Materials Science Forum Vol. 749

Abstract: The growth behavior of 300M steel was investigated on a Gleeble-3500 simulator at the heating temperatures ranging from 1273 K to 1453 K and the heating rates ranging from 0.83 K/s to 40 K/s. The grain size of austenite was measured by using SISC IAS V8.0 image analysis software on Olympus PMG3 microscope. The experimental results showed that the coarse grains of austenite occurred at the heating temperature above 1413 K and the grain size of austenite increased with the increasing of heating temperature and decreased with the increasing of heating rate. The grain boundaries of austenite became flat and the angel of grain boundaries tended to 120˚ with the increasing of heating temperature. The grain boundaries of austenite increased and changed from flat to bend with the increasing of heating rate.

Abstract: In this paper, in order to optimize the extrusion die action and to control quality of an industrial large Al-profile, the effects of die bearing band on the metal flowing behaviors are analyzed by applying some special methods, including designing the bearing band of the die as inclined plan and designing the geometry model as detachable structure. The velocity field showed that the metal flowing velocities in central and left-side locations were higher than that in groove location. The reasonable size-configuration of the extrusion die bearing band was obtained, and the defect of the right-direction bending was able to be removed effectively. The inclined bearing band structure, detachable geometry model and configuration method based on velocity balance proposed in this paper provided a valuable reference to design and to optimize this kind of solid Al-alloy large profile.

Abstract: In this paper, the compression stress-strain curves of 7050 Al-alloy under various deformation conditions were obtained, and the recrystallization structures were analyzed. The main parameters of dynamic recrystallization model were determined. The rationality of the model parameters were verified by hot compression and extrusion. The results show that the accuracy of the model is high enough for predicting the recrystallization of the 7050 Al-alloy during hot deformation. Comparison of the experiment and calculated results in hot compression shows that the maximum relative error of the recrystallization fraction is 11.4%. Comparison of the experiment and calculated results in hot extrusion shows that the maximum relative error of the recrystallization fraction is 13.0%, and that of the recrystallization grain size is 14.9%.

Abstract: The effect of heat treatment on the microstructure and properties of Cu-3Ti-1Al alloy was investigated. The microstructure was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM), and the hardness and electrical conductivity were tested as well. The results showed that the hardness and electrical conductivity of Cu-3Ti-1Al alloy increased significantly after solid solution and ageing treatment. The strengthening effect of Cu-3Ti-1Al alloy was attributed to the formation of intermetallic phase such as Ti₃Al and fine precipitates of coherent β-Cu₄Ti. With increase of the aging time and the temperature, the precipitates became coarse and incoherent with Cu matrix, and the discontinuous precipitate β started to grow from grain boundaries toward grain interior, which decreased hardness. As the formation of Ti₃Al, β-Cu₃Ti and β-Cu₄Ti phase can efficiently reduce Ti concentration in Cu matrix. The electrical conductivity of Cu-3Ti-1Al alloy increases. In the range of experiments, the optimal heat treatment process for Cu-3Ti-1Al alloy is solid solution at 850°C for 4h and ageing 500°C for 2h, and the hardness and electrical conductivity are 227HV and 12.3%IACS, respectively.

Abstract: The effect of Quenching-Partitioning-Tempering (Q-P-T) process on the microstructure and mechanical properties of 300M steel were investigated. X-ray diffraction, scanning electron microscopy and optical microscope were employed to characterize the evolution of microstructure as well as the volume fraction of retained austenite. The results showed that the samples treated by Q-P-T possess more retained austenite compared with traditional Quenching-Tempering process. After Q-P-T, the strength increased slightly, while the ductility and toughness improved markedly. Microstructure analysis indicated the difference between Q-T and Q-P-T process lies in the amount of the retained austenite as well as the size and size distribution of the martensite laths. The Q-T resulted in a little of thin film-like retained austenite in addition to wide martensite strip, while the Q-P-T resulted in large amount of thick-like austenite accompanying narrow martensite strip. Therefore, 300M steel with better ductility and toughness could be obtained by Q-P-T heat treatment.

