The effect of the chemical composition, austenitisation temperature and tempering temperature and time on the mechanical properties and on the ballistic performance of martensitic steel armour plates was studied. It was established in this study that the mechanical properties and the ballistic performance of martensitic steels can be optimised by controlling the chemical composition and the heat treatment parameters. However, it was observed that for a given chemical composition of the steel the heat treatment parameters to be applied to advanced ballistic performance armour plates were different from those required for higher mechanical properties. Such a contradiction rendered the relationship between mechanical properties and ballistic performance questionable. Systematic analysis of the microstructure and the fracture mechanism of some martensitic armour plate steels was carried out to explain the improved ballistic performance of steels whose mechanical properties were below that specificied for military and security applications. It was inferred from phase analysis and its quantification by X-ray diffraction, characterisation of the martensite using scanning electron microscopy, transmission electron microscopy and atomic force microscopy that the retained austenite located in the plate interfaces and on grain boundaries of the martensite was the main constituent resisting localised yielding during ballistic impact on thin steel plates. A part of the kinetic energy is transformed into adiabatic heat where a reaustenitisation of the plate martensite and the formation of new lath martensite was observed. Another part is used to elastically and plastically deform the ballistic impact affected region around the incidence point. Dislocation pile-ups at twinned plate interfaces suggest that the twin interfaces act as barriers to dislocation movement upon high velocity impact loading. The diameter of the affected regions, that determines the volume of the material deforming plastically upon impact, was found to vary as a function of the volume fraction of retained austenite in the martensitic steel. Upon impact, retained austenite transforms to martensite by Transformation Induced Plasticity, the “ TRIP ” effect. High volume fractions of retained austenite in the martensitic steel were found to yield low values of the ratio yield strength to ultimate tensile strength (YS/UTS) and a high resistance against localised yielding and, therefore, against ballistic perforation. A Ballistic Parameter was proposed for the prediction of ballistic performance using the volume fraction of retained austenite and the thickness of the armour plate as variables. Based on the martensite structure and the results of the ballistic testing of 13 armour plate steels a design methodology comprising new specifications was proposed for the manufacture of armour plates whose thicknesses may be thinner than 6mm. / Dissertation (MSc (Metallurgical Engineering))--University of Pretoria, 2007. / Materials Science and Metallurgical Engineering / unrestricted

Firstly, an existing thermodynamic model for the predicting of martensite start temperature of commercial steels has been improved to include more elements such as N, Si, V, Mo, Nb, W, Ti, Al, Cu, Co, B, P and S and their corresponding composition ranges for Martensitic transformation. The predicting ability of the existing model is improved considerably by critical assessment of different binary and ternary systems i.e. CALPHAD approach which is by wise selection of experimental data for optimization of the interaction parameters. Understanding the degree of variation in multi-component commercial alloys, various ternary systems such as Fe-Ni-X and Fe-Cr-X are optimized using both binary and ternary interaction parameters. The large variations between calculated and the experimental values are determined and reported for improvements in thermodynamics descriptions.Secondly, model for the prediction of Epsilon martensite start temperature of some commercial steels and shape memory alloys is newly introduced by optimizing Fe-Mn, Fe-Mn-Si and other Fe-Mn-X systems considering the commercial aspects in the recent development of light weight steels alloyed with Al and Si.Thirdly, the effect of prior Austenite grain size (pAGS) on martensite start temperature is introduced into the model in the form of non-chemical contribution which will greatly influence the Gibbs energy barrier for transformation. A serious attempt has been made to describe the dependency of transition between lenticular and thin-plate martensite morphologies on the refinement of prior Austenite grain size.Finally, the model is validated using a data-set of 1500 commercial and novel alloys. Including the newly modified thermodynamic descriptions for the Fe-based TCFE9 database by Thermo-Calc software AB, the model has the efficiency to predict the martensite start temperature of Multi-component alloys with an accuracy of (±) 35 K. The model predictability can be further improved by critical assessment of thermodynamic factors such as stacking faults and magnetism in Fe-Mn-Si-Ni-Cr systems.

