RESUMO Este estudo avalia o efeito da associação dos tratamentos térmicos de recozimento e desestabilização na precipitação e morfologia de carbonetos secundários e na dureza de uma liga de ferro fundido branco alto cromo (FFBAC) com adição de nióbio. O ferro fundido branco alto cromo é utilizado na fabricação de componentes envolvendo solicitações de desgaste, principalmente nas indústrias mineradoras, em equipamentos como chutes de transporte de minério, devido à sua elevada dureza e alta resistência ao desgaste abrasivo. Os tratamentos térmicos visam melhorar as propriedades mecânicas, por meio da precipitação de carbonetos secundários e da obtenção de matrizes martensíticas. A adição de cromo nos ferros fundidos tem como finalidade formar carbonetos e elevar a dureza da liga. A metodologia utilizada foi a fundição do material com adição de 0,5% de nióbio como elemento de liga e a caracterização metalográfica por meio de microscopias ópticas e eletrônicas, bem como a quantificação de carbonetos e a realização de ensaios de dureza. Foram analisadas amostras desestabilizadas sem e com recozimento prévio e foi verificado que o recozimento favoreceu uma maior precipitação de carbonetos secundários M7C3, possibilitando uma redução no tempo de tratamento térmico de desestabilização e a transformação da matriz austenítica em martensítica durante o resfriamento, promovendo um aumento da dureza.

ABSTRACT This study evaluates the effect of association annealing and destabilizing heat treatments in precipitation and morphology of secondary carbides and hardness in a high-chromium white cast iron alloy with the addition of niobium. White cast iron is used in the manufacture of components involving wear requests, mainly in the mining industries, in equipment such as ore transport chutes, due to its high hardness and high strength to abrasive wear. The heat treatments aim to improve the mechanical properties, through the precipitation of secondary carbides and the obtaining of martensitic matrices. The addition of chromium in cast irons aims to form carbides and increase the hardness of the alloy. The methodology used was the casting of the material with the addition of 0.5% niobium as an alloying element and metallographic characterization by means of optical and electronic microscopy techniques, as well as the quantification of carbides and performance of hardness tests. Destabilized samples were analyzed without and with previous annealing and it was verified that the annealing favored a greater precipitation of secondary carbides M7C3, allowing a reduction in the thermal treatment time of destabilization and the transformation of the austenitic matrix into martensitic during the cooling, promoting an increase in hardness.

Abstract High chromium cast iron (HCCI) with levels around 26% Cr - 2.9% C according to ASTM A 532 IIIA is widely used in systems where high abrasive wear resistance is required. To evaluate the niobium influence, HCCI with 0.5% Nb and 1% Nb additions were melted, besides one without niobium addition of for comparison. For characterization, X-ray diffractometry, scanning electron microscopy (SEM), Vickers microhardness tests, Rockwell C hardness, carbide quantifications, quantification of the retained austenite by Mössbauer spectroscopy, and rubber wheel abrasion test according to procedure A of ASTM G-65 were performed. The results indicated a reduction in the average size, average perimeter, and carbides volumetric fraction (CVF) due to the addition of niobium, also changing the microstructure of the HCCI from eutectic to hypoeutectic. In some regions, NbC0.75 carbides formed a coating around M7C3 carbides. The 0.5% Nb alloy showed the best performance in the rubber wheel abrasive wear test, with a 37% average volume loss reduction as compared to the sample without niobium addition with a 130N load and 34% with a 45N load. The 0.5% Nb alloy also had the highest content of retained austenite among the investigated alloys.

Abstract High Chromium Cast Iron (HCCI) is mostly utilised in environments under severe abrasion and corrosion wear conditions, and research on this material with added niobium obtained positive results regarding wear resistance. The casting process has surface finish and dimensional accuracy limitations compared to machining processes. There are too few studies about HCCI machinability and no one about HCCI with niobium additions. HCCI machinability studies may allow new applications for this material when excellent surface finish and high dimensional accuracy are required. This study analyses the influence of 0.5% Nb addition on the microstructure and machinability of a HCCI alloy with 25.6% Cr and 3.2% C. The samples were heat treated and subsequently machined in dry cutting conditions. Annealing was used to facilitate the pre-machining of the samples, which were later quenched and tempered. Microstructure and hardness were analysed at each stage of the heat treatment. The material was machined after tempered using polycrystalline cubic boron nitride (pcBN) tools. The alloys solidified in the hypereutectic condition. The addition of niobium reduced the hardness and carbide volumetric fraction, increased the service life of the cutting tools in all tests, provided better surface finishing and modified the wear mechanisms.

