Boilers have a great importance in the refinery and power plant industries. There are many factors that affecting the boiler’s performance. In this study, root case failure analysis of the superheater boiler tubes in the Gachsaran Gas and Liquefied Gas plant were carried out. Chemical composition of these boiler tubes was corresponding to a steel grade A213T12. A thin-lipped rupture at failed region was observed in the superheater tube. This was associated with a heavy oxide scale formation inside the tubes. Therefore, visual inspections, chemical analysis of the scale, and the microstructural examinations of the boiler tubes were performed. Moreover, the temperature distribution inside and outside of the boiler tubes was simulated by using Fluent 6.3 software. Investigation revealed that due to the heavy oxide layer formation inside the boiler tubes, the normal heat exchange between the steam and the metal surface was disturbed, leading to increase in temperature on metal surface beyond the designed temperature. Consequently, microstructural degradation has taken place leading to failure. The results of the experimental observations and analysis, matched well with the simulated temperature distribution. Based on the results, it can be concluded that oxidation of the tubes surface by the steam and the high temperature hydrogen attack are the main failure mechanisms.

In this study, aluminum based in situ composites reinforced by Al₃Zr and Al₃Ti nanoparticles were fabricated by Friction Stir Processing (FSP) technique. An wrought aluminum alloy of Al 3003- H14 was used as a base material, whereas elemental metallic powders of zirconium and titanium were used as reinforcments. Zirconium and titanium powders have size in the range of 20 and 45 micron respectively. Tensile strength and hardness of the FSPed composites were determined after applying 2, 4 and 6 passes of processing. Microstructural examinations were carried out by using optical and scanning electron microscopes (SEM). The assessment of phase formation was studied by X-Ray Diffraction (XRD) patterns. The effective Gibbs free energy change of formation (ΔG^e) model was used to thermodynamically predict the Al₃Zr and Al₃Ti phase formation at the interface between the base metal and the reinforcing particles. Results show that, by increasing the number of FSP passes, more aluminides are formed and distributed more uniformly throughout the matrix. It was also noticed that, the maximum tensile strength and hardness can be obtained after applying 6 passes of FSP in either of Al₃Zr or Al₃Ti reinforced composites. Nevertheless, the optimum mechanical properties with ultimate tensile strength 168 Mpa, elongation 23% and hardness 68.5 HV are obtained in hybrid composites, i.e. reinforced by both the mentioned alumineds. The alumindes phase formation at the interfaces of the base metal and reinforcing particles was predicted by (ΔG^e) model and confirmed by the experimental observations.

In supercritical steam boilers that are used in fossil fuel fired power plants, welding of superheater tubes to the collectors is very critical. these two section are made of austenitic stainless steel and  martensitic steel containing 9-12% Cr, respectively. while welding these alloys, microstructural features such as diffusion of carbon, type II grain boundary, carbon depleted zone, etc. have to betaken into account. In this study, effect of pulsed current on microstructure and mechanical properties of dissimilar joints made between AISI316 steel and X10CrMoVNb9-1(GR.91) steel by GTAW, was studied. Two filler metals of ER309 and Inconel 625 (ERNiCrMo-3) were used. Aging of weldments for short periods, showed a carburized zone at Gr.91 interface. Size of this zone was smaller for the ERNiCrMo-3 filler compare to ER309 one. Meanwhile, an amount of energy observed in the impact test was reduced by 20% when ER309 filler was used. Using pulsed current instead of constant current, led to modification of dendritic structure of ERNiCrMo-3 weldment and thereby higher impact energy. The other undesirable features related to dissimilar joints such as unmixed zone, and the transition zone was also reduced by pulsed current.

