Este trabalho está inserido em um projeto que procura estudar a viabilidade técnica da aplicação de um relativamente novo conceito de tratamento térmico, chamado de Têmpera e Partição (T&P), como alternativa para o processamento de ferros fundidos nodulares com alta resistência mecânica. O processo T&P tem por objetivo a obtenção de microestruturas multifásicas constituídas de martensita e austenita retida, estabilizada em carbono. A martensita confere elevada resistência mecânica, enquanto a austenita confere ductilidade. No processo T&P, após a austenitização total ou parcial da liga, o material é temperado até uma temperatura de têmpera TT entre as temperaturas Ms e Mf para produzir uma mistura controlada de martensita e austenita. Em seguida, na etapa de partição, o material é mantido isotermicamente em uma temperatura igual ou mais elevada (denominada temperatura de partição TP) para permitir a partição de carbono da martensita para a austenita. O carbono em solução sólida diminui a temperatura Ms da austenita, estabilizando-a à temperatura ambiente. O presente trabalho procurou estudar aspectos de transformações de fases -- com ênfase na evolução microestrutural e cinética das reações -- do tratamento térmico de Têmpera e Partição (T&P) aplicado a uma liga de ferro fundido nodular (Fe-3,47%C-2,47%Si-0,2%Mn). Tratamentos térmicos consistiram de austenitização a 880 oC por 30 min, seguido de têmpera a 140, 170 e 200 oC e partição a 300, 375 e 450 oC por até 2 h. A caracterização microestrutural foi feita por microscopia óptica (MO), eletrônica de varredura (MEV), difração de elétrons retroespalhados (EBSD) e análise de microssonda eletrônica (EPMA). A análise cinética foi feita por meio de ensaios de dilatometria de alta resolução e difração de raios X in situ usando radiação síncrotron. Resultados mostram que a ocorrência de reações competitivas -- reação bainítica e precipitação de carbonetos na martensita -- é inevitável durante a aplicação do tratamento T&P à presente liga de ferro fundido nodular. A cinética da reação bainítica é acelerada pela presença da martensita formada na etapa de têmpera. A reação bainítica acontece, a baixas temperaturas, desacompanhada da precipitação de carbonetos e contribui para o enriquecimento em carbono, e consequente estabilização, da austenita. Devido à precipitação de carbonetos na martensita, a formação de ferrita bainítica é o principal mecanismo de enriquecimento em carbono da austenita. A microssegregação proveniente da etapa de solidificação permanece no material tratado termicamente e afeta a distribuição da martensita formada na etapa de têmpera e a cinética da reação bainítica. Em regiões correspondentes a contornos de célula eutética são observadas menores quantidades de martensita e a reação bainítica é mais lenta. A microestrutura final produzida pelo tratamento T&P aplicado ao ferro fundido consiste de martensita revenida com carbonetos, ferrita banítica e austenita enriquecida estabilizada pelo carbono. Adicionalmente, foi desenvolvido um modelo computacional que calcula a redistribuição local de carbono durante a etapa de partição do tratamento T&P, assumindo os efeitos da precipitação de do crescimento de placas de ferrita bainítica a partir da austenita. O modelo mostrou que a cinética de partição de carbono da martensita para a austenita é mais lenta quando os carbonetos precipitados são mais estáveis e que, quando a energia livre dos carbonetos é suficientemente baixa, o fluxo de carbono acontece da austenita para a martensita. A aplicação do modelo não se limita às condições estudadas neste trabalho e pode ser aplicada para o planejamento de tratamentos T&P para aços. / The present work belongs to a bigger project whose main goal is to study the technical feasibility of the application of a relatively new heat treating concept, called Quenching and Partitioning (Q&P), as an alternative to the processing of high strength ductile cast irons. The aim of the Q&P process is to obtain multiphase microstructures consisting of martensite and carbon enriched retained austenite. Martensite confers high strength, whereas austenite confers ductility. In the Q&P process, after total or partial austenitization of the alloy, the material is quenched in a quenching temperature TQ between the Ms and Mf temperatures to produce a controlled mixture of martensite and austenite. Next, at the partitioning step, the material is isothermally held at a either equal or higher temperature (so called partitioning temperature TP) in order to promote the carbon diffusion (partitioning) from martensite to austenite. The present work focus on the study of phase transformations aspects -- with emphasis on the microstructural evolution and kinetics of the reactions -- of the Q&P process applied to a ductile cast iron alloy (Fe-3,47%C-2,47%Si-0,2%Mn). Heat treatments consisted of austenitization at 880 oC for 30 min, followed by quenching at 140, 170, and 200 oC and partitioning at 300, 375 e 450 oC up to 2 h. The microstructural characterization was carried out by optical microscopy (OM), scanning electron microscopy (SEM), backscattered diffraction (EBSD), and electron probe microanalysis (EPMA). The kinetic analysis was studied by high resolution dilatometry tests and in situ X-ray diffraction using a synchrotron light source. Results showed that competitive reactions -- bainite reaction and carbides