to understand how the widely-varying temperatures of outer space may affect device performance. Ultimately, the built-in performance, TID-tolerance, and now-developing SEU-hardness of the SiGe HBT make a compelling case for extreme environment electronics. The low-cost, high-yield, and maturity of Si manufacturing combine with modern bandgap engineering and modern CMOS to produce a high-quality, high-performance BiCMOS platform suitable for space-borne systems.

Multiple step tests under cyclic strain control have been performed using cylindrical specimens of cast polycrystalline Inconel 738LC superalloy at 23, 700, 500, 800 and 900 °C in laboratory atmosphere to obtain cyclic stress-strain curves. During cyclic straining of specimen were obtained cyclic hardening-softening curves. Their progress changed with temperature and strain amplitude. Evaluated cyclic stress-strain curves are shifted to lower stresses with increasing temperature. Surface relief was observed in fatigued specimens under SEM and metalography under optic microscopy. Slip markings were studied on specimen surface fatigued at 700 °C .Stress-strain response is compared and discussed in relation to the surface observations - persistent slip markings.

State-of-the-art SiGe BiCMOS technologies leverage the maturity of deep-submicron silicon CMOS processing with bandgap-engineered SiGe HBTs in a single platform that is suitable for a wide variety of high performance and highly-integrated applications (e.g., system-on-chip (SOC), system-in-package (SiP)). Due to their bandgap-engineered base, SiGe HBTs are also naturally suited for cryogenic electronics and have the potential to replace the costly de facto technologies of choice (e.g., Gallium-Arsenide (GaAs) and Indium-Phosphide (InP)) in many cryogenic applications such as radio astronomy. This work investigates the response of mixed-signal circuits (both RF and analog circuits) when operating in extreme environments, in particular, at cryogenic temperatures and in radiation-rich environments. The ultimate goal of this work is to attempt to fill the existing gap in knowledge on the cryogenic and radiation response (both single event transients (SETs) and total ionization dose (TID)) of specific RF and analog circuit blocks (i.e., RF switches and voltage references). The design approach for different RF switch topologies and voltage references circuits are presented. Standalone Field Effect Transistors (FET) and SiGe HBTs test structures were also characterized and the results are provided to aid in the analysis and understanding of the underlying mechanisms that impact the circuits' response. Radiation mitigation strategies to counterbalance the damaging effects are investigated. A comprehensive study on the impact of cryogenic temperatures on the RF linearity of SiGe HBTs fabricated in a new 4th-generation, 90 nm SiGe BiCMOS technology is also presented.

碩士
大同工學院
材料工程研究所
87
This research aims to study the relationship of die cast aluminum alloy Grade 356 between microstructure and mechanism properties. Using 356 aluminum alloys, which is one of the aluminum-silicon-magnesium systems to produce specimens. During the casting process, the castings rapidly cooled to room temperature, and get finer microstructure than specimens made by sand-cast or gravity cast ones. Then in the aging precipitation hardening heat treatment, the Mg-Si compounds are precipitation and the geometry and distribution of Si will be provided. Die-castings include a large quantity of pore, shrinkage hole, and shrinkage porosity, etc. which is one important factor of the effects of the mechanism properties. This research aims to study die cast aluminum alloy Grade 356 in as-cast condition directly apply the low temperature aging process (less then 300℃) at different temperature and time to attain precipitation hardening effect, and ignored the effect of porosity. The properties-structures relationship of the die cast aluminum 356 alloy subjected to the low temperature aging heat-treatments will be compared with that of the sand-cast / gravity cast ones. Though the as-cast specimens, which produced by die-casting, include high porosity, they still have good mechanism properties. And the tensile strength and elongation of 356 aluminum alloy with proper heat treatment were found to be up to 249.8MPa

碩士
國立高雄師範大學
化學系
90
Abstract Epoxy resin based on bisphenol-A(NPEL-128) , due to its excellent characteristics, has been widely applied in industrial field. By curing with phosphoric acid (PHA) and diluting with solvents, such as methyl ethyl ketone , acetone, isoamyl acetate or ethyl acetate, it achieves a very good effect for de-rust and anti-rust. Also, if epoxy is diluted with ether, it is properly used as a good room temperature cured paint because the high volatility of ether effectively lowers the curing temperature. Studies, using FT-IR and DSC, help us to understand how the solvents affect the linkage and the linking situation. The increase of phosphoric acid is helpful for the cross linkage. Such resin diluted with ether has been approved to have a longer pot life and its film is able to cure at lower temperature. Therefore, from practical view, epoxy/PHA/solvents/ether is a very good paint.

