Dissertations / Theses on the topic 'White cast-iron'

Laser-assisted machining has been considered as an alternative for difficult-to-machine materials such as metallic alloys and ceramics. Machining of some materials such as high chromium alloys and high strength steels is still a delicate and challenging task. Conventional machines or computer numerical control (CNC) machines and cutting tools cannot adapt easily to such materials and induce very high costs for operations of rough machining or finishing. If laser-assisted machining can be implemented successfully for such materials, it will offer several advantages over the traditional methods including longer tool life, shorter machining time and reduced overall costs. This thesis presents the results of the research conducted on laser assisted machining of hard to wear materials used in making heavy duty mineral processing equipment for the mining industry. Experimental set up using a high power Nd:YAG laser beam attached to a lathe has been developed to machine these materials using cubic boron nitride (CBN) based cutting tools. The laser beam was positioned so that it was heating a point on the surface of the workpiece directly before it passed under the cutting tool. Cutting forces were measured during laser assisted machining and were compared to those measured during conventional machining. Results from the experiments show that with the right cutting parameters and laser beam position, laser assisted machining results in a reduction in cutting forces compared to conventional machining. A mathematical thermal model was used to predict temperatures within the workpiece at depths under the laser beam spot. The model was used to determine the effect of various cutting and laser parameters on the temperature profile within the workpiece. This study shows that laser assisted machining of hard to wear materials such as high chromium white cast iron shows potential as a possible economical alternative to conventional machining methods. Further research is needed before it can be introduced in industry as an alternative to conventional machining.

Thesis (MEng) - Swinburne University of Technology, Industrial Research Institute Swinburne - 2006.
A thesis submitted in fulfillment of the requirement for the degree of Master of Engineering by Research, Industrial Research Institute Swinburne, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology - 2006. Typescript. Includes bibliographical references (p. 113-116).

In the work that is described in this thesis we studied a new cast iron based composite material that was produced by a double casting technique using sedimentation sand casting. The material is based on the high Cr white cast irons (WCI) 15, 20 % Cr-Mo- LC and 25 % Cr matrixes, according to ASTM A532-75a and was designed to exceed the wear life of wear resistant materials that are used in cement, mining, coal extraction and chemical and process industries currently. The working region of the material extends to a depth of 5-8 mm below the surface and is an in situ and ex situ particle reinforced composite. WC particles of different sizes (1-3 and 3-5 mm) were selected as the ex situ reinforcements. Thus, the composite consisted of a WCI matrix, which was reinforced with WC and other transition metal carbide particles. A sand casting method was developed during which the WC particles were directed to a specific location in the ingot and were distributed uniformly in the near surface area of the composite (the working region of the material) during casting/solidification of the ingot. This ensured chemical bonding between the high Cr WCI and the particle reinforced composite (the working region) at a well defined interface parallel to the working surface. Solidification of the melt started from the WC particles around which three reaction zones were formed. Owing to partial dissolution of the WC particles and the resulting interdiffusion of elements such as W, Co, Fe, C and Cr, carbides containing Fe, Cr, W and Co were formed in the reaction zones. It is shown that current models for the interaction between a reinforcing particle and an advancing S/L interface in liquid route MMCs cannot describe the present case successfully because the solidification of the melt starts around the WC particles after the latter have settled in the near surface region of the casting. Two new approaches have been studied; in the first the WC particles travel through the iron melt and settle at the bottom of the mould and in the second WC particles settled at the bottom of the mould experience the forces of the liquid flow. The solidification paths of the three WCI matrixes and MMCs have been simulated with the Scheil-Gulliver model using the Thermo-Calc software for different iron based alloy systems, by changing the W, Cr and C concentrations. In the WCI, carbides solidify after the austenite. The increase in Cr from 14 to 25 wt. %, for fixed C (at 2, 2.5 or 3 wt. %), causes the stabilization of phases according to cementite M7C3 FCC (stable). Only for Cr up to 30 wt. % and C up to 2.5 wt. % the cementite is replaced by the BCC phase. The increase in C content from 2 to 3 wt. % for fixed Cr (at 14, 20 or 25 wt. %) has the same effect. In the MMC, the increase in W from 15 to 25 wt. % causes the stabilization of M6C. As the C increases from 2.5 to 6 wt. %, first the M7C3 is stabilised and for C > 4 wt. % the MC phase is favoured. The increase in Cr leads in the formation of the M23C6 while simultaneous increase in Cr and C promotes the M7C3 phase and restrains the formation of the M6C that resulted from the increase of W. Finally the simultaneous increase of C, W and Cr stabilize the MC phase for > 4 wt. % C. The wear of the new materials was evaluated both in an industrial scale and in the laboratory (pin-on-disc). Segments of an industrial pulverising ash mill used in the cement industry were manufactured using the casting method and materials developed in this thesis. The results showed an improvement in the wear life of the component of several times compared to the standard high Cr WCI material used by the same industry to date.

