Search Results for gas tungsten arc welding

The objective of this study was to determine the typical range of weld metal cooling rates and phase transformations during multipass gas-tungsten arc (GTA) welding of Grade 23 (SA-213 T23) tubing, and to correlate these to the microstructure and hardness in the weld metal and heat affected zone (HAZ). The effect of microstructure and hardness on the potential susceptibility to cracking was evaluated. Multipass GTA girth welds in Grade 23 tubes with outside diameter of 2 in. and wall thicknesses of 0.185 in. and 0.331 in. were produced using Grade 23 filler wire and welding heat input between 18.5 and 38 kJ/in. The weld metal cooling histories were acquired by plunging type C thermocouples in the weld pool. The weld metal phase transformations were determined with the technique for single sensor differential thermal analysis (SS DTA). The microstructure in the as-welded and re-heated weld passes was characterized using light optical microscopy and hardness mapping. Microstructures with hardness between 416 and 350 HV 0.1 were found in the thick wall welds, which indicated potential susceptibility to hydrogen induced cracking (HIC) caused by hydrogen absorption during welding and to stress corrosion cracking (SSC) during acid cleaning and service.

There is a growing need to automate the gas tungsten arc welding process for fabrication and repair of nuclear components due to an increasing shortage of experienced welders. Therefore, a collaborative effort has been performed in this study to develop a fully autonomous gas tungsten arc welding system with adaptive capabilities. The system employs the application of two neural networks that have been presented in. The first utilizes a vision based convolutional neural network to perform real time control of the filler wire entry position into the weld pool. The second predicts optimal weld parameters and torch positioning for each weld pass deposited within a multi-pass groove. A commercialization path for the technology is in-progress, with the artificial intelligent algorithms currently being incorporated and tested on commercially available equipment.

In dissimilar welds between martensitic stainless steel F6NM and nitrogen-strengthened austenitic stainless steel FXM-19, type 209 austenitic welding consumables are used to align with the mechanical properties and chemical composition of FXM-19, with F6NM welds requiring post-weld heat treatment (PWHT) to restore ductility and toughness, raising concerns about sigma embrittlement in ER209 butter welds. This study investigated the mechanical properties and microstructure of F6NM+FXM-19 dissimilar welds, finding no detrimental sigma phase formation in the butter (PWHT) and groove weld metal (as welded) across various welding processes, indicating no sigma phase transformation due to PWHT. Submerged arc welding (SAW) and gas tungsten arc welding (GTAW) demonstrated good mechanical properties, while Gas Metal Arc Welding with 100% Ar gas shield (GMAW 100% Ar) could not be properly evaluated due to weld defects. SAW and GTAW were deemed suitable for this dissimilar weld joint, with several welding processes providing acceptable results using ER209 filler material for fabricating pressure vessels requiring F6NM to XM-19 joints.

The mechanical behavior of a cast form of an advanced austenitic stainless steel, CF8C-Plus, is compared with that of its wrought equivalent in terms of both tensile and creep-rupture properties and estimated allowable stress values for pressurized service at temperatures up to about 850°C. A traditional Larson-Miller parametric model is used to analyze the creep-rupture data and to predict long-term lifetimes for comparison of the two alloy types. The cast CF8C-Plus exhibited lower yield and tensile strengths, but higher creep strength compared to its wrought counterpart. Two welding methods, shielded-metal-arc welding (SMAW) and gas-metal-arc welding, met the weld qualification acceptance criteria in ASME BPVC Section IX for the cast CF8C-Plus. However, for the wrought CF8C-Plus, while SMAW and gas-tungsten-arc welding passed the tensile acceptance criteria, they failed the side bend tests due to lack of fusion or weld metal discontinuities.

This study addresses the welding challenges encountered when joining Haynes 282, a heat-resistant superalloy, to 3.5NiCrMoV high-strength low alloy steel (HSLA) for advanced power plant applications, particularly in thick-section components like rotors. The project demonstrated successful thick-section dissimilar metal welding up to 76 mm (3 in.) using two techniques: keyhole tungsten inert gas welding and conventional gas tungsten arc welding with Haynes 282 filler metal. Various groove weld geometries were evaluated, supported by computational weld modeling to predict and minimize weld distortion. The results validate these welding approaches for critical power plant components requiring both high-temperature performance and cost-effectiveness.

