Journal of Metallurgical and Materials Engineering
Year:2009 | Volume:21 | Issue:1|Papers Count:8

The Ni-base superalloy IN738LC is employed in manufacturing of the first stage blades of high power gas turbines. Its microstructure changes after 20000 hours in high temperature service. The microstructure degeneration is so that the blades loose their appropriate strength. Regarding their high price, microstructure recovery by heat treatment is a valuable job. Therefore, in this research, the effects of heat treatment parameters such as: time and temperature of homogenization, partial solution and aging temperatures, and cooling rate from homogenization and solution temperatures on the microstructure of the alloy are investigated. Microstructure of some specimens from used blades before and after applying of different heat treatment cycles was studied by SEM. Its results led to obtaining of the optimum cycle for producing optimum microstructure. Also, to verify the effect of the optimum cycle, creep test was carried out on the samples extracted from unused blades and on the samples extracted from used blades after applying the optimum heat treatment cycle. Comparison of the results confirmed the effectiveness of the optimum heat treatment cycle in rejuvenation of the blades.

Galvanizing kettle is one of the main components of hot dip galvanizing process. Welding is the final stage in galvanizing kettle fabrication. In the present research the effect of various welding parameters such as bevel pattern and chemical composition of filler metal on mechanical properties, microstructure and corrosion resistance of welded galvanized steel were investigated. Results showed that symmetry bevel pattern is more effective than the non-symmetry one in developing desired properties in welding zone. Although both patterns developed acceptable mechanical properties in welded coupons. NDT results confirmed that welding zone is free from defects such as porosity, inclusion trapping, lack of penetration and etc. Various tests on the electrode type showed Nilsil electrode is more effective than AMA 1531 and 2000 electrodes in producing sound joints. Corrosion tests showed that silicon is the most detrimental element on corrosion resistance of welded coupons.

In this paper a mechanism for fracture of GG25 Gray Iron has been presented under the non-constrained thermal shock. In order to constrain the residual stress on the Cast Iron matrix a notched sample with dimensions 5*5*20 mm from the GG25 were designed and machined. The notch had dimensions of 0.3 mm thickness and 2.5 mm depth. Then the samples were thermal shocked between cycles 900oC and 0oC in the ice-water and the non-oxidation slurry. Results of SEM metallographic and micro hardness show that thermal shocks can cause the Fe3C phase decomposition to carbon and ferrite. The precipitating of carbon on initial free graphite flakes can cause the growth and toughness of the graphite flakes. Then, due to difference of thermal expansion between the graphite and the ferrite, at the interface, micro cracks propagate into the ferrite phase and leads to fracture. However, dominant phenomenon for the quenched samples into the ice-water was the hot oxidation.

In production of magnesium by vacuum silicothermic reduction of calcined dolomite, Pidgeon process, experimental parameters such as temperature, time, density of briquettes and the ratio of stoichiometric requirement of silicon can be employed to control reduction efficiency and silicon efficiency. To model the effects of these parameters on reduction process, a basic statistical approach was utilized. Experimental data are inputs to the model. What about response of The model was compared with actual reduction data, in which temperature, time, density of briquettes and the ratio of stoichiometric requirement for reduction, were varied. A statistical "design of experiments" approach was undertaken in process experimentation and the optimium conditions of Pidgeon process were determined.

Understanding the thermal decomposition mechanism of titanium hydride, especially determining of the best conditions of the primary heat-treatment of this material, would be an important step on the process of production of Al foams. In this research, the STA diagram of titanium hydride powder with the heating ra te of 10oC/min in the presence of Argon gas with %99.99 purity and, also, STA diagrams of this powder with the heating rates 5, 10, 20, 25, 30oC/min at air were determined. In addition, the DSC diagram with the heating rate of 10oC/min at 500oC was studied and determined. Results showed that titanium hydride powder (TiH1.924) decomposes in two stages TiH1.924®d-TiHX®a-Ti, at 534oC and 624oC, respectively. The thermal decomposition mechanism of TiH2 complexes, in conventional atmosphere, is quite complex and different in comparison with the similar condition in the Argon gas. In the presence of air, hydrogen comes out from titanium hydride during eight stages. In the DTG diagram a minimum was noticed, at the temperature of 835oC, which can be the sign of reducing of the rate of oxidation and weight gaining of the powder. The thermal decomposition mechanism of TiH2 powder seems to be under control of both internal diffusion (diffusion of hydrogen and oxygen atoms within the layers of a solid solution, titanium oxide and titanium hydride) and the chemical reaction (titanium oxidation). The temperature of hydrogen excretion from TiH2, in the presence of air, increased relative to its excretion, in the presence of the argon gas at the first stage from 534oC to 550oC and at the second stage from 642oC to 660oC. Based on the results of this work it seems that the most appropriate temperature, for the primary heat treatment of TiH2 powder, is between 300-500oC.

In this paper, effect of cooling rate on dendrite arm space, porosities density and mechanical properties of composite has been investigated during degassing with H2. For this purpose a wedge sample was designed, and then the composite was cast under two degassing conditions: i. 100%Ar, ii. Before pouring, some thermocouples were placed into the wedge. Then thermal history of the wedge was saved in a computer using a data acquisition system which connected to the thermocouples. Then hardness and stiffness tests were conducted on the wedge. Finally, microstructures of the composites were investigated and relations between mechanical properties and cooling rate of composites were extracted. In the case of, results show effect of the micro-porosities on the mechanical properties will be omitted when the cooling rate controlled between.

MTDATA is a powerful software package for equilibrium conditions and phase diagram calculations. It can be applied in phase transformation or process simulations depending on the kind of databases they are connected to. The iron-carbon-aluminium-silicon (Fe-C-Al-Si) phase diagrams were calculated using MTDATA software and SGTE databases to determine the phases that are present at various temperature and composition ranges in equilibrium conditions. It was observed that the iron-carbon equilibrium could be modified by addition of aluminum. The austenite region decreased with increasing Al content, and in the case of cast irons containing about 4.88%Al or more, the austenite zone was disappeared. Further results obtained from heat treatment cycles and dilatometry examinations together with optical and electron microscopic investigations showed a good agreement with the calculated phase diagrams for low as well as high (6.16%) Al addition. However, there was a difference between the observations made for 4.88%Al. Experiments conducted on an alloy containing this Al content showed that the austenitic reaction occurred in the alloy, whereas no evidence was found for the-region in the corresponding phase diagram.