فهرست مطالب رضا دهملایی

In this study, the effect of temperature on the microstructure and reactive layer at the interface between the Ti interlayer and the base metal related to the diffusion bonding of Zr702 to A516 low alloy steel was investigated. The joining was done using the spark plasma sintering technique at temperatures of 900, 950 and 1000°C for 30 minutes. Field Emission Scanning Electron Microscope (FESEM) equipped with EDS analysis was used to investigate the microstructure of the interfaces in various joints. Investigations showed that at all temperatures, with the diffusion of atoms and the formation of a reactive layer between the Ti interlayer and Zr702, no intermetallic phases, cracks, porosity and discontinuities were formed at their interfaces. . It was found that increasing the bonding temperature did not cause the formation of new phases and compounds in the interface and only increased the thickness of the reaction layer. The measurement of the thickness of the reactive layer showed that the maximum and minimum amounts of diffusion were 84 microns at 1000 °C and 64 microns at 900 °C respectively

In this research, the corrosion resistance of weld metal in API-5L X90 micro-alloy steel joints was investigated. Welding was performed using filler metals ER120S-G, ER100S-G and ER90S-B3 by GTAW process. Microstructural studies were performed using light microscope and SEM scanning electron microscope. Microstructural studies showed that the base metal microstructure consists of ferrite and granular bentite. It was found that different weld metals all contain different amounts of accular Ferrite, polygonal ferrites and martensite / austenite (MA) islands. The highest amount of accular Ferrite was observed in ER 120 SG welding metal. The results of corrosion test showed that ER120SG welding metal and base metal had the lowest flow density and corrosion rate. And the highest corrosion rate was obtained for ER100SG welding metal.

In this research, the diffusion bonding of the stabilized zirconia ceramic and Nimonic 105 superalloy using Ti/Nb/Ni multi-interlayer was carried out. Joint was performed using the plasma spark technique in a vacuum atmosphere and at different temperatures and times. The microstructure of the different joint zones was studied using optical and FESEM microscopes equipped with an EDS analyzer. The results showed that the critical region is Ti/3YSZ interface and in all conditions diffusion bonding in Ti/Nb, Nb/Ni, and Ni/NI 105 interfaces were done. Microstructural observations showed that in the Ti/3YSZ interface at all temperature and time conditions, the connection of two separate regions including Ti3O and (Zr, Ti)2O was formed due to the difference in the diffusion depth of Ti, Zr, and O elements and with increasing temperature and time, the thickness of these regions increased. Microstructural studies showed that the bond at 900 ℃  and 30 minutes did not have any cracks and discontinuities and due to the better diffusion of atoms, a suitable reaction layer was formed. Microhardness observations and EDS analyses confirmed that the Ti3O reaction layer is the weakest zine.

In this study, the effect of input heat of SMAW process on the microstructure and corrosion behavior of welded metal API 5L X65 steel joints coated with INCOLOY 825 superalloy was investigated. Welding with input heat of 1.03, 0.99 and 0.94 kJ / mm was performed using E7018 filler metal by SMAW process. FESEM scanning light and electron microscopy was used to study the microstructure. Evaluation of corrosion behavior of weld metal was performed using TOEFL polarization test in 3.5% NaCl solution. The microstructural results showed that the needle ferrite in the weld metal microstructure decreased with increasing inlet heat. While the values of Wiedmann Statten ferrites are multifaceted and grain boundaries. It was found that increasing the inlet heat had a negative effect and reduced the corrosion resistance of the weld metal.

In the present paper, in order to improve the high temperature oxidation resistance of 316 AISI stainless steel, claddings of ER446 and ER446 + 4 wt. % Al filler wires were deposited on this steel using GTAW process. The chemical composition, phase types and microstructure of the claddings were determined by light microscopy (OM), electron microscopy (FESEM-EDS) and X-ray diffraction (XRD). The oxidation behavior of the cladded samples was evaluated and compared using isothermal oxidation test in air (at 1000 °C) and air + 1.5% SO2 (at 800 °C) environments. The adhesion of the interface between the claddings and the substrate was measured by shear strength test before and after oxidation according to ASTM A264 standard. Microstructural observations from the cross section showed that both claddings had excellent adhesion to the substrate and no cavities and discontinuities were observed in the interface between the claddings and the substrate. The results of shear strength test showed that both claddings before and after high temperature oxidation have higher shear strength than the minimum value specified (140 MPa) in the ASTM A264 standard. l Evaluation of the oxidation behavior of the base metal before and after the exert of the coating showed that the claddings (especially with present 4% Al) strongly reduced the oxidation rate (improving the oxidation resistance) in both oxidizing environments. In the early stages of oxidation, chemical reactions at the interface between the oxide scale and the metal controlled the oxidation reaction, and after primarily stages until the end of examination, the diffusion and transfer of oxygen and aggressive ions through the oxide scale toward the interface controlled the oxidation process. It was found that the oxidation kinetics of both claddings follow the parabollic law of oxidation and the cladding containing 4% Al had a constant oxidation rate (kp) 16 times lower than the coating without aluminum. The results showed that the presence of SO2 gas increased the oxidation rate of both claddings.

