Materials Science Forum Vols. 704-705

Abstract: By means of Gleeble-1500 dynamic thermo mechanical simulator using solidifying method, the simulation of continuous casting process for HRB335C steel was carried out and hot ductility and strength were determined. The test results indicate that there were three temperature areas of brittleness for billet produced by HRB335C within 700°C to solidification point, the first temperature area of brittleness was 1300°C to solidification point of the billet, the second temperature area of brittleness was 1200～1000°C, the third temperature area of brittleness was 700～850°C, the steel was plastic within 850～1000°C. The fractographs and micrographs were observed by optical microscope. The reason of embrittlement was analyzed. The cracking sensitivity studied under the different the temperature zone of the fragility for steel HRB335C was found out. The appropriate surface temperature (over 1000°C) of continuous cast billet during bending and straightening was beneficial to avoid and reduce surface cracks and the quality of billet was obviously improved. Key words: Brittle temperature zone; Tensile strength; Microstructure; High temperature mechanical properties; Continuous casting billet

Abstract: In order to study the effect of plastic deformation on microstructure and mechanical properties of as-cast AZ91 magnesium alloy, experiments of hot direct extrusion were performed at different extrusion temperatures and different extrusion ratios. The microstructure and mechanical properties of extruded billets and extrudate were measured. Experimental results show that the grain size of as-cast AZ91 magnesium alloy can be dramatically refined by extrusion. Hot extrusion can obviously improve the mechanical properties of as-cast AZ91 magnesium Alloy, comparing with the pre-extruded billet, the tensile strength, yield strength and elongation of extrudate can be improved by at least 69%, 117% and 150% respectively. As the extrusion temperature increases, the tensile strength and yield strength of extrudate will increase. As the extrusion ratio increases, the tensile strength and yield strength of extrudate will increase at first and then fall. At the time of extrusion temperature of 420°C and extrusion ratio of 45, the highest tensile strength of 381Mpa and yield strength of 303MPa can be achieved for the extrudate.

Abstract: In different conditions (time, temperature and coal particle size), oxygen physisorption of different coal samples were tested by a oxygen adsorption detection instrument with chromatograph, followed by the analysis of the relation between oxygen physisorption in coal self-heating and the propensity of coal to spontaneous combustion. The results indicate that the physisorption reaches 80% of the saturate amount in several seconds, and then with the rapid decline of the sorption rate takes about 1 min to get saturated. The physisorption decreases with the temperature rise, and increases then drops with decreasing coal particle size, only playing a role in oxygen transportation, physisorption heat generation and the first motivation for coal to self-ignite. The content of oxygen physisorption cannot reflect the capacity of chemisorption and chemical reaction during spontaneous combustion. It is concluded that the sole oxygen physisorption cannot identify the oxidation reactivity of coal effectively.

Abstract: The microstructure evolution of a medium-carbon Si-Mn steel during deformation of undercooled austenite at different degree of deformation, temperatures and strain rates has been investigated by means of a hot compression simulation test, metallographic microscope, scanning electron microscope and transmission electron microscopy. Also, the mechanism of carbide spheroidized during deformed process has been discussed. The experiment results demonstrate that the process of evolution experienced three stages: that is, strain-induced transformation, austenite eutectoid decomposed to carbides and ferrite matrix, and spheroidization of pearlite at the range of A₃-Ar₃. The austenitic grains would be refined for the extra-product of ferrite above the Ar₃. The eutectoid reaction was induced on the grain boundaries of ferrite and non-transformed austenite and deformation bands with the increasing volume of deformation. An optimum combination of deformation temperature and strain rate is important to obtian the dulplex microstructure consisting of ultrafine ferrites and dispersed carbide particles. The fine spheroidized microstructures are obtained while the deformed temperature reaches 650°C with ≥1.0, meanwhile, The carbides precipate in globular and shot-rod shapes. Keywords: Medium-carbon Si-Mn steel, Undercooled austentite, Microstructure evolution, Deformation induced transformation, Carbide spheroidization