Super Duplex stainless steels (SDSS) are alloys based on the Fe-Cr-Ni-N system. The chemical composition is tailored to achieve a balanced microstructure of 50% ferrite and 50% austenite. Hyper Duplex Stainless Steels (HDSS) are also duplex materials distinguished by their remarkable yield strength (≥700 MPa) and corrosion resistance (PREN>48). They have been developed as an alternative to the well-established SDSS when superior mechanical and corrosion performance is required. This enhanced performance is attributed to alloying additions, primarily Cr, Mo, and N. In this study, a comparison is conducted between filler metals of SDSS and HDSS for the root welding of SDSS plates. The gas tungsten arc welding (GTAW) process was used to carry out root welding passes and Gas Metal Arc Welding (GMAW) for filling passes on SDSS substrates arranged in a V groove to simulate a repair scenario. The heat input was controlled in both processes, keeping it below 2.0 kJ/mm in the GTAW and 1.2 kJ/mm in the GMAW. GTAW with constant current was used and the parameters achieved producing full penetration welds with SDSS and HDSS. In this case, Nitrogen was used as backing gas to avoid oxidation of the root. Thus, a special GMAW-Pulsed version was applied to achieve good wettability and defect-free joints. ASTM G48 tests were performed to evaluate the corrosion resistance through Critical Pitting Testing (CPT) analysis on the root pass, microstructural analysis via optical microscopy, and impact toughness. Consequently, a comprehensive examination of the welded joints outlines manufacturing conditions, limitations, and the applications of SDSS and HDSS filler metals.

Components such as tubes, pipes and headers used in power generation plants are operated in a creep regime and have a finite life. During partial replacement, creep exhausted materials are often welded to virgin materials with superior properties. The aim of this study was to identify a suitable weld filler material to join creep aged X20CrMoV12-1 to a virgin P91 (X10CrMoVNbV9-1) steel. Two dissimilar joints were welded using the gas tungsten arc welding (GTAW) process for the root passes, and manual metal arc (MMA) welding for filling and capping. The X20 and the P91 fillers were selected for joining the pipes. The samples were further heat treated at 755°C to stress relief the samples. Microstructural evolution and mechanical properties of the weld metals were evaluated. The average hardness of X20 weld metal (264 HV10) was higher than the hardness measurement of P91 weld metal (206 HV10). The difference in hardness was attributed to the high carbon content in X20 material. The characterisation results revealed that the use of either X20 or P91 weld filler for a butt weld of creep aged X20 and virgin P91 pipes material does not have a distinct effect on the creep life and creep crack propagation mechanism. Both weld fillers (X20 and P91) are deemed to be suitable because limited interdiffusion (<10 μm) of chromium and carbon at the dissimilar weld interface was observed across the fusion line. The presence of a carbon ‘denuded’ zone was limited to <10 μm in width, based on the results from local measurements of the precipitate phase fractions using image analysis and from elemental analysis using EDS. However the nanoindentation hardness measurements across the fusion line could not detect any ‘soft’ zone at the dissimilar weld interface. The effect of the minute denuded zone was also not evident when the samples were subjected to nanoindentation hardness testing, tensile mechanical testing, Small Punch Creep Test (SPCT) and cross weld uniaxial creep testing.

Inconel alloy 740, a precipitation-hardenable nickel-chromium-cobalt alloy with niobium addition, has emerged as a leading candidate material for ultra-supercritical (USC) boilers due to its superior stress rupture strength and corrosion resistance at operating temperatures near 760°C. While derived from Nimonic alloy 263, alloy 740's unique chemistry necessitates comprehensive weldability studies to address potential challenges including heat-affected zone liquation cracking, ductility-dip cracking, and post-weld heat treatment cracking. This ongoing investigation examines the alloy's weldability characteristics through material characterization studies comparing its cracking sensitivity to established aerospace alloys like Waspalloy and Inconel alloy 718. The research applies aerospace industry expertise to boiler applications requiring sections up to three inches thick, with gas tungsten arc welding and pulsed gas metal arc welding identified as the most promising processes for producing sound, crack-free welds.

