Carlos Alexandre dos Santos

Effect of Heat Treatments on Austempered Ductile Iron

This work presents the influence of austempering heat treatment carried out in one-step and two-step processes on the microstructures and mechanical properties of ductile cast iron. The samples were extracted from as-cast pieces and heat treated by austempering. For the one-step process the samples were heated at 910°C for 90 min for austenitization and cooled in salt bath at a temperature of 300°C for 30 min. For the two-step process the samples were cooled from 910°C to 245°C, kept at this temperature for 5 min in salt bath, then heated in another salt bath at a temperature of 300°C for 30 min. The samples were analyzed by optical microscopy and mechanical tests. After the one-step austempering, microscopic analysis of the samples showed ausferrite microstructure matrix and graphite in nodules surrounded by fine pearlite. For the two-step austempering, the presence of ausferrite matrix with graphite in nodules and retained austenite was observed. As to mechanical properties, the results showed that, with the two-step process there was gain (4.7%) in the average hardness and loss (3.5%) in the impact resistance. The microhardness of the ausferrite was 6.2% higher in the one-step austempering when compared to the two-step process.

The Interrelation between Casting Size, Steel Grade, and Temperature Evolution Along the Mold Length and at the Strand Surface during Continuous Casting of Steel

The present work focuses on the investigation of thermal profiles in copper molds and at the strand surface during continuous casting of different steel grades in an industrial plant. Thermocouples embedded in the mold walls were used to measure temperatures along the mold length. Noncontact pyrometers positioned at different locations along the machine monitored the strand surface temperatures. The experimental results permitted an interrelation between steel grade, mold section (240 × 240 mm, 180 × 180 mm, and 150 × 150 mm), and mold wall, and strand surface temperatures to be established as a function of casting operating conditions. It is shown that the mold outer face (external curved wall) has the highest temperature distribution from the meniscus to the bottom of the mold followed by the inner and side faces, respectively. The deepest meniscus is shown to occur for the 150-mold, and the 180-mold is shown to have the highest temperature profiles along the mold length in all faces examined. Low carbon steels present the highest strand surface temperatures along the machine when compared with those of medium and high carbon steels. When the steel composition is parameterized, it is shown that the 150-mold has the smaller strand surface temperature close to the mold exit when compared with the 180 and 240 molds, and it is shown that this behavior changes after the unbending point.

Microstructural Analysis of the Special Steels Produced by Continuous Casting Process

The objective of the present work is to obtain empirical equations allowing the correlation between secondary dendrite arm spacing (SDAS) (λ2) and cooling rates ( ) of the special steels, produced by continuous casting process. For that, samples were extracted from ingots with three different sections, i.e., 150 × 150 mm, 180 × 180 mm, and 240 × 240 mm, choosing steels with nine different chemical compositions (equivalent carbon contents). Metallographic techniques to reveal the structures, acquisition data system, and image analysis were employed to determine the SDAS in function of its position from surface to center of the ingots. Computer simulations generated by the continuous casting process software (InALC + Artificial Intelligence in the Continuous Casting), developed by the research group, were performed to obtain the relations between the solidification conditions and the cooling rates in operational conditions (chemical composition, ingot section, casting speed, and casting temperature). In the obtained equations for λ2 in function of the position, it was observed in the steels with similar chemical composition that the 150 mm section samples presented lower values when compared with 180 mm and 240 mm sections samples. In the comparison among the steels with similar equivalent carbon contents, the values of λ2 presented an increment in function of the increasing of the sections. In the comparison of steels with same section and different chemical compositions, the equations for λ2 in function of the simulated cooling rates presented an increasing of the λ2 with the increasing of the equivalent carbon contents.

The Influence of Austempering Conditions on the Machinability of a Ductile Iron

Austempering conditions such as temperature and time and their influence on austempered ductile iron machinability were analyzed. Austenitization at 910°C for 90 min and austempering into molten salt bath at 300°C, 360°C, and 420°C for 30, 60, and 90 min each were performed. Microstructures were analyzed by optical microscopy and hardness measurements. Samples were further machined in a lathe for machinability tests. The lathe was instrumented considering power and cutting time and machinability evaluation performed referring to cutting force and material removal. Microstructures at 300°C for 30 min showed ausferrite with retained austenite and martensite. Retained austenite decreased and acicular ferrite sheaves appeared at 60-min austempering time. Mixed bainite was also present at 90-min austempering. Ausferrite and retained austenite were observed in all austempering periods at 360°C, whereas at 420°C only bainite and fine pearlite were present. Hardness increased with increasing temperature at 30-min austempering and decreased with increasing time. However, an exception was observed at 420°C. The highest machinability performance was achieved at 360°C at 60-min austempering, and the lowest performance at 420°C at 90-min austempering.

