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Avaliação do tratamento criogênico na desestabilização da austenita retida no aço AISI D2 / Evaluation of the cryogenic treatment in the destabilization of austenite retained in AISI D2 steelRaúl Minaya Huamán 18 October 2017 (has links)
O processo de tratamento térmico à baixa temperatura é um dos métodos mais promissores para melhorar o desempenho dos materiais. O tratamento criogênico promove a transformação de austenita retida do aço em martensita, o que é atribuída para melhorar a dureza e resistência ao desgaste. Neste trabalho foram analisados os efeitos dos diferentes ciclos de tratamentos térmicos, comparando-se à tempera convencional (têmpera 1050°C + revenido simples e duplo a 200/530°C) respeito à adição do tratamento criogênico, (têmpera 1050°C + criogenia a -125°C + revenido simples e duplo a 200/530°C) com e sem tempo de espera de 24 horas, com a finalidade de avaliar a estabilização térmica da austenita retida no aço em relação a quantidade presente na microestrutura e consequentemente na influencia nas propriedades mecânicas do aço ferramenta para trabalho a frio AISI D2. As análises foram conduzidas através de testes de dureza, impacto, microscopia óptica, microscopia eletrônica de varredura e difração de raios-X. Os resultados encontrados foram uma variação pouco significativa na dureza entre 57 e 58 HRC. Foi evidenciada a baixa tenacidade ao impacto do aço AISI D2, independente das rotas dos ciclos de tratamento térmico, resultado da alta percentagem de carbonetos dispostos na microestrutura. A resistência ao impacto no aço em estudo após o tratamento criogênico, esses resultados foram relacionados à microestrutura do material. / The heat treatment process at low temperature is one of the most promising methods to improve the performance of materials. The cryogenic treatment promotes the transformation of retained austenite from the steel into martensite, which is attributed to improved hardness and wear resistance. In this work the effects of the different cycles of thermal treatments were analyzed, comparing to conventional tempering (tempering 1050°C + single and double annealing at 200/530°C) with respect to the addition of the cryogenic treatment (tempera 1050°C + cryogenics to - 125°C + single and double tempering at 200/530°C) with and without waiting time of 24 hours, in order to evaluate the thermal stabilization of the austenite retained in the steel in relation to the amount present in the microstructure and consequently in the influence on the mechanical properties of cold working tool steel AISI D2. The analyzes were conducted through tests of hardness, impact, optical microscopy, scanning electron microscopy and X-ray diffraction. The results found were a minor variation in hardness between 57 and 58 HRC. It was evidenced the low impact toughness of the AISI D2 steel, independent of the thermal treatment cycle routes, as a result of the high percentage of carbides disposed in the microstructure. The impact resistance in the steel studied after the cryogenic treatment, these results was related to the microstructure of the material.
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Desenvolvimento de ligas inoxidaveis com efeito de memoria de forma : elaboração e caracterização / Development of stainless alloys with shape memory effect : production and characterizationOtubo, Jorge 19 December 1996 (has links)
Orietadores: Paulo Roberto Mei, Sadamu Koshimizu / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-07-22T11:12:58Z (GMT). No. of bitstreams: 1
Otubo_Jorge_D.pdf: 18862199 bytes, checksum: 741b9894b9a5962f8f8467424dd40055 (MD5)