Moringa oleifera Lam. is a tree with great potential as it is fast growing and drought tolerant. Amongst the tree’s multitude of benefits, it can also be used to produce a biodiesel fuel. However, prior to the establishment of commercial plantations, all cultivation aspects of this promising tree have to be understood. Temperature is a significant climatic factor influencing both geographical plant distribution and growth, and since M. oleifera trees are naturally found in tropical climates around the world, the extent of their adaptability to cooler climates was the main objective of this study. Trees were cultivated from seed and germinated in a controlled greenhouse environment at the Experimental Farm of the University of Pretoria. After germination, 50% of the seedlings were hardened-off by placing them outside under ambient growing temperatures, while the rest remained inside the greenhouse. With trial commencement, 132 of both the hardened and non-hardened seedlings were planted into 10 ℓ plastic bags and randomly placed into three temperature-controlled greenhouses, each with a different fluctuating night/day temperature regime namely; 10/20°C ± 2°C, 15/25°C ± 2°C and 20/30°C ± 2°C. In addition, half the trees within each temperature regime were treated with the growth regulator paclobutrazol to determine its effect on growth/flowering at different temperatures. During the 224-day trial period, biweekly measurements of tree height, stem diameter and leaf area estimates of each individual tree within all three temperature regimes were taken. Despite germination percentages being slightly higher at the low 10/20°C regime, the MGT, germination rate, uniformity and seedling growth were superior at the higher 20/30°C regime. The temperature induced seed dormancy at the 20/30°C regime, could be overcome by an incubation period at lower temperatures, as fewer instances of seed dormancy were observed at the 10/20°C regime. The increase in temperature resulted in significant (P≤0.05) growth rate increases of over 650% between the 10/20°C and 20/30°C and 250% between the 10/20°C and 15/25°C night/day temperature regimes. In addition, the 20/30°C temperature treatment, although fluctuative, consistently had the highest leaf area over the entire trial period. Hardening-off of trees during the seedling stage, significantly (P≤0.05) increased the final tree height by 3.09X, 1.44X and 1.23X, compared to their non hardened-off counterparts under the 10/20°C, 15/25°C and 20/30°C temperature regimes respectively. Leaf thickness decreased by a significant (P≤0.05) 43.1% with increase in temperature between the 10/20°C and 20/30°C regime, mostly due to a thinner mesophyll layer. The efficacy of paclobutrazol on M. oleifera growth was found to be temperature dependant, reducing growth at 10/20°C, while increasing growth at both the higher 15/25°C and 20/30°C regimes. Flowering however remained unaffected by paclobutrazol. The highest instances of flowering and pollen viability were observed at the 15/25°C regime. The absence of inflorescence induction at the 20/30°C regime was responsible for the reduced flowering, signifying the necessity of vernalization prior to flowering. Even though all the results confirm the preference of M. oleifera trees towards a tropical climate, satisfactory growth with possibly improved flowering during the hot summer months in certain sub-tropical climates is achievable. Copyright
Dissertation (MSc(Agric))--University of Pretoria, 2010.
Plant Production and Soil Science
unrestricted

A continuum plasticity yield function is developed that captures tension/compression asymmetry and its evolution as exhibited by HCP materials such as magnesium alloy sheet. The model, referred to herein as “CPB06ex3ev”, is based upon the CPB06 [1] yield surface which is extended in this research to consider evolution of asymmetry and anisotropy under monotonic loading. The model is further modified to incorporate thermal softening and strain rate effects. Mechanical characterization experiments are performed to acquire uniaxial tensile and compressive stress-strain data along a range of in-plane and through-thickness loading orientations. Experiments are performed for a range of strain rates (0.001-1s-1) and temperatures (23-250°C). A strong, evolving asymmetry is observed at room temperature when comparing tensile and compressive flow stresses and r-values, while asymmetry and anisotropy are reduced dramatically as temperature is increased. AZ31B exhibits moderate strain rate sensitivity at room temperature, however, the rate sensitivity increases with temperature. The CPB06ex3ev model is applied to simulate AZ31B magnesium alloy sheet. An error minimization scheme is used to fit the yield function and evolution coefficients over the entire data set. The calibrated model is shown to capture the evolving asymmetric/anisotropic response of both flow stresses and r-values in tension and compression, while also fitting the flow stress at the biaxial tension and pure shear locations on the yield locus. The model, which uses three stress transformations, is implemented within a user defined material model (UMAT) and linked to the commercial finite element software LS-DYNA. In order to assess the finite element implementation of the CPB06ex3ev model, a series of validation experiments were performed and corresponding finite element models were developed: (i) room temperature three-point bending; (ii) elevated temperature (250°C) limiting dome height experiments; and, (iii) warm cup drawing experiments. The three point bend simulations demonstrated the importance of capturing material asymmetry and the associated shift in neutral axis. Comparison between the warm forming experiments and models revealed qualitative agreement between the predicted punch load-displacement and strain distributions. The CPB06ex3ev formulation was able to capture the anisotropy trends in terms of the differences in strains measured along the sheet rolling versus transverse directions. Beyond the constitutive characterization and modeling effort, the cup draw formability experiments have provided interesting insight into the effect of temperature and temperature distribution within the AZ31B sheet. The current work has served to show the existence of a process window in which the blank center temperature must lie below the die temperature but above the temperature for activation of non-basal slip systems (to avoid low temperature fracture). Two modes of failure have been identified at the process window boundaries in which the cup either fractures due to low temperature (brittle) failure or a high temperature (necking) failure.