A presente tese de doutorado avalia a Emissão Magnética Espontânea (SME), fenômeno descrito recentemente no grupo de pesquisa do professor Hélio Goldenstein e que já demonstrou ser uma ferramenta promissora para monitorar a transformação martensítica nos aços. O SME é utilizado para monitorar a velocidade de propagação de plaquetas de martensita; identificar a temperatura em que ocorre o inicio da primeira transformação (Ms); observar martensita induzida por deformação ou tensões durante a deformação de aços contendo austenita metaestável e também estudar a transformação isotérmica da austenita em martensita. A duração dos picos individuais magnéticos emitidos no início (Ms) e no final (Mf) da transformação martensítica durante o resfriamento contínuo foram medidos e comparados com os tamanhos médios das placas de martensita, obtidos através da metalografia quantitativa. O SME, um fenômeno mais sensível do que outras medidas globais como a resistividade, dilatometria, Ruído Magnético de Barkhausen (MBN), etc., não exige um volume crítico de transformação para ser detectado, e é capaz de detectar sinais de placas individuais crescentes. / This PhD thesis evaluates the Spontaneous Magnetic Emission (SME), a phenomenon recently described in the research group of Professor Hélio Goldenstein that has proved to be a promising tool to monitor the Martensitic Transformation in steels. The SME is used to monitor the speed of propagation of martensite plates; to identify the temperature at which the first transformation occurs (Ms); to observe martensite straininduced or stresses during deformation of steels containing metastable austenite and also study the isothermal transformation of austenite in martensite. The duration of individual magnetic peaks issued at the start (Ms) and in the end (Mf) of the martensitic transformation during the continous cooling were measured and compared with the average size of the martensite plates obtained by quantitative metalography. The SME, a phenomenon more sensitive than other global measures such as resistivity, dilatometry, Magnetic Barkhausen Noise (MBN), etc., does not require a critical volume of transformation to be detected, and is able to detect signs of growing individual plates.

The processes leading to the retention of small quantities of austenite following the bainite and martensite phase transformations have been examined, together with the influence of retained austenite on the properties of low alloy steels. It was found that the upper and lower bainite transformations are separate reactions, although both involve a displacive transformation mode, Growth seems to occur by the repeated nucleation of martensitic sub-units, and this leads to an apparently slow growth rate, The partitioning of carbon from bainitic ferrite into residual austenite was thermodynamically proven to occur subsequent to transformation, and was shown to be directly responsible for the 'incomplete reaction phenomenon'. The nature of sympathetic nucleation and of the limited size of bainitic sub-units was rationalised in terms of the relatively low driving force available for bainite transformations. It was shown that the retention, stability and morphology of austenite could be directly derived from the basic transformation mechanism. Under certain circumstances, the bainitic retained austenite conferred exceptional strength/toughness properties to silicon steels; these were· shown to be superior to the properties associated with tempered martensite microstructures, Using thermodynamics, a model was established which could predict the toughness behaviour of silicon steel bainites simply from a knowledge of the composition. The tempered martensite embrittlement phenomenon was not found to be directly linked to the decomposition of retained austenite films, but to the coarsening of inter- or intra-lath carbides. In dislocated martensites, it was found that the distribution and quantity of retained austenite could be rationalised in terms of the degree of accommodation between adjacent martensite variants. The incipient twins generally observed in lath martensites were shown to be accommodation defects such that the extent of twinning was the greatest when adjacent martensite units had twin-related lattices. The thermodynamics of dislocated martensites have been briefly examined, The inhomogeneous deformation behaviour of dual-phase steels has been analysed in terms of available models.