Previous issue date: 1996 / Resumo: É apresentado neste trabalho, pioneiro no Brasil, o desenvolvimento de ligas inoxidáveis com Efeito de Memória de Forma (EMF) assistido por transformação martensítica não termoelástica. Mostrou-se que a fusão por indução à vácuo é uma técnica adequada e que tanto o trabalho a quente como o trabalbo a frio são perfeitamente possíveis nas ligas em questão à medida que os parâmetros operacionais como temperatura de aquecimento, redução por passe, etc., sejam controlados. A eficiênciada técnica de treinamento na melhora do Efeito de Memória de Forma foi comprovada com a recuperação de forma em torno de 95 % da liga CrNi e de 89 % da liga CrNiCo após 5 ciclos com pré-deformação de 4% e aquecimento a 600 °e. Estes resultados são bastante promissores e comparáveis aos melhores resultados encontrados na literatura. O caráter inoxidável e a recuperação de forma de praticamente 4% das ligas desenvolvidas promovem-nas para aplicações práticas como conexões de tubo sem solda e mecanismo para liberação de painéis solares de satélite. Considerações sobre processamento, propriedades mecânicas e microestruturais são também analisadas / Abstract: It is presented in this work, pioneer in Brazil, the development of stainless alloys with shape memory effect assisted by non thermoelastic martensitic transformation. It was shown that Vacuum Induction Melting (VIM) is a suitable technique for melting these alloys and that hot working or cold working are perfectly possible as long as the heating temperature, reduction per pass, etc. are controlled. The efficiency of training technique in improving the Shape Memory Effect was shown by the 95 % shape recovery of the CrNi alloy and by 89 % of shape recovery of the CrNiCo alloy after five training cyc1es of 4 % pre-strain and annealing at 600°C. These results are very promising and comparable to the best ones published in the literature. The stainless character and the shape recovery of almost 4% promote these alloys for practical applications such as for pipe fitting and for satellite solar panel release mechanism.The alloys' processing, mechanicaland microstructural properties are also analyzed. / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica
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[en] QUENCHING AND PARTITIONING OF NI-ADDED HIGH STRENGTH STEELS: KINETICS MODELLING MICROSTRUCTURE AND MECHANICAL PROPERTIES / [pt] TÊMPERA E PARTIÇÃO EM AÇOS DE ALTA RESISTÊNCIA CONTENDO NI: MODELAGEM CINÉTICA, MICROESTRUTURA E PROPRIEDADES MECÂNICASANA ROSA FONSECA DE AGUIAR MARTINS 03 December 2007 (has links)
[pt] Aços de alta resistência contendo frações significativas de
austenita retida têm alcançado grande interesse comercial
principalmente quando associados ao fenômeno TRIP durante o
processo de conformação final. Recentemente, um novo
conceito de tratamento térmico, denominado Têmpera e
Partição, vem sendo estudado como mais uma alternativa no
desenvolvimento de aços multifásicos. Neste processo, o
controle da fração volumétrica da austenita retida é
possível uma vez que durante o tratamento de partição, a
supersaturação de carbono na martensita temperada é
utilizada para estabilizar a austenita não transformada,
evitando assim transformações futuras que poderiam ocorrer
em temperaturas mais baixas. A seqüência de processamento
térmico envolve o tratamento de têmpera numa faixa de
temperatura entre Ms e Mf, seguido de partição numa
temperatura igual ou superior à temperatura de têmpera. A
partição do carbono da martenista para a austenita é
possível caso reações competitivas, como por exemplo, a
precipitação de carbetos, sejam suprimidas pela adição de
elementos de liga tais como Si e/ou Al. Uma condição básica
para o modelo está relacionada à restrição de movimentação
da interface martensita/austenita, uma vez que a difusão em
temperaturas baixas está limitada aos átomos interticiais.
