Nanoindentation technique was used to investigate the surface properties of A2 and D2 tool steel subjected to different heat treatments. the mechanical characteristics of these two easily available tool steels were studied based on microstructural images obtained from SEM, the grain growth after heat treatment using X-ray diffraction method and nanoindentation technique. the investigation showed that a single nanoindentation result can explain how heat treatment influences reliability and failure in A2 and D2 tool steels. in this work, the causes and effects of these variations were studied to explain how they influence reliability and failure in A2 and D2 tool steel. Finally, a cube-corner indenter tip was used to determine the fracture toughness of silicon wafer. the emphasis of this research is on how nanoindentation technique is more extensive in material characterization.

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The continuous development of hot forming tool steels has resulted in steels with improved mechanical properties. A change in alloying composition, primarily a decreased silicon content, makes them tougher and more wear resistant at elevated temperatures. However, it is at the expense of their machinability. The aim of this study is to explain the mechanisms behind this negative side effect. Hot work tool steels of H13 type with different Si content were characterised mechanically, and evaluated analytically and by dedicated machining tests. Machining tests verified that materials with low Si content displayed reduced machinability due to their stronger tendency to adhere to the cutting edge. Three hypotheses were tested. The first hypothesis, that the improved toughness of the low Si steels is the reason behind their relatively poor machinability, was rejected after machining tests with one low Si steel heat treated to the same relatively low toughness as conventional hot work tool steels. The second hypothesis, that a change in oxidation properties, also associated with the change in Si composition, lies behind the reduced machinability was investigated by dedicated tests and evaluations. It was found that the oxide thickness increased with reduced Si content and that there was an enrichment of Cr at the oxide/steel interface. The differences in oxide thickness and the possible differences in oxidation properties may influence the machinability of the materials through their different abilities to adhere to the cutting edge. The third hypothesis, that a high enough temperature to initiate phase transformation from ferrite to austenite is generated during machining of the tool steels, was also investigated. This may lead to a reduced machinability because higher austenite content is directly related to higher compressive stresses and higher cutting forces. This causes accelerated tool wear. This hypothesis was verified by ThermoCalc calculation of austenite content in the steels, which showed a good agreement with Gleeble compression tests and cutting force measurements. This thesis confirms that a reduced Si content in conventional H13 steel improves the toughness, reduces the oxidation resistance and lowers the ferrite-to-austenite transformation temperature. The reduction in austenite temperature is probably the most important factor behind the reduced machinability.

An increasing number of engineering components are expected to have fatigue life in the range of 107 - 1010 load cycles. Some examples of such components are found in airplanes, automobiles and high speed trains. For many materials fatigue failures have lately been reported to occur well after 107 load cycles, namely in the Very High Cycle Fatigue (VHCF) range. This finding contradicts the established concept of a fatigue limit, which postulates that having sustained around 107 load cycles the material is capable of enduring an infinite number of cycles provided that the service conditions are unchanged. With the development of modern ultrasonic fatigue testing equipment it became possible to experimentally establish VHCF behaviour of various materials. For many of them the existence of the fatigue limit at 107 load cycles has been proved wrong and their fatigue strength continues to decrease with increasing number of load cycles. High performance steels is an important group of materials used for the components subjected to VHCF. This study explores the VHCF phenomenon using experimental data generated by ultrasonic fatigue testing of selected tool steels. The overall aim is to gain knowledge of VHCF behaviour of some common tool steel grades, while establishing a fundamental understanding of mechanisms for crack development in the very long life regime. The study demonstrates that VHCF cracks in tested steels initiate from microstructural defects like slag inclusions, large carbides or voids. It is established that VHCF life is almost exclusively spent during crack formation at below threshold stress intensity values which results in a unique for VHCF morphology on the fracture surface. Significant attention is devoted in the thesis to the ultrasonic fatigue testing technique, i.e. the validity and applicability of its results. FEM is employed to give an additional perspective to the study. It was used to calculate local stresses at fatigue initiating defects; examine the effect of material damping on ultrasonic stresses; and to evaluate various specimen geometries with respect to resulting stress gradient and maximum stressed material volume.

