Dissertations / Theses on the topic 'Steel alloys – Mechanical properties'

Thesis (M.S.)--University of Missouri--Rolla, 2007.
Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed November 29, 2007) Includes bibliographical references.

The penchant towards development of high performance materials for light weighting engineering systems through various thermomechanical processing routes has been soaring vigorously. Friction stir processing (FSP) - a relatively new thermomechanical processing route had shown an excellent promise towards microstructural modification in many Al and Mg alloy systems. Nevertheless, the expansion of this process to high temperature materials like titanium alloys and steels is restricted by the limited availability of tool materials. Despite it challenges, the current thesis sets a tone for the usage of FSP to tailor the mechanical properties in titanium alloys and steels. FSP was carried out on three near beta titanium alloys, namely Ti6246, Ti185 and Tiβc with increasing β stability index, using various tool rotation rates and at a constant tool traverse speed. Microstructure and mechanical property relationship was studied using experimental techniques such as SEM, TEM, mini tensile testing and synchrotron x-ray diffraction. Two step aging on Ti6246 had resulted in an UTS of 2.2GPa and a specific strength around 500 MPa m3/mg, which is about 40% greater than any commercially available metallic material. Similarly, FSP on an ultra-high strength steel―Eglin steel had resulted in a strength greater than 2GPa with a ductility close to 10% at around 4mm from the top surface of stir zone (SZ). Experimental techniques such as microhardness, mini-tensile testing and SEM were used to correlate the microstructure and properties observed inside SZ and HAZ's of the processed region. A 3D temperature modeling was used to predict the peak temperature and cooling rates during FSP. The exceptional strength ductility combinations inside the SZ is believed to be because of mixed microstructure comprised of various volume fractions of phases such as martensite, bainite and retained austenite.

