Papers by Author: Berenice Mendonça Gonzalez

Abstract: The use of Ferritic Stainless Steels has become indispensable due its lower cost and the possibility to replace austenitic stainless steels in many applications. In this study, cold rolled sheets of two stabilized ferritic stainless steels with 85% thickness reduction were annealed by applying a heating rate of 24 oC/s and a soaking time of 24 s. The niobium stabilized ferritic stainless steel type ASTM 430 (430Nb) was annealed at 880 oC while the niobium and titanium bi-stabilized steel ASTM 439 was annealed at 925 oC. The annealed samples were tensile tested and due to the smaller grain size, steel 430Nb, showed a higher yield stress and a higher total elongation. Concerning drawability the steel ASTM 439 presented a better performance with higher average R-value, lower planar anisotropy coefficient and a greater value for Limit Drawing Ratio (LDR). These results are explained in terms of the differences in size and volume fraction of precipitates between the two steels.

Abstract: The ferritic stainless steels are materials used in several segments due to the excellent combination of mechanical properties and corrosion resistance. The mechanical properties of these alloys are strongly dependent on the microstructural characteristics and crystallography texture. The aim of this experimental study is to investigate the roles of the grain size of the hot rolled sample on the development of the microstructure, texture and formability of ferritic stainless steel. The main elements of chemical composition of the steel under investigation were 16.0 %Cr, 0.021 %C, 0.024 %N and 0.35 %Nb. Coarse and fine grains samples were cold rolled up to 90% thickness reduction and annealed at 880°C with soaking time of the 24 s. The texture measurements were performed by Electron Backscattered Diffraction (EBSD) in the longitudinal section. The formability was evaluated by the R-value and planar anisotropy (Δr) in tensile tests. The final microstructure after annealed was more homogenous for smaller initial grain size sample. This condition was favorable to develop γ-fiber, with sharpness intensity in 111121 components. The highest R-value and smallest planar anisotropy was obtained for a {111}/{001} ratio around 5.37. On the other hand, coarser initial grain size sample had showed a heterogeneous microstructure and texture, performing badly in mechanical tests (anisotropy).

Abstract: ncreasing demand for automotive vehicles with reduced weight, improved crashworthiness and passengers safety has steamed the research of new Twinning Induced Plasticity (TWIP) steels. In this work the effect of annealing between 400 and 900°C on the microstructure and mechanical properties of hot and cold rolled 0.06C-24Mn-3Al-2Si-1Ni (wt%) steel with TWIP effect was investigated. The results have shown that steel exhibits fast recrystallization kinetics with a low amount of recovery, which results in a high driving force for the former. Mechanical properties were determined using Vickers microhardness and tensile tests. Tensile strength of 670 MPa with 54% of total elongation, and strain hardening exponent of 0.57 were reached after annealing at 900°C.

Abstract: Superelasticity is closely related to shape memory effect. It refers to the property presented by some materials submitted to large strains (usually up to about 8%) to restore their original shape immediately after unloading without the need of heating. This phenomenon results directly from a diffusionless transformation of the material from an austenitic to a martensitic phase (martensitic transformation). The recovering mechanism is the reverse transformation, from martensite to austenite. This paper compares fatigue live curves obtained in bending-rotation fatigue tests carried out on wires of NiTi alloys with three different microstructures, stable austenite, unstable austenite (superelastic), and stable martensite. These curves are also compared to data from the literature. The tests were strain controlled and the wires were submitted to strain amplitudes from 0.6% to 12.0%. To minimize changes in material properties, the wire temperature was monitored using a thermocouple and controlled by its rotation speed. For strain amplitudes up to 4%, the εa-Nf curve for superelastic wires was consistent with those reported in the literature, closely approaching the curve of the stable austenite wire. For higher strain amplitudes, fatigue life of superelastic wires increased with strain until it approached the fatigue life curve of stable martensitic wire. This unusual behavior results in a “Z-shaped” curve for high strain values. It is possibly linked to the changes in microstructure and fatigue properties that occur when the superelastic material is deformed.