Essa restrição leva a um novo conceito de equilíbrio
denominado Equilíbrio Constrito de Carbono, que é
caracterizado pela igualdade do potencial químico na
interface austenita-martensita apenas para o carbono. Nesse
trabalho foram desenvolvidos quatro aços, contendo
diferentes percentuais de C e Ni e com a presença dos
elementos Si, Mn, Mo e Cr. A adição desses elementos teve
finalidade reduzir a temperatura Bs, visando desacoplar o
tratamento de têmpera e partição de uma eventual
transformação bainítica. Um conjunto de condições para o
tratamento de têmpera e partição foi então desenhado,
envolvendo diferentes temperaturas de têmpera e diferentes
temperaturas e tempos de partição. A avaliação
microestrutural foi realizada utilizando recursos de
microscopia ótica e microscopia eletrônica de varredura e
de transmissão. A técnica de difração de raios-X foi
empregada para quantificar a fração de austenita retida e
seu enriquecimento em carbono. Foi modelado o processo de
partição do carbono utilizando o programa DICTRATM. Os
resultados dessas simulações foram analisados em termos dos
parâmetros microestruturais, do tempo e da temperatura, e
como essa combinação influência a cinética de partição do
carbono. Os resultados obtidos para as amostras ensaiadas
em tração indicaram uma vasta combinação de resistência
e ductilidade, confirmando o potencial do processo na
otimização das propriedades mecânicas. / [en] High strength steels containing significant fractions of
retained austenite have been developed in recent years and
are the subject of growing commercial interest when
associated with the TRIP phenomenon during deformation. A
new process concept, Quenching and Partitioning, has been
recently proposed for production of steel microstructures
containing carbon-enriched austenite. The heat treatment
sequence involves quenching to a temperature between the
martensite-start (Ms) and martensite-finish (Mf)
temperatures, followed by a partitioning treatment, above
or at the initial quench temperature, designed to enrich
the remaining untransformed austenite with the carbon
escaping from the supersaturated martensite phase, thereby
stabilizing the retained austenite phase during the
subsequent quench to room temperature. To enable the
austenite enrichment, competing reactions, principally
carbide precipitation, must be suppressed by appropriate
alloying elements, such as Si and/or Al. The concept
assumes a stationary martensite/austenite interface and the
absence of shortrange movements of iron and substitutionals
elements. The condition under which partitioning occur has
been called Constrained Carbon Equilibrium (ECC), due to
the restriction in movement of the interface and the
assumption that only carbon equilibrates its chemical
potencial at the interface. In this work, a group of four
alloys was investigated, containing different additions of
C and Ni and containing Si, Mn, Mo e Cr. These alloys were
designed to preclude bainite formation at the partitioning
temperatures of interest. Several heat-treatments, were
performed in these alloys, using the Q&P concept, to
evaluate its effect on the resulting microstructure and
mechanical properties. Each alloy was quenched at selected
temperatures and partitioned from 350 to 450°C for times
ranging from 10 to 1000s. Microstructural characterization
was performed by optical microcoscopy, scanning and
transmission electron microscopy, while X-ray diffraction
was used to determine both the fraction and the carbon
content of the retained austenite. Partitioning kinetics
were simulated with DICTRATM. The results were analyzed
taking into consideration the scale of the microstructure,
as well as the partitioning temperature. Tensile test
results indicated that very high levels of strength with
moderate toughness can be achieved confirming the potential
of the Q&P to produce a superior combination of mechanical
properties.
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Bone–Biomaterial Interface:the effects of surface modified NiTi shape memory alloy on bone cells and tissueMuhonen, V. (Virpi) 17 June 2008 (has links)
Abstract
Whenever a foreign material is implanted into a human body an implant–tissue interface area forms between them. In this microenvironment, interactions take place between the implant and the surrounding tissue. The implantation of a biomaterial into tissue results in injury and initiation of the inflammatory response. This host response to biomaterials is an unavoidable series of events that occur when tissue homeostasis is disturbed by the implantation process. In bone tissue, biocompatible implants must initially be capable of strong bone implant contact and subsequently, allow the normal bone remodeling cycle around the implant.
NiTi is a metal alloy composed of approximately a 50:50 ratio of nickel and titanium. It possesses shape memory and superelasticity properties, which make it an interesting biomaterial. NiTi has two phases: austenite and martensite. A decrease in temperature or applied stress induce the austenite-to-martensite transformation. Heating or removing the stress restores the parent austenite phase. The alloy in its martensite structure can be reshaped and strained several times more than a conventional metal alloy without irreversible deformation of the material. The alloy returns to its original shape as it changes from martensite-to-austenite. This transformation is seen as the macroscopic shape memory effect.
This study further investigated the biocompatibility of NiTi, especially the bone cell response to both austenite and martensite. Different surface treatments were investigated in order to improve and possibly even control NiTi's bioactivity as a bone implant material.