A presente tese de doutorado avalia a Emissão Magnética Espontânea (SME), fenômeno descrito recentemente no grupo de pesquisa do professor Hélio Goldenstein e que já demonstrou ser uma ferramenta promissora para monitorar a transformação martensítica nos aços. O SME é utilizado para monitorar a velocidade de propagação de plaquetas de martensita; identificar a temperatura em que ocorre o inicio da primeira transformação (Ms); observar martensita induzida por deformação ou tensões durante a deformação de aços contendo austenita metaestável e também estudar a transformação isotérmica da austenita em martensita. A duração dos picos individuais magnéticos emitidos no início (Ms) e no final (Mf) da transformação martensítica durante o resfriamento contínuo foram medidos e comparados com os tamanhos médios das placas de martensita, obtidos através da metalografia quantitativa. O SME, um fenômeno mais sensível do que outras medidas globais como a resistividade, dilatometria, Ruído Magnético de Barkhausen (MBN), etc., não exige um volume crítico de transformação para ser detectado, e é capaz de detectar sinais de placas individuais crescentes. / This PhD thesis evaluates the Spontaneous Magnetic Emission (SME), a phenomenon recently described in the research group of Professor Hélio Goldenstein that has proved to be a promising tool to monitor the Martensitic Transformation in steels. The SME is used to monitor the speed of propagation of martensite plates; to identify the temperature at which the first transformation occurs (Ms); to observe martensite straininduced or stresses during deformation of steels containing metastable austenite and also study the isothermal transformation of austenite in martensite. The duration of individual magnetic peaks issued at the start (Ms) and in the end (Mf) of the martensitic transformation during the continous cooling were measured and compared with the average size of the martensite plates obtained by quantitative metalography. The SME, a phenomenon more sensitive than other global measures such as resistivity, dilatometry, Magnetic Barkhausen Noise (MBN), etc., does not require a critical volume of transformation to be detected, and is able to detect signs of growing individual plates.

Abstract In this thesis, methods to process ultrafine ferritic (UFF) structures in steels, i.e. grain sizes below about 3 μm have been investigated. It is shown here, in accordance with the results in the literature, that a steel with a UFF grain size can be obtained by two methods, more or less convenient to mass production: deformation-induced ferrite transformation from fine-grained austenite (the DIF route) and the static recrystallization of various heavily cold-worked initial microstructures (the SRF/SRM route). In the present work, the influencing factors in the processing of UFF structure in the DIF route have been systematically studied in four low-carbon steels: one C-Mn steel and Nb, Nb-Ti and Nb-high Ti microalloyed steels. A high strain, a low deformation temperature close to Ar3 and a fine prior austenite grain size are beneficial to promote the formation of UFF grains. Especially by using complex pretreatments to refine the prior austenite grain size, cold rolling, repeating the low-temperature reheating cycle or using martensitic initial microstructure, a UFF grain size can be obtained in these steels at the strain of 1.2 (70% reduction) at 780 °C. By controlling the cooling rate, the type of the second phase can be adjusted. When using the static recrystallization route, it was found that UFF is difficult to obtain from a single-phase ferrite, but it is relatively readily obtained from deformed pearlite, bainite or martensite, especially in high-carbon steels with 0.3–0.8%C. In deformed pearlite, the cementite lamellae fragmented and spheroidised in the course of heavy deformation can provide numerous nucleation sites by the particle stimulated nucleation mechanism and retard the subgrain and recrystallized grain growth. Nucleation and retardation of grain growth are effective also in deformed bainite, martensite or high-carbon tempered martensite, as discussed in detail in the work. The thermal stability of UFF grained steels was tested and found to be generally excellent, but it varies depending on the processing method. The UFF structure obtained by the SRM route has a thermal stability somewhat weaker than that of the DIF route. For a given steel, UFF grains may show different grain growth modes, related to the dispersion of second phase particles. In the DIF structure, abnormal grain growth occurs at 700 °C after about 2.5 h, while in the SRM structure, normal grain growth takes place slowly at 600 °C. Carbides on the grain boundaries seem to play an important role in inhibiting grain coarsening. No coarse-grained zone was formed at the HAZ of electron beam or laser welded seams, as performed at low heat inputs (up to 1.5 kJ/cm) on thin strips. The hardness even increased from the base metal towards the HAZ and the weld metal in all seams as an indication that they were hardened during the rapid cooling.