O objetivo desta pesquisa é realizar um estudo sobre o desgaste abrasivo do ferro fundido branco alto cromo de acordo com a norma ASTM A532 II D, submetidas à diferentes faixas de temperatura de austenitização, decompondo a faixa de 950 a 1.150 ºC, amplamente usada nas fundições, em intervalos de 50 ºC. Esta liga foi escolhida para estudo em virtude de sua extensa aplicação na indústria de mineração, possuindo como principal característica a utilização em revestimentos de moinhos e martelos de britadores, onde a resistência ao desgaste abrasivo é exigida. Os corpos de prova foram caracterizados através de análise química, dureza, e estrutura metalográfica, avaliando as mudanças na microestrutura, e consequentemente, alterações nas propriedades de resistência ao desgaste abrasivo do material. Na avaliação das amostras, foi utilizada a microscopia ótica e, para simular a condição de desgaste abrasivo, determinando a perda de massa, foi empregando o abrasômetro do tipo roda de borracha, seguindo a norma ASTM G65. Os testes práticos demostraram que o ciclo de temperatura é uma etapa importante na determinação da dureza, que por sua vez, é uma característica fundamental à resistência ao desgaste abrasivo, visto que para o material estudado, quanto maior a dureza, maior foi a resistência ao desgaste abrasivo.
The objective of this research is to perform a study on the abrasive wear of high chromium white cast iron according to ASTM A532 II D, submitted to different austenitizing temperatures, in the range of 950 to 1150 ºC with 50 ºC steps, widely used in foundries. This alloy was chosen for study because of its extensive application in the mining industry, having as main characteristic the use in mill and hammer coatings of crushers where abrasive wear resistance is required. The specimens were characterized by chemical analysis, hardness, and metallographic analysis, evaluating the changes in the microstructure, and consequently changes in the abrasive wear properties of the material. In the evaluation of the samples, optical microscopy was used and, to simulate the abrasive wear condition, determining the mass loss, the rubber wheel type abrasive test was used, following the ASTM G65 standard. The practical tests demonstrated that the temperature cycle is an important step in determining the hardness, which in turn is a fundamental characteristic to the abrasive wear resistance, since for the material studied, the higher the hardness, the higher the resistance to abrasive wear.

No presente trabalho são relatados os resultados obtidos através de tratamentos térmicos e de ensaios de abrasão, em uma liga de ferro fundido branco alto cromo contendo nióbio, obtida por fundição. Os tratamentos térmicos de têmpera foram efetuados nas temperaturas de 950, 1000, 1050 e 1100ºC e o meio de resfriamento utilizado foi o ar soprado. Os tratamentos de revenidos foram efetuados nas temperaturas de 450, 500 e 550ºC. A liga no estado termicamente tratado foi comparada, nos ensaios de abrasão, com ligas comerciais utilizadas como revestimento duro por soldagem em peças desgastadas. Os ensaios de abrasão do tipo pino sobre lixa. Adicionalmente, também foram efetuados estudos de caracterização microestrutural para a identificação das diferentes fases presentes nas ligas. De maneira geral, a liga estudada apresentou as melhores taxas de desgaste para os tratamentos térmicos que resultaram em maiores durezas e o seu desempenho foi superior ao das ligas comerciais.
This work presents the heat treatments and abrasion tests results of a wnite cast iron with niobium alloy. The hardening heat treatment were made from 950, 1000, 1050 and 1100°C temperatures cooled by forced air. The tempering treatment were made at 450, 500 and 550°C temperatures. The heat treating alloy were compared, in the abrasive tests, with commercial alloys used as hardfacing by welding process in wear pieces. The abrasion tests was realized in pin on disk test. Additional tests were carried out for microstructural characterization to identify the differents phases presents in the alloys. In a general way, the alloy studied showed the best wear rate for the heat treatments that resulted in higher hardness. Its performance was superior than that of the commercial alloys.