A utilização de aços ferramenta fabricados com tecnologia da metalurgia do pó, bem como seu uso em aplicações que requerem resistência ao desgaste dos materiais em conformação mecânica de ligas, tem sido alvo de estudo e desenvolvimento pela indústria metalmecânica. Através dessa motivação o principal objetivo do trabalho é caracterizar os mecanismos de desgaste em aços ferramenta produzidos por metalurgia do pó (M/P), utilizados em matrizes de estampagem a frio, relacionando a resistência ao desgaste com a microestrutura temperada e revenida dos aços, tratados pelo processo de têmpera a vácuo. Para tanto, foram realizados experimentos utilizando um Forno a vácuo com capacidade de resfriamento de 12 bar, para realizar o processo de tratamento térmico de têmpera e revenimento em amostras dos aços DIN 1.2767, Vanadis® 4, Vanadis® 10, BOHLER S390 e Vancron® 40. As amostras dos aços foram submetidas a ensaios tribológicos para quantificar a resistência ao desgaste em 20N; ensaio de impacto Charpy, ensaio de dureza e a caracterização microestrutural em MEV. Os principais resultados mostraram que o parâmetro de desgaste (W) mostrou melhor desempenho para o aço S390 V, e pior desempenho para os aço DIN 1.2767 nos diferentes acabamentos ensaiados retificados e polidos. Os resultados de caracterização superficial em MEV mostraram o perfil de desgaste da trilha dos aços com melhor e pior desempenho e quais possíveis depósitos superficiais nos materiais após ensaio. As conclusões previas mostraram menor parâmetro (W) de desgaste para o aço ferramenta BOHLER S390 entre os materiais e maior (W) para o aço DIN 1.2767 nas condições de acabamento ensaiadas. / The use of tool steel fabricated with powder metallurgy technology, as well as its use in applications that require resistance to the wear of materials in mechanical alloying, has been studied and developed by the metalworking industry. Through this motivation the main objective of the work is to characterize the wear mechanisms in powder metallurgy (M / P) tool steel used in cold stamping dies, relating the wear resistance with the tempered and annealed microstructure of the steels, Treated by the vacuum quenching process. For this, experiments were carried out using a vacuum furnace with a cooling capacity of 12 bar to perform the tempering and tempering process in samples of DIN 1.2767, Vanadis® 4, Vanadis® 10, BOHLER S390 and Vancron® 40 steels. Samples Of the steels were subjected to tribological tests to quantify the wear resistance in 20N, Charpy impact test, hardness test and microstructural characterization in SEM. The main results showed that the wear parameter W showed better performance for S390 V steel, and worse performance for DIN 1.2767 steel in the different honed and polished tested finishes. The results of surface characterization in SEM showed the wear profile of the track of the steels with better and worse performance and what possible surface deposits in the materials after the test. The previous conclusions showed lower wear parameter W for tool steel BOHLER S390 between materials and higher W for steel DIN 1.2767 in the finishing conditions tested.

<p>Many engineering components reach a finite fatigue life well above 10⁹load cycles. Some examples of such components are found in airplanes, automobiles or high speed trains. For some materials the fatigue failures have lately been found to occur well after 10⁷ load cycles, namely in the Very High Cycle Fatigue (VHCF) range. This finding contradicted the established concept of fatigue limit for these materials, which postulates that having sustained 10⁷ load cycles the material is capable of enduring an infinite number of cycles provided that the service conditions are unchanged. With the development of modern ultrasonic fatigue testing equipment it became possible to experimentally establish VHCF behaviour of various materials. For most of them the existence of the fatigue limit at 10⁷ load cycles has been proved wrong and their fatigue strength continues to decrease with increasing number of load cycles.</p><p> </p><p>One important group of materials used for the production of high performance components subjected to the VHCF is tool steels. This study explores the VHCF phenomenon using experimental data of ultrasonic fatigue testing of some tool steel grades. The causes and mechanisms of VHCF failures are investigated by means of high resolution scanning electron microscopy, and in relation to the existing theories of fatigue crack initiation and growth. The main type of VHCF origins in steels are slag inclusions.</p><p>However, other microstructural defects may also initiate fatigue failure. A particular attention is paid to the fatigue crack initiation, as it has been shown that in the VHCF range crack formation consumes the majority of the total fatigue life. Understanding the driving forces for the fatigue crack initiation is a key to improve properties of components used for very long service lives. Finite element modelling of VHCF testing was added as an additional perspective to the study by enabling calculation of local stresses at the fatigue initiating defects.</p><p> </p><p> </p>

