Short time tempering has been applied to steels to improve toughness keeping the hardness at the same level. For AISI D2 tool steel, typical thermal cycles are quench and temper. The austenitizing temperature is around 1040 ºC and the subsequent double tempering at temperatures close to 500 ºC, for 2 hours each, in order to get the maximum secondary hardness. The aim of the present work is to verify the effects of short time tempering in the AISI D2. For such purpose the Hollomon-Jaffe tempering parameter was used to determine the equivalent time at 600 and 700 °C, being 500 °C for 2 hours the standard temperature and time. The thermal cycles were carried out at a dilatometer. All the specimens were austenitized at 1040 °C followed by double tempering at: 500 and 700 °C for 2 hours each; and 500 °C, 600 °C, and 700 °C for 10 s and 1 min each. The dilatometric curves were analysed. Scanning electron microscopy, X-ray diffraction and Vickers hardness were carried out to characterize the microstructure and mechanical properties. The short time tempering conducted at high temperatures led to the formation and growth of tempering carbides and to matrix recrystallization, which were responsible for greatly reducing the hardness of the material. The short time tempering up to the intermediate temperature of 600 ºC, for 1 minute or 10 seconds, produced results like the conventional tempering in microstructural and hardness terms, denoting that the Hollomon-Jaffe tempering parameter predictability close to the secondary hardness peak.

The aim of this investigation was to study and evaluate the fracture toughness (KICV) of an AISI M3:2 high speed steel that was prepared by powder metallurgical processing, which consisted of uniaxial cold compaction of irregularly shaped water atomized powders, without and with 0.3% of carbon in the form of graphite, followed by vacuum sintering to obtain compacts with densities close to its theoretical value. The sintered steels were then hardened by austenitizing, quenching and triple tempering. Chevron fracture toughness test samples were prepared from the compacts and the tests conducted to determine KICV. The microstructures of the specimens were examined by scanning electron microscopy (SEM), and the composition of the phases determined by x-ray diffraction analysis (XRD). The sizes of the primary carbides and of the austenite grains were determined using Quantikov digital analysis software. No significant difference in fracture toughness (KICV) between the two high speed steels AISI M3:2, austenitized at the different temperatures, was observed.

Resumo Simulações pelo método dos elementos finitos (MEF) foram realizadas para entender os efeitos das taxas de aquecimento/resfriamento na distribuição de tensões residuais. Dois parâmetros de material dos trilhos foram usados, enquanto as condições de contorno permaneceram constantes: tamanho da zona afetada pelo calor, temperatura máxima e taxa de extração de aquecimento. Para complementar a análise, foi realizada uma solda de topo por centelhamento de um trilho Premium com parâmetros de soldagem ajustado para obter uma baixa extensão de zona afetada pelo calor, sem resfriamento forçado, para examinar a microestrutura formada nas regiões críticas na alma e no extremo do patim. Os resultados mostraram que houve concentração de tensões residuais verticais na região da alma, enquanto a presença de tensão residual de compressão no sentido horizontal foi predominantemente superficial na região do trilho superior. Ao usar dois parâmetros de simulação de materiais semelhantes (trilhos), os resultados foram substancialmente diferentes. Os exames metalográficos mostraram que não havia presença de microconstituintes aciculares (martensita/bainita). Além disso, foi observada ferrita pro-eutetóide na região central, cementita em contorno de grão austenítico prévio na região próxima a AC3 e esferoidização quase completa na região de sítio de esferoidização máxima da alma. Em contrapartida, na região do extremo do patim, apresentou microestrutura completamente perlítica na região central e próxima a AC3 e menor volume de esferoidização na região de máxima esferoidização, provavelmente devido à maior taxa de resfriamento nesta região.