A constante busca por motores com maior eficiência energética, requer a introdução de novos materiais ou inovações nas rotas de fabricação dos materiais existentes. O presente trabalho busca, a partir de rotas de tratamento térmico, soluções para minimizar o desgaste originado no contato entre corpos metálicos em componentes submetidos a altas pressões de contato em regime lubrificado. A modificação microestrutural a partir de tratamentos térmicos originou matriz martensítica relativamente dura com uma dispersão refinada de nódulos esféricos de grafita, que atuariam como lubrificantes sólidos. O estudo baseou-se em dois ferros fundidos (cinzento e branco) submetidos a tratamentos de austenitização seguidos de tempera em óleo e revenimento em diferentes tempos. Ensaios de dilatometria foram efetuados nos dois materiais para estudar a cinética de grafitização secundaria sob diferentes condições de tratamento térmico. As propriedades mecânicas foram avaliadas por medidas de dureza Vickers das amostras tratadas termicamente. Amostras de ferro fundido cinzento e de branco modificado por tratamentos térmicos foram ensaiadas em teste de desgaste por deslizamento, com intuito de avaliar a influência de modificações na superfície no coeficiente de atrito.
The constant search for engines with higher energy efficiency requires the development of new materials or manufacturing innovations on the routes of existing materials. In the present work, heat treatment routs were investigated to minimize wear originated at the contact between metallic bodies into components subjected to high contact pressures in lubricated regime. Microstructural modification from heat treatments yielded relatively hard martensitic matrix with a fine dispersion of spherical graphite nodules, which act as solid lubricants. The study was based on two cast iron (gray and white) undergoing treatments of soaking (austenitizing) followed by quenching in oil and tempering at different times. Dilatometry tests were performed on both materials to study the kinetics of secondary graphitization under different heat treatment conditions. Mechanical properties were evaluated from Vickers hardness of the thermally treated samples. Samples of gray cast iron and white modified by heat treatments were tested in sliding wear test, in order to assess the influence of changes in the surface friction coefficient.

As ligas de ferro fundido branco alto cromo (FFBAC) da norma ASTM A-532 são comumente utilizadas em peças que necessitam de elevada resistência ao desgaste. Tal escolha acontece devido à elevada dureza apresentada por tais materiais, o que propicia maior vida útil dos componentes. Entretanto, uma vez que o teor de cromo utilizado nos FFBAC possui grande influência no custo das peças, é importante garantir que os tratamentos térmicos (TOTO’s) aplicados estejam otimizados para cada composição de liga. Buscando atuar nesse sentido, esta dissertação analisou a utilização de diferentes tempos e temperaturas de patamar em TOTO’s de desestabilização da austenita para duas ligas da norma ASTM A-532 (ligas com 20% e 25%Cr). Após tais procedimentos, foram estudadas a dureza e a resistência ao desgaste, tanto para algumas das amostras tratadas quanto para seis amostras de FFBAC fornecidas por diversas fundições nacionais. Os resultados demonstraram a existência de uma temperatura que otimiza a dureza da liga, possibilitando incrementos desta propriedade nas amostras tratadas em valores de até 37% (considerando a dureza inicial na condição “bruto de fusão”). Com relação ao tempo de patamar, verificou-se que o mesmo apresentou pouca influência no incremento de dureza, apresentando variações máximas inferiores à 4% para tratamentos variando entre 30 minutos à 6 horas de duração, resultado que destoou daquele indicado pela literatura. Além disso, verificou-se que uma liga com 19,6 %Cr pode apresentar uma dureza até 11% maior do que o verificado para ligas comerciais Nesta comparação, apesar da liga comercial possuir menor teor de carbono, ficou evidenciado o indicativo de que as peças consideradas neste estudo e oriundas de fundições nacionais não otimizam os resultados de seus TOTO’s. A resistência ao desgaste, por sua vez, demonstrou não depender apenas da macrodureza das amostras, mas também de outras características como tamanho, distância e continuidade dos carbonetos primários, o que explica a obtenção de resultados similares de perda de massa no ensaio abrasivo para amostras que possuem durezas diferindo em cerca de 12% porém possuindo tamanho de microconstituintes similares. Por fim, observou-se que ligas com maiores teores de cromo e menores teores de carbono apresentam menores valores de dureza e resistência ao desgaste, o que viabiliza uma aplicação com maior vida útil e menor custo.
The high chromium white cast iron alloys from ASTM A-532 are commonly used in parts which wear resistance are required. This choice happens due the great hardness showed by these alloys, what propitiates a higher lifetime for the components. However, since chromium content has great influence in the cost of the parts, it is important ensure that the apply heat treatments are optimized for each alloy composition. Thus this dissertation analyzed the application of different heat treatment cycles on the austenite destabilization for two ASTM A-532 alloys (20%Cr and 25%Cr). After the heat treatment, hardness, wear resistance and microstructure were analyzed, both for some of the treated samples as for six samples provided by different manufactures in Brazil. The results showed the existence of a temperature that optimizes the alloy hardness, allowing increases on this property in values up to 37% (considering the initial hardness of the “as cast” condition) Concerning the time of heat treatment, was verified that this parameter has low influence in the increase on hardness, showing maximum variations lower than 4% in treatments with times coming from thirty minutes until six hours, results that does not match that one’s indicated by the literature. Besides that, it was verified that an alloy with 19,6%Cr can present a hardness 11% greater than that verified in commercial. In this comparation, although the commercial alloy having lower carbon content, was evidenced that the parts from the Brazilians companies which were considered in this study, does not have an optimum heat treatment. The wear resistance was not only depend of the sample hardness, but also of other characteristics as size, distance and continuity of the primary carbides, which explain the results of similar mass loss in the abrasive test for samples with hardness differing about 12%, but with similar size of microconstituents. Finally, it was observed that alloys with higher chromium and low carbon content shows lower hardness and wear resistance values than alloys with low percentages of chromium, what enable an application with larger lifetime and lower cost.

