Volume 33, Issue 4

This investigation was conducted to examine the probable cause of failure of one pendent reheater tube of a 110 MW thermal power plant. Preliminary macroscopic examinations along with visual examination, dimensional measurement, chemical analysis, evaluation of mechanical properties, oxide scale thickness measurement were carried out to predict the probable cause/causes of failure. In addition, optical microscopy was a necessary supplement to understand the reason for the failure. It was concluded that due to progressive erosion caused by the high velocity flue gas streams on the fire side wall, the tube had thinned down and could not cope the prevailing working stress. As a result, the affected areas of the tube suffered plastic deformation. Apart from that, formation of micro creep cavities due to rise in tube metal temperature (on the steam side) at localized places led to initiation of minor cracks before culminating to final rupture. The final failure is due to both these mechanisms occurring simultaneously.

In the present work, studies of phase transitions and morphology evolutions during hydrogen reduction of MoO 3 to MoO 2 were carried out in the temperature range from 678 to 829 K. It was found that no intermediate phase was formed and MoO 2 was the unique detectable product during the reduction at 678 K. While above 713 K, an intermediate phase of Mo 4 O 11 was formed. The formation of Mo 4 O 11 was found to obey the consecutive mechanism under the experimental condition. Regarding the morphology evolution of MoO 2 during reduction, it was found that in the temperature range from 713 to 733 K, MoO 3 was firstly reduced to large spherical or oval Mo 4 O 11 grains, then to small spherical or oval MoO 2 ones. From 793 to 829 K, most of MoO 3 grains were firstly reduced to large plateshaped Mo 4 O 11 grains, subsequently to small plateshaped MoO 2 ones.

Phase equilibrium was investigated in the pseudo-ternary system of Na 2 MoO 4 -MoO 3 -La 2 O 3 at 1073 K in air.

Carbon dioxide is a greenhouse gas and substantially affects the global warming and climate change, so study on the adsorption of carbon dioxide is very urgent. As a new CO 2 captor, Ba 2 Fe 2 O 5 was prepared by the solid state reaction of Fe 2 O 3 with BaCO 3 , following formula Fe 2 O 3  + 2BaCO 3  = Ba 2 Fe 2 O 5  + 2CO 2 . The reaction kinetics in isothermal condition was investigated by using the method of thermo-gravimetric analyzer (TGA). It was found that the reaction of Fe 2 O 3 with BaCO 3 was controlled by the diffusion step in the product layer, and the kinetics process could be described by the RPP model (Real Physical Picture) with the apparent activation energy extracted to be 161.122 kJ/mol.

The sharp strength recession of silicate cement in high temperature is the crucial reason of casings damage and zonal isolation failure in heavy oil thermal recovery. Although aluminophosphate cement has a better high temperature resistance in comparison with silicate cement, its compressive strength recession in high temperature slightly recessed. The results show that adding slag into aluminophosphate cement can not only develop compressive strength of cement at low temperature, but it can also improve the high temperature resistance of the cement. After adding slag, the formation of C 2 ASH 8 conduces to develop cement strength at low temperature, and C 3 AS 2 H 2 conduces to high temperature resistance. To increase temperature resistance of aluminophosphate cement, C 3 ASH 4 generation and Al(OH) 3 decomposition should be avoided. Crystal structure of cement after high temperature is well developed with compactly and neatly arranged, allowing cement to maintain good mechanical properties to help protect the casing and improve zonal isolation performance.

The effects of ageing and hydrogen charging on the notch tensile properties and fracture behaviour of individual heat-affected zones (HAZ) and Ni-based weld metal (Ni WM) of T91/TP316H weldments were investigated. After the post-weld heat treatment at 750 °C for 1 h the weldments were annealed at 600 °C for 1000 h and 5000 h, respectively. All heat-treated states were studied in condition without as well as with hydrogen charging. Thermal expositions led to additional precipitation and microstructure coarsening but their influence on tensile strength was insignificant. In contrast, remarkable plasticity decrease and the fracture mode transition from ductile dimple tearing to transgranular cleavage were observed. The combined effects of thermal exposure and hydrogen charging were more complex. Whereas the regions of Ni WM and TP316H HAZ did not show any significant change in strength, the hydrogen effect caused the strength increase in T91 HAZ. Although the hydrogen embrittling effects were clearly manifested by decreasing plasticity, their significance was getting smaller with increasing annealing duration. The fracture behaviour of thermally exposed and hydrogen charged regions exhibited mixed fracture modes including transgranular cleavage, intergranular dimple fracture and intergranular decohesion.

