Volume 37, Issue 4 -

The effect of heat treatment on the microstructure and thermal fatigue properties were studied by means of optical microscope (OM) and scanning electron microscope (SEM). Energy dispersive X-ray detector (EDX) was used to analyze the role of phase composition in fatigue crack propagation. The results show that after heat treatment, the ultimate tensile strength increased from 285 MPa to 368 MPa and the elongation increased from 5.8 % to 6.5 %. During the initiation of fatigue crack, the crack was mainly propagated through eutectic Si area. With the long needles of eutectic Si particles spherodized after heat treatment, the split action from brittle Si particles to α-Al matrix was reduced and prolonged the fatigue crack initiation period. After aging for 6 h, the dispersed precipitation of secondary phases (Al 2 Cu, Mg 2 Si) elevated the driving force of crack propagation, blocked the spread of crack in the grain boundary, decreased the rate of fatigue crack growth and improved the fatigue resistance of alloy at the same time. In the process of crack initiation, the surplus-phase around the grain boundary fell off from α-Al matrix under thermal cycling stresses. The combination of interfaces was weaken by cycling stress and the fatigue crack was finally grown up in the weakness area between matrix and secondary phase.

Weld bowing distortions are detrimental to the assembly process, where laser process parameters such as laser power, welding speed, defocusing distance and gas flow rate play a significant role in determining the weld bowing distortion. Herein, weld bowing distortions in 1-mm-thick AA5052 aluminum were measured by the digital image correlation technique following laser welding. Two mathematical response models were developed to predict the laser weld bowing distortion according to the central composite rotatable design method. The optimized process parameters for minimum bowing distortion were obtained, and the influence of the laser process parameters on the weld bowing distortions was found.

To clarify the interaction mechanism between distinct tundish gunning materials (GM) and liquid steel, two kinds of gunning materials, namely MgO, Al 2 O 3 GM, were tested. SEM, EDS, XRD and chemical analysis were carried out for steel and GM to investigate the change of chemical composition of steel in contact with GM and the interface microstructure between steel and GM after high temperature holding experiments. The steel cleanliness in terms of inclusion number density, size and size distribution was evaluated. It was found that the reducible component, low-melting-point phases and the pores in GM were passageway for steel penetration. The Al 2 O 3 GM was less prone to steel penetration due to its poor wetting and the dense transition layer. MgO GM provided more oxygen and showed a stronger oxidizing capacity due to its higher content of reducible oxides (10.5 wt.% SiO 2 +2 wt.% Fe 2 O 3 ). The use of Al 2 O 3 GM resulted in an improved steel cleanliness and consequently could be a promising refractory in the tundish lining.

Ti-17 titanium alloy was treated with laser shock processing (LSP) and tested for impact toughness. Residual compressive stress, cross-section hardness, microstructure, and the impact toughness properties of the treated alloy samples were evaluated. The average impact toughness of the samples subjected to LSP increased by 10 J compared with that of the base material. The samples subjected to LSP remained connected after an impact test, whereas untreated samples completely ruptured. The location of fracture initiation shifted from the center to the side to the Charpy-V notch region because of the stress concentration induced by LSP. The stress concentration was induced by the uneven microhardness of the samples treated with LSP. The hardness of the irradiated surface increased compared with that of the base metal because of the higher dislocation density and smaller grains produced by LSP.

Microstructure evolution in Co-20Cr-15W-10Ni alloy with hot deformation and subsequent annealing has been studied using hot isothermal compression at different temperatures and strain rates followed by annealing at 1,423 K for 1 h. In the as-deformed condition, optical microstructure reveals deformation bands in specimens deformed at low temperature (1,323 K) and high strain rate (10 s –1 ). In deformation beyond 1,423 K, very fine recrystallized grains are observed at all the strain rates, which is seen to be fully developed at 1,473 K onwards especially at lower strain rates (0.1 and 0.01 s –1 ). EBSD maps reveal recrystallized grains and presence of deformation twins. Grain growth is observed at strain rate lower than 0.1 s –1 and temperature more than 1,473 K. After annealing of deformed samples, fully recrystallized grains and annealing twins have been observed. Samples subjected to prior deformation at higher strain rates showed mixed grains with small banding of recrystallized structure, whereas more uniform recrystallized microstructure is observed at slow strain rates. Microhardness decreases with increase in deformation temperature. However, annealed hardness is found to be increasing marginally with decreasing prior deformation temperature (especially for slower strain rates), attributing to relatively uniform and fine microstructure after annealing.