Neste trabalho, estudou-se a viabilidade da aplicação de revestimentos de ferro fundido branco multicomponente (FFBMC) pelo processo de aspersão térmica por chama hipersônica (HVOF). A melhor condição de aspersão, variando-se a vazão e razão dos gases oxigênio e propano, a distância de aspersão e a granulometria do pó, foi utilizada para aplicação dos revestimentos em substratos de aço e de FFBMC com e sem preaquecimento. Caracterizaramse os revestimentos de FFBMC em relação ao teor de carbono, temperatura final, espessura, porosidade, dureza, aderência, perda de massa, coeficiente de atrito e mecanismo de desgaste abrasivo. Os revestimentos com menor porosidade e maior dureza foram aqueles depositados com razão oxigênio/propano de 4,6, 200 mm de distância de aspersão e granulometria entre 20 e 45 μm. Com esses parâmetros obtiveram-se revestimentos com aderência de 84 MPa, em substratos de aço sem preaquecimento e temperatura final de 200 °C, sendo que o fator mais importante no valor da aderência foi a temperatura final do revestimento. Além disso, a perda de massa foi semelhante aos valores obtidos para amostras de FFBMC fundidas, temperadas e revenidas. Os fatores responsáveis por estes valores foram a eliminação da austenita retida e a sinterização das partículas do revestimento quando as amostras foram temperadas e revenidas. O mecanismo de desgaste predominante foi o microcorte associado à fratura das lamelas próximas das partículas não fundidas, poros de aspersão e dos vazios formados no processo de desgaste. Considerando os níveis de aderência e perda de massa obtidas neste trabalho é viável a aplicação de FFBM por aspersão térmica HVOF.
In this work, the viability of the application of multicomponent white cast iron coatings was studied using high velocity oxygen fuel thermal spray process. The best spray condition, varying the oxygen and propane flow rate, the oxygen/propane ratio, the spraying distance and the powder size, was used for the application of coatings in steel and multicomponent white cast iron substrates with and without preheating. Carbon content, final temperature, thickness, porosity, hardness, adherence, mass loss, friction coefficient and abrasive wear mechanism of the coatings were characterized. The coatings that presented lower porosity and greater hardness were those deposited with oxygen/propane ratio of 4.6, spraying distance of 200 mm and powder size between 20 and 45 μm. With these parameters, coatings with adherence of 84 MPa in steel substrate, without preheating and final temperature of 200 °C, were obtained. The final temperature was identified as the most important factor for the adherence value. Moreover, the coating mass loss was similar to the multi-component white cast iron substrate tempered and annealed. The factors responsible for the high adherence and lower mass loss were the elimination of the retained austenite and the sintering of particles of the coating when the samples were tempered and annealed. The predominant wear mechanism was microcutting associated with the fracture of the lamellae near to unmelted particles, pores of spraying process and the voids formed in the wear process. Considering the levels of adherence and mass loss verified in this work, the application of multicomponent white cast iron coatings using high velocity oxygen fuel thermal spray process is viable.

