Introduction: The mechanical properties and corrosion resistance of a material are dependent on its microstructure and can be modifi ed by phase transformation. When a phase transformation occurs in a material it usually forms at least one new phase, with physical-chemical characteristics that differ from the original phase. Moreover, most phase transformations do not occur instantly. This paper presents an evaluation of the phase transformation of martensitic stainless steels ASTM 420A and ASTM 440C when submitted to different thermal processes. Methods: Dilatometry tests were performed with several continuous heating and cooling rates in order to obtain the profi les of the continuous heating transformation (CHT) and continuous cooling transformation (CCT) diagrams for these two types of steel. Also, the temperature ranges for the formation of the different phases (ferrite and carbides; ferrite; austenite and carbides; non-homogeneous and homogeneous austenite phases) were identifi ed. Rockwell hardness (HRC) tests were performed on all thermally treated steels. Anodic and cathodic potential dynamic polarization measurements were carried out through immersion in enzymatic detergent as an electrolyte for different samples submitted to the thermal processes in order to select the best routes for the heat treatment and to recommend steels for the manufacture of surgical tools. Results: The martensitic transformation temperature tends to increase with increasing temperature for the initiation of cooling. The 440C steel had a higher hardness value than the 420A steel at the austenitizing temperature of 1100 °C. Above the austenitizing temperature of 1100 °C, the material does not form martensite at the cooling rate used, which explains the sharp decline in the hardness values. Conclusion: The study reported herein achieved its proposed objectives, successfully investigating the issues and indicating solutions to the industrial problems addressed, which are frequently encountered in the manufacture of surgical instruments.
The formation of grains and their growth based on the elimination of other grains, before thermal cycles are imposed, are essential for the correct conformation of a material and to control the microstructure, texture and orientation relationships between phases. However, when the boundary of a polycrystalline grain intersects the outer surface and the material is exposed to temperatures higher than half of its fusion temperature, a crack or groove appears. This research aims to apply a method to evaluate the energy of the grain boundary (GB) and also to compare the energy values of the faces of the GB, and to investigate commercially-pure grade 2 titanium in relation to the action of forces on the grain boundary of the free surface. The thermal attack under vacuum technique was used to reveal, at high temperatures, the titanium microstructure, which is preserved under a very thin oxide layer even when the sample is brought to room temperature. Thus, at room temperature, it was possible to analyze the Ti surface viewed at high temperature. The thermal cracks or grooves appearing due to the selective vaporization of atoms at the GB were measured geometrically using atomic force microscopy. The energy relationship of the GB was determined from the microstructural appearance at high temperatures and applying the equation g gb = 2·g s ·senj to the thermal crack formed between two grains.The results were compared experimentally with others reported in the literature. With the method applied in this study it was possible to evaluate the energy of the GB and it was verified that this energy was anisotropic for the material under study.Keywords Titanium, Grain boundary energy, Atomic force microscopy, Thermal attack under vacuum. Medida de energia de contorno de grão do titânio comercialmente purograu 2 -em alta temperaturaResumo A formação e o crescimento de grãos a partir de outros existentes, perante ciclos térmicos impostos, são fundamentais para a conformação adequada do material, para controlar a microestrutura, textura e relações de orientação entre as fases. Com o princípio da formação de uma fenda térmica á partir da exposição do material a temperaturas maiores que a metade de sua temperatura de fusão, esta pesquisa visa a comprovação de um método para avaliar a energia de contorno de grão (CG), comparar os valores da energia das faces do CG e investigar a ação das forças na superfície livre dos CGs do titânio comercialmente puro grau 2. A técnica de ataque térmico sob vácuo foi empregada para revelar, em altas temperaturas, a microestrutura do titânio, esta técnica permite a manutenção da estrutura "congelada" sob uma camada de óxido de pequena espessura mesmo quando a amostra é trazida à temperatura ambiente. Com este método foi possível analisar a superfície do Ti em temperatura ambiente, como se estivesse sendo visualizada em altas temperaturas. As trincas ou fendas térmicas provenientes da vaporização seletiva de átomos no CG foram medidas geometricamente utilizando Microscopia de Força Atômica. A energia d...
This research consisted of implementing and evaluating an empirical mathematical model to reproduce analytically the dilatometric behavior of ASTM 420A and ASTM 440C martensitic stainless steels, widely used for manufacturing surgical tools. Martensitic stainless steels can be subdivided into three subgroups: low-carbon, medium-carbon and high-carbon steels. The microstructure of each group is also characteristic as needlelike martensitic; very fine martensitic; and ultra-fine martensitic containing carbides. The proposed method was based on experimental data obtained from the dilatometric testing of the steel samples applying low heating rates. It was possible to determine the formation of phase fields near the equilibrium conditions. The method, being based on empirical data, ensured a greater approximation to the experimental values, verifying that it can be applied as a useful tool in the evaluation of industrial heat treatments for surgical tools.Keywords Dilatometric test, Mathematical modeling, Phase transformation, Martensitic stainless steels, Surgical tools. Modelamento matemático de ensaio dilatométrico em aços inoxidáveis martensíticos 420A e 440C utilizados em ferramental cirúrgico
Functionally graded materials (FGM) based on stainless steel and ceramic materials have inspired researchers to combine properties and features which are not present in conventional composites, and are considered to be an alternative in the production of motors parts, cutting tools and coatings for reactors. The addition of metal and ceramic in a graded structure allows the integration of distinct properties that combine advantages of metallic and ceramic materials. Ceramic components withstand high temperatures and present high corrosion resistance, while metallic ones provide higher mechanical resistance, in particular ultimate tensile strength and fracture toughness. In this work, composites with variable levels of 316 Stainless Steel and Ytria-stabilized Zirconia, were prepared and characterized, in order to determine the thermal behavior of each composition, aiming the optimization of sintering of pieces with chemical composition gradation.
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