 In the present work, microstructural and mechanical properties and corrosion behavior of Al5083/ CeO2 composites fabricated through mechanical alloying (MA) is investigated. Al5083 matrix composites reinforced with different amounts of 1, 2, 3 wt. % CeO2 with mean particle size of 40 nm. To ensure the formation of the base alloy, the composition of the powder was produced in three different milling times. The morphology and phase analysis of the produced powders have been studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicate, after 20 h milling the Al5083 alloy is composed. Subsequently, Alloy and composite powder mixtures were consolidate and sintered by employing both conventional and hot press techniques. after microstructural investigation, the hot press samples was chosen for more investigation. The scanning electron microscopy (FESEM) images indicate that CeO2 nanoparticles distribution in all samples is good. Mechanical studies that have been performed in this study include microhardness and compression tests. With increasing the cerium oxide weight percentage in composition, hardness significantly increased. Also, the maximum compressive strength of samples increased with increasing of reinforcement. But, in Al5083/ 3%CeO2 sample due to increase of porosity, strength decreased. The potentiodynamic polarization test was used to investigate corrosion behavior of samples in standard solution 3.5% NaCl. According to results of this test, addition of cerium oxide nanoparticles significantly increased pitting corrosion resistance. How these changes in properties, discussed and studied in this research

 Aluminum alloys of series 2xxx and 7xxx are used in aerospace and marine application due to their high strength. Therefore, welding of these alloys is very important. In this study, welding of aluminum alloy 7020-T6 was done by Gas Tungsten Arc welding (GTAW) by varying different heat inputs. In this case welding current in the range of 140 to 195 Amperes was implemented. This alloy was also friction stir welded at different tool rotations of 500-1400 rpm. In both cases, K-type thermocouples were inserted at different spaces from the fusion lines in order to record the temperature variations and exposure time. In these welding methods extension of Heat Affected Zone (HAZ), microstructural features and mechanical properties of the welded specimens were determined and compared with each other. Results showed that, HAZ region in both the welding methods was extend by increasing the heat input and a lot of microstructural changes were taking place. Optical, Scanning electron microscopes (SEM) equipped with EDX and Transmission Electron microscope (TEM) were used to reveal the microstructural features. Hardness and tensile testing were conducted for mechanical properties evaluations. Due to higher heat input in the case of GTAW, HAZ extension was extensive and the mechanical properties were poorer as compared to FSW.

<p style="text-align: justify"><span style="font-size: medium"><span style="font-family: Times New Roman">The function of Rotary Air Pre-heaters as a heat recovery device is to raise the temperature of the combustion air in boilers, recovering the heat from the outgoing combustion gases and as a result, increasing the efficiency of the boiler. In this study, the failure of heating elements of Rotary Air Pre-heater, in a steam power plant in Khuzestan province of Iran was investigated. The heating elements were made of either Corten steel or plane carbon steel. In the latter case, elements were enamel coated. Visual inspection, chemical analysis of sedimentation, analysis of fuel and its combustion products, microscopic examinations of the enamel coating, substrate and depositions were conducted. The results showed that the failure of the heating element plates was mainly attributed to the occurrence of acid dew point corrosion and the formation of sulfuric acid. The other damaging parameters were, under deposit pitting corrosion and the erosion caused by inappropriate cleaning system i.e. soot blowers. Based on the working conditions of Ramin power plant, sulfuric acid dew point temperature, calculated to be 135 - 141 &deg; C. Accordingly, the critical area mostly susceptible to condensation of sulfuric acid was identified. It was found that, the critical area starts from the cold end of the air pre-heater to the height of about 40 cm.In this study, the failure analysis of enamel coated heating elements was also carried out. The presence of large and inter connected bubbles as the main cause of coating was identified. In order to assess the failure mechanisms and select the proper materials suited to the working condition, corrosion coupons were prepared and placed inside the air pre-heater chamber. The temperature distribution was also determined by the simulation using the Fluent 6.3 software. It was found that, the calculated acid dew point is well matching with the temperature distribution determined by simulation. Finally, based on the results of visual observations, chemical and metallographic analysis, and the temperature distribution throughout the heating elements, the failure mechanisms were identified and the preventive solutions presented.</span></span></p>