There has been progress in the surface modification of low carbon steel in order to enhance its surface hardness. This study contributes to this by investigating the introduction of carbon nanotubes and amorphous carbon in the carburization of mild steel. In order to achieve the goal, carbon nanotubes were synthesized in a horizontal tubular reactor placed in a furnace also called the chemical vapor deposition process at a temperature of 700oC. Catalyst was produced from Iron nitrate Fe(NO3)3.9H2O and Cobalt nitrate Co(NO3)2.6H2O on CaCO3 support while acetylene C2H2 was used as the carbon source and nitrogen N2 was used as contaminant remover. The as-synthesized carbon nanotubes were purified using nitric acid HNO3 and characterized using scanning electron microscopy (SEM), thermo-gravimetric analysis (TGA) and fourier transform infrared spectroscopy (FTIR). It was found that as-synthesized carbon nanotubes had varying lengths with diameters between 42-52 nm from the SEM and the TGA showed the as-synthesized CNTs with a mass loss of 78% while purified CNTs had 85% with no damage done to the structures after using the one step acid treatment. The as-synthesized and purified carbon nanotubes were used in carburizing low carbon steel (AISI 1018) at two austenitic temperatures of 750oC and 800oC and varying periods of 10-50 minutes while amorphous carbon obtained by pulverizing coal was also used as comparison. The mild steel samples were carburized with carbon nanotubes and amorphous carbon in a laboratory muffle furnace with a defined number of boost and diffusion steps. The carburizing atmosphere consisted of heating up to the varying temperatures at a speed of 10oC/minute, heating under this condition at varying periods, performing a defined number of boost and diffusion processes at the varying temperatures and cooling to room temperatures under the same condition. The carburized surfaces were observed with the Olympus SC50 optical microscope and the hardness distribution of the carburized layer was inspected with a Vickers FM 700 micro-hardness tester. The as-synthesized and purified CNT samples showed higher hardness on the surface of the mild steel than the amorphous carbon. In the same vein, the change in the microstructures of vi the steel samples indicated that good and improved surface hardness was obtained in this work with the reinforcements but with purified CNT having the highest peak surface hardness value of 191.64 ± 4.16 GPa at 800oC, as-synthesized CNT with 177.88 ± 2.35 GPa and amorphous carbon with 160.702 ± 5.79 GPa which are higher compared to the values obtained at 750oC and that of the original substrate which had a surface hardness of 145.188 ± 2.66 GPa. The percentage hardness obtained for the reinforcement with the amorphous carbon, the CNT and the pCNT showed an increase of 5.47%, 10.04% and 15.77% respectively at 750oC when compared to that of the normal substrate carburized without reinforcements. Furthermore, at 800oC, the reinforcement with the amorphous carbon, the CNT and the pCNT show a percentage hardness increase of 7.04%, 14.68% and 22.05% when compared to that of the normal substrate carburized without reinforcements. Comparing the reinforcement potential of the amorphous carbon, the CNT and the pCNT at 750oC, the percentage hardness reveal that using pCNT displayed an increase of 10.89% over that of amorphous carbon and of 6.37% over that of CNT. In addition, the use of CNT as reinforcement at 750oC displayed a percentage hardness increase of 4.83% over that of the amorphous carbon carburized at the same temperature
Civil and Chemical Engineering
M. Tech. (Chemical Engineering)