Natural Gas with more than 80% methane has a liquefaction temperature around -165 ° C. Temperature at which the gas reduces its volume by a factor of 600/1. This operating temperature makes the use of ferritic materials unfeasible because they have a brittle fracture mode below a critical temperature, called transition temperature. For the construction of large containers, the most commonly used material is Steel A-553-T1 which has a nominal content of 9% nickel and whose crystalline structure is formed by a martensite matrix with some austenite reacted. This microstructure is achieved through double heat treatment; Tempering and tempering. To weld this steel, for this application, it is not possible to use materials of 9% Ni feed, in view of the impossibility of performing the necessary thermal treatments to achieve homogeneity of properties. On the other hand, the austenitic welding consumables present a ductile behavior with a high energy absorbed even to -196ºC and within these NiCrMo family nickel base alloys have a high mechanical strength, and a coefficient of thermal expansion close to the steel 9% Ni. Within this family of nickel base alloys, the Hastalloy C-276 alloy has been used, which increases its mechanical strength by solid solution, the main alloys being chromium and molybdenum both about 15% and with 2.5% of Tungsten and 5% iron. Although this is an alloy that is essentially single-phase Gamma, the last liquid is usually transformed into carbides or TCP phases as the pass Mu and P. These three phases have a very close composition so that their identification through the EDX is not possible . Currently, most of the tanks being built have a storage capacity between 150,000 and 200,000, so the sheet thickness of 1 was ferrule between 27 to 50mm, implying that the welds are multi-pass, Requiring between 16 and 30 passes to fill the joints of this first ferrule. Normally the vertical joints are welded with manual or semi-automatic processes while the horizontal joints are welded with automatic submerged arc process. According to the atmospheric tank design standards for cryogenic storage, the thickness of the sheets is determined by the Maximum Admissible Efforts that are calculated from the mechanical strength of the weakest structural element, the base metal or the weld joint. In the case of welding, the mechanical strength of the weld is determined from the cylindrical specimen tensile test obtained from the deposited metal, from the homologation coupon of the welding process. During the homologation of the manual or semi-automatic procedures, the results obtained in the tests of longitudinal and transverse traction are equivalent. In the case of automatic welding In addition to the low values obtained from the cylindrical tensions of the weld metal of the horizontal joints with respect to the transverse tensions, it is very often observed that an important difference in the resistance presented by the different tensile tests of the same welded specimen , These differences being much greater than the observed difference between two experimental conditions. Prior to this experimental work, 6 other tests and a few procedural approvals were carried out, in which sheets of 12, 21, 26.5 and 27 mm of thickness were used, with 2.4 mm and 1.6 mm threads always of the classification AWS A5.14 ER-NiCrMo-4, corresponding to the Hastalloy C-276 alloy, with different fluxes, stiffening levels, cylindrical probe diameter etc. This PhD work was carried out on the seventh test carried out in the summer of 2008 at Lincoln Electric Cleveland, where 4 fluxes, 2 wire diameters, alternating and continuous current and two voltage levels were tested, with an experimental design 23 with each flux. As all DOE tests were performed, 8 specimens were welded per flux, and a total of 32 specimens were welded. The purpose of this test was to select the best flux wire pair, and determine the optimum parameters to maximize the mechanical strength of the weld metal. The base material used in this experiment were A553 T1 steel sheets, with 9% Ni and annealed and tempered with a thickness of 21mm. The joint design of these specimens is asymmetrical and unbalanced "X" with a 1mm bead and a 2mm spacing. Following the actual joint design of the production plates. In order to prevent the melting bath from being picked up, a flux backing was placed. The tests performed on each specimen were as follows: Cylindrical traction welding metal: 4 per test pieces Charpy V Notch at -196 ° C Macro General Chemical Analysis, performed on the side faces of the macros. Chemical analysis on tensile specimens. Microhardness tests Vickers and Knoob. The wire-flux pair selected in these tests has been used for the welding of eight tanks: three in Spain: two in Gijón 2011-2013 and one in Bilbao 2014-2015; A tank in Chile, 2011-2013 and four other tanks in China, 2011-2013. With this pair, good results have been obtained in the approvals of welding procedures of these projects, both in the transversal tensions and in the cylindrical tensions, fulfilling the requisites of resistance necessary in each project. During the production, a welding metal with very few inclusions of slag has been deposited, presenting good degreasing and degassing. The objective of this research is to determine the factors that produce the variability of results in the tensile tests, correlating the structural and micro structural factors with the mechanical properties of the deposited metal, in order to maximize its mechanical resistance.
Hasta mediados del siglo XX, el gas era considerado como un residuo de la explotación petrolera con importantes barreras tecnológicas y económicas para su procesado y comercialización por lo que gran parte de este era quemado en los países de producción. Desde finales del siglo XX, el aumento de la demanda de energía sumado con los altos niveles de contaminación producido por la quema de petróleo y carbón hicieron que se desarrollen las tecnologías y normas para el transporte seguro y rentable de los gases derivados del petróleo. Desde entonces, El gas natural ha tenido una penetración muy importante en la cadena de consumo debido a su alto poder energético y a la baja cantidad, comparada con el petróleo y carbón, de residuos, sólidos y gaseosos, que han hecho que este se perciba como un combustible limpio. El transporte de este producto se realiza en estado líquido, por medio de 2 tecnologías, presurización o por enfriamiento, LPG y LNG. La primera requiere de plantas de presurizado y gasoductos. Las distancias económicamente rentables para la conducción por gasoducto rondan la docena de miles de kilómetros, requiriendo de plantas de re presurización a lo largo del gasoducto. Cuando la distancia entre los productores y los consumidores que muy grande la licuación por enfriamiento a presión atmosférica es la opción más económica y segura. En este caso en, el gas obtenido del pozo se conduce hasta la planta de licuefacción donde se realiza la separación de los distintos componentes sólidos, líquidos y gaseosos, por procesos de filtración y licuación diferencial. El Gas natural producto de este proceso se almacena temporalmente en un tanque de LNG mientras en cargado en el barco que lo transportará a destino. Una vez en destino el barco descarga a un tanque de LNG, de donde se suministra a la planta de regasificación. De esta el gas es canalizado a alta presión por los gasoductos de distribución o a las plantas de generación eléctrica. El Gas Natural, con más de un 80% de metano tiene una temperatura de licuefacción alrededor de los -165ºC. Temperatura a la cual el gas reduce su volumen por un factor de 600/1. Esta temperatura de operación hace inviable el uso de materiales ferríticos, debido a que estos presentan un modo de fractura frágil por debajo de una temperatura crítica, llamada de transición. Para la construcción de grandes contenedores el material más usado en es Acero A-553-T1 que tiene un contenido nominal de 9% de níquel y cuya estructura cristalina está formada por una matriz de martensita con algo de austenita revenida. Esta microestructura se consigue a través del doble tratamiento térmico; de temple y revenido. Para soldar este acero, para esta aplicación, no se pueden usar materiales de aportes similares al 9%Ni en vista de la imposibilidad de realizar los tratamientos térmicos necesarios para conseguir la homogeneidad de propiedades. Por otro lado, los consumibles de soldadura austeníticos presentan un comportamiento dúctil con una alta energía absorbida incluso a -196ºC y dentro de estos las aleaciones base níquel de la familia NiCrMo presentan una alta resistencia mecánica, y un coeficiente de expansión térmica cercano a del acero 9%Ni. Dentro de esta familia de aleaciones base níquel se ha usado la aleación Hastalloy C-276, la cual incrementa su resistencia mecánica por solución sólida, siendo los principales aleantes el cromo y molibdeno ambos alrededor de 15%, y con un 2,5% de tungsteno y un 5% de hierro. Aun que ésta es una aleación que en esencia es monofásica Gamma, el último liquido suele transformarse en carburos o fases TCP como la pase Mu y P. Estas tres fases tienen una composición muy cercana por lo que su identificación a través del EDX no es posible. Actualmente, la mayoría de los tanques que se están construyendo tienen una capacidad de almacenamiento entre los 150.000 y 200.000, por lo que el espesor de chapa de la 1 era virola de entre 27 a 50mm, lo que implica que las soldaduras son multi pasada, necesitándose entre 16 y 30 pasadas para rellenar las juntas de esta primera virola. Normalmente las juntas verticales se sueldan con procesos manuales o semi-automáticos mientras que las horizontales se sueldan con proceso automático de arco sumergido. Según las normas de diseño de tanques atmosféricos para almacenamiento criogénico, el espesor de las chapas está determinado por los Esfuerzos Máximos Admisibles que se calculan a partir de la resistencia mecánica del elemento estructural más débil, el metal base o la junta de soldadura. En el caso de la soldadura, la resistencia mecánica de ésta se determina a partir del ensayo de tracción con probeta cilíndricas obtenidas del metal depositado, a partir del cupón de homologación del procedimiento de soldadura. Durante la homologación de los procedimientos manuales o semi automáticos los resultados obtenidos en los ensayos de tracción longitudinal y transversal son equivalentes. En el caso de la soldadura automática de las juntas horizontales, los resultados obtenidos de las tracciones transversales siempre han sido muy superiores a los resultados de las tracciones cilíndricas. Además de los bajos valores que se obtienen de las tracciones cilíndricas del metal de soldadura de las juntas horizontales con respecto a las tracciones transversales, con mucha frecuencia se observa que una importante diferencia en la resistencia presentada los diferentes ensayos de tracción de una misma probeta soldada, siendo estas diferencias mucho mayores que la diferencia observada entre dos condiciones experimentales. Situación que dificulta la correcta interpretación de los resultados de los diferentes ensayos o pruebas realizadas. Antes de este trabajo experimental se realizaron otros 6 ensayos y unas cuantas homologaciones de procedimientos, en las cuales se usaron chapas de 12, 21, 26,5 y 27mm de espesor, con hilos de 2,4mm y 1,6mm siempre de la clasificación AWS A5.14 ER-NiCrMo-4, correspondiente a la aleación Hastalloy C-276, con diferentes fluxes, niveles de rigidización, diámetro de probeta cilíndrica etc. Este trabajo de doctorado se he realizado sobre el séptimo ensayo realizado en verano de 2008 en las instalaciones de Lincoln Electric Cleveland, en que se probaron 4 fluxes, 2 diámetros de hilo, corriente alterna y continua y dos niveles de voltaje, desarrollándose un diseño experimental 23 con cada flux. Como se realizaron todas las pruebas correspondientes al DOE, se soldaron 8 probetas por flux, y en total 32 probetas. El objetivo de este ensayo era seleccionar el mejor par alambre fundente, y determinar los parámetros óptimos para maximizar la resistencia mecánica del metal de soldadura. El material base usado en este experimento fueron chapas de acero A553 T1, con 9%Ni y templadas y revenidas con un espesor de 21mm. El diseño de junta de estas probetas es en “X” asimétrica y desbalanceada con un talón de 1mm y una separación de 2mm. Siguiendo el diseño de junta real de las chapas de producción. Con el fin de evitar que el baño de fusión se descuelgue se colocó un respaldo de flux. Los ensayos realizados a cada probeta han sido los siguientes: Tracciones Cilíndricas de metal de soldadura: 4 por probetas Charpy V Notch a -196ºC Macro Análisis Químico General, realizado sobre las caras laterales de las macros. Análisis Químico en las probetas de tracción. Ensayos de microdureza Vickers y Knoob. El par alambre-fundente seleccionado en estas pruebas ha sido usado para el soldeo de ocho tanques: tres en España: dos en Gijón 2011-2013 y uno en Bilbao 2014-2015; un tanque en Chile, 2011-2013 y otros cuatro tanques en China, 2011-2013. Con este par se han conseguido buenos resultados en las homologaciones de procedimientos de soldadura de estos proyectos, tanto en las tracciones transversales como en las tracciones cilíndricas, cumpliendo con los requisitos de resistencia necesarios en cada proyecto. Durante la producción se ha depositado un metal de soldadura con muy pocas inclusiones de escoria, presentando buen desescoriado y desgasificado. El objetivo de este trabajo de investigación es determinar los factores que producen la variabilidad de resultados el los ensayos de tracción, correlacionando los factores estructurales y micro estructurales con las propiedades mecánicas del metal depositado, con el fin de maximizar su resistencia mecánica.

The aim of the work was to create a model to simulate the evolution of the microstructure during welding. The model consists of heat transfer and heat input models, microstructure model and hardness model. The heat transfer and heat input models are used to model the arc welding and the temperature changes in the welded piece. A microstructure model has been coupled with the heat transfer model i.e. the microstructure evolution is modeled simultaneously with the heat transfer model. The microstructure model simulates phase transformations and grain growth. In addition, the model predicts the hardness based on the microstructure. A graphical user interface was also developed to ease the usage of the model. The developed model is numerical and is based on theories presented in the literature. Some parameters for theories have also been defined experimentally using thermomechanical simulator. Real welding experiments were also made to verify the model. The temperature model can predict the temperatures in the heat-affected zone quite reliably. The phase transformation model works also well. The phase fractions from the model correlate with those seen under a microscope and the model predicts the shapes of the heat-affected zone and fusion zone with relatively good accuracy. The grain growth model works well far from fusion line but is not as good near the fusion line. The hardness model is not as reliable as other models but is still able to predict the hardness quite well even though the model is rather simple
Työn tavoitteena oli kehittää malli hitsauksessa tapahtuvien mikrorakennemuutosten simuloimiseen. Malli koostu lämmönsiirto- ja lämmöntuontimallista, mikrorakennemallista sekä kovuusmallista. Lämmönsiirto- ja lämmöntuontimalleilla mallinnetaan kaarihitsausta ja sen aikaansaamia lämpötilamuutoksia teräksessä. Mikrorakennemalli on kytketty lämpötilamalliin eli mikrorakennetta mallinnetaan samanaikaisesti lämpötilojen kanssa. Mikrorakennemalli simuloi faasimuutoksia ja rakeenkasvua. Lisäksi malli pyrkii ennustamaan kovuutta mikrorakenteen perusteella. Malliin luotiin myös graafinen käyttöliittymä helpottamaan käyttöä. Työssä luotu malli on numeerinen ja se perustuu kirjallisuudessa esitettyihin teorioihin. Lisäksi teorioiden vaatimia parametreja on määritetty kokeellisesti termomekaanisella simulaattorilla. Lisäksi työssä tehtiin hitsauskokeita mallin verifioimiseksi. Lämpötilamalli ennustaa muutosvyöhykkeen lämpötilat melko luotettavasti. Faasimuutosmalli toimii myös hyvin. Kokeelliset ja mallinnetut faasiosuudet vastaavat toisiaan. Malli ennustaa myös suhteellisen hyvin sula-alueen ja muutosvyöhykkeen muotoa. Raekokomalli toimii hyvin kauempana sula-alueesta, mutta lähellä sula-aluetta malli ei toimi yhtä hyvin. Kovuusmalli ei ole yhtä luotettava kuin muut mallit, mutta ennustaa silti kovuuksia todella hyvin, vaikka onkin melko yksinkertainen