precipitation in martensite -- is unavoidable during the Q&P process. The bainite reaction kinetics is accelerated by the presence of martensite formed in the quenching step. The bainite reaction occurs at low temperatures without carbides precipitation and contributes to the carbon enrichment of austenite and its stabilization. Due to carbides precipitation in martensite, growth of bainitic ferrite is the main mechanism of carbon enrichment of austenite. Microsegregation inherited from the casting process is present in the heat treated material and affects the martensite distribution and the kinetics of the bainite reaction. In regions corresponding to eutectic cell boundaries less martensite is observed and the kinetics of bainite reaction is slower. The final microestructure produced by the Q&P process applied to the ductile cast iron consists of tempered martensite with carbides, bainitic ferrite, and carbon enriched austenite. Additionally, a computational model was developed to calculate the local kinetics of carbon redistribution during the partitioning step, considering the effects of carbides precipitation and bainite reaction. The model showed that the kinetics of carbon partitioning from martensite to austenite is slower when the tempering carbides are more stable and that, when the carbides free energy is sufficiently low, the carbon diffuses from austenite to martensite. The model is not limited to the studied conditions and can be applied to the development of Q&P heat treatments to steels.

Abstract not available

With the increased interest in bainitic steels, fundamental understanding of the bainite transformationis of major importance. Unfortunately, the research on bainite has been hampered by an oldcontroversy on its formation mechanism. Over the years two quite different theories have developedclaiming to describe the bainite transformation i.e. the diffusionless and the diffusion controlledtheory. In this thesis, attention is directed towards fundamental understanding of the bainitetransformation and both experimental and theoretical approaches are used in order to reveal its truenatureIn the first part of this thesis the symmetry in the Fe-C phase diagram is studied. It is based on ametallographic mapping of microstructures using light optical microscopy and scanning electronmicroscopy in a high carbon steel. The mapping revealed symmetries both with respect to temperatureand carbon content and an acicular eutectoid with cementite as the leading phase was found andidentified as inverse bainite. By accepting that all the eutectoid microstructures forms by diffusion ofcarbon, one may explain the existence of symmetries in the Fe-C phase diagram. Additional supportof its existence is obtained from an observation of symmetries in an alloyed steel. From the performedwork it was concluded that the existence of symmetries among the eutectoid microstructures fromaustenite supports the idea that bainite forms by a diffusion controlled transformation.In the second part the growth of bainite is considered. An experimental study using laser scanningconfocal microscopy was performed and growth rates of the transformation products from austenite ina high carbon, high chromium steel was analysed. The growth rate measurements reveals the kineticrelation between Widmanstätten cementite and the acicular eutectoid previously identified as inversebainite which confirms its existence and the conclusions drawn in the first part. In addition, in-situobservations of bainite formation below Ms provide additional support for the diffusion controlledtheory for bainite formation.The final part of the work is a study of the critical conditions for the formation of acicular ferrite.Based on experimental information found in the literature a thermodynamic analysis is performed inview of the two theories. The results demonstrate that the governing process for Fe-C alloys cannot bediffusionless but both kinds of processes can formally be used for predicting Bs temperatures for Fe-Calloys. / <p>QC 20130503</p>

The phenomenon of hydrogen embrittlement in steel has been known for over 150 years. Hydrogen-resistant alloys have been developed to mitigate this effect and three types of alloys with optimised structures have been enhanced over the years: nickel alloys, stainless steels, and quenched and tempered martensitic low alloy steels. Nevertheless, those alloys are limited in terms of strength and ductility. The aim of the work presented in this thesis was to design bainitic alloys with hydrogen embrittlement resistance, and with a better combination of strength and ductility than conventional alloys. In the novel alloys, two microstructural features were produced to mitigate the damaging effects of hydrogen: 1. A percolating austenite structure, in which hydrogen diffusion is orders of magnitude lower than in bainitic ferrite. This feature was introduced to impede the ingress of hydrogen through the structure. 