Este estudo apresenta uma abordagem para a avaliação do comportamento de um ferro fundido branco de alto cromo e um aço ferramenta AISI D2 submetido a ciclos térmicos. Para a realização do estudo foi desenvolvido um procedimento para ensaios de fadiga térmica submetidos a números de ciclos variados, usando para aquecimento um sistema indutivo e para resfriamento um tanque de água. Devido a complexidade dos fenômenos envolvidos, um modelo de elementos finitos foi elaborado para a solução do campo de temperaturas e tensões superficiais atuantes. O controle da tensão superficial atuante foi realizado por meio da mudança de geometria do corpo de prova, sendo esta geometria resultante de teste por simulação computacional. Os corpos de prova foram caracterizados quando a sua microdureza e fração volumétrica de carbonetos antes do ensaio e após ensaio foram caracterizados quanto a sua microdureza, número e profundidade das trincas. O regime de propagação durante os primeiros 50 ciclos foi controlado por mecanismo de fadiga de baixo ciclo, exibindo as maiores velocidade de propagação das trincas e o regime de propagação depois dos 50 ciclos foram controlados por mecanismos de fadiga de alto ciclo. A nucleação das trincas ocorreu predominantemente pela interface matriz/carboneto e pelo próprio carboneto, sendo a propagação das trincas predominantemente pela interconexão de carbonetos fraturados na superfície do material e predominantemente na interface matriz carboneto em camadas mais profundas do corpo de prova. Os resultados obtidos mostraram uma boa correlação entre o ensaio e o modelo numérico, permitindo uma maior confiabilidade para execução do modelo mecânico subsequente. Baseado nos resultados foi possível propor uma metodologia para a avaliação de ambos materiais submetidos a ciclos térmicos.
This study presents an approach to evaluate the behavior of high chromium white cast iron and AISI D2 tool steel submitted at thermal cycles. It was developed a procedure for a thermal fatigue test in different times of cycles were done, using induced heating and water cooling. Due to the complexity of the phenomena involved, a FEM study was performed for solving the temperatures and superficial stresses fields. The control of superficial stress was done by changing the geometry of the test specimens, what was defined by computational simulation. The test specimens were evaluated the microstructure, microhardness and carbides contends before the test. After the test were evaluated microhardness, amount and depth of thermal fatigue cracks. The propagation during the earlier cycles was defined by mechanics of low cycle instead of after 100 cycles that was defined by mechanics of high cycle fatigue. This mechanics was observed by crack velocity analyses in both periods. The nucleation of thermal fatigue cracks initiate mostly at the matrix/carbide interface or at the carbide itself, being the cracks propagation was mainly by the interconnection of fractured carbides at surface and mainly at the interface matrix/carbide in the inner layer of the specimens test. The results obtained showed an adherence between the test and the numerical model, allowed a greater reliability to the subsequent mechanical model. Based on the results was possible propose a methodology for evaluation of both materials subjected a thermal cycles.

Avaliou-se experimentalmente o ferro fundido branco de alto-cromo-molibdênio ao qual foi adicionado nióbio, a respeito de sua temperabilidade. Dilatometricamente traçou-se a curva TTT desse material, bem como a de um ferro fundido branco de alto cromo para efeito comparativo. Constatou-se por essa comparação e pelos resultados da literatura a alta temperabilidade obtida pela liga experimental. A partir da temperatura de austenitização utilizada, podem-se usar taxas de resfriamento de até 0,12°C/s sem transformação de fase, enquanto que na liga comparativa essa taxa aumenta para 5,8°C/s. Foram realizadas análises de desgaste abrasivo, de dureza e microdureza, de difração por raios-X , de fase magnética e de microanálise como forma de caracterizar as transformações de fase observadas por dilatometria. As variações estruturais decorrentes dos tratamentos térmicos realizados nas duas ligas foram acompanhados por microscopia ótica. É indicativo pelo presente trabalho que tal material além de suas características inerentes de resistência à abrasão e à corrosão oferece maior flexibilidade nos projetos que envolvam paredes espessas.
In this work we studied the hardenability of high chromium- molibdenum white cast iron with niobium. By dilatometry we obtained the TTT curve of this material as well as the high chromium white cast iron for comparison. This comparison and available results in literature confirm the high temperability of experimental material. We can use slow rate cooling 0,12°C/s or higher without phase transformation, while in the comparative alloy this rate is around 5,8°C/s or higher from specific used austenitic abrasive temperature. We also realized analysis of abrasive wear, hardness and microhardness, raios-X difraction, magnetic phase and microanalyse for verifing the phase transformation as observed by dilatometry. The structure variation as function of termic treatment in both alloys is monitored by optical microscope. The present work indicate that the material under study can offer more flexibility for projects with thick materials apart from intrinsic characteristics of abrasive wear and corrosion.