Multi-pass fusion welding by a filler wire (welding electrode) is normally carried out to join thick steel sections used in most engineering applications. Welded joints in an installation, is the area of critical importance, since they are likely to contain a higher density of defects than the parent metal and their physical properties can differ significantly from the parent metal. Fusion arc welding process relies on intense local heating at a joint where a certain amount of the parent metal is melted and fused with additional metal from the filler wire. The intense local heating causes severe transient thermal gradients in the welded component and the resulting uneven cooling that follows produces a variably distributed residual stress field. In multi-pass welds, multiple thermal cycles resulted in a variably distribution of residual stress field across the weld and through the thickness. These complex thermal stresses generated in welds are undesirable but inevitable during fusion welding. Presence of such tensile residual stresses can be detrimental to the service integrity of a welded structure. In addition to a complex distribution of residual stress state, multi-pass welds also forms dendritic grain structure, which are repeatedly heated, resulting in segregation of alloying elements. Dendritic grain structure is weaker and segregation of alloying elements would result in formation of corrosion microcells as well as reduction in overall corrosion prevention due to depletion of alloying elements.

Equal-channel angular pressing (ECAP) is one of the severe plastic deformation (SPD) to produce ultra-fine grain (UFG) material, and its principle and microstructural developments. The majority of papers on SPD materials have been devoted to the face centered cubic (FCC) structure materials such as Al, Cu and Ni. The UFG of high alloy ECAP processing has been difficult up to now, but we were successful in this study. Fe-20%Cr steel with extremely low C and N has different slip behavior from the FCC. The mechanical properties and corrosion resistance were investigated in term microstructural evolution during ECAP processing.
博士(工学)
Doctor of Philosophy in Engineering
同志社大学
Doshisha University

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico
Superferritic stainless steels are widely used in heat exchangers and in petrochemical power plant because of their excellent mechanical properties coupled to corrosion resistance in chloride rich environments. In this research new superferritic alloys with high Cr, Mo, and Ni contents were produced for oil industry applications. Therefore, two superferritic stainless steels model alloys were developed, one with 0.07% B addition and another one without this alloying element. The phase diagrams for both materials were carried out by Thermo-Calc computacional program. Microstructure characterization was performed using the following techniques: optical and scanning electron microscopy, energy-dispersive spectroscopy, and Xray diffraction. The mechanical properties were assessed by hardness measurements, Charpy impacts and tensile tests. Corrosion resistance of the model alloys in aqueous solution 0.6 M NaCl was investigated by open circuit potential, potentiodynamic cyclic anodic polarization, and electrochemical impedance spectroscopy. Furthermore, critical pitting temperature was determined. The results showed that the superferritic stainless steel model alloy without B addition achieved better mechanical properties than the one of the two commercial superferritic stainless steels alloys. The corrosion resistance of this alloy was similar to the one of the superaustenitic stainless steels, and superduplex in aqueous solution 0.6 M NaCl. However, there was the formation of eutectoid lamellar constituent like perlite for the superferritic stainless steel model alloy with B addition. This eutectoid constituent was deleterious to mechanical properties, as well as to corrosion resistance for the material developed and studied in this thesis.
Os aÃos inoxidÃveis superferrÃticos sÃo amplamente utilizados em trocadores de calor e em plantas petroquÃmicas, devido a suas excelentes propriedades mecÃnicas associadas à resistÃncia à corrosÃo em ambientes ricos em cloreto. Nesta pesquisa foram produzidas novas ligas superferrÃticas com elevados teores de Cr, Mo e Ni para aplicaÃÃo em plantas petrolÃferas. Portanto, foram desenvolvidas duas ligas modelo de aÃo inoxidÃvel superferrÃtico, uma com 0,07% de B e outra sem adiÃÃo deste elemento de liga. Os diagramas de fases para os materiais foram construÃdos a partir do programa computacional Thermo-Calc. A caracterizaÃÃo microestrutural foi realizada usando as tÃcnicas; microscopia Ãptica, microscopia eletrÃnica de varredura, espectroscopia de energia dispersiva de raios-X e difraÃÃo de raios-X. As propriedades mecÃnicas foram avaliadas por medidas de dureza, ensaios de impacto Charpy e ensaios de traÃÃo. A resistÃncia à corrosÃo das ligas modelo, em soluÃÃo aquosa de 0,6M NaCl, foi avaliada por monitoramento do potencial de circuito aberto, polarizaÃÃo potenciodinÃmica cÃclica e espectroscopia de impedÃncia eletroquÃmica. AlÃm de determinar a temperatura crÃtica de pite. Os resultados obtidos demonstram que, a liga modelo de aÃo inoxidÃvel superferrÃtico sem boro apresentou melhores propriedades mecÃnicas, comparada com duas ligas de aÃo inoxidÃvel superferrÃtico comerciais. A resistÃncia à corrosÃo dessa liga foi similar a um aÃo inoxidÃvel superaustenÃtico e a um aÃo inoxidÃvel superduplex, em soluÃÃo aquosa de 0,6M de NaCl. Entretanto, para a liga modelo com boro, ocorreu a formaÃÃo de um constituinte eutetÃide lamelar semelhante à perlita. Este constituinte eutetÃide à deletÃrio para as propriedades mecÃnicas, assim como, na resistÃncia à corrosÃo do material desenvolvido e estudado nesta tese.

The present study focuses on developing a relationship between process variables, mechanical properties and post weld microstructure in Friction Stir Welded HSLA 65 steel. Fully consolidated welds can be produced in HSLA 65 steel by PCBN Convex-Scrolled-Shoulder-Step-Spiral (CS4) tool over a wide range of parameters. Microstructures in the nugget center (NC) are dominated by lath bainite and a few polygonal/allotriomorphic grain boundary ferrites. FSW dependent variables are related to FSW independent variables by non-linear relationship. Heat input is identified to be the best parameter index to correlate with microstructures. With increasing heat input, the volume of bainite is reduced, the shape of bainite is more curved and grain/lath size become coarser. A linear relationship was established between heat input and semi-quantitative post-weld microstructures based on the optical microstructures. Further analysis has been applied on the NC to obtain more fundamental understanding of FSW. The new approach via Orientation Imaging Microscopy (OIM) was developed to acquire quantitative microstructural data including bainite lath/packet and prior austenite grain size (PAG). A linear relationship between heat input and quantitative microstructural features in the NC have been established. Mechanical properties exhibits linear relationship with heat input. These correlations can be utilized to determine FSW weld parameter to get desired mechanical properties welds.