Osteoclasts grew and attached well on the austenite NiTi phase, but the results indicated that the biocompatibility of martensite NiTi was compromised. Oxidation of the NiTi surface improved osteoblast attachment and viability. This was due to the formation of a TiO2 surface layer of moderate thickness. Coating the NiTi surface with the extracellular matrix protein fibronectin was shown to enhance osteoblast proliferation and increase the number of cells in the G1 cell cycle stage. Austenite was more prone to show these effects than martensite. A sol-gel derived titania-silica surface treatment was observed to increase the bone implant contact of functional NiTi intramedullary nails. The surface treatment was most effective with the constant bending load provided by the NiTi nail.
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Modelling and Characterisation of the Martensite Formation in Low Alloyed Carbon SteelsGyhlesten Back, Jessica January 2017 (has links)
The current work contains experimental and theoretical work about the formation of martensite from the austenitic state of the steel Hardox 450. Simulation of rolling and subsequent quenching of martensitic steel plates requires a model that can account for previous deformation, current stresses and the temperature history, therefore dilatometry experiments were performed, with and without deformation. Two austenitization schedules were used and in the standard dilatometry the cooling rates varied between 5-100 °C/s, in order to find the minimum cooling rate that gives a fully martensitic microstructure. Cooling rates larger than 40°C/s gave a fully martensitic microstructure. The cooling rate of 100 °C/s was used in the deformation dilatometry tests where the uniaxial deformation varied from 5-50 %. The theoretical work involved modelling of the martensite formation and the thermal/transformation strains they cause in the steel. Characterizations were done using light optical microscopy, hardness tests and electron backscatter diffraction technique. The parent austenite grains of the martensitic structure were reconstructed using the orientation relationship between the parent austenite and the martensite. Kurdjumov-Sachs orientation relationships have previously been proven to work well for low-carbon steels and was therefore selected. The standard implementation of the Koistinen-Marburger equation for martensite formation and a more convenient approach were compared. The latter approach does not require the storage of initial austenite fraction at start of martensite formation. The comparison shows that the latter model works equally well for the martensite formation. The results showed that the use of martensite start and finish temperatures calibrated versus experiments for Hardox 450 works better when computing thermal expansion than use of general relations based on the chemistry of the steel. The results from deformation dilatometry showed that deformation by compressive uniaxial stresses impedes the martensite transformation. The simplified incremental model works well for deformation with 5 % and 10 %. However, the waviness in the experimental curve for deformation 50 % does not fit the model due vi to large barrelling effect and the large relative expansion for the material that the sample holders are made of. Crystallographic reconstruction of parent austenite grains were performed on a hot-rolled as-received reference sample and dilatometry samples cooled with 60 °C/s and 100 °C/s. The misorientation results showed that the samples match with the Kurdjumov-Sachs orientation relationship in both hot rolled product and dilatometry samples. When misorientation between adjacent pixels are between 15° and 48°, then the boundary between them was considered as a parent austenite grain. The austenitic grain boundaries of the sample cooled at 100 °C/s is in general identical with the hot rolled sample when considering high angle boundaries (15°-48°). The results from the hardness tests showed that the rolled product exhibits higher hardness as compared to samples cooled by 100 °C/s and 60 °C/s. This can be attributed to the formation of transition-iron-carbides in the hot rolled product due to longer exposure of coiling temperature.