The oxidation behavior of Super 304 H, Sanicro 25, HR3C and HR6W steels, which are recommended for use in ultra-supercritical power plants, as well as corrosion resistant X2CrNiMo17-12-2 steel was studied in this work. Oxidation tests were carried out under thermal shock conditions in an oxygen-rich environment (containing 50 vol. % water vapor) at a temperature equal to 750 °C. The investigated steels (excluding X2CrNiMo17-12-2 steel) are characterized by good oxidation resistance under thermal shock conditions. A highly protective Cr 2 O 3 layer was formed in the internal part of scales growing on the surfaces of investigated steels. The X2CrNiMo17-12-2 steel has worse oxidation resistant properties than the other grades of steels.

NiO/YSZ anode layers for metal-supported solid oxide fuel cells (MS-SOFCs) were fabricated by spark plasma sintering (SPS). SPS parameters were optimized in order to achive anodes of the desired microstructure. The effect of sintering conditions on microstructure of NiO/YSZ was studied by scanning electron microscopy and X-ray diffractometry. Also NiO/YSZ layers were formed on porous metal supports by a screen-printing method and sintered in inert atmosphere and vacuum by conventional sintering technique. At temperatures above 1,200 °С in inert atmosphere and vacuum nickel oxide dissociation and its massive agglomeration are observed during conventional sintering. SPS process allows sintering of NiO/YSZ granules without NiO dissociation, Ni agglomeration and the metal substrate oxidation at 1,100 °С. SPS sintered anodes demonstrate sufficiently homogeneous distribution of NiO and YSZ making a conduction path for electrons and ions. Well-bonded metal support/anode interface was obtained.

With the increase of hot metal ratio in electric arc furnace (EAF) steelmaking process, physical sensible heat and chemical latent heat of gas increased significantly. As EAF raw material condition is similar to basic oxygen furnace (BOF), and the condition of BOF gas waste heat utilization technology is mature, waste heat utilization technology in EAF steelmaking will be getting more and more attention. Scrap preheating and steam production as mature technology is the main way of EAF gas waste heat utilization. Power generation converted high temperature steam to electricity will further improve the EAF gas utilization value. The previous ways are to recycle physical sensible heat of EAF gas. To use chemical latent heat of gas, the secondary combustion technology is usually adopted to make CO fully burn into CO 2 . Coal gas production can fully recycle the chemical latent heat of gas theoretically, which is higher efficiency than other ways. Coal gas production needs a stable steelmaking process to stabilize high temperature gas. And the way need to develop EAF sealing technology, oxygen removal technology and gas purification technology, to make gas content meet the requirements of coal gas production in EAF steelmaking process.

The electrolysis of CO 2 was investigated using a simple Pt/YSZ/Pt cell (yttria-stabilized zirconia (YSZ)). The cell performance increased drastically after applying the overvoltage of 4.0 V for ~5 min. The current density increased 30 times at 1.4 V and 11 times at 2.0 V. Using a laser microscope and field emission scanning electron microscopy (FE-SEM), the structural changes of the surface of the Pt electrode and the interface between the YSZ and Pt electrode were investigated to understand the reaction mechanism at the cathode for increasing the cell performance. The results showed that the Pt electrode surface became smooth after the experiment. Moreover, the interface between YSZ and the Pt cathode greatly changed after the electrolysis experiment, while that of the anode showed only a minor change. The structure of the cathode interface consisted of a 1-μm-thick nano-porous Pt layer and a micro-porous structure comprising YSZ and Pt. These structures are involved in CO 2 electrolysis. Possible elementary reactions are also presented in this paper.

In the present work, a coupled three-dimensional numerical model of fluid flow, heat transfer, and inclusion motion during the solidification of molten steel in slab continuous casting mold has been developed. Based on the model, this paper has studied the inclusion capture during the process. The influence of the primary dendrite arm spacing on inclusion capture has been considered. The inclusion distributions, total masses, and average diameters at different depth from the slab surface have been given out in the present paper. The simulation results revealed the inclusion concentration existed in the solidification process, and the inclusion capturing area varies with the depth from the slab surface.

The oxidation of stainless steel with Fe-Sn intermetallics adhesion in 300 °C water was studied by analyzing the morphologies and phase structures of surface oxide films. The oxidation behavior of stainless steel was discussed in order to illustrate the effect of Fe-Sn intermetallics on the oxidation characteristic. Larger size of faceted spinel and small size of SnO 2 particles are observed on the samples with Fe-Sn intermetallics adhesion, whereas smaller faceted hematite and spinel grow on the samples without Fe-Sn intermetallics adherence. The surface intermetallics aggravates the spalling of oxide particles and decreases the corrosion resistance of stainless steel. Such deterioration is attributed to the loose microstructure characteristic and the poor corrosion resistance of SnO 2 particles formed in high temperature water.