Orientador: Paulo Roberto Mei
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica
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Resumo: Os efeitos da adição de níquel e da temperatura em ligas ferro-cromo foram estudados. Sete ligas de base ferro, com 18% Cr e teores de níquel variando entre zero e 60%, foram utilizadas no estudo (0, 10, 20, 30, 40, 50 e 60% Ni). Foram realizados ensaios de tração em temperatura ambiente, 350 e 700 ºC; simulação por termodinâmica computacional; microscopia ótica no estado recozido, nas amostras de quick stop, após deformação a frio, a 350 e a 700 ºC; difração de raios X no estado recozido, após deformação a 350 e a 700 ºC; ensaio de dureza a quente a 100, 200, 300 e 400 ºC; ensaio de impacto Charpy em temperatura ambiente e a -196 ºC; e ensaio de quick stop; análise térmica por DTA; ensaios de dureza no estado recozido, nos cavacos obtidos em torneamento, após deformação a 350 e 700 ºC. Os resultados dos ensaios de tração a 350 ºC foram correlacionados com os dados obtidos por Marques (2007), visando relacionar propriedades mecânicas e usinabilidade. A tendência geral de redução da usinabilidade com o aumento do teor de níquel foi relacionada com o aumento da resistência mecânica e da ductilidade a quente, proporcionados pelo níquel. A alta usinabilidade observada na liga 30% Ni foi relacionada com a presença de inclusões com cálcio nesta liga, o que proporcionou ganhos muito expressivos em usinabilidade, mas sem prejudicar as propriedades mecânicas avaliadas.
Abstract: The effects of nickel addition and temperature in iron-chromium alloys have been studied. Seven iron-base alloys with 18% Cr and nickel content varying from zero to 60% were used in the study (0, 10, 20, 30, 40, 50 and 60% Ni). The alloys were submitted to tension tests at room temperature, 350 and 700 ºC; computational thermodynamics simulations; optical metallography observations in annealed state, after deformation at room temperature, 350 and 700 oC, and in quick stop samples; x-ray diffraction, annealed and after deformation at 350 and 700 ºC; hot hardness test at 100, 200, 300 and 400 ºC; Charpy impact tests at room temperature and at -196 ºC; quick stop test; differential thermal analysis tests; Vickers hardness tests: annealed, on chips, after deformation at 350 and 700 ºC. The results of the tension tests at 350 ºC were correlated with the data obtained by Marques (2007), aiming at reaching a relationship between mechanical properties and machinability. The general tendency of reduction in machinability with the increase of nickel content was related to the increase in mechanical strength and hot ductility provided by nickel. The high machinability observed in the 30% Ni alloy was related with the presence of calcium inclusions in this alloy, which allowed for significant gains in machinability, but without loss to the evaluated mechanical properties.
Mestrado
Materiais e Processos de Fabricação
Mestre em Engenharia Mecânica

High grade galvanized steel in large amounts is needed to match the increasing demand of automotive industry both in our country and in the world. Stainless steels, used in fabrication of zinc bath hardware of continuous galvanizing lines, lose their corrosion resistance due to various mechanisms in such mediums containing molten metals like zinc and aluminum. Consequently they corrode to the levels where they should be taken to maintenance or replaced. In this study, corrosion performance and the effect of typical galvanizing and age treating heat treatments to mechanical properties of 4 newly developed austenitic stainless steels and AISI 316L grade stainless steel were investigated and compared with each other. Experimental studies involved immersion corrosion tests for 168 and 504 hours followed by weight loss determinations and comparisons of corrosion performances of age treated and solution annealed stainless steels. Parallel with corrosion testing, delta ferrite content v determinations with 3 different methods, tensile tests and v-notch impact tests at 4 different heat exposure conditions were carried out and discussed. 2 new stainless steel compositions were selected to be used in fabrication of galvanizing hardware based on the comparisons of corrosion &
mechanical performances of candidate steels.

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

Thesis (Ph.D.); Submitted to Iowa State Univ., Ames, IA (US); 19 Dec 2004.
Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2322" Yoon-Jun Kim. US Department of Energy 12/19/2004. Report is also available in paper and microfiche from NTIS.

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

O presente trabalho de pesquisa avalia os efeitos das temperaturas de austenitização plena e intercrítica, associado ao tratamento térmico de têmpera interrompida e partição, realizada em banho de sais fundidos e sua influência no comportamento mecânico e microestrutural do aço de alta resistência e baixa liga AISI300M. Para os ensaios de caracterização mecânica foram realizados ensaios de dureza cujos resultados variaram de (35HRC a 55HRC) dependendo do tratamento; ensaios de tração, que atingiram o limite de resistência máximo de 2272 MPa com 6% de alongamento; ensaios de tenacidade ao impacto Charpy que superaram os tratamentos convencionais de têmpera atingindo 38J de energia absorvida; além dos ensaios de tenacidade à fratura linear-elástica, método (KIC) que ficaram entre (35 e 67 MPa√m). Para a caracterização microestrutural foram realizadas análises por microscopia óptica, microscopia eletrônica de varredura (MEV), além da investigação microestrutural pela técnica de microscopia eletrônica de transmissão (MET) que elucidaram os microconstituintes multifásicos presentes como martensita, ferrita e bainita. Para quantificar as frações de fases presentes utilizou-se a técnica de difração por raios-X e método de refinamento Rietveld, mostrando até 20% de austenita retida na condição de partição em 400ºC. Para auxiliar e mapear as fases presentes foram utilizadas as técnicas de MEV-EBSD e EBSD acoplada ao microscópio eletrônico de transmissão MET-EBSD com indexação via software ASTAR. Após os ensaios mecânicos, as superfícies de fratura foram examinadas via MEV e exibiram em geral um modo misto de fratura om a presença de dimples, coalescimento de microcavidades e quase-clivagem.
The present research work evaluates the effects of the complete and intercritical austenitization temperatures associated with interrupted quenching and partitioning performed in molten salt baths and its influence in the mechanical behavior and microstructural of AISI 300M high strength low alloy steel. Hardness tests with range from 55HRC to 35HRC were obtained depending on the heat treatment. Tensile tests have shown maximum strength limit of 2272 MPa and 6% elongation. Toughness parameters by instrumented Charpy impact tests exceeded values obtained in the conventional heat treatments providing 38J of absorbed energy. In addition, results from linear-elastic fracture toughness tests (KIC), values among 67 and 35MPa√m were found. Microstructure characterization was performed by optical microscopy, scanning electron microscopy (SEM) and microstructural research by transmission electron microscopy (TEM), which optimized the multiphase microconstituents with the presence of martensite, ferrite and bainite. In order to quantify the phases volume fractions present, it was used a X-ray diffraction technique and \"Rietveld\" refinement method that obtained up to 20% of retained austenite in the partitioning condition at 400°C. To assist and map the present phases, it was used MEV-EBSD and EBSD techniques coupled to the transmission electron microscope MET-EBSD with indexing via ASTAR software. After the mechanical tests, fracture surfaces were analyzed by SEM and showed in general, a mixed mode of fracture with the presence of dimples, microcoalescence and quasi-cleavage.