L’objectif de ce travail est d’optimiser la résistance au revenu d’un acier martensitique à 0,6% de carbone, faiblement allié en Cr, V, et Si/Al. Une série d’alliage modèle ternaire de haute pureté (Fe-C-X, X=Cr, V, Si, Al) a été élaborée à l’EMSE par fusion en creuset froid.. L’étude du comportement des alliages a été réalisée au moyen d’une caractérisation fine de la précipitation (Diffraction RX, MEB-FEG, MET) complétée par des mesures dilatométriques. Le comportement des alliages modèles a été comparé à celui de l’alliage industriel. Le vanadium est responsable de deux effets : (i) les carbures de vanadium bloquent la croissance des grains au cours de l’austénitisation ; (ii) le vanadium dissous permet la précipitation d’une cémentite (V,Cr,Fe)3C à l’origine d’un pic de durcissement secondaire pour un revenu vers 550°C. Le silicium a un effet particulier : il stabilise la matrice martensitique et retarde la précipitation de la cémentite vers des températures de revenu plus élevées, augmentant ainsi la résistance au revenu. Cependant, le silicium se révèle nocif pour le procédé industriel de mise en forme utilisé. Deux nouveaux alliages ont alors été élaborés pour chercher un élément de substitution à l’aluminium : un alliage ou le silicium est remplacé par l’aluminium, et un alliage ou le silicium est absent. L’aluminium reste neutre dans l’alliage mais l’étude de ces deux nuances a permis de mettre en évidence la possibilité d’utiliser une nuance ne contenant pas de silicium mais présentant une résistance au revenu comparable a celle de la nuance industrielle, du fait de la présence conjointe du chrome et du vanadium. / The purpose of this work is to optimise the strength of a tempered martensitic medium C steel, containing low-level of Cr, V and Si/Al. A series of high purity "model" ternary alloys (Fe-C–X, X=Cr, V, Si, Al) was prepared (EMSE) by the cold crucible method. The initial structures of quenched alloys were optimised through appropriate austenitizing conditions. Analysis of alloys behaviour on tempering was performed through carbide precipitation characterisation (X-ray diffraction, SEM and TEM), completed by hardness and dilatometric measurements. The behaviour of "model" alloys was compared to that of the industrial material.Vanadium addition is responsible for two effects: (i) vanadium carbides lock the grain growth during the austenitizing; (ii) dissolved vanadium promotes the precipitation of (V,Cr)3C, that is at the origin of a secondary hardening peak near 550°C. Silicon addition has an interesting effect, as it stabilises the martensitic matrix and increases the temperature of cementite precipitation. Therefore, a high value of hardness is conserved up to high tempering temperatures.Unfortunately, because of some noxious properties, the industrial process in concern excludes the presence of Si. That’s why two new model alloys have been prepared and tested, in which the silicon addition (i) was absent, or (ii) replaced by Al.Aluminium has been shown to be "neuter" vs tempering behaviour. Moreover, the interesting effect of secondary hardening (550°C) has been observed in both Si-free steels, showing that alloying with only Cr and V can be enough to get the expected behaviour of the studied steel.

No description available.

Nuclear reactor pressure vessels are constructed by welding low alloy steel forgings together. Welding processes can leave residual stresses which affect the RPV's resistance to fracture. Welding also induces microstructural changes and these changes have a number of associated consequences, including inelastic strains and altering mechanical properties. The extent to which these microstructural changes influence residual stress evolution during welding is not fully understood. The aim of this project is to characterise the microstructural and mechanical response of SA-508 Gr.3 Cl.1 pressure vessel steel to thermal cycles and develop representative models that can be used to determine how these effects influence stress predictions. There is insufficient materials data to inform the models used to predict how phase transformations influence residual stresses. Using the recently developed Gleeble thermo-mechanical simulator, previously unmeasured data characterising the response of the material to weld-like thermal cycles was generated. Variations in the kinetics of austenite formation and decomposition were investigated using dilatometry. It was found that when the steel is subjected to multiple thermal cycles that exceed the austenisation temperature, the behaviour during the first thermal cycle is different to that of subsequent cycles. In the subsequent thermal cycles, two observations were made: 1) the austenite formation rate increases on heating, and 2) for a given cooling rate, the austenite will decompose at lower temperatures into harder phases. It is explained how these changes in behaviour can affect the residual stress distribution in this thesis. Bainitic, austenitic and martensitic samples were generated. The stress-strain behaviour of these phases is presented and has been used to inform mechanical constitutive models. Finite element simulations of autogenous edge welded beams have shown how microstructural changes can affect the residual stress predictions. The extent of the transformed region of the HAZ and the yield stress of the material surrounding this region influences the location and magnitude of the peak tensile residual stress after a weld pass. Changes in mechanical properties induced by tempering bainitic and martensitic samples were quantified experimentally. The reductions in yield stress in bainite and martensite during short tempering heat treatments were found to be significant. A new approach to integrate the observed tempering behaviour into existing models is presented. The data and models presented in this thesis can provide guidance to structural integrity engineers and help produce more accurate and less conservative residual stress predictions for use in structural integrity assessments.

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.