Advanced Materials Research Vols. 634-638

Abstract: The delayed fracture resistance of pre-stressed concrete (PC) steel bar with different vanadium contents was investigated by using tensile sustained load delayed fracture test and optical microscope. The experimental results show that delayed fracture resistance of vanadium containing steels is higher than that of conventional steel 30MnSi at the same strength level and it increases with the increase of vanadium content. The improvement is resulted mainly from the irreversible traps for hydrogen and the prior austenite grain size refinement by vanadium carbide particles.

Abstract: In the present investigation, 45CrMoV alloy steel was gas oxynitrocarburized at 530°C, 550°C and 570°C for 1h by applying a low-temperature gas multi-elements penetrating system. Microstructure, surface, composition, case depth, microhardness, wears and corrosion resistance were analyzed. The properties of samples treated at 570°C are optimum.

Abstract: Lead-silver-cobalt was electrodeposited on the aluminum matrix, which was used as anode for zinc electrowinning. Scanning electron microscopy (SEM), linear sweep voltammetry(LSV), Tafel curve were used to study the effect of different current density on the surface morphology, electrocatalytic activity and corrosion resistance of Al/Pb-Ag-Co anode. The results showed that the Ag content change slightly with the current density change, but the Co content change big with the current density change; the value of a and b oxygen evolution of Pb-Ag-Co composite coating electrode obtained under 1 Adm-2were the smallest, respectively, was 1.436 V, 0.930 V; and the corrosion potential was 0.272 V; surface microstructure was smooth and dense.

Abstract: Effect of Ti on the microstructure evolution and element partition in  and  phase after heat treatment were investigated in single crystal superalloy with different contents of Ti addition (0, 0.5, 1.0wt.%) by using scanning electron microscope (SEM) and 3-D atom probe (3DAP). The results show that with the increase of titanium addition, the size of r’phase increase after full heat treatment and the amount of TCP phase increase during long term thermal exposure. Ti makes more r matrix forming elements such Re, W, Cr, Co, Mo partition into r matrix and more Ni element distribute to the r’phase. The electronic structure and binding energy of a number of Ni+X alloy systems were calculated using the discrete variational cluster method based on the local density approximation of the density functional theory

Abstract: The silicon steel was rolled to 95% reduction at 20°C, 400°C and 600°C, and subsequently annealed at different temperatures to obtain complete recrystallization microstructure without appreciable grain growth. The effects of rolling temperature on through-thickness deformation and recrystallization textures were investigated by ODF analysis. The deformation textures are all composed of α- and γ-fiber, whereas α-fiber and {111}﹤110﹥ decrease and {111}﹤112﹥ increases with the increasing rolling temperature. Through-thickness recrystallization texture varied significantly, a strong partial γ-fiber spreading from {111}﹤112﹥ or {554}﹤225﹥ to {111}﹤134﹥ and {114}﹤481﹥ are developed in steel sheet rolled at 20°C and 400°C, while a dominated η-fiber peaked at {310}﹤001﹥ is formed between surface and quarter thickness in steel sheet rolled at 600°C. The different recrystallization textures can be ascribed to the profuse shear band at 600°C compared with the microstructures at 20°C and 400°C.

Abstract: Aluminum foam is a new type of material that can be used in many fields. Compaction conditions are the most important parameters that have influence on foam preparation process. So in this paper, detailed researches about extrusion methods and powder carriers are conducted. The results show that: compare with direct extrusion, pre-pressing can effectively eliminate the influences of hydrogen, obtain high density precursors. However, when the flank of the precursor is wrapped with copper, H2 can escape from the combination parts of Al and Cu, in early foaming stage. The minimum density is only 0.75g/cm3, pore structures are almost round and nearly no plateau borders exist, so the quality of aluminum foam is still poor. When there is no copper wrapped, an oxide layer can be formed in the whole body of the precursor and limit the escaping of H2. The minimum density can reach 0.45g/cm3, pore structures are polygonal with thin cell walls about 0.08mm. Thus high quality aluminum foams can be obtained by using pre-pressing and then extruding method and precursor sheet powder carrier.