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No trabalho foi investigado o efeito da carga e do tamanho da partícula abrasiva na resistência ao desgaste da liga de ferro fundido branco alto cromo (FFBAC) com composição química de 2,9% de carbono e 19,9% de cromo em massa. A microestrutura da liga é composta por uma matriz de martensita revenida a 200 ºC com carboneto eutético. No ensaio abrasivo pino sobre lixa foram utilizadas lixas de sílica e de granada como abrasivo, com três granulometrias distintas: 63 μm, 149 μm e 177 μm, sob a aplicação de duas cargas: 5 N e 10 N. Em abrasão no roda-deborracha utilizou-se como abrasivo a areia normal brasileira (ANB) com três tamanhos médios: 87 μm, 115 μm e 170 μm e três cargas: 44 N, 80 N e 130 N. As superfícies de desgaste foram analisadas com um analisador tridimensional de superfícies e os micromecanismos de desgaste foram caracterizados usando microscopia eletrônica de varredura. Os resultados obtidos nos ensaios mostraram, de maneira geral, um aumento no volume perdido no desgaste com o aumento da carga e do tamanho da partícula abrasiva e permitiram, para a roda-de-borracha, a observação de um tamanho crítico do abrasivo. No ensaio pino sobre lixa de sílica, para o maior tamanho de abrasivo, o volume perdido no desgaste triplicou com o aumento da carga de 5 N para 10 N
In this research, the effects of different applied load and abrasive size on the martensitic high-chromium write cast iron (with a chemical composition: 2,9% carbon and 19,9% chromium) abrasive wear was investigated. The alloy s microstructure consists of: a matrix martensite tempered at 200 ° C with eutectic carbide. Abrasive wear tests using a pin test on silica and garnet paper were carried out using three different abrasives sizes: 63 μm, 149 μm e 177 μm and two applied load: 5 N e 10 N. And the rubber wheel abrasion test (ASTM standard G-65) was carried out using three abrasives sizes: 87 μm, 115 μm e 170 μm and three distinct applied loads: 44 N, 80 N e 130 N. The specimens wear surface was examined by scanning electron microscopy (SEM) for identifying the wear mechanism. The rubbers wheel abrasion test results show that: the mass loss of the write cast iron alloy increases linearly with the increase of particle size until the critical particle size is reached. In pin test on silica paper, to the largest abrasive size, the lost volume in abrasion tripled with applied load increasing of 5 N to10 N. In general, the mass loss increases with the increase of the load applied and the abrasive size

As mudanças microestruturais, causadas pela adição de elementos de liga e pelos tratamentos térmicos, modificam as propriedades mecânicas e tribológicas dos materiais. Foi desenvolvido então um estudo em amostras de um ferro fundido branco de alto cromo com molidbênio e nióbio, procurando determinar os efeitos dos elementos de liga molibdênio e nióbio, na microestrutura deste material, através de medidas de dureza, dilatometria, ataque químico, microscopia eletrônica de varredura e,principalmente, a utilização de metalografia em altas temperaturas. A técnica de metalografia em altas temperaturas permite o estudo das mudanças microestruturais durante os ciclos térmicos, que foram programados, tendo como referência a curva TTT previamente traçada para este material, focalizando regiões onde não ocorrem mudanças de fase. Deste modo foram obtidas as aparências metalográficas em altas temperaturas e feito um estudo comparativo com as outras técnicas usadas. Com isto é possível melhorar o conhecimento sobre a microestrutura do material em questão e obter ligas com propriedades pré-determinadas.
The microstructural changes caused by the addition of alloying elements and by thermal treatments modify the mechanical and tribological properties of the materials. A study was develop on samples of molybdenum and niobium high chromium white cast iron, trying to determine the effects of molybdenum and niobium alloys elements on microstructure of this material through hardness measurements, dilatometry, chemical etching, scanning electron microscopy, and mainly, utilizing metallography at high temperatures. The metallography at high temperatures technic allows the study of microstructural changes during the thermal cyc1es, which were scheduled, having as reference the TTT curve previously traced to this material, focusing regions where changes of fase don\'t occur. So the metallographic appearances at high temperatures were obtained and it was done a comparative study using other technics. Because of that is possible to improve the knowledge of the material microstructure, which it has been studied, and to obtain alloys with predetermined properties.