RESUMO: A alta demanda por concreto faz com que estudos sejam desenvolvidos para amenizar os impactos causados pela produção do cimento Portland, como emissões de gases do efeito estufa e demanda energética, além de viabilizar a redução de custos. A utilização de materiais em substituição parcial do cimento vem sendo empregada como alternativa para minimizar os danos causados pela indústria do cimento. Nesse escopo, se faz necessário utilizar materiais que consigam substituir de maneira eficiente o clínquer de cimento. O presente trabalho utilizou o rejeito resultante da produção de magnésio metálico como substituição parcial do cimento Portland. A substituição visa diminuir a quantidade de clínquer utilizada, tendo em vista que sua produção requer um elevado consumo de energia e emite grandes quantidades de CO2 na atmosfera. A caracterização do rejeito foi feita por meio de fluorescência de raios X (FRX), difração de raios X (DRX), microscopia eletrônica de varredura (MEV) e análise granulométrica. Com intuito de avaliar o comportamento mecânico e a porosidade, a dosagem do compósito resultante foi realizada com substituição parcial do cimento pelo rejeito no teor de 25% em massa, e foram moldados corpos de prova cilíndricos. Após a cura de 28 e 91 dias, todos os corpos de prova foram ensaiados à compressão. Os resultados da caracterização física mostraram que mais de 65% dos grãos do rejeito são menores que 45 µm, o que contribui para o efeito de empacotamento. Quanto à composição química e mineralógica, o rejeito possui altos teores de cálcio e foi possível identificar as fases predominantes. A resistência mecânica à compressão da argamassa com substituição apresentou valores superiores a 40 MPa. A convergência entre os resultados de diferentes técnicas de caracterização evidencia a eficiência do rejeito para uso como material cimentício suplementar.

ABSTRACT: The high demand for concrete has triggered studies on the mitigation of Portland cement production impacts, such as greenhouse gas emissions and energy demands, in addition to enabling cost reduction. Partial replacement of cement with other materials has been employed as an alternative to minimize the damage caused by the cement industry. In this regard, it is necessary to use materials that efficiently replace cement clinker. This study uses waste generated from the production of metallic magnesium as a partial replacement for Portland cement. The substitution is aimed at reducing the amount of clinker used, as its production necessitates high energy consumption and results in emission of large quantities of CO2 into the atmosphere. The tailings were characterized via X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), and granulometric analysis. For evaluating the mechanical behavior and porosity, 25% of the cement (by mass) was replaced with tailings, and the resulting composite was molded into cylindrical specimens. After curing for 28 and 91 days, all specimens underwent compression testing. The results of the physical characterization showed that more than 65% of the tailing grain was lesser than 45 μm in size, which contributes to the packaging effect. In terms of the chemical and mineralogical composition, the tailing had high levels of calcium, and the predominant phases could be identified. The compressive strength of the mortar with substitution was higher than 40 MPa. The convergence observed between the results of the different characterization techniques demonstrates the efficiency of using the waste as a supplementary cementitious material.

ABSTRACT High chromium cast iron (HCCI) is usually employed in equipment that experience high abrasive wear and therefore requires high resistance. Niobium could be added to these alloys to improve their resistance to wear. This study consisted in addressing the influence of 0.5% and 1.0% of niobium on the microstructure of a 26% Cr and 2.8% C HCCI alloy. Standard samples – without niobium – and samples with 0.5% and 1.0% Nb were melted, cut and then machined into adequate dimensions for tests. Two groups of samples – with and without heat treatment – were analyzed through X-ray Diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), Vickers and Rockwell-C hardness. Carbides Volume Fraction (CVF) was quantified in the heat-treated samples. The majority of NbC carbides were observed to be thin and presented a hook shaped morphology. In addition, it was noted a precipitation of Nb-rich layers around the M7C3 carbides, which could act as a higher hardness coating. There was significant reduction in carbide volume fraction, from an initial 33.3% to 28.1% for the 0.5 % Nb alloy, and further reduction to 24.9% for the 1.0% Nb alloy, it was also noted an alteration on the quasi-eutectic microstructure to a hypoeutectic microstructure. The NbC carbides prevented hardness reduction as a result of lowering the CVF in heat-treated alloys; however, the alloys without heat treatment experienced reductions in hardness.