Many engineering components reach a finite fatigue life well above 109 load cycles. Some examples of such components are found in airplanes, automobiles or high speed trains. For some materials the fatigue failures have lately been found to occur well after 107 load cycles, namely in the Very High Cycle Fatigue (VHCF) range. This finding contradicted the established concept of fatigue limit for these materials, which postulates that having sustained 107 load cycles the material is capable of enduring an infinite number of cycles provided that the service conditions are unchanged. With the development of modern ultrasonic fatigue testing equipment it became possible to experimentally establish VHCF behaviour of various materials. For most of them the existence of the fatigue limit at 107 load cycles has been proved wrong and their fatigue strength continues to decrease with increasing number of load cycles.   One important group of materials used for the production of high performance components subjected to the VHCF is tool steels. This study explores the VHCF phenomenon using experimental data of ultrasonic fatigue testing of some tool steel grades. The causes and mechanisms of VHCF failures are investigated by means of high resolution scanning electron microscopy, and in relation to the existing theories of fatigue crack initiation and growth. The main type of VHCF origins in steels are slag inclusions. However, other microstructural defects may also initiate fatigue failure. A particular attention is paid to the fatigue crack initiation, as it has been shown that in the VHCF range crack formation consumes the majority of the total fatigue life. Understanding the driving forces for the fatigue crack initiation is a key to improve properties of components used for very long service lives. Finite element modelling of VHCF testing was added as an additional perspective to the study by enabling calculation of local stresses at the fatigue initiating defects.

The recent development of tool steels and high-speed steels has led to a significant increase in alloy additions, such as Co, Cr, Mo, N, V, and W. Knowledge about the phase relations in these multicomponent alloys, that is, the relative stability between different carbides or the solubility of different elements in the carbides and in the matrix phase, is essential for understanding the behaviour of these alloys in heat treatments. This information is also the basis for improving the properties or designing new alloys by controlling the amount of alloying elements. Thermodynamic calculations together with a thermodynamic database is a very powerful and important tool for alloy development of new tool steels and high-speed steels. By thermodynamic calculations one can easily predict how different amounts of alloying elements influence on the stability of different phases. Phase fractions of the individual phases and the solubility of different elements in the phases can be predicted quickly. Thermodynamic calculations can also be used to find optimised processing temperatures, e.g. for different heat treatments. Combining thermodynamic calculations with kinetic modelling one can also predict the microstructure evolution in different processes such as solidification, dissolution heat treatments, carbide coarsening, and the important tempering step producing secondary carbides. The quality of predictions based on thermodynamic calculations directly depends on the accuracy of the thermodynamic database used. In the present work new experimental phase equilibria information, both in model alloys containing few elements and in commercial alloys, has been determined and was used to evaluate and improve the thermodynamic description. This new experimental investigation was necessary because important information concerning the different carbide systems in tool steels and high-speed steels were lacking. A new thermodynamic database for tool steels and high-speed steels, TOOL05, has been developed within this thesis. With the new database it is possible to calculate thermodynamic properties and phase equilibria with high accuracy and good reliability. Compared with the previous thermodynamic description the improvements are significant. In addition the composition range of different alloying elements, where reliable results are obtained with the new thermodynamic database, have been widened significantly. As the available kinetic data did not always predict results in agreement with new experiments the database was modified in the present work. By coupling the new thermodynamic description with the new kinetic description accurate diffusion simulations can be performed for carbide coarsening, carbide dissolution and micro segregation during solidification. / QC 20100929