2. Iron carbide traps, which can form at the bainite transformation temperature. This feature was introduced to trap diffusible hydrogen and prevent it from causing damage. The alloys, designed with the aid of computer models and phase transformation theory, contained a volume fraction of retained austenite above its percolation threshold, theorised as 0.1, which was proven to form an effcient barrier to hydrogen ingress. The effective diffusivity of hydrogen, measured using an electrochemical permeation technique, was shown to decrease with increasing austenite fraction up to the percolation threshold. It was seen to plateau for austenite fractions comprised between 0.1 and 0.18, and to decrease further for fractions above 0.18. The compositions of the alloys were precisely selected to allow for iron carbides to precipitate during the bainitic transformation reaction. Until the present work, only alloy carbides V4C3, TiC and NbC had been reported to strongly trap hydrogen. The literature was very inconsistent regarding the trapping ability of cementite, with reported trap binding energies ranging from 11 to 66 kJ mol−1. The carbides produced in the alloys were identified as cementite. The cementite fraction was measured to be 0.001 ± 0.0001 for one of the designed alloys, which is the lowest ever reported carbide fraction in steel measured using a simple X-ray diffraction technique. Experimental thermal desorption spectroscopy data were used to determine the binding energy of hydrogen to cementite to be 37.5 kJ mol−1, suggesting that cementite is not a strong hydrogen trap. Further tests performed after room temperature hydrogen degassing displayed insignifcant amount of trapped hydrogen, thus confrming the reversible nature of cementite traps. The comparison of two successive transients using the electrochemical permeation technique confirmed that result. The influence of the heat treatments on the microstructures and on the mechanical properties of the designed alloys was extensively studied. The novel alloys met all the set requirements, and successfully outperformed conventional alloys in terms of strength and ductility. They did not meet the NACE TM0316-2016 standard requirement for operation in hydrogen-rich environments, likely owing to the inadequate trapping ability of cementite. Future work should focus on exploring the possible use of alternative carbides for hydrogen trapping in bainitic structures.

This research aims to model the microstructure development of Si-containing bainitic free-machining steel, including allotriomorphic ferrite, idiomorphic ferrite, pearlite, Widmanstatten ferrite, bainite and martensite. The effect of recalescence has been included to give a better estimation of the cooling curve under natural cooling conditions. A model for estimating retained austenite size distribution in the carbide-free bainitic microstructure has been developed. Manganese sulphide particles are used in the free-machining steel to break chips during machining; its effect on the prior austenite grain size has been investigated, taking account of the sulphide shape. The theories of all the major solid state phase transformations involved in steel are reviewed in chapter 2. The theory of the simultaneous transformation model is presented in chapter 3.uu A recalescence model dealing with the heat of reaction has been developed in chapter 5 for bar-shaped products. The model is based on the integration of a heat transfer model, considering latent heat generation, into the simultaneous transformation framework. It has been found that latent heat can greatly affect the transformation, especially in the case of pearlite and Widmanstatten ferrite. Chapter 6 presents the model for estimating the size distribution of retained austenite regions. The model builds on the random division of an austenite grain by bainite sheaves, which means the sizes of the two new compartments generated by the division of an austenite grain by a bainite sheaf are allocated randomly. The next compartment to be divided is also chosen at random. Good agreement between prediction and experiment has been achieved for high carbon carbide-free bainitic microstructures. The transition temperature from upper to lower bainite is modelled in chapter 7. The model compares the time required for decarburising a supersaturated bainitic ferrite platelet and that for cementite precipitation within the ferrite platelet. Manganese, silicon and chromium are considered in the model. It is suggested that carbon and manganese favour lower bainite, whereas silicon promotes upper bainite. The effect of manganese sulphide particles on austenite grain boundary motion has been studied in chapter 8. These rod-shaped particles span many austenite grains; the result shows that the long rod-shaped particles are more effective in pinning the austenite grain boundary than spheres of the same volume, or even strings of identical spheres with the same total volume. Experimental work is presented in chapters 9 and 10. In situ synchrotron X-ray study of the bainite transformation reveals that the distribution of carbon in the residual austenite becomes heterogeneous as transformation progresses. Low carbon regions transform preferentially into martensite during cooling after isothermal bainite transformation. The partitioning of carbon was found to lag behind the bainite transformation; more time is needed as the transformation temperature is reduced. Tetragonality was not observed in either the bainitic ferrite or martensite, because the carbon content of the alloy is relatively low, and the Zener ordering temperature is below the bainite and martensite transformation temperature. No significant difference was observed in the kinetics of bainite transformation between the high sulphur and low sulphur steel.