Abstract: The precipitates, mechanical properties and strengthening effect of Cu-Ni-Si,Cu-Ni-Si-Co0.8 and Cu-Ni-Si-Co1.9 alloys are investigated. It is concluded from the TEM and XRD analysis that the strengthening of Cu-Ni-Si-Co alloy is mainly attributed to precipitates of both Ni₂Si and Co₂Si phases, with the same structures and very close lattice parameters. The addition of 1.9 wt% Co barely affects the precipitation process of Ni₂Si or Co₂Si, and may not be beneficial for enhancing the strength. The strengthening of Cu-Ni-Si-Co alloy is determined by the Orowan mechanism, and a critical precipitate radii r_c about 1.5 nm corresponding to the peak strength is obtained through the theoretical analysis. It can be deduced that the peak strength of aged Cu-Ni-Si-Co alloy is obtained with the microstructures containing metastable phase and Ni₂Si (or Co₂Si) precipitates.

Abstract: Fused magnesia is an essential basic material for metal making and construction industries. Fused magnesia is usually produced with mineral arc furnace. In China, 1600 kVA arc furnaces are widely used as fused magnesia production facility. The unit power consumption for magnesia production is about 3000 kWh/t, higher than that in developed countries. In this research, a 3000 kVA new arc furnace was used to produce fused magnesia, and the unit consumption decreased to 2560kWh/t. The experimental results showed that the new furnace has good energy saving effect and market prospects.

Abstract: The microstructure and properties of Q&P steel were studied by means of tensile test, OM and SEM after simulating heat treatment process in salt bath furnace. The results showed that the main microstructure of Q&P steel was lath martensite and retained film austenite. With the increase of partitioning time, the morphology of the parallel martensite lath became clear and ordered. With the trivialness and disorder with massive martensite appearing, the yield strengths and tensile strength decreased initially and then increased. On the other hand, the elongation increased initially and then decreased. This was because of that the retained austenite is unstable at the beginning for low carbon content, and the carbide precipitated after a long partitioning time. Therefore, there was an optimum partitioning time to obtain the best properties combination. Under 250 quenching temperature and 350 partitioning temperature, partitioning time was 60s, the tensile strength and elongation were 1027MPa and 27%, respectively. The product of strength and elongation was up to 27729MPa·%.

Abstract: Sodium silicate solution-impregnated carbon/copper composites were prepared from electrolytic copper powder, graphite particulate and pitch coke powder through compression molding, carbonization and impregnation. The effect of different concentrations of sodium silicate solution on their mechanical and tribological properties was studied. The results showed that the density of the composites increased monotonically with increasing the concentration of the sodium silicate solution. As did the bending strength and compressive strength, and they reached maxima of 47.78 MPa and 170.06 MPa at 30 wt. % of sodium silicate, respectively. The friction coefficient and wear rate of composite decreased monotonically with increasing the concentration of sodium silicate due to the formation of a lubricating film. The wear mechanism of the impregnated composites was abrasive wear accompanied by slight adhesive wear. The netlike silicon dioxide skeleton reduced the probability of abrasive wear, forming an integrated lubricating film.

Abstract: In this study, TiH₂ and WO₃ powders were co-milled together, then the ultrafine powders were reduced at H₂ atmosphere, and W-Ti alloys were prepared by the solid phase sintering. The main purpose of WO₃ powder instead of the W powder was to use the transition of the lattice type of WO₃, and the lattice distortion and defects in the lattice of W would promote Ti atom diffusing into W. It was easy to form a W-rich solid solution and reduce the effect of Ti-rich phase. The results showed that when the milling time of WO₃-TiH₂ was 24h, the particle size of mixed powder reached nanoscale, and WO₃ particles were coated on the surface of TiH₂ particles. The particle size changed unobviously with the increase of the milling time. The XRD analysis showed that the milled WO₃ and TiH₂ were not decomposed in the milling process. When WO₃-TiH₂ milled powders were decomposed in the H₂ atmosphere at 800, WO₃ reduction was not sufficient, and the middle phase of WO₂ was existed. When the reduction temperature was 850 for 2h, WO₃ was reduced to W, and the phase of WO₂ was disappeared. A small amount of TiO₂ was formed by the decomposed Ti and decomposed O from WO₃. W-10Ti alloys were prepared by the solid-phase sintering with the reduced powders at 850. It was found that the amount of W-rich solid solution in W-10Ti alloy was decreased, because the diffusion of Ti to W was inhibited by a small amount of TiO₂ in grain boundaries during the sintering process.