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Soltani N.,Sharif University of Technology | Soltani N.,CINVESTAV | Jafari Nodooshan H.R.,Shanghai JiaoTong University | Bahrami A.,Razi Metallurgical Research Center | And 4 more authors.
Materials and Design | Year: 2014

Al-15wt.% Mg2Si composites were prepared by in situ casting and characterized in wear tests. Previous to the extrusion of specimens at 470°C - varying extrusion ratio (7.4, 14.1 and 25), the as-cast composites were homogenized at 500°C for 5h, followed by slow furnace cooling. The microstructure, hardness and sliding wear behavior were characterized for both, the as-cast and hot extruded composites. Results show that increasing the extrusion ratio causes a significant improvement in hardness and wear resistance. This is ascribed to the observed decrease in average size and better distribution of Mg2Si particles, in tandem with a remarkable decrease in porosity percentages, which goes from 5.63 in the as-cast condition, to 0.47 at the extrusion ratio of 25. It was found that abrasion is the dominant wear mechanism in all extruded composites, whilst a combination of adhesion and delamination appears to be the governing mechanism for as-cast composites. © 2013 Elsevier Ltd.


Bahmani A.,Razi Metallurgical Research Center | Bahmani A.,Sharif University of Technology | Eisaabadi G.B.,Arak University | Davami P.,Razi Metallurgical Research Center | And 4 more authors.
Russian Journal of Non-Ferrous Metals | Year: 2014

The present study investigated the effects of initial Hydrogen level and cooling rate on ultimate tensile strength of commercial Al-A319 alloys. Three hydrogen levels (0.01, 0.2, and 0.41 mL/100 grams of melt) and five cooling rate were studied. Total of 45 tensile test bars was prepared (three hydrogen levels × five cooling rate × three repeats). The UTS of the samples was determined though uniaxial tension tests. Furthermore, the microstructures of the samples were studied by standard metallographic technique and image analysis software. Finally the relationship between UTS and microstructurai features—SDAS and fraction of porosity (Fp%)—of the alloys was investigated.Results of tensile test revealed: (i) UTS of the alloy decreased with increasing of hydrogen level or decreasing of cooling rate and (ii) Increasing of cooling rate beyond a certain value increased the UTS of the alloy significantly. Results of image analysis showed that the Fp% increased with increasing of hydrogen level and decreasing of cooling rate.Finally a Matrix Index [= −SDAS (μm) − 68.7 Ln (Fp%) + 275] was defined to correlate the tensile strength and microstructurai features of the alloy. It was shown that the UTS of the alloys had a linear dependence on matrix index according the below equation: UTS(MPa) = 0.916M.I. + 265.17R2 = 0.95. © 2014, Allerton Press, Inc.


Molaei S.,Shiraz University | Molaei S.,Razi Metallurgical Research Center | Alizadeh R.,University of Tehran | Attarian M.,Razi Metallurgical Research Center | And 2 more authors.
Case Studies in Engineering Failure Analysis | Year: 2015

The purpose of this study was to investigate the main causes of the co-fracture of sixteen connecting bolts of a filter press cylinder-piston system. Stress state of the bolts during the service conditions has been analyzed and the failure reasons were determined from the fractography analysis and gathered information. According to the obtained results, it was concluded that the bolts had failed by the fatigue mechanism. It seems that insufficient torque was used during assembly. © 2015.