Abstract: The grain boundary character distribution (GBCD) and its effect on the corrosion resistance of copper and brass (H65, H80) after solid solution and cold-rolled annealing were investigated by means of static weight loss method, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and Electron back-scattered diffraction (EBSD).The possible mechanism of the effect of proportion of low ∑ coincidence site lattice (ΣCSL) on corrosion resistance were discussed preliminary. The results show that the proportion of low ΣCSL of the three kinds of samples after cold rolling and annealing all increase obviously compared with samples in solid solution. The proportions of low ΣCSL( such as Σ3,Σ9 and Σ27 ) of H65 brass,H80 brass and copper increase in turn in the same heat treatment conditions. In addition, it can increase the grain boundary ratio of low ΣCSL up to 67.9% by cold rolling of 6% and annealing at 650°C for 10min; The corrosion experiments show that corrosion resistances of samples after heat treatment are much better than that in solid solution. In the three samples, the corrosion resistance of copper is the best and that of H80 brass is better than that of H65 brass. Key words: copper; brass; GBCD; corrosion resistance

Abstract: The polymer was prepared by the radical copolymerization. Tensile tests were conducted on a N-isopropylacrylamide (NIPA)-acrylamide (AAm) comonomer gel sensitive to variations in environmental temperature. and salinity Mechanical properties (Young’s modulus) were determined at different levels of swelling equilibrium. Shifts in the mechanical properties were observed between samples tested in the swollen and unswollen states. Additional tests were conducted to determine the effect of salinity on the LCST. Increasing the salinity from 0M to 1.0M resulted in a decrease in LCST (55centigrade to 28centigrade).It is found that adjustment of the salinity of the environment solution is an effective method of controlling the LCST of the gel. Keywords: Salinity, hydrogels, mechanical properties

Abstract: Mo-Cr alloy cast iron is the most important material often used to make auto panel dies. To study high-speed machining process of auto panel dies, the material’s elastic modulus and fracture critical values of Mo-Cr alloy iron at 20°C-800°C were studied based on the high temperature elongation test. The material’s stress-strain relation curves at various temperatures (20°C-500°C) and various strain-rates (500/s-5000/s) were studied and the dynamic tensile yield strength values were obtained by dynamic SHPB(Split Hopkinson Pressure Bar) high-speed compression test. The test observation showed that Mo-Cr alloy iron has heat resistance and its behavior is between toughness and brittleness materials. Its toughness is enhanced with temperature increasing. At 20°C-500°C and strain-rates (500/s-5000/s) the dynamic tensile yield strength is decreased about 17％-24％ by temperature softening effect and is increased about 56％-70％ by strain-rate strengthening effect. The strain-rate strengthening effect prevails over temperature softening effect. Keywords: Auto panel die; Mo-Cr alloy cast iron; SHPB; Temperature softening effect; Strain-rate strengthening effect

Abstract: The paper regard pillaring agent which is the most important research content of pillared-montmorillonite as subject investigated. By changing the pH value of solution to form different pillaring agent which is used to prepare pillared-montmorillonite. Adsorption capacity of pillared-montmorillonite on SO₄^2-ion as standards, the effection of main process factors—pH value during the preparation of the pillaring agent on adsorption capacity of pillared-montmorillonite is explored.

Abstract: Hot deformation behavior of Fe-3%Si steel within temperature range of 1073~1473K and strain rate range of 0.01~5s⁻¹ was investigated by isothermal compression test using thermo-simulation method. Over the applied deformation conditions, steady state flow behavior was well described by the power law relationship with dislocation climb as the rate-controlling mechanism, and the high apparent activation energy can be attributed to the high yield stress. A modified Bergström model was proposed by introducing yield stress, and consequently the whole stress-strain curves can be accurately predicted.

Abstract: Strain-rate sensitivities of 55-65vol.% aluminum 2024-T6/TiB₂ composites and the corresponding aluminum 2024-T6 matrix were investigated using split Hopkinson pressure bar. Results showed that 55-65vol.% aluminum 2024-T6/TiB₂ composites exhibited significant strain-rate sensitivities, which were three times higher than that of the aluminum 2024-T6 matrix. The strain-rate sensitivity of the aluminum 2024-T6 matrix composites rose obviously with reinforcement content increasing (up to 60%), which agreed with the previous researches. The aluminum 2024-T6/TiB₂ composites showed hybrid fracture characteristics including particle cracking and aluminum alloy softening under dynamic loading. The flow stresses predicted by Johnson-Cook model increased slowly when the reinforcement volume fraction ranged in 10%-40%. While the reinforcement volume fraction was over 40%, the flow stresses of aluminum matrix composites increased obviously and the strains dropped sharply. Keywords: Composite materials; Dynamic compression; Stress-strain relationship