Ho, Ching Man = 可柔韌的白鑄鐵 / 何靜雯.
Thesis submitted in: November 2007.
Thesis (M.Phil.)--Chinese University of Hong Kong, 2008.
Includes bibliographical references (leaves ).
Text in English; abstracts in English and Chinese.
Ho, Ching Man = Ke rou ren de bai zhu tie / He Jingwen.
Chapter Chapter 1: --- Introduction --- p.1
Chapter 1.1 --- Introduction of Composites --- p.1
Chapter 1.1.1 --- Dispersion-Strengthened composites --- p.2
Chapter 1.1.2 --- True Particulate Composites --- p.2
Chapter 1.1.3 --- Fiber-Reinforced Composites --- p.2
Chapter 1.1.4 --- Laminar Composites --- p.3
Chapter 1.2 --- Mechanical Properties of Metal Matrix Composites --- p.4
Chapter 1.2.1 --- Stress-Strain Test --- p.4
Chapter 1.2.2 --- "Stiffness, Strength and Ductility" --- p.4
Chapter 1.2.3 --- Hardness --- p.5
Chapter 1.3 --- Fabrication of Metal Matrix Composites --- p.5
Chapter 1.3.1 --- Liquid ´ؤ´ؤ State Processing --- p.6
Chapter 1.3.1.1 --- Infiltration Processes --- p.6
Chapter 1.3.1.2 --- Dispersion Processes --- p.7
Chapter 1.3.1.3 --- Spray Processes --- p.7
Chapter 1.3.1.4 --- In - Situ Processes --- p.8
Chapter 1.3.2 --- Solid - State Processing --- p.8
Chapter 1.3.2.1 --- Diffusion Bonding --- p.8
Chapter 1.3.2.2 --- Deformation Processing --- p.9
Chapter 1.3.2.3 --- Deposition Techniques --- p.9
Chapter 1.4 --- Fabrication of Metal Matrix Composites by Spinodal Decomposition --- p.10
Chapter 1.4.1 --- Phase Transformation --- p.10
Chapter 1.4.2 --- Nucleation and Growth --- p.12
Chapter 1.4.2.1 --- Kinetics of Nucleation and Growth --- p.12
Chapter 1.4.2.2 --- Phase Separation by Nucleation and Growth --- p.14
Chapter 1.4.3 --- Spinodal Decomposition --- p.14
Chapter 1.4.3.1 --- Phase Separation by Spinodal Decomposition --- p.14
Chapter 1.4.3.2 --- The Diffusion Equation for Spinodal Decomposition --- p.15
Chapter 1.4.4 --- Methods to obtain large undercooling --- p.17
Chapter 1.5 --- Aim of This Project --- p.18
Chapter Chapter 2: --- Experimental --- p.26
Chapter 2.1 --- Preparation of fused silica tube --- p.26
Chapter 2.2 --- Preparation of Sample --- p.26
Chapter 2.2.1 --- Weighing and Alloying --- p.26
Chapter 2.2.2 --- Fluxing --- p.27
Chapter 2.3 --- Slow Cooling --- p.28
Chapter 2.4 --- Microstructure Analysis --- p.28
Chapter 2.4.1 --- Optical Microscope (OM) Analysis --- p.28
Chapter 2.4.2 --- Sample Preparation for Scanning Electron Microscope (SEM) Analysis --- p.29
Chapter 2.4.3 --- Sample Preparation for Transmission Electron Microscope (TEM) Analysis --- p.29
Chapter 2.4.3.1 --- Specimen Requirement --- p.29
Chapter 2.4.3.2 --- "Cutting, Grinding and Polishing" --- p.30
Chapter 2.4.3.3 --- Ion Milling --- p.31
Chapter 2.5 --- Microstructure Characterization by TEM --- p.31
Chapter 2.5.1 --- Indexing Diffraction Pattern --- p.31
Chapter 2.5.2 --- Energy Dispersive X-Rav (EDX) Analysis --- p.32
Chapter 2.6 --- Mechanical Properties --- p.33
Chapter 2.6.1 --- Hardness Testing --- p.33
Chapter 2.6.2 --- Compression Testing --- p.33
Chapter 2.7 --- Characterizations of Non-spinodal Samples --- p.34
Chapter Chapter 3: --- Study of the Relationship between Microstructures and Undercooling of Fe81C14Si5 --- p.41
Chapter 3.1 --- Abstract --- p.41
Chapter 3.2 --- Introduction --- p.42
Chapter 3.3 --- Experiment --- p.42
Chapter 3.4 --- Results --- p.44
Chapter 3.5 --- Discussion --- p.46
Chapter 3.6 --- Conclusion --- p.46
Chapter Chapter 4: --- Ductile white cast iron --- p.56
Chapter 4.1 --- Abstract --- p.56
Chapter 4.2 --- Introduction --- p.57
Chapter 4.3 --- Experimental --- p.58
Chapter 4.4 --- Results --- p.60
Chapter 4.5 --- Discussions --- p.66

