Volume 33, Issue 3

Molybdenum and molybdenum alloys are considered to be attractive structural materials for high-temperature applications. However, molybdenum alloys are sensitive to gas impurities and have the characteristics of low temperature embrittlement and less resistance to oxidation at elevated temperature. The toughness and strength of welded joint is not easy to be ensured by traditional technology. Recently, many efforts have been made to join molybdenum and its alloys. In this paper, we present the result of investigations on welding methods of molybdenum and its alloys and overview the practical applications in engineering. The key of joining molybdenum alloys is to improve the toughness of welded joint and prevent the generation of pores and cracks.

Accelerated creep or stress rupture data is used for remaining life assessment for life management studies of elevated temperature components, e.g. for reformer tubes where packed nickel catalysts are used for synthesis of hydrogen, ammonia etc. This research has become a regular task because of large range of time for failure (3 to 15 years) compared to designed life (11.4 years or 100,000 hours) and huge loss associated to damage, production and safety hazards. Utilization of appropriate inspection during plant shut down has been strategic short term life assessment. Tests have been typically done by high temperature mechanical properties, microstructure analysis and accelerated creep. Inspection of micro-cracks, hot spot formation, carburization/metal dusting for inner wall and oxidation, tube diameter increment for outer wall inspection have been traditional symptoms of expiry of tubes after service exposure. Aim of this review has been to study damage analysis of reformer tube in response to so wide time frame for failures and accidents involved, even after stipulation of designed time schedules.

High temperature mechanical properties of service exposed reformer tubes are compared and revalidated with those of virgin material for the sake of health assessment studies. Technical interpretations of creep and stress rupture (accelerated creep) data usually are in close proximity in absence of defects. Results of extrapolation of accelerated creep data were optimized. This review article is aimed at studying life prediction methodology of reformer tubes operating at high temperature and aggressive chemical environment, in response to the designed time frame for preventing failures and accidents, even before stipulated design time schedules in some cases.

In this study, T5 heat treatment was applied to AA6063 alloy aged at 455 K for 2 hours after extrusion at 686 K. T6 heat treatment was also carried out by ageing at 455 K for 2 hours after solution heat treatment at 794 K for 1 hour. Heat treated T5 and T6 specimens were tested by pin-on-disc type wear equipment. Wear test was carried out by using 10, 20, 30 N loads and 400, 800, 1200 and 1600 m wear distance. T5 and T6 heat treated specimens were characterized with scanning electron microscope, X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and hardness measurements. Worn surfaces of the specimens was also characterised with SEM. The results indicated that small and homogenously dispersed Mg 2 Si precipitates formed in AA6063 aluminium alloy with T6 heat treatment were compared to the T5 heat treatment. As a result of increment precipitate size, wear resistance decreased. T6 heat treated specimens showed higher hardness compared to the T5 heat treated specimens. In addition wear resistance and friction coefficient of both T5 and T6 heat treated specimens decreased with increasing applied load.

Mo-MoSi 2 functionally graded coating on Mo substrate is prepared by siliconizing technology. The characters of Mo-MoSi 2 functionally graded coating are analyzed by SEM, EDS, GDS and XRD. The result shows that Mo-MoSi 2 functionally graded coating is dense and smooth, without pore and crack defects, gradient coating and Mo matrix achieve good metallurgy bond. The silicon molybdenum atom ratios (Si/Mo) in functionally graded coating appear three changing regularities. When the Si/Mo of outer layer are from 6:1 to 2:1, the surface phase is constituted by MoSi 2 and Si; when the Si/Mo of interlayer is about 2:1, the interlayer phase is constituted by MoSi 2 ; when the Si/Mo of transitional layer are from 2:1 to 0:1, the transitional layer phase is constituted by Mo 5 Si 3 and Mo. The phase structure of Mo-MoSi 2 functionally graded coating from outside to inside is outer layer MoSi 2  + Si, interlayer MoSi 2 and transitional layer Mo 5 Si 3  + Mo 3 Si three-layer structure. The thermodynamic calculation results are consistent with the experimental results substantially.

Defect structure of NiS 2 disulphide has been studied using marker technique and sulphidation rate measurements of NiS to NiS 2 , as a function of temperature and sulphur vapor pressure. It has been found that cation sublattice of the discussed compound is predominantly disordered and the prevailing defects are doubly ionized interstitial cations and quasi-free electrons. Using kinetic results, the self-diffusion coefficient of cations in NiS 2 has been calculated basing on Wagner's theory of metal oxidation.

The oxidation of Fe-36Ni invar alloy at 1273–1473 K in 15%CO 2 -(0, 3, 5, 6)%O 2 -N 2 atmosphere and the effect of the grain structure on the compressed cracks were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and a microcomputer control universal material testing machine. The results showed that the oxygen content and the oxidation temperature had an obvious influence on the morphology of the external scales, which changed into only two-layer structures from three stratified structures when the oxygen content was 0% or the oxidation temperature was below or equal 1423 K. However, the subscales still consisted of a typical structure of the intergranular subscale and the intragranular subscale with different oxygen contents or oxidation temperatures. The growth rate of the scales did not depend on the partial pressure of oxygen when oxygen existed. The oxidation temperature had an obvious influence on the growth rate of the scales, the growth rate of the scales was slower below or equal 1423 K and faster over 1423 K. A linear relationship existed between the thickness of the scales and the square root of oxidation time, moreover, the estimated activation energy for the growth of the external scale, the intragranular scale and the intergranular scale were 165, 159 and 131 kJ/mol, respectively. The higher apparent activation energy corresponded to the thinner scale at each oxidation temperature. In addition, the compressed cracks were hard to propagate and grow in the specimen with equiaxed grain structures, which would result in the narrower and shallower compressed cracks and the better surface quality during the compressed process after oxidation.