Dissimilar metal welds (DMWs) between ferritic and austenitic materials at elevated temperatures have long posed challenges for boiler manufacturers and operators due to their potential for premature failure. As the industry moves towards higher pressures and temperatures to enhance boiler efficiencies, there is a need for superior weld metals and joint designs that optimize the economy of modern boilers and reduce reliance on austenitic materials for steam headers and piping. EPRI has developed a new filler metal, EPRI P87, to enhance the performance of DMWs. Previous work has detailed the development of EPRI P87 for shielded metal arc welding electrodes, gas-tungsten arc welding fine-wire, and its application in an ultra-supercritical steam boiler by B&W. This study examines the weldability of EPRI P87 consumables through various test methods, including Varestraint testing (both trans and spot), long-term creep testing (approximately 10,000-hour running tests), procedure qualification records for tube-to-tube weldments between traditional/advanced austenitic steels and creep-strength enhanced ferritic steels, and elevated temperature tensile testing. Macroscopic examinations from procedure qualification records using light microscopy confirmed the weldability and absence of cracking across all material combinations. The findings demonstrate that EPRI P87 is a weldable alloy with several advantages for DMW applications and highlight that specific weld joint configurations may necessitate the use of high-temperature tensile data for procedure qualifications.

This work describes the repair procedure conducted on the High Pressure/Intermediate Pressure (HP/IP) and generator rotors of a 180 MW steam turbine General Electric (GE) - STAG207FA type D11 installed at La Sierra Thermoelectric Power Plant in Puerto Nare, Colombia. A lubricant supply failure at base load caused severe adhesive damage to the shafts in the bearing support areas and a permanent 3.5 mm bow at the HP/IP rotor mid span section, which required a complex intervention. The repair process began with the identification of the rotors manufacturing material through in-situ metallographic replicas, handheld XRF analysis and surface hardness measurements. Volumetric manual Gas Tungsten Arc Welding (GTAW) welding reconstruction of cracked areas followed by a surface overlay using GTAW and Plasma Arc Welding (PAW) welding processes were applied with a modular mechanized system, where a stress relief treatment through vibration was implemented with the help of computational simulations carried out to determine the fundamental frequencies of the rotors. Geometric correction of the HP/IP rotor mid span section was achieved thanks to the excitation of the rotor at some fundamental frequencies defined by the dynamic modeling and the use of heat treatment blankets at specific locations as well. Finally, after machining and polishing procedures, the power unit resumed operation eleven months after the failure and remains in service to the present date.

Haynes 282 alloy is a relatively new Ni-based superalloy that is being considered for advanced ultrasupercritical (A-USC) steam turbine casings for steam temperatures up to 760°C. Weld properties are important for the turbine casing application, so block ingots of Haynes 282 alloy were cast for properties studies. Good, sound welds were produced using Haynes 282 weld-wire and a hot gas-tungsten-arc welding method, and tensile and creep-rupture properties were measured on cross-weld specimens. In the fully heat-treated condition (solution annealed + aged), the tensile properties of the welded specimens compare well with as-cast material. In the fully heat-treated condition the creep-rupture life and ductility at 750°C/250MPa and 800°C/200MPa of the cross-weld specimens are similar to the as-cast base metal, and repeat creep tests show even longer rupture life for the welds. However, without heat-treatment or with only the precipitate age-hardening heat-treatment, the welds have only about half the rupture life and much lower creep ductility than the as-cast base metal. These good properties of weldments are positive results for advancing the use of cast Haynes 282 alloy for the A-USC steam turbine casing application.

A new nickel-base superalloy GH750 has been developed as boiler tube of advanced ultrasupercritical (A-USC) power plants at temperatures about and above 750°C in China. This paper researched the weld solidification of GH750 filler metal, microstructure development and property of GH750 welded joint by gas tungsten arc weld. Liquid fraction and liquid composition variation under non-equilibrium state were calculated by thermo-dynamic calculation. The weld microstructure and the composition in the dendrite core and interdendritic region were analyzed by SEM(EDX) in detail. The investigated results show that there is an obvious segregation of precipitation-strengthening elements during the weld solidification. Titanium and Niobium are the major segregation elements and segregates in the interdendritic region. It was found that the changing tendency of the elements’ segregation distribution during the solidification of GH750 deposit metal is agree with the thermodynamic calculation results. Till to 3,000hrs’ long exposure at 750°C and 800°C, in comparison with the region of dendrite core of solidification microstructure, not only the coarsening and the accumulation of γʹ particles are remarkable in the interdendritic region, but also the small quantity of the blocky and needle like η phases from. The preliminary experimental results indicate that the weakening effect of creep-rupture property of the welded joint is not serious compared with GH750 itself.

