Fabiane Roberta Freitas da Silva

Experimental Analysis of Forming Limits and Thickness Strains of DP600-800 Steels

In this work, the plastic behavior of cold-rolled zinc coated dual-phase steel sheets DP600 and DP800 grades is firstly investigated by means of uniaxial tensile and Forming Limit Curve (FLC) testing. The uniaxial tensile tests were carried out at 0o, 45o and 90o angular orientations with respect to the rolling direction to evaluate the mechanical properties and the plastic anisotropy Lankford r-values. The forming limit strains are defined according to Nakajima’s procedure. Thickness measurements of tested Nakajima’s samples cut perpendicular to the fracture allowed to identify a rapid decrease of the strain, which governs the plastic instability that preceded the fracture in the drawing region of the FLC. Optical metallographic and scanning electron microscopy techniques helped to characterize and distinguish the orientation of rotated grains and flat fractured surface (ductile shear failure in blank specimens close to plane-strain tension) from no grain rotations and rough fractured surface (ductile tensile fracture in blank geometries in the biaxial stretching domain).

EBSD Texture Analysis of NdFeB Magnets

The crystallographical texture is relevant information for NdFeB magnets, since the maximum energy product is directly related to orientation of the crystals. EBSD (Electron Backscattered Diffraction) is a very suitable tool for preferred orientation measurement of NdFeB magnets. The advantages of EBSD against X-ray Diffraction (XRD) pole figures for texture determination are discussed. EBSD identifies misaligned grains, and this is not feasible with XRD pole figures. EBSD is also helpful on the identification of oxides.

Micromechanical Modeling of Dual-Phase DP600 Steel Sheet Plastic Behavior Based on a Representative Volume Element Defined from the Real Microstructure

Dual-phase steels offer very attractive combinations of strength and ductility owing to the coexistence of different microstructures components and their interactions. These steels are suitable to the automotive industry due to their improved impact resistance increasing the passenger safety along with the vehicle weight reduction. The properties of the dual-phase steels are attributed to the chemical composition, type, size, amount and spatial distribution of different phases that can be obtained during thermomechanical treatments, namely, ferrite and martensite. In this work, the microstructure of as-received DP600 cold rolled steel sheet with 1.2 mm nominal thickness was firstly characterized by means of scanning electron microscopy technique. Then, a representative volume element was obtained from the DP600 microstructure and a micromechanical finite element model is proposed considering the steel chemical composition, average ferrite grain size, martensite volume fraction and mechanical properties of both ferrite and martensite phases. The uniaxial tension loading was simulated by assuming either plane-stress and plane-strain conditions. The numerical predictions corresponding to the plane-strain model are in good agreement with the experimental true stress-strain curve determined along the sheet rolling direction. The proposed finite element micromechanical approach based on the real microstructure proved to be an important tool to evaluate both local and overall behaviors of DP600 steel grade.

Correlation between Microstructure and Mechanical Properties of Austempered Ductile Irons

Austempered ductile cast irons (ADI) have received great attention in last years because their combined properties of good ductility, high strength and fracture toughness, good fatigue strength, good wear properties and low production cost. Such combination of properties can be reached because of their microstructures consist of a mixture acicular ferrite (bainite), residual austenite with a high carbon content and nodular graphite. In this work, the effect of austempering heat treatment on the microstructure of a commercial alloy to produce three different grades of ADI, with different strength level, is analyzed. Microstructure characterization has been performed using techniques of optical microscopy, scanning electron microscopy and x-ray diffraction. Mechanical properties were evaluated from tensile and impact tests at room temperature. In addition, the residual stress due to heat treatment was evaluated. The results of this study show that there is a strong relationship between austempering temperatures and mechanical properties. The highest tensile and yield strength obtained were 1599 and 1427 MPa, respectively, for the sample austempered at 280°C. The sample austempered at 320°C presented the highest Charpy absorption energy (99,90 J) and highest volume fraction of austenite (27%).

