Papers by Keyword: Grain Size Refinement

Abstract: Reduction in grain size of bcc based structure steel is still highly concerned in the cold rolled sheet to attain superior mechanical properties. As long as, the reduction of weight is much considered in the structure purposes, the strength/weight ratio of steel is highly demanded. In this study, an innovative technique was applied to attain ferrite grain size with hundreds of nanometer, in tandem with preserving the mechanical properties. In this approach, the micro-alloyed low carbon steel resulted from the thermomechanical process was followed by subcritical annealing regime prior to the first critical transformation temperature. To identify the effect of a micro-alloying element as vanadium, and the effect of subcritical annealing regime on the low carbon steel, two low carbon steel was subjected to studying in this research. The results refer that applying a subcritical annealing regime for the micro-alloyed low carbon steel after hot compression at intercritical annealing temperature can lead for attaining hundreds of nanometer ferrite grain size, which has a powerful effect on promoting the strength of the steel to exceed 1200 Mpa, in one hand with preserving the formability up to 20% as uniform elongation. Unexpectedly, the fine grain size obtained after the innovative technique promotes the impact toughness at room temperature, which is attributed to the fineness and the spheroid morphology of the secondary phase in conjugation with bcc ferrite structure.

Abstract: This study demonstrates applying local laser heat treatment to produce ultrafine-grained austenite (UFGA) structures in an AISI 301LN type commercial austenitic steel. Pieces of 50% cold-rolled sheets containing more than 90% strain-induced martensite were heated locally by a laser beam to various peak temperatures to obtain different degrees of martensite reversion to austenite. Mechanical properties and formability of grain-refined and coarse-grained structures were measured by tensile and Erichsen cup tests. In addition to standard Erichsen cup test, additional interrupted tests were carried out, where cups were first stretched close to the critical strain. Drawn cups were then heated locally by a laser beam to revitalize the structure and thereby enhance the formability in the following cupping test until failure. Results showed that local laser heat treatment is suitable for the reversion treatment to refine the austenite grain size. Various structures were produced: completely reverted microstructures (T > 700 °C) with grain sizes 0.9 - 2 µm in addition to partially reverted structure (T < 700 °C) containing nano- and ultrafine-grained austenite (0.6 µm) with some martensite. The grain refinement by local annealing improved the strength properties. The Erichsen cup tests showed that the formability was equal in the completely reverted ultrafine-grained structures to that of the coarse-grained sheets. It was demonstrated that the local laser treatment restored formability of the drawn cups, allowing stretching to be continued. The second forming step after the laser-treatment provided an enhancement of 19 and 14% in the cup depths in coarse-grained and ultrafine-grained steels, respectively, even though the laser-treatment parameters were not optimized yet.

Abstract: The effect of friction stir processing (FSP), on the microstructure and mechanical properties of a magnesium alloy AZ61 has been analyzed. This is a widely used wrought magnesium alloy provided in the form of rolled and annealed sheets with a grain size of 45 μm. The FSP was performed with an adequate cooling device in order to increase the heat extraction and reduce the processing temperature. The final microstructure showed a noticeable grain size refinement down to values close to 1.8 μm and an important change in texture. The change in texture favors basal slip during tensile testing leading to an increase of ductility and a decrease in yield stress. The stability of the grain size and the creep behavior at high temperatures were investigated. The optimum conditions for superplastic forming were determined; however, the presence of a large amount of cavities precludes the achievement of high superplastic elongations. Additionally, these results are compared with those obtained by severe hot rolling.

Abstract: Friction Stir Processing (FSP) has attracted much interest as a tool for refining grain size and achieving high angle boundary misorientation in magnesium alloys. These characteristics have a great influence in key engineered properties such as strength and ductility, which could be markedly improved by means of this technique. The main objective of this work is to study the microstructural modifications produced when FSP is applied to homogenized cast AZ91 and wrought AZ61 magnesium alloys. Several attempts were made for achieving a homogenous microstructure without defects and enhancing the refinement of the grain size in the stir zone. It was revealed that is of great importance to break the initial microstructure, of coarse grains unfavourably oriented for deformation, in order to facilitate the process, particularly in the case of cast AZ91 alloy. It is highlighted that, after breaking up the initial microstructure, is possible to process the material, in subsequent passes, Furthermore, the use of different backing materials as heat sink and a previous heating treatment of the sample were evaluated. Changing the backing plate can improve more the reduction of the grain size during a second pass. Using a copper plate instead of a steel one can promote a refinement up to 700 nm in AZ91 and 1 μm in AZ61. A coolant agent can be used for inhibiting the grain growth causing a little more reduction of the grain size.