In this study, welding of high strength low alloy steel, HSLA-100 was performed using three fillers metals, cut from base metal (HSLA-100), ER100S-G and ER120S-G by GTAW procedure. Microstructural studies were conducted using optical and scanning electron microscopes. Tensile, impact and hardness tests were also used to evaluate the mechanical properties of the joint. The results showed that the microstructure of HSLA-100 weld metal included granular bainite and polygonal ferrite, ER100S-G weld metal consisted of acicular, Widmannstatten and grain boundary ferrites and ER120S-G weld metal comprised of acicular, polygonal and quasi-polygonal ferrites. Furthermore, the formation of a secondary phase (constituent) of martensite / austenite (M / A) was observed in the microstructure of all weld metals. The predominant form of this phase in HSLA-100 and ER100S-G weld metals was blocky type and formed along the prior austenite grain boundries and in ER120S-G weld metal was in the form of stringer type. The results of mechanical tests demonstrated that among weld metals, ER120S-G weld metal had the highest tensile strength (859 MPa), percent elongation (22%), impact toughness (45 joule) and hardness (294.7 HV30). whilst, the ER100S-G weld metal had the lowest tensile strength (775 MPa) and hardness (268.4 HV30) and the HSLA-100 weld metal had the lowest impact toughness (25 Joule).

The microstructure and mechanical properties of HSLA-100 steel weld joints was investigated. Welding with three heat input of 0.820, 1.176 and 1.392 kJ / mm was performed using E12018 electrode. Microstructural studies were performed using scanning electron and optical microscopes. The mechanical properties of welded joints were evaluated by impact and microhardness tests. Microstructural studies showed that with increasing the heat input, the amount of acicular ferrite in the weld metal decreased and the amount of polyhedral and quasi-polygonal ferrite increased. It was found that with increasing the heat input, the amount of layered bainite in the heat affected zone increased and the amount of granular bainite decreased. Due to the decrease in the amount of acicular ferrite in the weld metal microstructure with increasing inlet temperature, the amount of hardness and impact energy decreased. The results showed that the increase in heat input due to the reduction of the acicular ferrite of the weld metal and the dissolution of precipitates in the coarse grain heat affected zone has caused a decrease in hardness in these zones. It was found that with increasing the heat input due to decreasing the acicular ferrite, the impact energy of the weld metal decreased by 29% (from 45 joules at an heat input of 0.82 to 32 joules at an heat input of 1.392 kJ / mm). It was found that at all heat inputs, the impact energy of the base metal is greater than the impact energy of the weld metal.

In the current research, the welding of HSLA-100 steel was accomplished with three different heat inputs using the GTAW process. The microstructure of various joint zones was investigated utilizing an optical microscope and scanning electron microscope. For the purpose of assessing the impact of thermal cycles upon the extension and alterations in the heat-affected zone, the thermocouples were placed at distinct distances from the fusion line. The results indicated that the weld metal microstructure comprised of acicular ferrite and a number of polygonal and quasi-polygonal Ferrite with M/A constituent. An epitaxial growth was observed in the fusion line in all heat inputs. It was determined that the heat-affected zone microstructure consisted of two regions, namely a coarse-grained heat-affected zone with a granular and lath bainite microstructure and a fine-grained heat-affected zone with the prevailing microstructure of granular Bainite. The results indicated that increasing the heat input leads to a decrease in the amount of acicular ferrite and M/A constituent volume fraction in the weld metal and an increase in lath bainite in coarse-grained zone. It was found that by increasing the heat input from 0.78kJ/mm to 1.42 kJ/mm, the grain size increases from 29.2μm to 76.5 μm and from 15.7μm to 32.5μm in coarse-grained and fine-grained zone, respectively. The thermal cycles evaluation pertinent to the implanting of a thermocouple disclosed that the heat-affected zone width is extended from 3.6 mm to 4.5mm by increasing the heat input from 0.78kJ/mm to 1.42 kJ/mm.