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Modelling and Characterisation of the Martensite Formation in Low Alloyed Carbon SteelsGyhlesten Back, Jessica January 2017 (has links)
The current work contains experimental and theoretical work about the formation of martensite from the austenitic state of the steel Hardox 450. Simulation of rolling and subsequent quenching of martensitic steel plates requires a model that can account for previous deformation, current stresses and the temperature history, therefore dilatometry experiments were performed, with and without deformation. Two austenitization schedules were used and in the standard dilatometry the cooling rates varied between 5-100 °C/s, in order to find the minimum cooling rate that gives a fully martensitic microstructure. Cooling rates larger than 40°C/s gave a fully martensitic microstructure. The cooling rate of 100 °C/s was used in the deformation dilatometry tests where the uniaxial deformation varied from 5-50 %. The theoretical work involved modelling of the martensite formation and the thermal/transformation strains they cause in the steel. Characterizations were done using light optical microscopy, hardness tests and electron backscatter diffraction technique. The parent austenite grains of the martensitic structure were reconstructed using the orientation relationship between the parent austenite and the martensite. Kurdjumov-Sachs orientation relationships have previously been proven to work well for low-carbon steels and was therefore selected. The standard implementation of the Koistinen-Marburger equation for martensite formation and a more convenient approach were compared. The latter approach does not require the storage of initial austenite fraction at start of martensite formation. The comparison shows that the latter model works equally well for the martensite formation. The results showed that the use of martensite start and finish temperatures calibrated versus experiments for Hardox 450 works better when computing thermal expansion than use of general relations based on the chemistry of the steel. The results from deformation dilatometry showed that deformation by compressive uniaxial stresses impedes the martensite transformation. The simplified incremental model works well for deformation with 5 % and 10 %. However, the waviness in the experimental curve for deformation 50 % does not fit the model due vi to large barrelling effect and the large relative expansion for the material that the sample holders are made of. Crystallographic reconstruction of parent austenite grains were performed on a hot-rolled as-received reference sample and dilatometry samples cooled with 60 °C/s and 100 °C/s. The misorientation results showed that the samples match with the Kurdjumov-Sachs orientation relationship in both hot rolled product and dilatometry samples. When misorientation between adjacent pixels are between 15° and 48°, then the boundary between them was considered as a parent austenite grain. The austenitic grain boundaries of the sample cooled at 100 °C/s is in general identical with the hot rolled sample when considering high angle boundaries (15°-48°). The results from the hardness tests showed that the rolled product exhibits higher hardness as compared to samples cooled by 100 °C/s and 60 °C/s. This can be attributed to the formation of transition-iron-carbides in the hot rolled product due to longer exposure of coiling temperature.
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Analýza deformačně indukovaných změn fázového složení oceli TRIP metodou EBSD / Analysis of Strain - induced Variations of Phase Composition of the TRIP Steel using EBSD MethodPešina, Zbyněk January 2008 (has links)
The diploma thesis deals with phase composition measurement of the TRIP steel, using EBSD method. The steel was delivered as thermo-mechanically treated via two different routes. The phase composition of the steel was examined during gradual plastic deformation in the range 0 to10.99%. One route of thermo-mechanical treatment exhibited good agreement with the literature in terms of measured fraction of the retained austenite (15.6%) as well as its decrease during the deformation (to 8.9% at the maximum imposed strain). The samples of the second route did not show any agreement in either of the parameters spoken.
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Casting and Characterization of Advanced High Strength SteelsHedman, Daniel January 2020 (has links)
The Latin American steel making company Ternium S.A. aims to develop and produce a new type of advanced high strength steel (AHSS) in which the main alloying elements are carbon, aluminium, manganese, and silicon. The present work is the first phase of the development project and it involves casting and characterization of four steel compositions with varying amounts of the aforementioned elements. The results revealed that the Mn-content had a large impact on the development of hard phases during solidification. A steel with a Mn-content of 2 %wt. had almost completely transformed to pearlite during cooling, while a steel with a 4 %wt. Mn-content consisted of primarily martensite and retained austenite. Only the impact of the Mn-content is evaluated. The columnar grain size for two of the four steel compositions were in the range of 20-30 mm, which is similar to those observed from continuous casting. This indicate that the heat transfer rate was slow enough to allow these grains to grow. Measurements during casting showed an initial cooling rate of 10-20°C/min at a distance of 10 mm inside the ingot, which is much slower than the surface cooling rate during continuous casting (100-150°C/min). It was assumed that the cooling rate was similar for all castings since the methodology was identical. However, the steel used for cooling rate measurements was not characterized, why a correlation between cooling rate and composition could not be obtained. A heat transfer model was developed to gain further knowledge of the solidification process. As a reference to the heat transfer model, a eutectic Bi-42Sn alloy was cast with temperaturemonitoring using a casting setup identical to that of the steel castings. A similar cooling rate tothe Bi-42Sn reference casting was obtained where the cooling was faster from above of the ingot than below. Thus, the last part of the metal to solidify during the simulation was situated in the lower half of the ingot. This provides a model for testing future steel compositions.