A series of vanadium micro-alloyed high strength low alloy steels were tested to evaluate the effect thermo-mechanical treatments had on their mechanical properties. The properties under consideration were the mechanical strengths and strain ageing propensity, as defined by the standard tensile test, and the fracture mode transition temperature, as defined by the Charpy impact test. Ferrite grain size was also evaluated, as was the portion of the interstitial nitrogen in the steel which was present in the form of a micro-alloy precipitate. Cooling rate was found to be the most significant factor in determining the ferrite grain size of the steel. Steel chemistry was found to be the primary factor in determining precipitation strengthening, this being independent of thermo-mechanical history. Carbon rich vanadium carbo-nitride precipitates in the steel were found to coarsen more rapidly than nitrogen rich precipitates, and a vanadium to nitrogen atomic ratio of less than 2:1 was, therefore, required to avoid loss of precipitation strengthening as a result of this effect. The effect of vanadium carbo-nitride precipitates on the fracture mode transition temperature of the steel was found to be essentially independent of cooling rate. It was dependent on the vanadium concentration of the steel, and on whether the carbo-nitride precipitates were coherent or non-coherent with the ferrite lattice. Strain ageing was found to be significantly reduced (less than 15 MPa) if the atomic ratio of vanadium to nitrogen was greater than 1.8:1. The fine precipitate present in the samples were found to be dissolved by standard acid dissolution nitrogen analysis techniques. As a result of this an alternate analysis technique was developed and shown to be more accurate in determining the concentration of nitrogen rich precipitates in the steel samples.

Whereas considerable progress has been reported on the quantitative estimation of the microstructure of steels as a function of most of the important determining variables, it remains the case that it is impossible to calculate all but the simplest of mechanical properties given a comprehensive description of the structure at all conceivable scales. Properties which are important but fall into this category are impact toughness, fatigue, creep and combinations of these phenomena. The work presented in this thesis is an attempt to progress in this area of complex mechanical properties in the context of steels, although the outcomes may be more widely applied. The approach used relies on the creation of physically meaningful models based on the neural network and genetic programming techniques. It appears that the hot-strength, of ferritic steels used in the powerplant industry, diminishes in concert with the dependence of solid solution strengthening on temperature, until a critical temperature is reached where it is believed that climb processes begin to contribute. It is demonstrated that in this latter regime, the slope of the hot-strength versus temperature plot is identical to that of creep rupture-strength versus temperature. This significant outcome can help dramatically reduce the requirement for expensive creep testing. Similarly, a model created to estimate the fatigue crack growth rates for a wide range of ferritic and austenitic steels on the basis of static mechanical data has the remarkable outcome that it applies without modification to nickel based superalloys and titanium alloys. It has therefore been possible to estimate blindly the fatigue performance of alloys whose chemical composition is not known. Residual stress is a very complex phenomenon especially in bearings due to the Hertzian contact which takes place. A model has been developed that is able to quantify the residual stress distribution, under the raceway of martensitic ball bearings, using the running conditions. It is evident that a well-formulated neural network model can not only be extrapolated even beyond material type, but can reveal physical relationships which are found to be informative and useful in practice.

Les Alliages à Haute Entropie (AHEs ou HEAs en anglais) sont un nouveau type d'alliages multi-élémentaires. Ils contiennent au moins cinq éléments de teneur comprise entre 5 et 35 at %. L'entropie de configuration élevée, qui est une raison du nom de cette famille d'alliages, ainsi que d'autres paramètres, tels que l'enthalpie de mélange, la différence de taille atomique, la différence d'électronégativité ou la concentration d'électrons de valence, stabilisent une solution solide plutôt que des composés intermétalliques. L'attention de la communauté scientifique a été attirée par les propriétés prometteuses et les microstructures intéressantes des HEAs.Dans ce travail, une nouvelle famille de HEAs Al-Cr-Fe-Mn-Mo a été étudiée. Les analyses microstructurales et chimiques ont été menées par DRX, spectrométrie Mössbauer, MEB, MET, EDX, EBSD. Dans un premier temps, des calculs basés sur une approche paramétrique ont été réalisés pour optimiser la composition chimique. Les compositions sélectionnées ont été préparées par mécanosynthèse dans différents types broyeurs. Les conditions optimisées garantissant une homogénéité chimique maximale de la poudre et une faible contamination par les matériaux des billes et des jarres ont été déterminées. Deux phases cubique centrée (cc) se forment pendant la mécanosynthèse avec les paramètres de maille 3,13 Å (cc#1) et 2,93 Å (cc#2). Le traitement thermique de la poudre entraîne plusieurs transformations de phase (la formation de la phase χ). Le recuit à 950 °C/1 h favorise l'augmentation de la fraction volumique de la phase cc#2, tandis que les cc#1 et χ disparaissent. Néanmoins, de petites fractions de carbures et d'oxydes ont été trouvées.Les échantillons massifs ont été fabriqués par frittage à chaud des poudres mécanosynthétisées. Les conditions de consolidation ont été évaluées et optimisées pour favoriser la formation de la phase cc et réduire la formation de carbures et d'oxydes résultant de la contamination. Les échantillons massifs optimisés présentent une phase majoritaire cubique centrée (> 95 % volumique) avec un paramètre de maille de 2,92 Å et une très petite quantité de carbures (M6C, M23C6) et d'oxydes (Al2O3). La phase cc est stable après recuit à 950 °C pendant 10 h. De plus, l'alliage présente une dureté très élevée jusqu'à 950 HV2N. Les essais de compression de l'échantillon massif optimisé, entre la température ambiante et 800 °C, révèlent des propriétés prometteuses, en particulier entre 600 et 700 °C. L'alliage présente un comportement fragile entre la température ambiante et 400 °C. Cependant, l'alliage commence à démontrer un certain degré de plasticité à 500 °C. À 600 °C, la limite d'élasticité est de 1022 MPa, tandis que la déformation à la rupture est d'environ 22 %
High Entropy Alloys (HEAs) are a new type of multicomponent alloys. They contain at least five elements with the content of each between 5 and 35 at. %. The high configuration entropy, which is the source of the name of the whole family of alloys, together with other parameters, such as mixing enthalpy, atomic size difference, electronegativity difference, or valence electron concentration, stabilize a solid solution instead of complex intermetallic compounds. Promising properties and interesting microstructures focus the attention of the scientific community to HEAs.In this work, the novel Al-Cr-Fe-Mn-Mo high entropy alloy family was studied. The microstructural and chemical analyses were performed by XRD, Mössbauer spectrometry, SEM, TEM, EDX, EBSD. In the first stage, parametric approach calculations were carried out to optimize the chemical composition of the alloy. The selected compositions were prepared by mechanical alloying in different devices. The optimized conditions that ensure maximum chemical homogeneity of powder and the small contamination from balls and vial materials were chosen. In most of the powders, two bcc phases form during mechanical alloying with the lattice parameters about 3.13 Å (bcc#1) and 2.93 Å (bcc#2). The heat treatment of powder results in several phase transformations (e.g., the formation of the χ phase). The annealing at 950 °C for 1 h promotes the significant increase of volume fraction of the bcc#2 phase, while the bcc#1 and χ disappear. Nevertheless, small fractions of carbides and oxides were found. The bulk samples were fabricated by hot press sintering of the optimized mechanically alloyed powders. The conditions of consolidation were evaluated and optimized to promote the formation of the bcc phase and reduce the formation of carbides and oxides resulting from the contamination during mechanical alloying and sintering. The optimized bulk samples present a major disordered body-centered cubic phase (> 95 % of volume fraction) with a lattice parameter of 2.92 Å and a very small fraction of carbides (M6C, M23C6) and oxides (Al2O3). The bcc phase is stable after annealing at 950 °C for 10 h. Moreover, the alloy presents very high hardness up to 950 HV2N. The compression tests of the optimized bulk sample from room temperature to 800 °C reveal promising properties, especially between 600 and 700 °C. The alloy shows brittle behavior between room temperature and 400 °C. However, the alloy starts to demonstrate some degree of plasticity at 500 °C. At 600 °C, the yield strength is 1022 MPa, while strain to failure is about 22 %