Soltani N.,Sharif University of Technology | Bahrami A.,Razi Metallurgical Research Center | Moghimi F.M.,Islamic Azad University at Sāveh | Pech-Canul M.I.,Islamic Azad University at Sāveh | Hajaghasi A.,National Polytechnic Institute of Mexico
HTM - Journal of Heat Treatment and Materials | Year: 2012

This article investigates the simultaneous effect of extrusion ratio and T6 heat treatment on microstructure and mechanical properties of in-situ Al-15wt.%Mg2Si composite. his composite has already been introduced as a new class of light materials but the brittle structure of the primary Mg2Si which is formed during solidification limits its application. As-cast composite was directly extruded as rod by using three different dies. After T6 heat treatment on extruded samples, microstructure was studied by optical and scanning electron microscopy. Results demonstrated that extruded and heat treated composite possesses considerably higher strength and enhanced ductility in comparison with the as-cast samples. It was also found that heat treatment and extrusion processes change the primary Mg2Si morphology considerably and its size increases as extrusion ratio decreased. Fracture surface examinations revealed a transition from brittle fracture mode in as-cast composite to ductile fracture in heat treated and extruded specimens. his can be attributed to the changes in size and morphology of Mg2Si intermetallics and porosity content.


Fardi Ilkhchy A.,Sharif University of Technology | Fardi Ilkhchy A.,Razi Metallurgical Research Center | Varahraam N.,Sharif University of Technology | Varahraam N.,Razi Metallurgical Research Center | And 2 more authors.
Iranian Journal of Materials Science and Engineering | Year: 2012

During solidification and casting in metallic molds, the heat flow is controlled by the thermal resistance at the casting-mold interface. Thus heat transfer coefficient at the metal-mold interface has a predominant effect on the rate of heat transfer. In some processes such as low pressure and die-casting, the effect of pressure on molten metal will affect the rate of heat transfer at least at initial steps of solidification. In this study interfacial heat transfer coefficient at the interface between A356 alloy casting and metallic mold during the solidification of casting under pressure were obtained using the IHCP (Inverse Heat Conduction Problem) method. Temperature measurements are then conducted with the thermocouples aligned in the casting and the metallic mold. The temperature files were used in a finite-difference heat flow program to estimate the transient heat transfer coefficients. The peak values of heat transfer coefficient obtained for no pressure application of A356 alloy is 2923 (W/m 2k) and for pressure application is 3345 (W/m 2k). Empirical equation, relating the interfacial heat transfer coefficient the applied pressure were also derived and presented.


Soltani N.,Sharif University of Technology | Soltani N.,CINVESTAV | Pech-Canul M.I.,CINVESTAV | Bahrami A.,CINVESTAV | Bahrami A.,Razi Metallurgical Research Center
Materials and Design | Year: 2013

A zirconia/alumina nanocomposite stabilized with cerium oxide (Ce-TZP/Al2O3 nanocomposite) can be a good substitute as reinforcement in metal matrix composites. In the present study, the effect of the amount of 10Ce-TZP/Al2O3 particles on the microstructure and properties of Al/(10Ce-TZP/Al2O3) nanocomposites was investigated. For this purpose, aluminum powders with average size of 30μm were ball-milled with 10Ce-TZP/Al2O3 nanocomposite powders (synthesized by aqueous combustion) in varying amounts of 1, 3, 5, 7, and 10wt.%. Cylindrical-shape samples were prepared by pressing the powders at 600MPa for 60min while heating at 400-450°C. The specimens were then characterized by scanning and transmission electron microscopy (SEM and TEM) in addition to different physical and mechanical testing methods in order to establish the optimal processing conditions. The highest compression strength was obtained in the composite with 7wt.% (10Ce-TZP/Al2O3) sintered at 450°C. © 2013 Elsevier Ltd.


Bahmani A.,Sharif University of Technology | Hatami N.,Razi Metallurgical Research Center | Varahram N.,Sharif University of Technology | Davami P.,Sharif University of Technology | Shabani M.O.,Iranian Materials and Energy Research Center
International Journal of Advanced Manufacturing Technology | Year: 2013

In the present work, a mathematical model was developed based on finite difference method to predict the microporosity distribution in A356 aluminum alloy casting. Heat, mass, and gas conservation equations were solved in this model. Moreover, Darcy's equation was considered in the mushy zone. Results show that the distribution and concentration of microporosities in cast parts vary with both cooling rate and initial gas content. Simulation results were compared with experimental data where proportionally good agreement with experimental results was found. Finally, a complex cast part was simulated presenting the ability of the model to predict the porosities in industrial cast parts. © 2012 Springer-Verlag London Limited.