Abstract: The flow behavior of Al-Zn-Mg-Sc-Zr alloy during hot compression deformation was studied by thermal simulation test at strain rate of 0.001 to 10s-1 and deformation temperature of 340 to 500°C on the Gleeble-1500 thermal mechanical simulator. The results show that the flow stress increases with increasing strain rate, and decreases with increasing deformation temperature. The flow stress of the alloy during the elevated temperature deformation can be represented by a Zener-Hollomon parameter with the inclusion of the Arrhenius term. The values of A, n, α in the analytical expression of flow stress are fitted to be 1.49×10¹⁰s⁻¹, 7.504 and 0.0114MPa⁻¹, respectively. The hot deformation activation energy of the alloy during hot deformation is 150.25kJ/mol.

Abstract: High-cycle fatigue fracture behavior of microalloyed bainitic steels with three different carbon and vanadium contents were studied using rotating-bending fatigue test and compared with the ferrite-pearlite type microalloyed steel F38MnVS. The results indicated that the fatigue properties of the microaIloyed bainitic steels had a significant relation to the microstructures in forging condition. Compared with the ferrite-pearlite type microalloyed steel F38MnVS, the bainitic steels possessed higher fatigue strength and lower fatigue limit ratio σ-1/Rm. It was found that the bainitic transformation temperature was decreased and the hardness of the bainitic ferrite was enhanced, at the same time, the fatigue strength was increased, however, the fatigue limit ratio was lower. Furthermore, according to the SEM images of the fracture surface of fatigue specimens, it was revealed that the fatigue cracks mainly initiated along the bainitic ferrite laths in the specimen surface and preferred to propagate along the length direction of laths.

Abstract: A computational fluid dynamic (CFD) model was developed to study the fluid flow phenomena taking place in an industrial tundish. Numerical results showed spatial distributions of the velocity vectors, the residence time and fields of turbulence kinetic energy. Selected computer simulation results were validated with experimental data. The effect of the impact pad and interior dams on the hydrodynamics of liquid steel flow were studied numerically and optimized to reduce the fraction of dead volume zones and augment nonmetallic inclusions to float into the slag. A novel design of a turbo-stopper was proposed and its function to decelerate the ladle shroud jet and direct the flow back to reduce slag entrapment was discussed. Such numerical results improved our understanding of the hydrodynamics of liquid steel flow in the tundish and contribute to an optimized operation.

Abstract: Advanced ferritic steels containing 9 wt% Cr are widely used in the construction of supercritical and ultra supercritical boiler components. Grade 91 is one of the most common alloys used in this application. The microstructure of the as supplied 91 materials consists of a tempered martensite matrix, a fine dispersion of intergranular chromium rich M23C6 precipitates and intragranular carbonitrides MX particles rich in V and Nb. This steel requires post weld heat treatment (PWHT) to produce a tempered microstructure after welding to develop excellent creep strength for high temperature service. Based on past experience, situations may arise whereby the components are subjected to an accidental ‘overshoot’ in temperature during PWHT. The consequence is the formation of deleterious phases which will result in undesirable changes in material property. In this research, P91 base metal specimens were heated to various peak temperatures in a laboratory furnace. Heat treatment parameters, as practiced at site, were applied. Peak temperatures applied were below Ac1, between Ac1 and Ac3, and above Ac3. Hardness measurement demonstrated a significant reduction once the Ac1 temperature was exceeded, due to the presence of soft α-ferrite matrix. As the temperature was increased towards Ac3, newly transformed fresh martensite which is hard and brittle in nature would form the dominant matrix. The phase transformation and precipitate morphology changes were studied using optical microscopy and scanning electron microscopy techniques. Three factors were identified to determine the phase transformation: (1) the homogeneity level and amount of precipitates dissolved in austenitic matrix upon heating; (2) slow cooling rate that may shift the cooling curve to enter ferrite nose and (3) deviation in chemical composition.