The usage of cobalt (Co) as binder phase material in cemented carbides has been questioned becauseof the potential health hazards associated with cobalt particle inhalation. Cobalt is used because ofits excellent adhesive and wetting properties, combined with adequate mechanical properties. Thepurpose of this work is to investigate the mechanical properties of Fe-Ni bulk alloys and WC-Cocemented carbides using Integrated Computational Materials Engineering (ICME) methods com-bined with FEM data. The report investigates the mechanical properties of several bulk alloys inthe Fe-Ni system as a function of void size and fraction. FEM indentation and FEM fracture datais interpolated and used to model the hardnessHand fracture toughnessKIc. A precipitationhardening model based on the Ashby-Orowan’s equation is implemented to predict the e↵ect on theyield strength from precipitated particles. A model for solid solution hardening is also implemented.Existing models are used to simulate the properties of WC-Co cemented carbides together with thesolid solution hardening model. Results show that the simulated properties of the Fe-Ni bulk alloysare comparable to those of cobalt. However, the results could not be confirmed due to a lack ofexperimental data. The properties of WC-Co cemented carbides are in reasonable agreement withexisting experimental data, with an average deviation of the hardness by 11.5% and of the fracturetoughness by 24.8%. The conclusions are that experimental data for di↵erent Fe-Ni bulk alloys isneeded to verify the presented models and that it is possible to accurately model the properties ofcemented carbides.
Anv¨andandet av kobolt (Co) som bindefas-material i h°ardmetall har blivit ifr°agasatt som en f¨oljdav av de potentiella h¨alsoriskerna associerade med inhalering av koboltpartiklar. Kobolt anv¨ands p°agrund av dess utm¨arkta vidh¨aftande och v¨atande egenskaper, kombinerat med tillr¨ackliga mekaniskaegenskaper. Syftet med detta arbete ¨ar att unders¨oka de mekaniska egenskaperna hos Fe-Ni bulklegeringarochWC-Co h°ardmetall genom att anv¨anda Integrated Computational Materials Engineering(ICME) metoder kombinerat med FEM-data. Rapporten unders¨oker de mekaniska egenskapernahos flera bulklegeringar i Fe-Ni systemet. FEM-indentering och FEM-fraktur data interpoleras ochanv¨ands f¨or att modellera h°ardheten H och brottsegheten KIc. En modell f¨or utskiljningsh¨ardningbaserad p°a Ashby-Orowans ekvation implementeras f¨or att f¨oruts¨aga e↵ekten p°a brottgr¨ansen av utskiljdapartiklar. ¨Aven en modell f¨or l¨osningsh¨ardning implementeras. Existerande modeller anv¨andsf¨or att simulera egenskaperna hos WC-Co h°ardmetall tillsammans med modellen f¨or l¨osningsh¨ardning.Resultaten visar att de simulerade egenskaperna hos Fe-Ni bulklegeringar ¨ar j¨amf¨orbara medde f¨or kobolt. Dock kan de inte bekr¨aftas p°a grund av avsaknad av experimentell data. Egenskapernahos WC-Co h°ardmetall st¨ammer rimligt ¨overens med existerande experimentell data, meden genomsnittlig avvikelse av h°ardheten med 11.5% och av brottsegheten med 24.8%. Slutsatserna¨ar att det beh¨ovs experimentell data f¨or Fe-Ni bulklegeringar f¨or att kunna verifiera modellernasnoggrannhet och att det ¨ar m¨ojligt att f¨oruts¨aga egenskaperna hos h°ardmetall.

Thesis (M.S. in Applied Physics)--Naval Postgraduate School, September 2006.
Thesis Advisor(s): Craig Smith, Xavier Maruyama. "September 2006." Includes bibliographical references (p. 57-59). Also available in print.

Thesis (M.S. in Mechanical Engineering and Mechanical Engineer)--Naval Postgraduate School, December 1990.
Thesis Advisor(s): Fox, Alan G. "December 1990." Description based on title screen as viewed on April 2. 2010. DTIC Identifier(s): Steel, Microstructure, Mechanical Properties, Copper, Quenching, Tempering, Processing, Naval Vessels, HSLA-100 Steel, Theses, Age Hardening, Modulus of Elasticity, Charpy V Notch Tests. Author(s) subject terms: HSLA-100, Mechanical Properties, Copper Precipitation, Carbide. Includes bibliographical references (p. 66-68). Also available in print.

At the time of the 50th anniversary of the Kohn-Sham method, ab initio calculations based on density functional theory have formed an accurate, efficient, and reliable method to work on the properties of engineering materials. In this thesis, we use the exact muffin-tin orbitals method combined with the coherent-potential approximation to study the mechanical properties of high-technology materials. The thesis includes two parts: a study of long-range chemical order effects and a study of alloying effects on the mechanical properties of alloys. In the first part, we concentrate on the impact of chemical ordering on the mechanical properties. The long range order effect on the elastic constants behaves in a very different way for non-magnetic materials and ferromagnetic materials. For a non-magnetic Cu3Au, the long-range order effect on the elastic constants is very small. The Debye temperature does not show a strong chemical order dependence either. For a ferromagnetic material, on the other hand, the long-range chemical order produces considerable influence on C' in the ferromagnetic state, but negligible effect on C' in the paramagnetic state. The lattice parameter, bulk modulus $B$, and shear elastic constant C44 change slightly with the degree of long-rang order for both magnetic states. The Young's modulus E and the shear modulus G increase significantly with the degree of order in the ferromagnetic state, but the effect becomes weak as the system approaches the random regime.In the second part, the alloying effect of Mn/Ni on the lattice parameter, elastic constants, surface energy, and unstable stacking fault energy of bcc Fe is examined. The calculated results show that the lattice parameter of ferrite Fe is slightly altered upon Ni/Mn alloying the trend of which can be explained by the magnetism-induced pressure. Nickel addition decreases C' but has a negligible effect on C44, whereas manganese addition increases C44 and has a weak influence on C'. In both systems, the bulk modulus B shows a smooth second order variation. On the other hand, the surface energy and the unstable stacking fault (USF) energy decrease by adding Mn or Ni to Fe. For both planar fault energies, Ni shows a stronger effect than Mn. Segregation seems to have a minor effect on the surface and USF energies for dilute Fe-Ni and Fe-Mn alloys. The ductility can be estimated using available physical parameters via traditional phenomenological criteria like the Pugh ratio B/G, the Poisson ratio ν, the Cauchy pressure C12-C44, and the Rice ratio γs/γu .According to dislocation theory, a dislocation can not cross a grain boundary. Therefore, the study of dislocations is assumed to be limited to single-crystals. Several theoretical studies indicate that the cleavage plane is {001} in bcc crystals. Based on these information, we suggest that the resolved single-crystal tensile strength E[001] and the resolved single crystal shear strength G{110}<111> should be used to describe brittle cleavage and dislocation movement rather than polycrystalline parameters such as B and G. We demonstrate that all shear moduli G{lmn}<111> associated with the <111> Burgers vector take the same value 3C44C'/(C'+2C44), which could in fact explain the observed multiple slip in bcc systems. Due to the discrepancy between the resolved single-crystal elastic constants and the averaged polycrystalline elastic constants, the Pugh ratio B/G and the traditional criteria based on polycrystalline elastic constants lead to large differences for magnetic systems. Finally, we propose a new measure of the ductile-bittle behavior based on the ratio σclevage/Gresolved which gives the right experimental trend for Fe-Mn and Fe-Ni system.

