Showing papers on "Shielding gas published in 2015"

Development and evaluation of SUS 304H — IN 617 welds for advanced ultra supercritical boiler applications

Abstract: At moderately high temperature sections of Advanced Ultra Super Critical (AUSC) boilers, welding of superalloys to austenitic steels is inevitable owing to economic aspects of boiler. Welding of SUS 304H and Inconel 617 (IN 617) was attempted using IN 617 filler material employing conventional Gas Tungsten Arc Welding (GTAW) process and the procedure was successfully established along with optimized welding parameters. Microstructural characterization was carried out to identify various zones on either side of the fusion boundaries. Unmixed Zone and Heat Affected Zone (HAZ) were observed towards SUS 304H fusion boundary while no distinct HAZ was observed towards IN 617 fusion boundary. Micro-hardness profiling indicated decrease in hardness at the HAZ towards SUS 304H fusion boundary. Mechanical properties evaluation at both ambient and elevated temperatures was carried out and data obtained was compared with those of base metals. The tensile strength of the cross weld specimens at high temperatures were observed to be marginally lower than that of IN 617 but significantly more than that of SUS 304H, hence, tolerable. Stress-rupture properties of the cross-weld specimens as tested in this study were found to be intermediate to the base metals’ data, thus, suitable for AUSC power plants' boiler applications. Hence, this work gives an insight into welding procedure establishment, microstructural development, variation of mechanical properties at elevated temperatures and stress-rupture properties of the dissimilar metal welds at elevated temperatures.

Welding of 316L Austenitic Stainless Steel with Activated Tungsten Inert Gas Process

Abstract: The use of activating flux in TIG welding process is one of the most notable techniques which are developed recently. This technique, known as A-TIG welding, increases the penetration depth and improves the productivity of the TIG welding. In the present study, four oxide fluxes (SiO2, TiO2, Cr2O3, and CaO) were used to investigate the effect of activating flux on the depth/width ratio and mechanical property of 316L austenitic stainless steel. The effect of coating density of activating flux on the weld pool shape and oxygen content in the weld after the welding process was studied systematically. Experimental results indicated that the maximum depth/width ratio of stainless steel activated TIG weld was obtained when the coating density was 2.6, 1.3, 2, and 7.8 mg/cm2 for SiO2, TiO2, Cr2O3, and CaO, respectively. The certain range of oxygen content dissolved in the weld, led to a significant increase in the penetration capability of TIG welds. TIG welding with active fluxes can increase the delta-ferrite content and improves the mechanical strength of the welded joint.

Improvement of Microstructure and Mechanical Behavior of Gas Tungsten Arc Weldments of Alloy C-276 by Current Pulsing

Abstract: Superalloy C-276 is known to be prone to hot cracking during fusion welding by Gas Tungsten Arc method. Microsegregation occurring during cooling of fusion zone with consequent appearance of topologically close-packed phases P and µ has been held responsible for the observed hot cracking. The present work investigated the possibility of suppressing the microsegregation in weldments by resorting to current pulse. Weldments were made by continuous current gas tungsten arc welding and pulsed current gas tungsten arc welding using ERNiCrMo-4 filler wire. The weld joints were studied with respect to microstructure, microsegregation, and mechanical properties. Optical microscopy and scanning electron microscopy were employed to study the microstructure. Energy-Dispersive X-ray Spectroscopy was carried out to evaluate the extent of microsegregation. Tensile testing was carried out to determine the strength and ductility. The results show that the joints fabricated with pulsed current gave rise to narrower welds with practically no heat affected zone, a refined microstructure in the fusion zone, reduced microsegregation, and superior combination of mechanical properties.

Studies on Effects of Welding Parameters on the Mechanical Properties of Welded Low-Carbon Steel

Abstract: In this work, the effect of heat input on the mechanical properties of low-carbon steel was studied using two welding processes: Oxy-Acetylene Welding (OAW) and Shielded Metal Arc Welding (SMAW). Two different edge preparations on a specific size, 10-mm thick low-carbon steel, with the following welding parameters: dual welding voltage of 100 V and 220 V, various welding currents at 100, 120, and 150 Amperes and different mild steel electrode gauges of 10 and 12 were investigated. The tensile strength, hardness and impact strength of the welded joint were carried out and it was discovered that the tensile strength and hardness reduce with the increase in heat input into the weld. However, the impact strength of the weldment increases with the increase in heat input. Besides it was also discovered that V-grooved edge preparation has better mechanical properties as compared with straight edge preparation under the same conditions. Microstructural examinations conducted revealed that the cooling rate in different media has significant effect on the microstructure of the weldment. Pearlite and ferrite were observed in the microstructure, but the proportion of ferrite to pearlite varied under different conditions.