Coal burning power companies are currently considering FeAlCr weld overlay claddings for corrosion protection of waterwall boiler tubes located in their furnaces. Previous studies have shown that these FeAlCr coatings exhibit excellent high-temperature corrosion resistance in several types of low NOx environments. In the present study, the susceptibility of FeAlCr weld overlay claddings to hydrogen cracking was evaluated using a gas-tungsten arc welding (GTAW) process. Microsegregation of alloying elements was determined for the FeAlCr welds and compared to a currently used Ni-based superalloy. Long-term gaseous corrosion testing of select weld overlays was conducted along with the Ni-based superalloy in a gaseous oxidizing/sulfidizing corrosion environment at 500°C. The sample weight gains were used along with analysis of the corrosion scale morphologies to determine the corrosion resistance of the coatings. It was found that although there were slight differences in the corrosion behavior of the selected FeAlCr weld coatings, all FeAlCr based alloys exhibited superior corrosion resistance to the Ni-based superalloy during exposures up to 2000 hours.

This paper summarizes recent efforts to improve creep performance in Grade 91 (Mod. 9Cr-1Mo, ASTM A387) steel weldments via non-standard heat treatments prior to welding. Such heat treatments offer a potential solution for minimizing Type IV failures in creep strength enhanced ferritic (CSEF) steels. A lower temperature tempering (LTT, 650°C) of the 9Cr steels prior to gas tungsten arc welding (GTAW) resulted in improved creep-rupture life at 650°C compared to the samples tempered at a standard condition (HTT, 760°C) before welding. From detailed characterization of precipitation kinetics in the heat affected zone, it was hypothesized that M 23 C 6 carbides in the fine-grain heat-affected zone (FGHAZ) in the LTT sample were fully dissolved, resulting in re-precipitation of strengthening carbides during post weld heat treatment (PWHT). This was not the case in the HTT sample since M 23 C 6 in the FGHAZ was only partially dissolved prior to welding, which caused coarsening of existing M 23 C 6 after PWHT and premature creep failure in the FGHAZ. However, it was also found that the LTT raised the ductile-brittle transition temperature above room temperature (RT). Two different thermo-mechanical treatments (TMTs); two-step tempering and aus-forging/aus-aging, of the modified 9Cr-1Mo steels were attempted, in order to control the balance between creep properties and RT ductility, through control of precipitation kinetics of the M 23 C 6 carbides and/or MX carbo-nitrides. The hardness map of the TMT samples after GTAW and PWHT were evaluated.

Inconel alloy 740 is a precipitation-hardenable nickel-chromium-cobalt alloy with niobium, derived from Nimonic 263, and is considered a prime candidate for the demanding conditions of advanced ultrasupercritical boilers. It offers an exceptional combination of stress rupture strength and corrosion resistance under steam conditions of 760°C (1400°F) and 34.5 MPa (5000 psi), surpassing other candidate alloys. Initially, Inconel alloy 740 was prone to liquation cracking in sections thicker than 12.7 mm (0.50 in), but this issue has been resolved through modifications in the chemical composition of both the base and weld metals. Current concerns focus on the weld strength reduction factor for direct-age weldments. This has led to further development in welding Inconel alloy 740 using Haynes 282, which has higher creep strength and may mitigate the weld strength reduction factor. This study details successful efforts to eliminate liquation cracking and compares the properties of Inconel alloy 740 and Haynes 282 filler materials using the gas tungsten arc welding process.

Coke drums experience failures in through-wall cracking throughout their operating life, resulting from low cycle fatigue. Coke drums are typically fabricated from Chrome Moly (CrMo) steels. This study was performed on P4 (1.25Cr-0.5Mo) base material using ER70S-B2L and Alloy 625 (ERNiCrMo-3) filler materials. Specimens were welded with the temper-bead/controlled deposition welding technique. The weld processes used were HP-GTAW, GMAW and SMAW. The fatigue performance, HAZ hardness and toughness of the weld samples was evaluated. The HP-GTAW welds exhibited an order of magnitude improvement in fatigue performance when compared to the other weld processes using ER70S-B2L filler material. The HP-GTAW welds also exhibited improved HAZ hardness and toughness when compared to the other weld processes. This presentation will introduce the HP-GTAW process, its features, and benefits and where it is applied in Coke drum repair welding. Comparative test results of the different weld processes for fatigue performance, HAZ tempering, and toughness will also be presented.