<p style="text-align: left;">Pitting corrosion is a phenomenon in which a lot of complicated factors are involved, and is considered as an unexpected behavior, especially in aluminum alloys. Aluminum alloy 5083 is prone to pitting corrosion due to the presence of intermetallic compounds. In this research, the rate of pitting initiation, and the uniform corrosion of this alloy, in synthesized seawater were studied electrochemically using potentiodynamic and potentiostatic polarization tests at various pH, temperatures and chlorine ion concentrations. The rate of pitting initiation was assessed on the bases of corrosion potential, corrosion rate, pitting potential ( Epit), and the number of metastable pits per unit time and exposed area. The surface of samples after polarization tests was examined by an optical and field emission scanning electron microscopes (FESEM), equipped with EDX. The results showed that, there is a meaningful relationship between pH, chlorine concentration, temperature and the rate of pitting initiation. The results revealed that the pitting initiation rate decreases with increase of pH, and decrease of chlorine ion concentration. Also, the pitting initiation rate was decreased up to 30˚C and it was increased with further rise of temperature. Finally, in order to predict the pitting initiation and the uniform corrosion rate, two appropriate models were designed by Artificial Neural Network using the Data obtained from electrochemical tests. It was noticed that, models can predict the pitting initiation and general corrosion rate with 91 and 95 percent accuracy respectively.</p> <p>&nbsp;</p>

In the present investigation, Friction Stir Processing (FSP) was utilized to incorporate Multi Walled Carbon NanoTubes (MWCNT) and nanosized cerium oxide (CeO₂) particles into the matrix of Al5083 alloy to form surface reinforced composites. Threaded cylindrical hardened tool steel with rotation speed of 600 and 800 rpm and travel speed of 35 and 45 mm/min and a tilt angle of 5^o was used. Mechanical properties and corrosion resistance of FSPed samples were evaluated and compared with the base alloy. The maximum tensile strength and hardness value was achieved for the hybrid composite containing a mixture of CNTs and cerium oxide in the volume ratio of 75-25 respectively, whereas, a significant increase in pitting resistance of the base alloy was obtained when cerium oxide alone is incorporated. Corrosion behaviour of the samples was investigated by potentiodynamic polarization tests and assessed in term of pitting potential and passivation range. Microstructural analysis carried out by using optical and electron microscopes showed that reinforcements are well dispersed inside the Nugget Zone (NZ), and remarkable grain refinement is gained. It was also observed that high wear resistance exhibited in the Al5083/ 50% CNT/ 50% CeO₂ surface hybrid composites due to presence of Carbon Nanotube and Cerium Oxide acted as load bearing elements and solid lubricant respectively. Wear assessment was utilized by Disk on Pin tset method. Where the disk was made of AISI E52100 steel with hardnessof 63 HRC.

In thepresentstudy, hybridsurfacenanocompositescontainingdifferentratiosof silicon carbide(SiC)andcerium oxide(CeO2) reinforcement particleson5083aluminumalloybyfriction stirprocessing(FSP)andthe last threepasswas created.By optical and electron microscope images as well as tests of energy-dispersive x-ray spectroscopy(EDS), vickers macrohardness, tention, pin on disc wear and potentiodynamic polarization corrosion,microstructure, mechanical, wear and corrosion properties ofhybrid compositesevaluation and compared with base metal, sample of under the process without reinforcement particles andAl5083/100%SiC and Al5083/100%CeO2 composites.Microstructural analysis of the composites showed a uniform distribution of reinforcement particles inside the processed zone and good bondingbetweenthesurfacenanocomposites layerand thesubstrate.Maximumhardness,yield strengthand tensile strengthof the madecomposites,for sample of Al5083/100%SiCand respectively1.5, 2.81and1.2timesof hardness, yield strengthandtensile strength ofthe base metalwas obtained. Hardness and strength ofhybridsamples increased with increasing volume percent of silicon carbide.Compared with the base metal, all of thecompositesamplesshowed higher wear properties in wear test, and the best wear properties obtained for sample of Al5083/(50%CeO2+50%SiC).Maximum and minimum corrosion resistance, related to Al5083/100%CeO2(the corrosioncurrent density was95% lower thanthe base metal) and al5083/100%SiC (the corrosioncurrent density was78% higher thanthe base metal) composites respectively and in hybrid composites, with the increases of CeO2ratio, improved corrosion resistance.