Austempering heat treatments of ductile iron using molten metal baths

ABSTRACT This work aims to evaluate the use of two different zinc–tin and zinc–aluminum molten metal baths on austempering heat treatments performed in ductile cast iron. Samples were extracted from as-cast standard Y-blocks for austempering heat treatments. The samples were heated for austenitization at 910°C for 90u2009min and further cooled in two different molten metal baths for austempering: zinc–tin and zinc–aluminum alloys at 370 and 400°C, respectively, for 30, 60 and 90u2009min. The Zn–50u2009wt% Sn hypoeutectic alloy and the Zn–5u2009wt% Al eutectic alloy were chosen for the molten metal baths. After heat treatments, the samples were analyzed by optical and scanning electron microscopy, Brinell hardness, Vickers microhardness, Charpy impact and tensile tests, and fracture mode analysis. The results indicated the viability of using Zn–Al and Zn–Sn molten metal baths as a substitute of molten salts. When the austempering temperature was increased from 370 to 400°C, the hardness, tensile strength, and elongation decreased, while impact energy increased. The ideal processing parameters were obtained for austempering at 370°C for 60u2009min, where the austempered ductile cast iron presented a microstructure completely formed by finer ausferrite.

Sliding Wear Behavior of an AISI 440B Martensitic Stainless Steel Lubricated with Biodiesel and Diesel–Biodiesel Blends

The aim of this work was to analyze the effect of biodiesel and diesel–biodiesel blends on the sliding wear behavior of AISI 440B martensitic stainless steel. Lubricated tests were performed on AISI 440B martensitic stainless steel samples using Brazilian commercial diesel (containing 7 vol.% biodiesel), pure biodiesel, and diesel–biodiesel blends with biodiesel additions of 20% v/v, 30% v/v, and 50% v/v. Non-lubricated tests were also performed. The stainless steel was analyzed in the as-received condition (annealed) and after heat treatments (quenched and tempered at different temperatures), using a pin-on-disk device with an alumina pin, at a 1.8xa0m/s sliding speed, 14.7xa0N load, and 4400xa0m sliding distance, following the ASTM G99-04 Standard. Wear track widths, wear coefficients, and wear track surfaces were analyzed by optical and scanning electron microscopy. The results showed that AISI 440B presented the worst wear behavior in the dry condition with a microstructure characterized by a ferritic matrix and dispersed carbides (annealed condition). The wear resistance increased with the increase in biodiesel content due to the matrix strengthening by the martensitic transformation (heat-treated condition). When the biodiesel content was superior to 50% v/v, a reverse result was found. The observed wear mechanism was abrasive in all conditions. A −u20091.35 power law coefficient characterized the wear coefficient as a function of biodiesel content for the annealed condition and a −u20090.95 power law coefficient for the heat-treated conditions.

Aplicação do processo de consolidação de pós metálicos por constrição radial (CCR) para a produção de pré-formas de ferro

The Radial Constriction Consolidation (RCC) process can be considered a powder metallurgical technique that allows for the fabrication of components with complex geometries, near-net-shape and full density at low production costs when compared to the traditional process. In some aspects, the RCC process is similar to the hot isostatic pressing process, but it uses a ceramic mold confectioned by investment casting and a powder as a secondary pressing media, all of which are confi ned in a metallic container. In this paper, a RCC experimental system was developed aiming to produce iron billets with the characteristics of near-net

Influência da taxa de solidificação na morfologia das fases da liga eutetóide Zn-22%Al

A liga Zn-22%Al apresenta uma microestrutura complexa, tanto no estado bruto de fusao, como apos envelhecimento, devido a transformacao eutetoide (β → α + η). O objetivo desse trabalho e relacionar a morfologia das fases presentes em lingotes brutos de fusao e apos envelhecimento com a taxa de solidificacao. Amostras dos lingotes da liga Zn-22%Al foram analisadas por microscopia eletronica de varredura (MEV) e EDS (Energy Dispersive Spectroscopy). A estrutura α + η com morfologia lamelar em amostras brutas de fusao transforma-se em uma textura refinada tendendo a globular-equiaxial apos o envelhecimento. Observou-se que a dureza das amostras esta fortemente relacionada com a morfologia da estrutura α + η. Por outro lado, a composicao e a morfologia das fases sao consequencias da taxa de solidificacao e resfriamento.