The hardness, elastic modulus, and friction coefficient of the CuZn alloy before and after treatment at 700 °C and 3 GPa for 10 mins were measured by nanoindenter. Effects of high pressure heat treatment on its microstructure and micro-mechanical properties were discussed. The results show that high pressure heat treatment can refine the microstructures of the CuZn alloy, and its hardness, elastic modulus, ratio of hardness, and elastic recovery rate after the 3 GPa treatment increased by 15.44 %, 4.89 %, 8.82 % and 35.90 %, respectively. And the friction coefficient is also decreased.

The effect of high density polyethylene (HDPE) addition on the pre-reduction of Mn 3 O 4 to MnO by metallurgical coke (Coke) has been investigated through experiments conducted in a laboratory horizontal tube furnace coupled with off-gas analysis through an infrared (IR) gas analyser. Composite pellets of Mn 3 O 4 with Coke, HDPE and blends of Coke with HDPE (in three different proportions) were rapidly heated at 1150 °C under pure argon gas and the off gas was analysed continuously for CO, CO 2 and CH 4 . The extent of pre-reduction of Mn 3 O 4 to MnO was then calculated by mass balance for removable oxygen. The results showed improvements in the extent of pre-reduction of Mn 3 O 4 to MnO when coke is blended with HDPE. The time for complete pre-reduction was found to decrease with an increase in the amount of HDPE that was blended with coke. A decrease in CO 2 emissions was observed with HDPE addition.

To investigate the optimum conditions of the CuInO 2 (CIO) preparation by cation exchange reaction method, the standard entropy, thermal capacity at constant pressure, standard enthalpy and standard Gibbs free energy of formation were estimated. The relations of temperature ( T ) and thermodynamic equilibrium constants ( K ), the conditions for three independent reactions were analyzed in CuInO 2 preparation. The curves of Gibbs free energy- T , K  −  T and P CuCl  −  T for those reactions have been plotted. The results showed that the CIO powder could be synthesized by cation exchange reaction. There were two main reactions through the way of NaInO 2 reacting with CuCl. One was NaInO 2(s) reacting with CuCl (s) at T  ≥ 1698 K. The cost was higher in the way. The other was NaInO 2(s) reacting with CuCl (g) at T  < 1698 K. However, the yield of CIO would be decreased because CuInO 2 powder decomposed into Cu 2 O (s) and In 2 O 3(s) with the temperature reduced, especially when T  ≤ 573 K. And the higher vapor pressure of CuCl (g) was necessary for the process, which needs the higher temperature since the vapor pressure of CuCl (g) increases with increasing temperature, especially when T  > 973 K. It was recommended that NaInO 2(s) react with CuCl (g) for preparing CIO under conditions of 973 <  T  ≤ 1698 K and the vacuum in the reactor.

The interactions between carbon and nitrogen in liquid silicon have been studied experimentally. High purity silicon was melted in silicon nitride crucibles under an Ar atmosphere with a graphite slab inserted in the crucible prior to melting as a carbon source. The system was thus simultaneously equilibrated with Si 3 N 4 and SiC. Samples were extracted in the temperature range 1695–1798 K and analyzed using Leco. It was observed that the simultaneous saturation of nitrogen and carbon caused a significant increase in the solubilities of both elements. The interaction parameters were derived as The solubility of carbon in liquid silicon as a function of temperature and nitrogen content was found to follow: And the solubility of nitrogen in liquid silicon found to follow:

The deformation behavior of 7005 alloy was studied by hot compression tests. The processing map was constructed by superimposing the instability map over the power dissipation map at a strain of 0.7 using the corrected flow stress data to eliminate the effect of friction. Microstructural examination was performed for validation. It can be found that the flow stresses increase with the decrease of deformation temperature or the increase of strain rate. At the relatively high strain rates, the material exhibits flow instability manifesting as adiabatic shear bands or flow localization. A large volume of coarse precipitations distributing in the grain boundaries in one of the peak efficiency domains: 275–325 °C/0.0005–0.001 s −1 , which may result in inter-granular corrosion and spalling layer, should be avoided in the final deformed alloy. The optimum hot working domain is the temperature range of 400–450 °C and strain rate range of 0.0005–0.005 s −1 , at which DRX is identified.

In order to prepare FeTi alloy using ilmenite concentrate in molten salt by electrolytic method, the electrochemical deoxidation process was studied by cyclic voltammetry and AC impedance spectroscopy. The electrolytic experiments at different time verified the electrochemical behavior. It is shown that the electrodeoxidation process for ilmenite concentrate mainly consists of two steps. Fe is reduced firstly from the ilmenite concentrate. The second step is the reduction of titanium dioxide with an intermediate product perovskite. At low voltage, the speed of electro-deoxidation process is mainly dependent on the electrochemical reaction. As the potential increases, the diffusion of ion becomes the main rate-controlling step for the electro-deoxidation of ilmenite concentrate.