Abstract Simulations using the finite element method (FEM) were done to understand the effects of heating/cooling rates on the distribution of residual stresses. Two material parameters from rails were used while the boundary conditions remained constant: heat-affected zone size, maximum temperature and heating extraction rate. To complement the analysis, a flash-butt weld of a Premium rail was done with welding parameters adjusted to obtain a narrow HAZ, without forced cooling, to examine the microstructure formed in the critical regions in the web and the edge of the rail foot. The results showed that there was a concentration of vertical residual stresses in the web region, while the presence of horizontal compression residual stresses was mostly superficial in the rail head region. The main result from the simulation sets was that when using two simulation parameters with similar materials (rails) substantially different results were obtained. Metallographic examinations showed that there was no presence of acicular microconstituents (martensite/bainite). In the rail web, proeutectoid ferrite was observed in the central region, cementite in a previous austenitic grain boundary, in the region that reached temperatures close to AC3, and almost complete spheroidization in the region of maximum spheroidization. In contrast, in the rail foot edge region, there was a completely pearlitic microstructure, in the central region and in the zone that reached temperatures close to AC3, and a lower volume of spheroidization in the region where maximum spheroidization is typically observed, probably due to the higher cooling rate in this region.

Heavy haul transportation (load over 30 tons/axle), as well as the axle load, has been more and more used in Brazil and worldwide. The stress generated in the wheel-rail contact with loads up to 30 tons/axle is around 760 MPa, which causes premature wear and cracks of conventional wheels (AAR (Association of American Railroads) class C). Microalloyed wheels are fundamental on heavy haul transport, whose main function is to combine high hardness, ductility, and yield strength of the material in order to prevent shelling. The main purpose of this research is to develop a new microalloyed wheel steel with niobium addition that meets all the requirements of the AAR class D material with mixed microstructure composed of pearlite and bainite. The 0.71C/0.020Nb steel developed in this study (Nb material) for railroad achieved the standards required for AAR class D in all mechanical properties, with fracture toughness higher than the usual vanadium microalloyed steel used in comparison. The Continuous Cooling Transformation (CCT) diagram showed the presence of bainite even at very low cooling rate, in the range between 0.3 - 2 °C/s. These cooling rates to form bainite are much lower than those observed in other steels with similar composition.

ABSTRACT The Multiphase steels are materials with exceptional mechanical properties including high strength, excellent ductility and hardening coefficient and good conformability. These properties are related with the microstructure, which can be a combination of ferrite and martensite (Dual phase steels) or more complex microstructures with ferrite, bainite, martensite and retained austenite as in TRIP steels. One of the most important stages on the fabrication of multiphase steels is the intercritical austenization step which is performed between the A1 and A3 temperatures. This step determines the amount and chemistry (especially carbon) of the high temperature austenite. Aiming to study the intercritical austenization step, steels with variations in carbon (0.2-0.5wt%) and silicon (1.4-2.0wt%) were manufactured and dilatometric studies were conducted. The results allowed to stablish the intercritical austenization temperature (IAT) according with the needed amount of high temperature austenite. Also, the variation in chemistry of the high temperature austenite was studied as a function of the intercritical austenization temperature. (IAT) The results were analyzed according with the possibilities of the step after austenization, i.e., quenching or austempering and its impact on the microstructure and properties of the heat treated alloys.

RESUMEN Los aceros multifásicos son una familia de materiales con propiedades mecánicas excepcionales que incluyen alta resistencia mecánica, excelente ductilidad e índice de endurecimiento y buena conformabilidad. Estas propiedades están relacionadas con la microestructura, que puede ser desde combinaciones de ferrita y martensita (aceros dúplex) hasta microestructuras complejas con presencia de ferrita, bainita, martensita y austenita retenida como en el caso de los aceros TRIP. Una de las etapas fundamentales en la fabricación de estos aceros es la austenización intercrítica que se produce entre las temperaturas A1 y A3, de la cual depende la fracción de austenita de alta temperatura y la composición química de las fases, en especial el contenido de carbono. Con el fin de estudiar esta etapa del tratamiento térmico se fabricaron aceros con variaciones en la concentración de carbono (0.2-0.5%) y silicio (1.4-2.0%) y se realizaron ensayos dilatométricos. Los resultados permitieron establecer las temperaturas adecuadas de tratamiento térmico intercrítico de acuerdo con la fracción de austenita de alta temperatura deseada. Además, se realizó un estudio de las variaciones en la composición química de la austenita en función de la temperatura de austenización intercrítica (TAI). Los resultados se analizaron a la luz de las posibilidades para las etapas posteriores que incluyen temple o austemperado y su posible impacto en la microestructura y las propiedades mecánicas de las aleaciones tratadas térmicamente.