This study investigated the creep rupture strength and microstructure evolution in welded joints of high-boron, low-nitrogen 9Cr steels developed by NIMS. The welds were fabricated using the GTAW process and Inconel-type filler metal on steel plates with varying boron content (47-180 ppm). Creep rupture tests were conducted at 923K for up to 10,000 hours. Despite their higher boron content, these steels exhibited good weldability. Welded joints of the boron steel displayed superior creep properties compared to conventional high-chromium ferritic steel welds like P92 and P122. Notably, no Type IV failures were observed during creep testing. Welding introduced a large-grained microstructure in the heat-affected zone (HAZ) heated to the austenite transformation temperature (Ac3 HAZ). This contrasts with the grain refinement observed in the same region of conventional heat-resistant steel welds. Interestingly, the grain size in this large microstructure was nearly identical to that of the base metal. Analysis of the simulated Ac3 HAZ revealed crystal orientation distributions almost identical to those of the original specimen. This suggests a regeneration of the original austenite structure during the alpha-to-gamma phase transformation. Simulated Ac3 HAZ structures of the boron steel achieved creep life nearly equivalent to the base metal. The suppression of Type IV failure and improved creep resistance in welded joints of the boron steels are likely attributed to the large-grained HAZ microstructures and stabilization of M 23 C 6 precipitates. The optimal boron content for achieving the best creep resistance in welded joints appears to lie between 90 and 130 ppm, combined with minimized nitrogen content.

The use of high-nickel superalloys has greatly increased among many industries. This is especially the case for advanced coal-fired boilers, where the latest high temperature designs will require materials capable of withstanding much higher operating temperatures and pressures than current designs. Inconel alloy 740H (UNS N07740) is a new nickel- based alloy that serves as a candidate for steam header pipe and super-heater tubing in coal-fired boilers. Alloy 740H has been shown to be capable of withstanding the extreme operating conditions of an advanced ultra-super-critical (AUSC) boiler, which is the latest boiler design, currently under development. As with all high nickel alloys, welding of alloy 740H can be very challenging, even to an experienced welder. Weldability challenges are compounded when considering that the alloy may be used in steam headers, where critical, thick-section and stub-to-header weld joints are present. This paper is intended to describe the proper procedures developed over years of study that will allow for ASME code quality welds in alloy 740H with matching composition filler metals.

This paper presents results and analyses from long-term creep-rupture testing of alloy CCA617 (also known as alloy 617B) in wrought and welded forms at temperatures and stresses relevant to power generation under advanced steam conditions. The refined controlled chemical composition of CCA617 resulted in increased creep-rupture strength compared to the conventional alloy 617 chemistry at applied stress levels of ~150 MPa and above. Long-term creep rupture testing of weldments (in one case, over 100,000 h) showed that their creep-rupture lives were dependent on the welding process. Gas-tungsten-arc and shielded metal-arc weldments of CCA617 performed nearly equivalent to standard alloy 617 base metals in creep, but there was some debit in creep-rupture resistance when compared to CCA617 base metal. Submerged arc welding produced weldments that were notably weaker than both versions of alloy 617 base metal under creep conditions, possibly due to lack of optimization of filler wire composition and flux.

A new martensitic steel was developed for power generation applications. Tenaris High Oxidation Resistance (Thor) is an evolution of Grade 91, designed to have improved steam oxidation resistance and better long-term microstructural stability, with equal or better creep strength. Based on consolidated metallurgical knowledge of microstructural evolution mechanisms, and extensive development performed in the last decade, Thor was engineered to overcome temperature limitations of Grade 91, yet it can be processed in the same fashion, permitting the use of existing best practices for Grade 91 boiler fabrication. Welding trials were performed on Thor tubes and pipe using welding procedures that are routinely employed in the construction of Grade 91 steel components. A summary of relevant results is presented, demonstrating the applicability of long-established and tested welding procedures to components manufactured with Thor steel.