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[en] The technique, which allows fast and efficient identification of the parameters in the phase transformation model, is described in the paper. This technique is based on inverse calculations performed for experimental tests. The method combines simulation of experiments with measurements of selected parameters. The results of constant cooling rate experiments, in which transformation start and end is monitored, are used for identification. The cost function for optimization is defined as a distance between measured and calculated transformation start and end temperatures. An example of an application of the technique to the eutectoid steels is presented in the paper. The model with optimum coefficients is implemented into the finite element code and simulations of various cooling processes involving complex time-temperature profiles are performed. (author)
[en] The effects of alloying elements, especially Cr and Si, and transformation temperature on the microstructures and mechanical properties of hyper-eutectoid steels were investigated. Increases in Si and Cr changed the morphology of cementite in upper bainite, while the morphology of pearlite seemed to be unchanged by the amount of alloying elements, except for variation in interlamellar spacing. Increasing Si and Cr content also caused increases in tensile strength and a reduction of area of the steels containing pearlitic microstructure, while decreasing tensile strength and reduction of area in steels transformed into upper bainitic microstructure. The tensile strength of steels consisting of pearlite, upper bainite and a mixture of pearlite and upper bainite, can be expressed with the equation σ= (σpo+kp·λp-1/2)·Vp+(σubo+kub·λub-1/2)·Vub. The values calculated using the above equation coincided well with the measured tensile strength of hyper-eutectoid steels containing Cr and Si.
[en] Highlights: • A multiscale lamellar structure was produced by warm rolling a carbon steel with an initial pseudoeutectoid structure. • The multiscale lamellar steel exhibited high strength identical to that of ultrafine-grained steel. • Uniform plastic deformation was stabilized by the multiscale lamellar structure and ductility was retained. • The strain hardening was enhanced as a result of the strain gradient and stress state change under tension. In order to ameliorate the ductility and stabilize the uniform deformation of ultrafine-grained (UFG) ferrite-cementite steel while retaining its high strength, a multiscale lamellar structure was developed and compared with the UFG structure. Microstructures were characterized and tensile behaviors were analyzed in the samples with multiscale lamellar and UFG structures; the mechanisms underlying the different strain-hardening capabilities were studied. Plastic instability dominated during the tensile deformation of spheroidized UFG carbon steel, because of its low strain-hardening owing to the uniform UFG structure with high dislocation density. However, the steel with multiscale lamellar structure exhibited a marked improvement in the balance between strength and uniform deformation, ascribing to the enhanced strain-hardening capability. This result was primarily attributed to the extra strain-hardening generated by the strain gradient and stress state change caused by the multiscale lamellar structure.
[en] Observations of the structure of commercial zircaloy-2 have been made in the microscope showing that the high temperature beta phase is transformed isothermally at lower temperatures into alpha plus secondary precipitate. The alpha occurs mainly as Widmanstaetten plates developed by a shear mechanism. The secondary precipitate is formed from the beta - alpha structure at the phase boundary between these phases. This precipitation of particles of secondary phase occurs on account of a eutectoid reaction, alpha also being formed. A time-temperature transformation diagram has been constructed from the observations
[en] The consequences of the microstructural in an eutectoid steel has undergone a cold drawing process are studied. This manufacturing technique produces changes in the steel microstructure a progressive orientation and slenderizing of the pearlite colonies and an orientation and packing of the pearlite lamellae, thus reducing the inter lamellar spacing. In addition, the experimental results of the stress corrosion cracking tests show that cold drawing produces resistant anisotropy in the steel and thus the stress corrosion cracking resistance is a directional property which seems to depend on the orientation of the microstructure in relation to the cold drawing direction. As a consequence, a transversal crack tends to change its propagation direction approaching the wire axis and a crack growth in mode I evolves to a mixed mode propagation. (Author) 5 refs
[en] Uranium-zirconium alloy system has been studied for over half a century for nuclear applications. Uranium rich part of its binary phase diagram has frequently been investigated mainly due to its promising performance as metallic nuclear fuel. However, the agreement on its phase diagram has not been completely made since experimentally reported peritectoid reaction of (β↔α+γ2) and eutectoid reaction of (β+γ1↔α) are conflicting to each other. In this study transformation start temperatures (Ts) and enthalpies (ΔH) of annealed U-Zr alloys (U-2, 5, 10, 20 and 50wt.%Zr) were measured by differential scanning calorimetry (DSC) and discussed with phase diagrams including either peritectoid or eutectoid reaction