博士
國立臺灣大學
機械工程學研究所
101
This research studied the difference of wear resistance between high chromium white cast iron and multi-component white cast iron under two optimum conditions (including optimal alloy composition and optimal heat treatment) by using Pin-On-Disk wear test and solid particle erosion test. The composition of cast iron has a close relationship with its phase change of mold status and it directly influences its wear resistance and erosion properties. The test results found that wear resistance of high chromium white cast iron has linear relationship with testing time. There were obvious scratching grooves on the surface of specimen and various sheared pits. The depth of pits is about 4.5~8.0 mm by observing its sub surface. Crack deformation path after quenching was observed clearly at the martensite crystalline grain boundaries .After tempering treatment, Toughness was increased by changing its state to tempered martensite. Tempered martensite has a better resistance toward the formation of crack. The deformation path occurs only along the martensite grain boundaries and has no any sign of spreading .On the other hand , the wear resistance of multi-component white cast iron showed non-linear relationship with testing time. There were obvious shallow scratches on the surface of specimen. The depth of pits is about 4.0 mm by observing its sub surface. After tempering (570¬oC) and erosion treatment, the hardness of alloy was lowered, however, the larger matrix, is able to resist ductile fracture. As the amount of tungsten (W) is low (2.15%), thus the amount of secondary carbide separated out is not much. It implies that the amount of hole on the surface of secondary carbide is smaller and able to resist brittle fracture. Therefore, no obvious deformation path was observed. The deformation path occurs only in crater region and has no any sign of spreading. The establishment of phase diagram is important in expand the application scope of high chromium white cast iron and multi-component white cast iron. Thus , we have conducted high temperature differential scanning calorimeter (DSC) using a sample from our designated alloy and measured the temperature point of phase change using thermal analysis .The experimental results are also consistent with the simulation results by commercial software Thermo Calc. Keywords：High chromium white cast iron, multi-component white cast iron, phase diagrams.

Chow, Wing Hei = 白鑄鐵特殊的微結構--網絡圖案 / 周穎熙.
Thesis submitted in: November 2007.
Thesis (M.Phil.)--Chinese University of Hong Kong, 2008.
Includes bibliographical references.
Text in English; abstracts in English and Chinese.
Chow, Wing Hei = Bai zhu tie te shu de wei jie gou -- wang luo tu an / Zhou Yingxi.
Abstract --- p.i
Acknowledgements --- p.iii
Chapter Chapter 1 --- Introduction
Chapter 1.1 --- Introduction --- p.1
Chapter 1.2 --- Background of Thermodynamic --- p.3
Chapter 1.3 --- Nucleation in pure metals --- p.4
Chapter 1.4 --- Introduction of Growth --- p.7
Chapter 1.5 --- Solidification of the Single Phase Binary Alloy --- p.9
Chapter 1.6 --- Solidification of the Binary Eutectic Alloy --- p.12
Chapter 1.7 --- Phase Separation by Nucleation and Growth --- p.15
Chapter 1.8 --- How to obtain large undercooling --- p.19
References --- p.20
Figures --- p.22
Chapter Chapter 2 --- Experiment
Chapter 2.1 --- Experimental Preparation --- p.31
Chapter 2.2 --- Experimental Procedures --- p.32
Chapter 2.3 --- Sample Analysis --- p.33
References --- p.36
Figures --- p.37
Chapter Chapter 3 --- Network patterns in a white cast iron
Result --- p.38
Reference --- p.58
Figures --- p.64

碩士
國立臺灣大學
機械工程學研究所
96
The objective of this study is to obtain the optimal heat treatment conditions (tempering treatment in specific) for achieving the required wear resistant property for both high-Cr (~15%Cr) cast iron used in the outer shell of bi-metal rolls and multi-component white cast iron used in hot milling rolls. The experimental results indicate that the quenched and tempered microstructures consist of temper martensite (contains numerous fine global secondary M7C3 carbides), eutectic M7C3 carbides, and retained austenite. The amount of retained austenite decreases with increasing tempering temperature, and the amount is reduced to below 3% as the tempering temperature exceeds 500oC. In addition, the hardness value increases first with increasing tempering temperature, reaches peak hardness at around 500oC, and then decreases with further increase in tempering temperature. Furthermore, the change in wear resistance of the treated alloys as affected by tempering temperature shows a reverse trend as that of hardness. Finally, the optimal heat treatment condition has been obtained to be: Destabilization: (1000~1050)oC-2hr/ Quench: forced air cooling/ Tempering: 500oC-6hr. For multi-component white cast irons, minor modifications of C and W contents from the base alloy composition of Fe-2%C-5%Cr-5%Mo-6%V-2%Co-2%W were made to evaluate their effects on the response of heat treatment and wear resistance property. The results show that the peak tempered hardness (HTmax) was achieved when tempered within 525~545oC with minor variations in different alloys. In addition, the occurrence of the minimum wear rate (Rw) coincides with the HTmax and nil retained austenite (Vγ) in the tempered state. Furthermore, an improved wear resistance was obtained by increasing austenitizing temperature, or by reducing the W content, or by increasing the C content. Finally, the optimal heat treatment condition has been obtained to be: Austenitization: 1100oC-70min/ Quench: forced air cooling/ Tempering: 525~545oC-3hr.