Abstract Duplex stainless steels have a structure normally composed of austenite and ferrite in approximately equal proportions. In order to attain control of its fabrication processes and performance, it is important to understand its microstructural evolution, due to the formation of intermetallic phases such as sigma (σ) and chi (χ), which may cause a severe deterioration of mechanical properties. In the present study, the evolution of sigma phase during heat treatments at temperatures in which intermetallic phases can be formed (800ºC-900°C) was studied using magnetic analyses on a SAF2205 (DIN 1.4462/UNS S31803) steel. A significant reduction of the intensity of Magnetic Barkhausen Noise (MBN) was observed with the increase of heat treatment time, indicating a decrease in the quantity of ferromagnetic phases. For 24-hour-long treatments, the Barkhausen Noise signal is almost completely enclosed by the background noise, indicating the existence of a very small volume fraction of ferrite. If proper calibration samples are to be produced, this technique may be a viable method for non-destructive evaluation of field components working under thermal conditions that may cause the formation of intermetallic phases.

A commercial ductile iron alloy was submitted to a quenching and partitioning heat treatment. Samples were austenitized at 900 ºC for two hours, quenched at 160 ºC and kept at this temperature for 2 minutes and finally were re-heated at temperatures between 300 and 450 ºC during time intervals between 2 and 180 minutes. The microstructural evaluation was performed with SEM and X-ray diffraction and the mechanical properties were measured using tensile strength and Charpy tests. In general, the quenching and partitioning treatment is viable to achieving expressive fractions of retained austenite in ductile cast irons. Generally, higher partitioning temperatures produce a higher fraction of retained austenite after shorter times. This behavior can be explained by the increase on diffusion rate of carbon at higher temperatures. For all tested conditions it was possible to see a well-defined process window and the combination of mechanical properties is very similar to the austempered ductile irons.

Aluminum nitride is a relatively new engineering ceramic and its mechanical properties have not been extensively studied. In this work, the effects of porosity (~0 to 38%) on sonic velocities, Poisson's ratio, and elastic moduli of partially sintered A1N, both pure and with additives, were investigated. The elastic constants, determined by the ultrasonic pulse-echo method, were significantly influenced by the porosity, but not by the different types of additives used (2 wt. (%) of CaCO3, CaO, and Y2O3). All elastic constants evaluated decreased with increasing porosity, but the rates of decrease were higher for elastic moduli compared do sonic velocities and Poisson's ratio. The results were analyzed in the light of stress concentration and loading bearing area models proposed in the literature.

An Al-5SiO2 (5 wt% of SiO2) aluminium matrix fiber composite was produced where the reinforcement consists of fossil silica fibers needles. After being heat-treated at 600 °C, the original fiber morphology was retained but its microstructure changed from solid silica to an interconnected (Al-Si)/Al2O3 interlaced structure named co-continuous composite. A technique of powder metallurgy, using commercial aluminium powder and the silica fibers as starting materials, followed by hot extrusion, was used to produce the composite. The co-continuous microstructure was obtained partially or totally on the fibers as a result of the reaction, which occurs during the heat treatment, first by solid diffusion and finally by the liquid Al-Si in local equilibrium, formed with the silicon released by reaction. The internal structure of the fibers was characterized using field emission electron microscope (FEG-SEM) and optical microscopy on polished and fractured samples.