QC 20150616

The aim of the thesis is to investigate the alloying and temperature effects on the mechanical properties of body-centered cubic (bcc) random alloys. We employ the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The second-order elastic constants reflect the mechanical properties of materials in the small deformation region, where the stress-strain relations are linear. Beyond the small elastic region, the mechanical properties of defect-free solids are described by the so called ideal strength. These two sets of physical quantities are the major topic of my investigations. In part one (papers I and II), the elastic constants and the ideal tensile strengths (ITS) are investigated as a function of Cr and Ti for the bccV-based random solid solution. We find that alloys along the equi-composition region exhibit the largest shear modulus and Young’s modulus, which is a resultof the opposite alloying effects obtained for the two cubic shear elastic constants C′ and C44. The classical Labusch-Nabarro solid-solution hardening (SSH) model extended to ternary alloys predicts a larger hardening effect in V-Ti than in V-Cr alloy. By considering a phenomenological expression for the ductile-brittle transition temperature (DBTT) in terms of Peierls stress and SSH, we show that the present theoretical results can account for the observed variations of DBTT with composition. Under uniaxial [001] tensile loading, the ITS of V is 12.4 GPa and the lattice fails by shear. Assuming isotropic Poisson contraction, the ITSs are 36.4 and 52.0 GPa for V in the [111] and [110] directions, respectively. For the V-based alloys, Cr increases and Ti decreases the ITS in all principal directions. Adding the same concentration of Cr and Ti to V leads to ternary alloys with similar ITS values as that of pure V. We show that the ITS correlates with the fcc-bcc structural energy difference and explain the alloying effects on the ITS based on electronic band structure theory. In part two (paper III), the alloying effect on the ITS of four bcc refractory HEAs based on Zr, V, Ti, Nb, and Hf is studied. Starting from ZrNbHf, we find that the ITS decreases with equimolar Ti addition. On the other hand, if both Ti and V are added to ZrNbHf, the ITS is enhanced by about 42%. An even more captivating effect is the ITS increase by about 170%, if Ti and V are substituted for Hf. We explain the alloying effect on the ITS based on the d-band filling. We explore an intrinsic brittle-to-ductile transition, which arises due to an alloying-driven change of the failure mode under uniaxial tension. Our results indicate that intrinsically ductile HEAs with high intrinsic strength can be achieved by controlling the proportion of group four elements to group five elements. In part three (papers IV and V), the ITS of bcc ferromagnetic Fe-based random alloys is calculated as a function of compositions. The ITS of Fe is calculated to be 12.6 GPa under [001] direction tension, which is in good agreement with the available theoretical data. For the Fe-based alloys, we predict that V, Cr, and Co increase the ITS, while Al and Ni decrease it. Manganese yields a weak non-monotonic alloying behavior. We show that the previously established ideal tensile strengths model based on structural energy differences for the nonmagnetic V-based alloys is of limited use in the case of Fe-bases alloys, which is attributed to the effect of magnetism. We find that upon tension all investigated solutes strongly alter the magnetic response of the Fe host from the unsaturated towards a stronger ferromagnetic behavior. In part four (paper VI), the temperature effect on the ITS of bcc Fe and Fe0.9Co0.1alloy is studied. We find that the ITS of Fe is only slightly temperature dependent below∼500K but exhibits large thermal gradients at higher temperatures. Thermal expansion and electronic excitations have an overall moderate effect, but magnetic disorder reduces the ITS with a pronounced 90% loss in strength in the temperature interval∼500 - 920K. Such a dramatic temperature effect far below the magnetic transition temperature has not been observed for other micro-mechanical properties of Fe. We demonstrate that the strongly reduced Curie temperature of the distorted Fe lattices compared to that of bcc Fe is primarily responsible for the onset of the drop of the intrinsic strength. Alloying additions, which have the capability to partially restore the magnetic order in the strained Fe lattice, push the critical temperature for the strength-softening scenario towards the magnetic transition temperature of the undeformed lattice. This can result in a surprisingly large alloying-driven strengthening effect at high temperature as illustrated in our work in the case of Fe-Co alloy

QC 20150616

The automobile industry has set the demand regarding Powder Metallurgy (PM) parts for decades, since this near-net shape technology is a cost-effective manufacturing process allying good mechanical properties with dimensional and geometrical precision. Aiming at the future of the electric automobiles high production and demand, many changes are on the way to guarantee the competitiveness of PM against other manufacturing process. The high costs of alloying elements such as Ni and Cu, the changes in health and safety regulations as well as light weighting of components are the topics of major importance in the field of PM and focus of main R&D around the globe. The use of high temperature sintering and different alloying elements are possible solutions to overcome properties obtained by using Ni as an alloying element sintered at conventional temperatures. Materials with Cr, Mo and Si were investigated using high temperature sintering (1180°C and 1250°) in comparison to traditionally high Ni materials sintered at conventional temperature (1120°C). The dimensional stability, geometrical precision, density, and microstructure of ring-shaped specimens were studied by using a coordinate measuring machine (CMM) and the effect of HTS on the mechanical properties were estimated through the fraction of the load bearing section. The effect of HTS on the dimensional precision and geometrical stability was later investigated in real parts manufactured by industrial partners through an EPMA Club Project. The 4%Ni material sintered at 1120°C was also compared to Ni-less/Ni-free materials sintered at 1250°C using tensile testing, impact testing, and hardness. The use of HTS to improve the mechanical properties without impairing the dimensional and geometrical stability was confirmed in parts with both low and high complexity designs. This project sets the blueprint for future material developments using HTS as manufacturing process.

Cold drawn eutectoid steel wires have been widely used for a variety of applications, such as suspension bridges, steel cords for automobile tires, and springs. Much research has been done to increase their mechanical strength. With advances in modern production technology, both the drawing speed and the quality of drawn steels have been enhanced. After a careful literature survey, it is obvious that some issues are still controversial. As Y.S. Yang, J.G. Bae and C.G. Park mentioned, the lamellar spacing, thickness and volume fraction of cementites have all reached the nanometer regime, and the conventional theory is not enough to explain it. Besides, the cementite dissolution is a huge problem to the performance, according to Y.S. Yang and C.G. Park. To address the above issues, a systematic study has been taken on the wire drawing process under the conditions of the industrial production. Through the morphology, microscopic, mechanical and comprehensive analysis, a clear understanding of the microstructures and associated professing conditions of the high-strength carbon steel wires has been obtained. This project aims to clarify why the mechanical properties improve with the increasing strain. The project will be carried out in four stages: 1) characterisation of the microstructure of the cold drawn steel wires; 2) measurements of the modulus, hardness and toughness of steel wires; 3) modelling the deformation behaviour of the cold drawn steels. The techniques involved in the project include X-ray diffraction (XRD), focus ion beam (FIB), scan electron spectrum (SEM), Nanoindentation. A deep understanding of the relationship between composition, structure and performance will be achieved in this project. The results may provide the basis for improving cold-drawn steel wire designs.

Selective laser melting (SLM) of metallic powders is an additive manufacturing technique that is widely employed to produce 3D components, and is fast becoming an important method for manufacturing near-net shape and complex metallic parts. In this thesis, a comprehensive investigation on the effect of SLM on the mechanical and corrosion properties of the Al-12Si (AS), 316L stainless steel (SS), and 18(Ni)-300 grade managing steel (MS) is investigated, with particular emphasis on the developing (micro- as well as mesa-)structure -property correlations. Detailed microstructural characterization combined with quasi-static tensile, fracture toughness, fatigue crack growth, and unmatched fatigue tests were conducted. The effect of post-SLM heat treatment as well as the scanning strategy (linear vs. checker board hatch style) was examined and the results are compared with those of conventionally manufactured (CM) counterparts. The SLM alloys exhibit a mesostructured, in addition to the fine cellular structure along the boundaries. In a case of SLM-AS, the fine cellular structure imparts higher strength at the cost of ductility, while the mesostructured, which arises due to the laser track hatching, causes the crack path to be tortuous, and in turn leads to substantial increase in fracture toughness. This imparts significant anisotropy to the toughness while tensile properties are nearly-isotropic. The experimental results of SLM-SS also show that higher tensile strengths properties with a marked reduction ductility. In spite of these, the fracture toughness, which ranges between 63 and 87 MPa.m0.5, of the SLM-SS is good, which is a result of the mesostructured induced crack tortuousity.Both tensile and toughness properties of SLM-SS were found to be anisotropic in nature. Upon aging SLM-MS, nanoscale precipitation of intermetallic compounds occurs within the cells that, in turn, lead in marked improvements in tensile strengths properties, but substantial reductions in ductility and fracture toughness. Overall, the mechanical performance, except ductility, of the SLM-MS after aging is found to be similar to that of CM-MS. Importantly, the lack of ductility does not lead to a reduction in toughness. Although the SLM-MS alloy possesses a mesostructured, no significant anisotropy in the mechanical behaviour is observed. The unnoticed studies on SLM-AS, -SS, and -MS reveal that the tensile residual stresses, gas-pores, and unmelted powder particles, can degrade the unmatched highest fatigue properties considerably and hence need be eliminated for high fatigue strength. Room temperature, electrochemical corrosion resistances (CRs) of SLM-AS, -SS and -MS in 0.1M NaCl solution were also evaluated and compared with those CM counterparts. While SLM improves CRs of AS and SS, it degrades that of MS. The results are discussed in terms of microstructural refinement and porosity that are common in SLM alloys.