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Investigation of Microstructural Effects in Rolling Contact FatigueDallin S Morris (11185158) 30 July 2021 (has links)
<p>Rolling
contact fatigue (RCF) is a common cause of failure in tribological
machine
components such as rolling-element bearings (REBs). Steels selected for RCF applications are
subject to various material processes in order to produce martensitic
microstructures. An effect of such
material processing is the retention of the austenitic phase within the steel
microstructure. Retained austenite (RA)
transformation in martensitic steels subjected to RCF is a well-established
phenomenon. In this investigation, a
novel approach is developed to predict martensitic transformations of RA in steels
subjected to RCF. A criteria for phase
transformations is developed by comparing the required thermodynamic driving
force for transformations to the energy dissipation in the microstructure. The method combines principles from phase
transformations in solids with a damage mechanics framework to calculate energy
availability for transformations. The
modeling is then extended to incorporate material alterations as a result of RA
transforming within the material. A continuum
damage mechanics (CDM) FEM simulation is used to capture material
deterioration, phase transformations, and the formation of internal stresses as
a result of RCF. Crystal lattice
orientation is included to modify energy requirements for RA transformation. Damage laws are modified to consider residual
stresses and different components of the stress state as the drivers of energy dissipation. The resulting model is capable of capturing
microstructural evolution during RCF.</p>
<p>The development and stability of
internal stresses caused by RA transformation in bearing steel material was
experimentally investigated. Specimens
of 8620 case carburized steel were subjected to torsional fatigue at specific
stress levels for a prescribed number of cycles. X-ray diffraction techniques were used to
measure residual stress and RA volume fraction as a function of depth in the
material. A model is set forth to
predict compressive residual stress in the material as a function of RA
transformation and material relaxation.
Modeling results are corroborated with experimental data. In addition, varying levels of retained austenite (RA) were
achieved through varying undercooling severity in uniformly treated case
carburized 8620 steel. Specimens were
characterized via XRD and EBSD techniques to determine RA volume fraction and
material characteristics prior to rolling contact fatigue (RCF). Higher RA volume fractions did not lead to
improvement in RCF lives. XRD
measurements after RCF testing indicated that little RA decomposition had
occurred during RCF. The previously
established RCF simulations were modified to investigate the effects of RA
stability on RCF. The results obtained
from the CDM FEM captured similar behavior observed in the experimental
results. Utilizing the developed model,
a parametric study was undertaken to examine the effects of RA quantity, RA
stability, and applied pressure on RCF performance. The study demonstrates that the energy
requirements to transform the RA phase is critical to RCF performance.</p>
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Strain Path Effect on Austenite Transformation and Ductility in TBF 1180 SteelGibbs, Parker Kenneth 01 March 2019 (has links)
TBF 1180 steel was studied under various conditions focusing on the correlation of ductility and amount of retained austenite. Samples were prepared from sheet stock and then strained using limiting dome height tooling (LDH), a standard uniaxial test frame, and a tensile stage for use in an electron microscope. The steel was observed in plane, biaxial, and uniaxial strain to determine its effect on retained austenite transformation and ultimately, ductility. Retained austenite was observed using a scanning electron microscope (SEM) equipped with an electron backscatter detector (EBSD) to distinguish the different phases present. Initial austenite levels were around 5% by volume and was quickly reduced as the sample was strained. The biaxial samples were the slowest to transform, having about 2.5% austenite at .05 effective strain, which allowed the specimen to reach an effective strain of .3 with 1.1% austenite remaining. In contrast, the plane strain samples had the fastest rate of transformation having only 1.2% austenite at .05 effective strain and .7% austenite at a final effective strain of .18. Both forms of uniaxial, (in-situ and ex-situ), were near identical, as expected, and exhibited an austenite transformation curve between that of the plane and biaxial curves. The uniaxial austenite level at .05 strain was 2.1% and was able to reach about .15 strain with a final austenite percentage around 1%. It was concluded that the biaxial strain path had the greatest ductility due in part to its slower austenite transformation rate while plane and uniaxial strain paths were not as ductile with their faster austenite transformation rates.
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