Influence of Current and Shielding Gas in TiO2 Flux Activated TIG Welding on Different Graded Steels

Abstract: In tungsten inert gas (TIG) welding, limited depth of penetration can be achieved during single pass welding. To achieve the desired depth of penetration, the speed of welding needs to be significantly reduced and hence, the productivity decreases. In the present work, the effect of TiO2 activated flux on penetration is evaluated for different workpieces namely AISI 1020, AISI 304, AISI 316, and Duplex 2205 steels at different currents and shielding gas compositions. The results show a significant increase in the depth of penetration and reduction in the width-to-penetration ratio using the activated flux for all the workpiece materials considered here. Current increases the depth of penetration, however, the influence of flux becomes more significant with higher welding current. Maximum of 37.8%, 44.3%, 47%, and 124% increase in depths of penetration is measured for AISI 1020, AISI 304, AISI 316, and Duplex 2205 steels, respectively, when activated flux is used. Also, maximum of 70% increase in the depth...

Weldability of Duplex Stainless Steel

Abstract: Duplex stainless steels (DSSs) have many advantages due to the unique structural combination of ferrite and austenite grains. The structural change of these materials is very complex during welding, and it deteriorates the functional properties. This research investigates different welding processes such as laser beam, resistance, tungsten inert gas, friction stir, submerged arc, and plasma arc weldings considering the research available in the literature. The welding mechanism, change of material structure, and control parameters have been analyzed for every welding process. This analysis clearly shows that DSS melts in all most all welding processes, but the thermal cycle and maximum heat input are different. This difference affects the resulting structure and functional properties of the weld significantly.

Effect of shielding gas on welding performance and properties of duplex and superduplex stainless steel welds

Abstract: The influence of shielding gases on welding performance and on properties of duplex and superduplex stainless steel welds was studied Using argon as the reference gas, helium, nitrogen and carbon dioxide were added and five mixtures evaluated Bead-on-plate welds and circumferential pipe welds were produced using mechanised GMA welding in the downhand position Welding performance, corrosion resistance, mechanical properties, microstructural features and weld imperfections were assessed and related to the shielding gas Shielding gases containing 30 % helium showed excellent results; whilst pure argon showed unstable arc and poor weld pool fluidity and Ar + 2 %CO2 resulted in underfill and porosity Mixtures containing helium resulted in higher ductility welds and higher impact toughness values than welds produced with Ar + 2 %CO2 Sound and balanced duplex microstructures free from intermetallics were found with suitable ferrite contents for all the shielding gases studied All the duplex pipe welds passed the corrosion test regardless of the shielding gas used, and the best results in the corrosion test for superduplex pipe welds were found when using Ar + 30 %He + 05 %CO2 + 18 %N2 as shielding gas

Study for the mechanism of TIG-MIG hybrid welding process

Abstract: On TIG-MIG hybrid welding process (TIG: DCEN and MIG: DCEP), the stability of MIG arc can be kept even in pure inert shielding gas by the effect of hybridization with TIG arc, and it becomes possible to achieve the new welding method which has both merits of high quality as TIG welding process and high efficiency as MIG welding process. In this report, we performed two experiments mainly as follows to solve two problems: (1) Analysis of output from TIG power source in hybrid welding state to prove experimentally the existence of direct current path which flows between TIG cathode and MIG anode via in hybrid arc plasma. (2) Measurement of actual heat input based on evaporation amount of liquid nitrogen to investigate detailed heat source property of this process. As results, it was shown that output of TIG power source in hybrid welding condition is lower than it in single TIG welding, and the heat efficiency in hybrid welding condition decreases about 10 % rather than it in single heat source of TIG or MIG arc. Moreover, in specific condition that value of TIG current is lower than value of MIG current, output of TIG power source becomes entirely zero even though TIG arc shape is observed clearly, and thus, the stability of MIG arc is lost then. It can be said that these findings reveal the existence of direct current and explain one of mechanisms that TIG arc can control the arc stability and heat input of MIG arc.

Numerical simulation of metal transfer in argon gas-shielded GMAW

Abstract: The gas metal arc welding (GMAW) process combines aspects of arc plasma, droplet transfer, and weld pool phenomena. In the GMAW process, an electrode wire is melted by heat from an arc plasma, and molten metal at the wire tip is deformed by various driving forces such as electromagnetic force, surface tension, and arc pressure. Subsequently, the molten droplet detaches from the tip of the wire and is transferred to the base metal. The arc plasma shape changes together with the metal transfer behavior, so the interaction between the arc plasma and the metal droplet changes from moment to moment. In this paper, we describe a unified arc model for GMAW, including metal transfer. In the model, we do not account for heat transfer in the metal, but the wire melting rate is determined by the arc current. The developed model can show transition from globular transfer at low currents to spray transfer at higher currents. It was found that electromagnetic force is the most important factor at high currents, but surface tension is more important than electromagnetic force at low currents in determining the transfer mode.

