Papers by Keyword: Pinning Force

Abstract: The grain coarsening temperature in Nb microalloyed steels is investigated by multiphase-field model. In this study, the pinning force is treated as time-dependent using mean-field kinetics of precipitates including volume fraction and their size. The grain size is calculated with time under various temperature range. The grain coarsening temperature is determined by the ratio of the largest radius of grain vs. the average grain radius criteria (R_max / R_avg > 2.94) in two-dimensional growth. Through this model, it is possible to simulate grain growth in microalloyed steels more precisely.

Abstract: The superconducting characteristics, such as the critical current density and the critical magnetic fields, of MgB2-based materials, which in fact belong to the Mg-B-O system because of the high concentration of admixed oxygen (up to 17 wt. %), depend on the inhomogeneity of the oxygen and boron distribution, which can be controlled by the synthesis temperature (600-1200 oC) and pressure (up to 2 GPa) as well as by SiC and Ti additions (10 wt%). With increasing manufacturing temperature grain boundary pinning transforms into point pinning, which is well correlated with the transformation of discontinuous oxygen enriched layers into separately located Mg-B-O inclusions in the MgB2 nanostructure and with a reduction of the size and amount of inclusions of higher magnesium borides MgBX (X>2). Ti or SiC additions can influence the oxygen and boron distribution as SEM and Auger structural studies showed.

Abstract: The static recrystallization and precipitation behavior of weathering steel were studied using thermal simulation technology. The influence of temperature and interval time was analyzed and the softening percentage of static recrystallization was calculated. The morphology and distribution of precipitates were studied, and the driving force for static recrystallization and pinning force of precipitation were calculated. Results show that the higher the deformation temperature is, the faster static recrystallization proceed. Retardation of recrystallization could occur even in the early stage of precipitation. The precipitation pinning force showed a peak in the intermediate stage, and finally decreased as particles coarsened.

Abstract: We present the fabrication and electric properties of MgB2 ceramic samples doped with nanosized spheres, 4-8 nm, of graphite with a metallic core. The samples were prepared using the spark plasma sintering technique. The size of the additive is comparable to the superconducting coherence length. The short processing time limits the diffusion of the carbon while keeping the core intact. Therefore, in addition to the doping with carbon, the metallic core, which has the size smaller than the superconducting coherence length, create pinning centers which might improve the dissipationless electric transport. The results are analyzed in the framework of different pinning models.

Abstract: The austenite static recrystallization kinetics at several temperatures and the recrystallization-precipitation-time- temperature (RPTT) diagrams of a medium-carbon vanadium microalloyed steel have been determined for a strain ε = 0.35. Unlike many other studies carried out previously on V microalloyed steels, the recrystallized fraction against time curves showed the formation of a double plateau that indicates two stages of inhibition of recrystallization due to the formation of different types of strain induced precipitates. This work makes use of transmission electron microscopy to study the nature and size distribution of these precipitates capable of inhibiting recrystallization. The values of driving and pinning forces for static recrystallization are calculated and an analysis of the relationship between the net balance of these forces, the precipitation state and the progress or inhibition of the recrystallization is accomplished. A value of driving force that decreases as recrystallized fraction grows during isothermal holding time is estimated and helps to interpret the behavior of austenite after deformation.

Abstract: In this work the pinning forces exerted by TiN particles in the austenitic phase in two Ti microalloyed steels have been determined and compared with the driving forces for austenite grain growth and for static recrystallisation between hot rolling passes, respectively. TiN precipitate sizes were measured by transmission electron microscopy (TEM) and the precipitated volumes were calculated. These results were then used to calculate pinning forces. The driving forces for recrystallisation were found to be approximately two orders of magnitude higher than the pinning forces, which explains why the austenite in these steels barely experiences hardening during rolling and why the accumulated stress prior to the austenite→ferrite transformation is insufficient (low dislocation density) to refine the ferritic grain.

Abstract: The values of recrystallisation driving (FR) and pinning forces (FP) during the hot rolling of a low Nb-microalloyed steel were calculated using several expressions found in the literature. A comparative study of the hypothesis into the interaction between precipitates and migrating grain boundaries was carried out, and the methods for estimating volume fractions of precipitates and dislocation density were assessed. Though the criterion selected greatly influences the values obtained for both forces, FP grows faster than FR as the rolling temperature drops.