RESUMO A soldagem de aços inoxidáveis duplex é uma operação crítica, devido aos ciclos térmicos aos quais o material é submetido, que podem favorecer a precipitação de fases deletérias. Nesse contexto, objetivo dessa pesquisa é analisar a influência da energia de soldagem na susceptibilidade à formação de fase sigma no aço duplex SAF2205. As chapas de SAF2205 foram soldadas pelo processo de soldagem GMAW, variando a energia de soldagem em 0,5kJ/mm, 1,0kJ/mm, 1,5 kJ/mm. O material foi analisado por microscopia óptica, DRX e microdureza Vickers. A análise da microestrutura das chapas evidenciou que quanto maior o aporte térmico utilizado na soldagem, maior é a quantidade de austenita e maior o tamanho do grão austenítico e ferrítico. Por meio do perfil de microdureza verificou-se que ocorre a redução da dureza na ZTA. Através do DRX foi determinada a presença de fase sigma nas regiões de solda para amostras soldadas com energia de soldagem de 1,0kJ/mm e 1,5 kJ/mm. Concluiu-se que pode-se utilizar nos processos de soldagem GMAW de aços inoxidáveis duplex SAF2205 com 4 mm de espessura aporte térmico próximo de 0,5 kJ/mm quando o objetivo for evitar a formação de fase sigma e aproximadamente igual a 1,5 kJ/mm quando o objetivo for manter percentual das fases na ZTA e ZF próximo ao do metal.

ABSTRACT Welding of duplex stainless steels is a critical operation, due to the thermal cycling to which the material is subjected, which can favor the precipitation of deleterious phases. The purpose of this research is to analyze the influence of welding energy on susceptibility to sigma phase formation in SAF2205 duplex steel. The SAF2205 sheets were welded by the GMAW welding process, varying the welding energy by 0.5 kJ/mm, 1.0 kJ/mm, 1.5 kJ/mm. The material was analyzed by optical microscopy, XRD and Vickers microhardness. The analysis of the microstructure of the plates showed that the higher the thermal input used in the welding, the greater the amount of austenite and the larger the size of the austenitic and ferritic grains. By means of the microhardness profile it was verified that the reduction of the hardness in the ZTA occurs. Through the XRD, the presence of sigma phase was determined in the weld regions for samples welded with welding energy of 1.0 kJ/mm and 1.5 kJ/mm. It was concluded that in the GMAW welding processes of SAF2205 duplex stainless steels with a 4 mm thick thermal input close to 0.5 kJ/mm when the objective is to avoid the formation of sigma phase and approximately equal to 1.5 kJ/mm when the objective is to maintain percentage of the phases in the ZTA and ZF next to the metal.

Os aços inoxidáveis duplex possuem uma microestrutura na qual a composição de ferrita e austenita, tem frações volumétricas próximas a 50 %. Esses são utilizados na fabricação de diversos componentes e produtos, principalmente para aplicações na indústria petrolífera. Nestas aplicações muitas vezes são utilizados processos de soldagem. Para mitigar as alterações provocadas por esse processo nas propriedades do material é necessária uma escolha adequada dos parâmetros de soldagem. Neste contexto, o objetivo desse trabalho é avaliar o efeito do aporte térmico sobre a morfologia da austenita e na quantidade das fases, além de identificar qual condição de soldagem proporciona um equilíbrio das frações volumétricas de ferrita e austenita. Os materiais analisados são chapas de aço inoxidável duplex SAF2205, soldadas pelo processo de soldagem GMAW, na ESAB, variando-se o aporte térmico em 0,5, 1,0 e 1,5 kJ/mm. Após a soldagem, as chapas foram cortadas, preparadas para a análise metalográfica da morfologia de austenita formada e para a quantificação das fases. Os resultados mostraram a presença da austenita alotriomórfica de contorno de grão, a austenita de Widmanstätten e a austenita intragranular. Foi verificado que o aporte térmico que menos alterou o balanceamento das fases ferrita e austenita foi o de 1,5 kJ/mm, concluindo que se deve utilizá-lo quando o objetivo for manter a proporção volumétrica das fases no aço duplex SAF2205.

The duplex stainless steels have a microstructure wherein the composition of ferrite and austenite volume fraction is near 50%. These are used in the manufacture of various components and products, mainly for applications in the oil industry. In these applications often welding processes are used. To mitigate this process changes due to the material properties a suitable choice of the welding parameters is needed. In this context, the aim of this study was to evaluate the effect of heat input on the morphology of the austenite and the amount of the phases, and identify which welding condition provides a balance of volume fractions of ferrite and austenite. The materials analyzed are stainless steel duplex sheet SAF2205, welded by GMAW welding process, the ESAB, varying the heat input in 0.5, 1.0 and 1.5 kJ / mm. After welding, the sheets were cut, prepared for metallographic examination of the morphology formed and austenite phases for quantification. The results showed the presence of alotriomórfica austenite grain boundaries, the austenite Widmanstätten and intragranular austenite. It was found that the heat input less altered the balance of ferrite and austenite phases was 1.5 kJ / mm, concluding that one should use it when the goal is to keep the volume ratio of the phases in duplex steel SAF2205.