The effect of titanium on the homogenizing process of alloy 2618 and the microstructure of the Ti-containing alloy before and after homogenization has been investigated using optical microscope (OM), scanning electron microscope (SEM), energy dispersive X-ray (EDX), differential thermal analysis (DTA). The results show that the addition of 1.5 mass% Ti to alloy 2618 could refine its grain size remarkably. A large amount of rod-shape Al 3 Ti particles with the size range of 2–12 μm distribute evenly and dispersively in the alloy. It is θ (Al 2 Cu) phase instead of S' (Al 2 CuMg) obtained in eutectic structures due to the non-equilibrium solidification in casting of 2618-Ti alloy, which is the same as alloy 2618. The optimal homogenization temperature (773 K) for Ti-containing alloy 2618 (2618-Ti alloy) is the same as that of alloy 2618. Combining the results of kinetic analysis and experiments, the optimized homogenizing time for Ti-containing alloy 2618 at 773 K is only 12 h, which is shorter than that for alloy 2618. The shortening of the optimized homogenizing time results mainly from the decrease of the grain size of 2618-Ti alloy.

The corrosion of structural materials is a great challenge for the applications of a molten salt reactor using molten fluorides. In this paper, electrochemical behavior of pure Fe, Ni and Cr has been investigated in molten (Li,Na,K)F at 973 K by potentiodynamic polarization and electrochemical impedance spectroscopy. The experimental results indicated that three metals are all in active state at the corrosion potential, with an increase in the corrosion potential by the following order: Cr, Fe, Ni. The free corrosion current density of Ni is more than one order of magnitude lower than that of Fe and Cr. A Warburg impedance was observed in the impedance plots for the corrosion of Cr, but not for Ni and Fe. Ni is the most stable metal, then Fe and Cr, and the corrosion of Cr is controlled by the diffusion of oxidants in the melt.

In this study, the effects of thixoforming, both equal channel angular pressing (ECAP) and thixoforming on high cycle fatigue and fatigue surface morphology of AA7075 have been examined. Experiments are carried out with the same sample materials (AA7075) at a constant temperature (483 K) and the ``C'' route for 4 passes at ECAP process. In the process of thixoforming is 20 min at 888 K for waiting and 1 min at 673 K for pressing implemented. 140 MPa, 120 MPa and 100 MPa strength values were used at fatigue tests. The microstructural characterizations of the samples were carried out by using optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). This study is an attempt in detail to transformation fine and spherical grain structure with thixoforming process of minimized grain structure by ECAP. As a result of this study, it was seen that ECAP (1 pass) + semi-solid processing (SSP) applied samples have the highest hardness value (171 HV). When the values that are obtained after fatigue strength analyzed, SSP applied materials' property gave the best results and ECAP (1 pass) + SSP applied samples' results were second. When the both process applied materials' optimum values are investigated, it was observed that ECAP 1 pass + SSP applied material is more appropriate in terms of high hardness and fatigue life.

The high-temperature oxidation behavior of Fe-Si-Y alloys has been studied at 1173 and 1273 K in 0.1 MPa flowing pure O 2 . Results show that as the content of rare-earth yttrium raises, the grain size of Fe-Si-Y alloys decreases obviously, which increases grain boundaries in the alloys. The grain size of the oxides formed on the grain-fined alloys is also decreased. These enhance the diffusion rate of the alloying elements through the alloy matrix and oxide scales, and promotes the formation of SiO 2 and Y 2 O 3 . However, a single and continuous SiO 2 or Y 2 O 3 scale still does not form on yttrium-containing alloys when being oxidized at 1173 or 1273 K, but their rapid formation and partial lateral connection due to the grain refinement can restrain the element diffusion in the alloys to some extents, and thus the oxidation resistance of the alloys is improved. The mechanism related with the effect of the grain refinement resulting from the addition of rare-earth yttrium on high-temperature oxidation behavior of Fe-Si alloys is also discussed.

Abstract: Tungsten-copper (W-Cu) composite and 1Cr18Ni9 stainless steel were brazed with Ag-28wt%Cu brazing filler metal in a vacuum furnace, via controlling brazing temperature 855–865 °C and holding time 30 min. Microstructure, microhardness, four-point bend strength, element distribution and phase constituents near interface of the brazed joint, were investigated by a series of standard technique. Results indicated that Ag-Cu eutectic phase near W-Cu composite side and the bulk phase near 1Cr18Ni9 steel side were formed in the brazing seam region. Microhardness of the eutectic phase was low, while that of the bulk copper-rich phase was high. Four-point bend strength was as high as 576 MPa, and the fracture occurred at interface near 1Cr18Ni9 steel side. On the brazed interface, Cu and a little W element were transited from W-Cu composite into the brazing seam region, as same as Fe from 1Cr18Ni9 steel. Ag contained in the brazing filler metal was diffused into the W-Cu composite. Element concentration of Ag was high at interface of both two base metals, which was helpful to promote the metallurgical reaction. Phase constituents of the brazed joint were Cu(Ag, Fe), Ag(Cu) and a little of Ag 0.7 Fe 0.3 and FeCu 4 , formed in the metallurgy of Fe from 1Cr18Ni9 steel and the Ag-Cu brazing filler metal.