Diversos componentes mecânicos como engrenagens, rolamentos, cilindros de laminação, trilhos e rodas de trem sofrem uma determinada solicitação conhecida por fadiga de contato, que consiste em uma solicitação causada pela tensão gerada entre o contato de dois corpos sob a ação de uma carga cíclica. Este trabalho teve como objetivo avaliar a resistência a esta solicitação para um aço ferramenta com microestruturas martensíticas e bainíticas. Para isto, foi utilizado um equipamento na configuração esfera contra plano, onde as esferas foram confeccionadas em material comercial ABNT 52100 e os discos em aço ferramenta tratados termicamente para obtenção de microestruturas martensíticas e bainíticas. Os ensaios foram realizados com a aplicação de uma pressão máxima de contato de 4,8 GPa até que ocorresse a falha por microlascamento da superfície do disco. Os resultados de vida em fadiga destes materiais foram apresentados por uma distribuição de Weibull que demonstraram uma maior resistência do material bainítico. Os resultados apresentados para ambas as microestruturas demonstraram que as falhas se iniciaram predominantemente na sub-superfície, atendendo as premissas iniciais do trabalho que buscava uma baixa ocorrência de falhas superficiais prematuras ocasionadas por possíveis inclusões, defeitos ou poros superficiais que agiriam como um concentrador de tensões, levando a uma falha antes que efetivamente as características físicas e metalúrgicas das microestruturas sejam testadas. / Several mechanical components such as gears, bearings, rolling mill rolls, rails and train wheels suffer a particular request known to contact fatigue, which consists of a request caused by the tension generated between the contact of two bodies subjected to a cyclic loading. This study aimed to evaluate the resistance to this request for tool steel with bainitic and martensitic microstructures. For this, was used a machine in the configuration ball against flat washer, where balls were made of commercial material ABNT 52100 and flat washer in tool steel heat-treated to obtain martensitic and bainitic microstructures. The tests were performed by applying a maximum contact pressure of 4.8 GPa until the failure by micro-spalling on the disk surface or subsurface. The results of the fatigue life of those materials were presented by a Weibull distribution that showed a better resistance to the bainitic material to this request. The results presented for both microstructures showed that the failures were initiated predominantly sub-surface, given the initial goals of the work, as evidenced by the low occurrence of premature superficial failures possibly caused by inclusions, pores or surface defects that would act as a stress concentrator leading to a premature failure before the physical and metallurgical microstructures characteristics were really tested.