The research work in this thesis involves an experimental and theoretical investigation for high speed machining of AISI 4140 medium carbon steels and AISI D2 tool steels which are classified as being difficult to machine materials. An experimental program was carried out to determine the cutting forces, chip formation, the secondary deformation zone thickness and surface roughness at different cutting speeds using a 0.4mm and 0.8mm nose radii ceramic tools and -7?? rake angle for annealed (virgin) AISI 4140 and heat treated AISI 4140 steel. Another series of experiments was carried out on the annealed (virgin) and heat treated AISI D2 with 0.4mm, 0.8mm and 1.2mm nose radii CBN (Cubic Boron Nitride) tools under various cutting conditions. A theoretical model is developed by taking into account the flow stress properties of the AISI 4140 (0.44% carbon content) to use with the Oxley Machining approach. To find the flow stress data for AISI D2 tool steel, the Johnson and Cook empirical constitutive equation is used as the constitutive model. In addition, the magnitude of tool radius should be also considered to determine the prediction of cutting performances. To account for the effect of nose radius edge in hard machining, a simplified geometrical method is used to model the parameters for application in the Oxley Model and works for the cutting conditions considered here. These extensions to the Oxley machining theory were verified by experimental results. These results show a good agreement between the Oxley machining theory and hard machining experiment at data. The research work described in this thesis provides useful data for hard machining conditions.

This experimental program investigated the effects of cryogenic treatment on the mechanical properties and microstructures of AISI 4340 steel and aluminum alloy 7075. Mechanical tests including fatigue resistance, impact energy and hardness were carried out on both alloys, after they had undergone various heat treating conditions and the results were compared. Experimental results showed that the hardness and fatigue resistance of the cryogenically treated 4340 steel, in general, were a little higher than those of the conventionally treated steel. However, the toughness of the cryogenically treated steel was lower when compared to that of the conventionally treated steel. The quantitative assessment of retained austenite using neutron diffraction technique was also explored. It is concluded that the transformation of retained austenite to martensite is the determining factor which improved hardness and fatigue resistance of the cryogenically treated 4340 steel. In aluminum alloy 7075, the results indicated that after cryogenic processing the fatigue resistance was lower than for conventionally solution and precipitation heat-treated samples. However, the hardness and toughness of the cryogenically treated samples were slightly higher in comparison to the conventional T6 treated specimens. The quantitative optical and electron microscopy has shown an increase in second phases after cryogenic treatment which was in agreement with x-ray diffraction results and electron dispersive spectroscopy (EDS)

It is well known that NiTi alloys are the most widely used shape memory alloys nowadays, with a large number of possible applications, such as in the aerospace, auto motive, medical and civil industries. This large versatility makes them a very difficult alloy to replace. However, there is a group of shape memory alloys which has been receiving ev ermore attention as a candidate to replace NiTi alloys, with many of the same applica tions. These are the Cu-Al-Mn alloys, a lightweight group of alloys recognized for their large versatility and lower cost, while managing to achieve the same or even better results than NiTi in certain scenarios, specially the Cu-17Al-11.4Mn (at. %) alloy. However, little is known about how this alloy’s properties behave when welded to other materials, which leaves a lack of knowledge regarding this alloy’s versatility on this issue. In this dissertation, this alloy laser welded to stainless steel to study the welding process’s effects on it together with the influence of the 316L low carbon stainless steel it is welded to. For this effect, the welded alloy was subjected to microstructural and mechanical testing, such as Scanning Electron Microscopy, Electron Backscatter Diffraction, Energy-Dispersive X-ray Spectroscopy and X-ray Diffraction, as well as Microhardness Mapping, Mechan- ical Cycling and Uniaxile Tensile Testing. This study shows that the welding process heavily influences the microstructural composition of the fusion zone, such as grain size and orientation while, on a mechanical level, also reinforcing the alloy’s superelastic properties together with the steel.
É sabido que as ligas NiTi são as ligas de memória de forma mais amplamente utilizadas atualmente, com o seu grande número de possíveis aplicações, como é o caso nas indústrias aeroespacial, automóvel, médica e civil. Esta grande versatilidade torna estas ligas bastante difíceis de substituir. No entanto, existe um grupo de ligas de memória de forma que tem recebido cada vez mais atenção como candidato a substituir as ligas NiTi, com muitas das mesmas apli cações. Estas são as ligas Cu-Al-Mn, um grupo de ligas leves reconhecidas pela sua grande versatilidade e custo reduzido, capazes de atingir os mesmos ou até melhores resultados do que NiTi em certos cenários, especialmente a liga Cu-17Al-11.4Mn (at. %). No entanto, pouco se sabe sobre como as propriedades desta liga se comportam quando soldadas a outros materiais, deixando uma falta de conhecimento sobre a versatilidade desta liga neste assunto. Nesta dissertação, esta liga foi soldada a laser a um outro material, aço inoxidável, para estudar os efeitos do processo de soldadura nela juntamente com a influência do aço inoxidável ao qual se encontra soldada. Para este efeito, a liga soldada foi sujeita a ensaios microestruturais e mecânicos, como Microscopia Eletrónica de Varrimento, Difração de Eletrodispersão Eletrónica, Espectroscopia de Energia Dispersiva e Difração de raios X, bem como Mapeamento de Microdurezas, ensaios de Ciclagem Mecânica e de Tensão Uniaxial. Este estudo mostra como o processo de soldadura influencia fortemente a composição microestrutural da zona de fusão, como o tamanho de grão e sua orientação enquanto, mecanicamente, também reforça as propriedades superelásticas da liga juntamente com o aço.

碩士
國立虎尾科技大學
材料科學與綠色能源工程研究所
101
Powder metallurgy (PM) steels have been extensively used in the automobile industry. The strength and toughness of PM products are lower than those of wrought ones due to the presence of about 10 vol.% porosity in the PM products. Thus, the method to increase the sintered density is the key issue in the PM field. The aim of this study was to investigate the effect of boron on the liquid phase sintering and sintered density of PM steels. The results showed that, after 1120℃ sintering, the sintered density is only 87%~88%. When the sintering temperature was increased to 1250℃, the sintered density achieve 94%~95%. The influences of Mo, Cr, and Ni on the liquid phase sintering were also examined. Ni is found to increase the temperature for liquid formation. To investigate the alloying distribution, EPMA was used to analyze the alloying diffusion and homogeneity. The results indicated that Ni atom starts to diffuse into the iron powder after 1120℃ sintering. After 1200℃ sintering, the distribution of Ni in the Fe grain is homogeneous. Furthermore, the distributions of Cr, Mo, and C matched that of B. The compositions of boride in various alloy systems are different, as identified by quantitative analyses. The addition of boron can generate many borides with high microhardness of Hv1600~2600. These borides result in the highest apparent hardness of HRC. However, these borides were continous and located primarily at the grain boundary. During tensile test, the intergranular fracture occurs along the network of boride and results in low tensile strength.