In this study, Inconel 617 alloy was welded to A387-Gr.11 low-alloy steel using ER309L filler metal via gas tungsten arc welding (GTAW). First, the corrosion behavior of Inconel 617, A387-Gr, and the weld metal was evaluated by the Tafel polarization test and electrochemical impedance spectroscopy (EIS) in acidic (H2SO4), neutral (NaCl), and combined (H2SO4 + NaCl) solution at ambient temperature. The results of polarization and EIS measurements in all corrosive solutions indicate that the corrosion resistance decreases from 617 alloy to weld metal and from weld metal to low-alloy steel, respectively. The Comparison of the polarization curves of the base metals and the weld metal showed susceptibility to galvanic corrosion between Inconel 617 / weld metal in 1M NaCl solution. The behavior of galvanic corrosion of this pair was evaluated using the mixed potential theory and the electrochemical noise measurement. The results showed that in a galvanic couple of alloy 617 / weld metal, the weld metal acts as anode and corrodes in such a way that its corrosion rate increases from 0.22 μA/cm2 before joining to 1 μA /cm2 after joining.

Dissimilar welding of API 5L X80/DSS 2250 steels was carried out using PCGTAW process. The welds were produced using three types of filler metals (ER2209, ER309L and KJS-124). The microstructural investigations showed that the microstructure of ER2209 weld metal is duplex (ferrite and austenite) and ferrite content changes from root pass (35%) to cap pass (41%). It was revealed that the solidification mode of ER309L weld metal was ferritic-austenitic mode and its microstructure has an austenitic matrix with skeletal ferrites and smaller amount of vermicular ferrites along the grain boundaries. Ferrite scope results revealed that ferrite content of root pass (6%) is less than the final pass (10%). Microscopic results showed that the microstructure of KJS-124 weld metal consists of grain-boundary, Widmanstätten and acicular ferrites with martensite-austenite islands. Epitaxial growth was detected in the interfaces of ER309L/DSS 2209 ، ER2209/DSS 2205 and KJS-124/API 5L X80 and unmixed zones and type II boundaries were detected in the ER2209/API 5L X80- ER309L/API 5L X80 and KJS-124/DSS 2205 interfaces. The results of impact and hardness tests revealed that ER2209, ER309L and KJS-124 weld metals have the best toughness and the KJS-124, ER2209 and ER309L filler metals got the highest hardness, respectively.

In this study butt joining of 7020-T6 alloy was performed using tungsten inert gas (TIG) welding with continuous (constant) and pulsed current. Effect of heat input on microstructure and mechanical properties of the joints was investigated. Microstructural examinations were conducted using optical and scanning electron microscopy. Differential scanning calorimetry (DSC) was used to identify the precipitates phase evolution. Temperature at the welding region was recorded by inserting thermocouples. Hardness and tensile strength of the joints was evaluated. Results showed that current pulsing leads to relatively finer grain size in weld metal compare to the one welded by constant current. In either cases of constant and pulsed current, the grain size and the width of heat affected zone increased with increasing heat input. Unlike earlier studies in precipitation hardening aluminum alloys, strength of the weld metal were increased as compare to base metal in T6 condition. The enhancement in mechanical properties of weld metal was attributed to the solution annealing and re-formation of strengthening precipitates on cooling from welding temperature.

The effect of electromagnetic vibration on the microstructure and impact toughness of API-X70 steel weld metal was investigated. The welding was carried out by gas tungsten arc welding process and using filler metals including ER80S-G and ER309L. Electromagnetic vibration under voltages 0- 30 volts concurrent with welding was applied. The microstructure of the base and weld metals using optical microscope and SEM was studied. The impact energy of weld metals at TR, 0°C and -20℃ using charpy impact test was measured. Microstructural investigations revealed that applying electromagnetic vibration have caused the finer and more uniform of Martensitic-Austenitic (MA) islands in the ER80S-G weld metal . The results showed that, due to electromagnetic vibration, the microstructure became finer and the average size of dendrites in the ER309L weld metal decreased from 18.19 to 8.01 μm.. The impact test result, illustrated that the applied vibration cause improvement of impact energy of weld metals at different temperature (TR, 0°C and -20℃). It was found that with vibration under 30V, impact energy of ER80S-D weld metal in TR, 0°C and - 20℃ was increased equal to 53, 29 and 36 percents, respectively. Besides for ER309L weld metal, the impact energy in TR and -20℃ was increased equal to 18 and 5 percents, respectively. (TR= Room Temperature).

The aim of this research was to study the effect of simultaneous vibration application during welding under the voltage range of 0-30 volts on the mechanical properties and hot cracking susceptibility of 304 stainless steel welded joints. The microstructural investigations by optical and scanning electron microscopes showed that electromagnetic vibration application resulted in fragmentation of dendrite tips, increasing of nucleation sites and alteration of solidification structure from columnar to equiaxed structure. Therefore, the final microstructure undergoes grain refinement and extension of equiaxed dendriteic region. Hardness measurement of welded region showed increasing value due to vibration application. The results of Charpy test done on welded specimens revealed that impact energy has been increased from 112 J to 136 j under different electromagnetic vibration intensities. It was also determined that under electromagnetic vibration, weldability was improved, hot cracking susceptibility was decreased and fracture toughness of welded joints under electromagnetic vibration was increased.