Foram realizados ensaios de dilatometria em ligas não comerciais que apresentam microestrutura bainítica após o resfriamento contínuo. As variações de composição química foram realizadas sobre o aço destinado para construção mecânica AISI 5120 com adições de molibdênio, boro e nióbio. Os ensaios foram conduzidos no dilatômetro com atmosfera e temperatura controlada. No dilatômetro, foram aplicados resfriamentos contínuos em diferentes velocidades a partir da temperatura em que normalmente peças forjadas são reaquecidas. Tradicionalmente, a classe dos aços apresentados neste trabalho é exposta ao tratamento térmico de têmpera e revenimento e apresentam microestrutura martensítica. Com o objetivo de eliminar o tratamento térmico realizado pós conformação, foi proposto como substituição os aços bainíticos. Aços bainíticos não exigem tratamento térmico pós conformação e, apenas com a aplicação de um resfriamento controlado, é possível obter uma microestrutura que apresenta propriedades (tensão de escoamento e tenacidade) iguais ou melhoradas em relação ao material temperado e revenido. As microestruturas obtidas nas diferentes ligas resfriadas de maneira contínua foram caracterizadas de modo a estabelecer relações entre a velocidade de resfriamento e produtos formados, morfologia e fração de microconstituíntes. A caracterização microestrutural foi realizada de maneira intensiva de modo a relacionar desde propriedades magnéticas com padrões de difração de raios X das amostras para medição da fração de austenita retida. O objetivo deste trabalho foi investigar qual é a influência do molibdênio, boro e nióbio no resfriamento contínuo de aços bainíticos, bem como estabelecer o intervalo de velocidades de resfriamento em que é possível obter de maneira homogênea a estrutura bainítica. Após o resfriamento, os corpos de prova foram caracterizados por metalografia (microscopia óptica e eletrônica de varredura), dureza, saturação magnética, difração de raios x e EBSD. De fato foi verificado o efeito do molibdênio, boro e nióbio na cinética de decomposição da austenita no resfriamento contínuo e estabelecido relações entre a microestrutura obtida, velocidade de resfriamento e composição química. Foi observado também o efeito do molibdênio, boro e nióbio em evitar a transformação ferrítica para baixas velocidades de resfriamento de modo a obter uma estrutura bainítica sob um maior intervalo de resfriamento. / Dilatometry tests were carried out in a non commercial alloy that showed bainitic microstructure after continuous cooling from the austenitization temperature. The chemical composition variations were performed on a base chemical composition of a commercial steel (AISI 5120), additions were of molybdenum, boron and niobium. The tests were conducted on the dilatometer with atmosphere and temperature control. In the dilatometer, continuous cooling was carried out at different rates from the temperature in which the reheating of forged parts is usually performed. Traditionally, the steels used for this application are quenched and tempered and present a predominantly tempered martensite microstructure; bainitic steels were proposed as a substitution in order to eliminate further heat treatments after forging. The bainitic steels do not require post-conformation heat treatment: only with the application of a controlled continous cooling is possible to obtain a homogenous bainitic microstructure which has equal or improved properties (yield strength and toughness) comparing to quenched and tempered material. The microstructures obtained from the different alloys continuously cooled were characterized in order to establish relations between the cooling rate and formed products, morphology and volume fraction of phases. The microstructural characterization was carried out intensively and correlated with magnetic properties and X-ray diffraction patterns of the samples. The objectives of this work were to investigate the influence of molybdenum, boron and niobium on the continuous cooling of bainitic steels, as well as to establish the range of cooling rates needed in order to obtain an homogeneous bainitic structure. After cooling, the specimens were characterized by metallography (optical and scanning electron microscopy), hardness, magnetic saturation, x-ray diffraction and EBSD. The effect of molybdenum, boron and niobium on the kinetics of austenite decomposition in the continuous cooling was verified and relationships established between the microstructure, cooling rate and chemical composition. It was also observed the effect of molybdenum, boron and niobium in avoiding ferritic transformation at low cooling rates in order to obtain a bainitic structure under a longer cooling interval.