Jabbari Behnam M.M.,Sharif University of Technology | Davami P.,Sharif University of Technology | Davami P.,Razi Metallurgical Research Center | Varahram N.,Sharif University of Technology | Varahram N.,Razi Metallurgical Research Center
Materials Science and Engineering A | Year: 2010

This paper presents the results obtained and the deductions made from a series of microstructural studies and mechanical tests involving gray cast iron which was sand cast using a variety of modules. The effect of cooling rate on the primary dendrite arm spacing (DAS), secondary dendrite arm spacing (SDAS), thickness of ferrite-cementite layer (λe) and the hardness (HB) were evaluated. Results show that the both DAS and SDAS and also λe are highly dependent on the cooling rate, and they decreases as the cooling rate increases. More attempts were also done to correlate the HB with DAS, SDAS and λe. It was found that HB increases as DAS, SDAS and λe decreases. © 2010 Elsevier B.V.


Attarian M.,Sharif University of Technology | Karimi Taheri A.,Sharif University of Technology | Jalilvand S.,Razi Metallurgical Research Center | Habibi A.,Pardis Petrochemical Co
Engineering Failure Analysis | Year: 2016

In this paper, the failure of the reformer furnace tube, made of HP-Nb micro alloyed heat resistant steel is investigated. The failure was due to a longitudinal crack developing in the circumferential weld joint close to the outlet end of the tube, where the mean wall temperature had reached the highest value during service. According to the temperature records during the period of 7 years of being in service, the tube had experienced temperatures higher than the tubes designed temperature. This situation besides the occurrence of several sudden shutdowns caused the mechanical properties degradation.Macroscopic and microscopic examinations revealed that the failure had occurred due to the extensive fissuring initiated and developed from the heat affected zone (HAZ) adjacent to the root weld. The results of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) analysis, and the crack growth rate calculations indicated that the crack propagation rate was controlled by the overheating and the secondary thermal stress due to the sudden shutdowns.Moreover, the ductility at the tip of the crack had dwindled due to the nitrogen uptake at high temperature, which promoted the crack growth rate. Quantitative microstructural analysis performed at the uncracked and cracked sides of the weld joint showed that the area fraction of HAZ creep cavities were 0.3% and 0.7%, respectively, which were interpreted in turn as the accumulated creep damage level D and E. Finally, it was concluded that the creep cavities alignment and fissuring that took place at HAZ are the main causes of the cracking and failure of the tube. © 2016 Elsevier Inc.


Movahedi-Rad A.,University of Tehran | Plasseyed S.S.,Razi Metallurgical Research Center | Plasseyed S.S.,Sharif University of Technology | Attarian M.,Razi Metallurgical Research Center | Attarian M.,Sharif University of Technology
Engineering Failure Analysis | Year: 2015

The failure analysis of a ruptured superheater tube after 20. years service in the oil-fueled boiler, as the typical problems in power plants, was investigated. A thin-lipped rupture at failed region was observed in superheater tube. By measuring the tube's wall thicknesses far from failed region, non-uniformity was seen. The suggested main root cause of failure was fireside corrosion of the tube during the service. Because of low grade of used fuel, sodium, sulfur, and vanadium elements were observed at the outer surface, which caused continuously scale formation and reduction of wall thickness, by metal consumption. In addition, it seems that it has been worsened by occurrence of long-term overheating. Coagulation of carbides at both outer and inner regions of tube was observed that could prove the occurrence of overheating during the service. In addition, the formation of sigma-phase particles was revealed because of being in the susceptible temperature after 20. years in service. At the end, in order to prevent or decelerate such failure, some recommended remedies were suggested. © 2014.

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