MODARES TECHNICAL AND ENGINEERING
Year:2001 | Volume:- | Issue:5|Papers Count:15

The influences of roll pass design and thermomechanical treatment parameters on the development of surface cracks in hot rolled low alloy steel (41Cr4) bars were studied. Continuously cast slabs of (41Cr4) steel were solutionized at 1000°C and rolled down to 45 mm rounds. Various types of surface cracks in direction, length, depth and width were observed. It was found that the extent of defects depends on both mechanical and metallurgical factors. By a judicious design of roll pass it was possible to reduce extensively the number of surface cracks present after rolling. Also by controlling rolling parameters such as interpass time, deformation per pass and finish rolling temperature and deformation, the optimum hot rolling conditions were obtained. For each rolling schedule the microstructure and the mechanical properties were studied. Finally, for studying the effects of surface defects on formability during the next hot forging process, cylindrical samples of rolled bars in different conditions Le: as rolled, as rolled and peeled, as rolled and machined were prepared and hot deformed between 30 to 60%. The influence of processing parameters and the extent of cracks on the forgeability was determined. The results are interpreted in the framework of classical nucleation and phase transformation theories.  

Kinetics of nucleation and growth of metallic precipitate from n-butylamine complex containing nickel chloride via tetra-bytyl-ammonium tetra-fleuro-borate electrolyte dissolved in anisole-phenanthrene on copper and platinum plates was investigated by cyclic voltametry, chronoamperometry and electrochermical impedance spectroscopic methods. Results showed that a three-dimensional electrocrystallization reaction consisting of an instantaneous nucleation process combined with a diffusion-controlled growth regime prevailed the system. Further studies indicated that the precipitation reaction was irreversible with a specific rate constant of 1-1.4× 10-3cm/sec for crystallization of Ni on Cu and 1.8-5.4×10-3 cm/sec for crystallization of Ni on Pt. The diffusion coefficient of nickel in aromatic electrolyte solution was 2.2-2.8×10-6 cm2/sec for crystallization of Ni on Cu and 1.2-1.8×10-6 cm2/sec for crystallization of Ni on Pt. These relatively small coefficients seem to be due to the formation of nickel ion complex with large ligands present in the solution.        

Application of colored concrete has been the subject of interest of many researchers for the last three decades. The main reason of this interest is the application of colored concrete in building, bridge, tunnel, dom, blocks, and different concrete structures. In the past, concrete was colored more with organic colored material and mineral dust which though has a beautiful and appropriate appearance, its durability was very low and its surface was changed after being exposed to light and variable climate. In this research, alternatively different ceramic pigments were used to produce colored concrete. Pigments were added to concrete mixture and finally the compressive strength of all samples was measured. The results showed that these types of colored concrete have compressive strength of up to twice more than that of standard ones. Furthermore the results showed that particle size and volume fraction of ceramic pigments have a dominant effect on Compressive strength.    

The effects of thermomechanical treatments in high temperature on the morphology of inclusions and mechanical properties (yield strength, tensile strength and elongation) of D6AV steel have been studied. The EDAX system in SEM was used to identify different inclusions existing at the fracture surfaces of tensile specimens. For determining of the amount of the aspect ratio of inclusions, an image analysis system was used. Results showed that, with increasing the rate of the thickness reduction from 75% to 87% (by increasing the number of rolling passes), the aspect ratio of inclusions increased from 1.53 to 2.40. This type of variation in the increase of the aspect ratio of inclusions can be attributed to CaS and oxide inclusions with plasticity near to zero, which drastically reduces the aspect ratio of inclusions. The results of mechanical tests showed that the mechanical properties have slightly decreased with increasing reduction. This would be due to grain growth of austenite because of an increase in the time between rolling passes, and also due to presence of elongated inclusions in the plates of lower thicknesses.      

In this article the effects of alloying elements on dispersion of α- SiC particulates into aluminum melt were investigated. The time required for incorporation of α - SiC particulate into aluminum melt was defined as incorporation time. The incorporation time of SiC can be shortened by adding Mg or by adding Ti and Sn. These alloying elements have a strong affinity for SiC and are able to form silicide as a result of reaction with SiC. Magnesium silicide was detected by EPMA, although the existence of .titanium silicide could not be confirmed. The incorporation time of SiC was prolonged by adding Zn, Cu and Sn which have no affinity for SiC. Moreover, particulate dispersion is considerably influenced by surface - active elements. The incorporation time of SiC particulate was shortened by adding Li which is a surface - active element with strong affinity for SiC, whereas it was prolonged by adding surface - active elements with weak affinity for SiC such as Pb and Bi. This is probably because Pb and Bi adsorb at the interface and hinder the wetting of SiC particulate by aluminum melt. The addition of Si to aluminum prolonged the incorporation time of SiC, because the dissociation of SiC, that might be necessary for good wetting, was retarded by the existence of Si.      

The in situ formation and dispersion behavior of TiC particles in a liquid aluminum alloy have been investigated to develop the novel technique for fabrication of aluminum matrix composites utilizing the spontaneous reaction among materials. Fine TiC particles were in situ formed by the reaction between SiC or Al4C3 particles and an Al-Ti alloy. The effects of processing parameters, such as the kind and size of raw carbide on the in situ formation rate of TiC have been quantitatively estimated by means of ICP analysis. The experimental results were analyzed by the in situ reaction kinetic model based on the assumption that the overall reaction rate was controlled by both the interfacial reaction and the diffusion through the generated carbide particle. At the initial stage of the TiC formation, the interfacial reaction rate is the main controlling step. However, the process is gradually controlled by the diffusion from the middle stages onwards.      

Nitriding of steels, using plasma environments has been practiced for many years. A lot of efforts have been put on developing new methods, such as plasma immersion ion implantation (PI3) and radio frequency (RF) plasma nitriding, for mass transfer of nitrogen into the surface of the workpiece. This article presents the results obtained from an in depth investigation of the surface morphology of the treated samples, carried out using an atomic force microscope (AFM). Samples from microalloyed steel were treated by both methods for 5 hours at different temperatures ranging from 350 to 550 °C in 75% N2 -25% H2 atmospheres. It has been found that the surface of the samples treated by PI3 technique, although having morefavorable properties, were rougher than the surfaces treated by RF plasma nitriding.      

In this article the energetics of cluster formation in undercooled liquids is evaluated by using a simple field-theoretical approach where the solid / liquid interface is taken to be diffuse. The interface is represented by a monotonic and smooth variation of a single order parameter between the two phases, and the free energy change upon cluster formation is calculated as a function of this order parameter, for a model system with typical properties. The numerical results suggest that the energy barrier for crystal nucleation can vanish, contrary to the predictions of the classical nucleation theory, at sufficiently high undercoolings. This behaviour is also shown to be characteristically associated with the nucleation models which are incorporated with a curvature-dependent interfacial energy. The model predictions are found to be consistent with the existing nucleation data.