The purpose of the work presented in this thesis was to explore and understand the mechanisms governing toughness, ductility and ballistic performance in a class of nanostructured carbide-free bainite-austenite steels, sometimes known as ‘superbainite’. The mechanical properties of these alloys have been extensively reported, but their interpretation is not clear. The thesis begins with an introduction to both the relevant nanostructures and some of the difficulties involved in explaining observed properties, alongside a summary of the role of mixed- microstructures in alloy development. An overview of the debate regarding the mechanism of bainite formation is pre- sented in Chapter 2, in the form of a literature survey encompassing the period of explicit recognition of the bainite microstructure. Of note is the role played by the displacive theory of formation in the development of the alloy structures investigated in this thesis. A characterisation of a commonly available bainitic alloy forms the basis for Chapter 4. Observations confirm the nanoscale nature of the structure, although additional phases are found to be present, namely: cementite and martensite. This is explained as resulting from relatively low alloying additions and chem- ical segregation effects, which are modelled using thermodynamic and kinetic approaches. Chapters 5 and 6 contain a comprehensive study of the response of this alloy to the stress concentration present at the notch root of a Charpy impact sample. The work provides evidence of notch root embrittlement due to stress-induced martensite transformation. Results from synchrotron and laboratory X-ray experiments in particular reveal that machining, as well as applied stress, can initiate the austenite-martensite transformation, and methods to mitigate this effect are suggested. An innovative approach is harnessed in Chapter 7, in order to identify exper- imentally the volume fraction at which three-dimensional connectivity (‘percolation’) of austenite is lost in a superbainitic steel. Hydrogen thermal desorption techniques are applied to this problem, inspired by the tendency of such alloys to undergo tensile failure with limited or zero necking. The striking result sheds light on the importance of austenite morphology in restricting the diffusion of hydrogen into a mixed structure. The final set of experimental work is directed towards understanding the damage mechanisms that occur during projectile penetration of a coarser bainitic armour- plate alloy. The formation of adiabatic shear bands is found to be a dominant factor governing the ballistic failure of the plate. The sheared material undergoes severe high-temperature deformation, but does not change phase upon cooling, leading to the proposal of certain methods that could be implemented to improve ballistic resistance of the steel. The totality of the research presented herein is summarised in Chapter 9, which draws attention to new areas of interest that have arisen from the current work, proposing several future directions of investigation. The broader issue of understanding, common to all studies performed thus far, is that of the causes, effects, and extent, of stress-induced transformation to martensite experienced by the retained austenite that is a key feature of superbainite and similar steels.

The aim of the work presented in this thesis is to clarify one of the most fundamental aspects of fatigue damage in bearings steels through critical experiments, in particular whether damage in the form of cracks precedes hard “white-etching matter" formation, which is carbon supersaturated nanoscaled ferrite. Heat treatments have been designed to create four different crack types and distributions: scarce martensite plate cracks, fine grain boundary cracks, abundant martensite plate cracks, and surface cracks. Subsequent rolling contact fatigue experiments showed that the amount of hard white-etching matter is higher in pre-cracked samples compared to those without prior damage and that its formation mechanism is the frictional contact of disconnected surfaces within the bulk that elevate the temperature and localise deformation. These key experiments indicate that hard white-etching matter is the consequence, not the cause, of damage. Therefore, one way to avoid white-etching matter is by increasing the toughness of the material. The macroscopically homogenous distribution of microcracks proved also to be a useful rolling contact fatigue life enhancer due to damage deflection via crack branching and a powerful trap for diffusible hydrogen. Successful trapping was corroborated by the inability of hydrogen to cause crack propagation via embrittlement or accelerate white-etching matter generation during rolling contact fatigue. By also studying the behaviour of a nanostructured bainitic steel under rolling contact fatigue, it was found that its degradation mechanism is ductile void formation at bainitic ferrite/stress-induced martensite interfaces, followed by growth and coalescence into larger voids that lead to fracture along the direction of the softer phase as opposed to the conventional damage mechanism in 52100 steel of crack initiation at inclusions and propagation. Given the relevance of phase quantification in nanobainite and the possible surface artefacts introduced by preparation, alternative methods to X-ray diffraction such as magnetic measurements were also investigated. The lack of hard white-etching matter obtained in the carbide-free nanostructured bainite led to conclude that an alternative route to mitigate hard white-etching matter could be by eliminating pre-eutectoid carbides from the microstructure, therefore restricting their dissolution and ultimate carbon supersaturation of the mechanically deformed and homogenised nanoferrite.

This diploma thesis takes into account the possibility of repairing high-strength steels using coldspray method. The theoretical part is focused on the coldspray method, austenite transformation, bainitic steels and Rmat contact fatigue tests. In the experimental part evaluate the hardness, the structure of the supplied substrate. The hardness, adhesion and porosity of the test sprays are ranked. Furthermore, tests for contact fatigue are evaluated. Finally, the results of the experiments are summarized and measures are proposed to improve adhesion, or to continue this issue.