Keyhole-induced porosity in laser-arc hybrid welded aluminum

Abstract: Keyhole-induced macro-porosity, which results from the collapse of the keyhole that formed by the reaction forces of metal vapors, is a major problem limiting laser and laser-arc hybrid weldability of age-hardenable aluminum alloys, such as AA2024-T3 The mechanism of porosity suggests that the weld metal solidifies more rapidly than the possible rise velocity of the gas bubbles that formed during keyhole collapse, resulting in severe porosity The porosity behavior of AA2024-T3 during laser-arc hybrid welding was studied using microscopy and X-ray radiography techniques Porosity-free welding of the alloy is attainable in the conduction mode welding, whereas porosity increased significantly with increased laser intensity during keyhole mode welding Porosity was mostly severe when the beam was focused at the surface of the workpiece The laser beam and the arc decouple from each other with increased laser-wire distance, affecting keyhole depth and porosity In order to control porosity during laser-arc hybrid welding of aluminum alloys, the role of various welding parameters on the material’s response should be balanced with the required weld geometry

Low-cost open-source voltage and current monitor for gas metal arc weld 3D printing

Abstract: Arduino open-source microcontrollers are well known in sensor applications for scientific equipment and for controlling RepRap 3D printers Recently low-cost open-source gas metal arc weld (GMAW) RepRap 3D printers have been developed The entry-level welders used have minimal controls and therefore lack any real-time measurement of welder voltage or current The preliminary work on process optimization of GMAW 3D printers requires a low-cost sensor and data logger system to measure welder current and voltage This paper reports on the development of a low-cost open-source power measurement sensor system based on Arduino architecture The sensor system was designed, built, and tested with two entry-level MIG welders The full bill of materials and open source designs are provided Voltage and current were measured while making stepwise adjustments to the manual voltage setting on the welder Three conditions were tested while welding with steel and aluminum wire on steel substrates to assess the role of electrode material, shield gas, and welding velocity The results showed that the open source sensor circuit performed as designed and could be constructed for <$100 in components representing a significant potential value through lateral scaling and replication in the 3D printing community

An Experimental and Numerical Approach for the Welding Effects on the Duplex Stainless Steel Microstructure

Abstract: Key microstructural changes that occur when Duplex Stainless Steels (DSS) are welded could be evaluated when bead-on-plate welding was carried out on a 2205 DSS by the GMAW process. By using numerical simulations, it was possible to calculate locally the heating and cooling rates taking place during the 2205 DSS welding and discuss its correlation to the microstructural changes experimented by the parent metal. Results showed that increasing heat input has promoted the ferritic grain growth with a slight reduction in the austenite content present at the high temperature heat affected zone (HTHAZ), whereas the cooling rates remained above from those reported as critical for sigma phase precipitation in 2205 DSS. Furthermore, nitrogen has proved to be an effective austenite former at the fusion zone (FZ), which can contributes to get a balanced microstructure in DSS welds in contrast to the effects from the elevated cooling rates.

Laser welding method

Abstract: A laser welding method includes: projecting a laser beam onto irradiation regions on plural metallic workpieces such that a weld section is formed when the workpieces are joined by laser welding, the weld section being formed of plural nuggets, and each of the irradiation regions being formed with each of the nuggets. The nuggets are sequentially formed by sequentially projecting the laser beam onto the irradiation regions that respectively correspond to the nuggets. The laser beam is projected onto each of the irradiation regions such that an amount of input heat from the laser beam that is projected onto each of the irradiation regions to the workpiece is reduced as the nuggets are sequentially formed.

Hybrid Laser-arc Welding of 17-4 PH Martensitic Stainless Steel

Abstract: PH stainless steel has wide applications in severe working conditions due to its combination of good corrosion resistance and high strength The weldability of 17-4 PH stainless steel is challenging In this work, hybrid laser-arc welding was developed to weld 17-4 PH stainless steel This method was chosen based on its advantages, such as deep weld penetration, less filler materials, and high welding speed The 17-4 PH stainless steel plates with a thickness of 19 mm were successfully welded in a single pass During the hybrid welding, the 17-4 PH stainless steel was immensely susceptible to porosity and solidification cracking The porosity was avoided by using nitrogen as the shielding gas The nitrogen stabilized the keyhole and inhibited the formation of bubbles during welding Solidification cracking easily occurred along the weld centerline at the root of the hybrid laser-arc welds The microstructural evolution and the cracking susceptibility of 17-4 PH stainless steel were investigated to remove these centerline cracks The results showed that the solidification mode of the material changed due to high cooling rate at the root of the weld The rapid cooling rate caused the transformation from ferrite to austenite during the solidification stage The solidification cracking was likely formed as a result of this cracking-susceptible microstructure and a high depth/width ratio that led to a high tensile stress concentration Furthermore, the solidification cracking was prevented by preheating the base metal It was found that the preheating slowed the cooling rate at the root of the weld, and the ferrite-to-austenite transformation during the solidification stage was suppressed Delta ferrite formation was observed in the weld bead as well no solidification cracking occurred by optimizing the preheating temperature