Three-Dimensional Microstructure Reconstruction and Phase Boundary by Serial Sectioning and Numerical Approach

The microstructure and pores of the consolidated powder materials are important to determine the final properties of the pieces produced. Therefore, the control of their volume fractions and morphologies represent actual technological challenges. Besides the quantification of the phase and pores volume fractions there are important features related with the morphological aspects and the phase boundary structure that reveals the mechanisms of properties improvements, which are crucial for development of materials obtained by powder metallurgy techniques. In this paper, stereological measurements of pores present in the material stainless steel (316L) are obtained by combining serial sectioning technique and optical microscopy (OM). Dilatometric experiments were carried out in order to simulate heat treatment for sintering the material. The 3D microstructures are obtained by images reconstruction using numerical pixels treatment and stereological results of the porosities are presented.

Evolution of Sintering of Particles that Compose Iron Ore Agglomerates

The increasing demand for new technologies in the ironmaking/steelmaking field has been motivating several studies towards pelletizing process improvement. Within this context, evaluate the reduction of iron ore pellets using the dilatometer technique constitutes a promising approach for optimizing this process. This paper aims the metallurgical characterization through the sintering of particles in iron ore pellets. With this purpose, some experimental procedures are of concern as follows. Firstly, the kinetic densification of the iron ore pellets is measured using a dilatometer, which heats the samples up at 30 K/min until high temperatures about 1473 K and an isotherm at 10 minutes have been done. Then, the sample is cooled back to room temperature and undergoes a microstructural characterization, with the aid of a scanning electron microscope. At last, the density of the pellets is evaluated, using an Arquimedes Principle and consequently the porosity of the agglomerates. The results indicate the sintering progress of the particles that comprise the pellets as well as reduction the porosity. This behavior is due to the fact that the heat arising from gas induces the partial liquid phase formation and involves the agglomerate particles aiding in the sintering process.

ANÁLISE TERMODINÂMICA E EXPERIMENTAL DA FORMAÇÃO DA FASE SIGMA EM UM AÇO INOXIDÁVEL DUPLEX SAF 2205

Ana Carolina Martins Silva Fabiane Roberta Freitas da Silva Wesley Luiz da Silva Assis Gláucio Soares da Fonseca Resumo Os aços inoxidáveis dúplex ferríticos-austeníticos fazem parte de uma classe de materiais com microestrutura bifásica, composta por uma matriz ferrítica e ilhas de austenita, com frações volumétricas aproximadamente iguais dessas fases. Essa classe de materiais é caracterizada por apresentar interessante combinação de elevadas propriedades mecânicas e de resistência à corrosão e, por isso, é considerada bastante versátil, sendo utilizados nas indústrias química e petroquímica, de papel e celulose, siderúrgicas, alimentícias e de geração de energia. Este trabalho objetiva caracterizar as microestruturas do Aço Inoxidável Duplex SAF 2205, revelando as fases que se apresentam após tratamento térmico a 800oC, em diferentes tempos, estudar a relação entre a dureza do material e a precipitação da fase sigma, além de determinar a cinética de precipitação dessa fase. Após obtidos os dados experimentais, dados de VV e composição de equilíbrio da fase sigma foram obtidos via Thermo-Calc. Ao final do trabalho, os resultados experimentais e de simulação são comparados e conclui-se que a nucleação da fase sigma é por difusão de Cromo na matriz. Palavras-chave: Aço Inoxidável duplex; fase sigma; cinética; Thermo-Calc.

Analysis of the Y-TZP Ceramic Substucture in Blasting with Aluminum Oxide before and after Sintering

The objective of this study was to evaluate the microstructure of a ceramic based on yttria stabilized zirconia (Y-TZP) in blasting with aluminum oxide and its effect on the sintering. 25 pre-sintered Y-TZP blocks were obtained. Ten samples were blasted with alumina, and then all samples were sintered and divided into the groups: Control-no treatment; alumina G1-blasted with alumina and sintered; alumina and Rocatec G1-blasted, sintered, and Rocatec; alumina G2-sintered, alumina blasting; alumina and Rocatec G2-sintering, alumina blasting, and Rocatec. The samples were evaluated by a scanning electron microscopy. The qualitative analysis showed that the treated samples had an increase in the surface texture and that group 1– alumina and Rocatec– presented the silica incorporation to be regular and homogeneous. It was concluded that the treatment of pre-sintered surfaces is a good alternative in the bonding strength between the Y-TZP and the resin cements.