A utilização de aços ferramenta fabricados com tecnologia da metalurgia do pó, bem como seu uso em aplicações que requerem resistência ao desgaste dos materiais em conformação mecânica de ligas, tem sido alvo de estudo e desenvolvimento pela indústria metalmecânica. Através dessa motivação o principal objetivo do trabalho é caracterizar os mecanismos de desgaste em aços ferramenta produzidos por metalurgia do pó (M/P), utilizados em matrizes de estampagem a frio, relacionando a resistência ao desgaste com a microestrutura temperada e revenida dos aços, tratados pelo processo de têmpera a vácuo. Para tanto, foram realizados experimentos utilizando um Forno a vácuo com capacidade de resfriamento de 12 bar, para realizar o processo de tratamento térmico de têmpera e revenimento em amostras dos aços DIN 1.2767, Vanadis® 4, Vanadis® 10, BOHLER S390 e Vancron® 40. As amostras dos aços foram submetidas a ensaios tribológicos para quantificar a resistência ao desgaste em 20N; ensaio de impacto Charpy, ensaio de dureza e a caracterização microestrutural em MEV. Os principais resultados mostraram que o parâmetro de desgaste (W) mostrou melhor desempenho para o aço S390 V, e pior desempenho para os aço DIN 1.2767 nos diferentes acabamentos ensaiados retificados e polidos. Os resultados de caracterização superficial em MEV mostraram o perfil de desgaste da trilha dos aços com melhor e pior desempenho e quais possíveis depósitos superficiais nos materiais após ensaio. As conclusões previas mostraram menor parâmetro (W) de desgaste para o aço ferramenta BOHLER S390 entre os materiais e maior (W) para o aço DIN 1.2767 nas condições de acabamento ensaiadas. / The use of tool steel fabricated with powder metallurgy technology, as well as its use in applications that require resistance to the wear of materials in mechanical alloying, has been studied and developed by the metalworking industry. Through this motivation the main objective of the work is to characterize the wear mechanisms in powder metallurgy (M / P) tool steel used in cold stamping dies, relating the wear resistance with the tempered and annealed microstructure of the steels, Treated by the vacuum quenching process. For this, experiments were carried out using a vacuum furnace with a cooling capacity of 12 bar to perform the tempering and tempering process in samples of DIN 1.2767, Vanadis® 4, Vanadis® 10, BOHLER S390 and Vancron® 40 steels. Samples Of the steels were subjected to tribological tests to quantify the wear resistance in 20N, Charpy impact test, hardness test and microstructural characterization in SEM. The main results showed that the wear parameter W showed better performance for S390 V steel, and worse performance for DIN 1.2767 steel in the different honed and polished tested finishes. The results of surface characterization in SEM showed the wear profile of the track of the steels with better and worse performance and what possible surface deposits in the materials after the test. The previous conclusions showed lower wear parameter W for tool steel BOHLER S390 between materials and higher W for steel DIN 1.2767 in the finishing conditions tested.

A utilização de aços ferramenta fabricados com tecnologia da metalurgia do pó, bem como seu uso em aplicações que requerem resistência ao desgaste dos materiais em conformação mecânica de ligas, tem sido alvo de estudo e desenvolvimento pela indústria metalmecânica. Através dessa motivação o principal objetivo do trabalho é caracterizar os mecanismos de desgaste em aços ferramenta produzidos por metalurgia do pó (M/P), utilizados em matrizes de estampagem a frio, relacionando a resistência ao desgaste com a microestrutura temperada e revenida dos aços, tratados pelo processo de têmpera a vácuo. Para tanto, foram realizados experimentos utilizando um Forno a vácuo com capacidade de resfriamento de 12 bar, para realizar o processo de tratamento térmico de têmpera e revenimento em amostras dos aços DIN 1.2767, Vanadis® 4, Vanadis® 10, BOHLER S390 e Vancron® 40. As amostras dos aços foram submetidas a ensaios tribológicos para quantificar a resistência ao desgaste em 20N; ensaio de impacto Charpy, ensaio de dureza e a caracterização microestrutural em MEV. Os principais resultados mostraram que o parâmetro de desgaste (W) mostrou melhor desempenho para o aço S390 V, e pior desempenho para os aço DIN 1.2767 nos diferentes acabamentos ensaiados retificados e polidos. Os resultados de caracterização superficial em MEV mostraram o perfil de desgaste da trilha dos aços com melhor e pior desempenho e quais possíveis depósitos superficiais nos materiais após ensaio. As conclusões previas mostraram menor parâmetro (W) de desgaste para o aço ferramenta BOHLER S390 entre os materiais e maior (W) para o aço DIN 1.2767 nas condições de acabamento ensaiadas. / The use of tool steel fabricated with powder metallurgy technology, as well as its use in applications that require resistance to the wear of materials in mechanical alloying, has been studied and developed by the metalworking industry. Through this motivation the main objective of the work is to characterize the wear mechanisms in powder metallurgy (M / P) tool steel used in cold stamping dies, relating the wear resistance with the tempered and annealed microstructure of the steels, Treated by the vacuum quenching process. For this, experiments were carried out using a vacuum furnace with a cooling capacity of 12 bar to perform the tempering and tempering process in samples of DIN 1.2767, Vanadis® 4, Vanadis® 10, BOHLER S390 and Vancron® 40 steels. Samples Of the steels were subjected to tribological tests to quantify the wear resistance in 20N, Charpy impact test, hardness test and microstructural characterization in SEM. The main results showed that the wear parameter W showed better performance for S390 V steel, and worse performance for DIN 1.2767 steel in the different honed and polished tested finishes. The results of surface characterization in SEM showed the wear profile of the track of the steels with better and worse performance and what possible surface deposits in the materials after the test. The previous conclusions showed lower wear parameter W for tool steel BOHLER S390 between materials and higher W for steel DIN 1.2767 in the finishing conditions tested.