Multi-pass Gas Tungsten Arc Research Articles

Characterization of the spin arc welding technique for dissimilar materials Hastelloy C-2000 and C-276

Hastelloy C-2000 and C-276 are primarily employed in the petrochemical and pharmaceutical industries. Finding an effective way to combine these alloys is still a complex task in industry. The main focus of this study is to investigate the potential efficacy of the spin arc welding process on Hastelloy C-2000 and C-276. The welding process was carried out under various conditions, such as weld current, spinning speed, and spin diameter. ERNiCrMo-4, with a diameter of 1.2 mm, was employed as a filler. Optical microscopy, XRD and FESEM were used for microstructural examination. The mechanical properties of the welded samples were critically analyzed. The results of the ultimate tensile strength (UTS), hardness value (HV), and impact tests of spin arc welding were compared with those of the multipass pulsed current gas tungsten arc welding process (MPCGTAW). In spin arc welding, Hastelloy C-2000 and C-276 exhibited higher UTS, HV, and toughness compared to MPCGTAW because there was no secondary TCP phase formation in the weld centre and hot cracking in spin arc weldments. Additionally, compared to MPCGTAW, the elongation with spin arc welding was noted to be lower. Moreover, microstructural analysis revealed no evidence of grain segregation, which may have contributed to the improved UTS and HV of the materials during spin arc welding. Thus, spin arc welding proves to be superior for dissimilar Hastelloy joints of C-2000 and C-276.

Structural integrity and corrosion behavior assessment of the dissimilar gas tungsten arc welded joint of sDSS 2507/ IN625 superalloy

This study investigates the effect of filler materials on the structural integrity of dissimilar welded joints (sDSS 2507/IN625) employed in marine subsea manifold applications. Multi-pass gas tungsten arc welding (GTAW) using ER2594 and ERNiCrMo-3 fillers investigates microstructure evolution, solidification mechanisms, mechanical characteristics, residual stresses, and corrosion behavior. Optical and scanning electron microscopy revealed an unmixed zone (peninsula/island-shaped) at the weld interface and weld zone (WZ) in both filler weldments. Segregation of Mo, Nb, and Ni was observed in the interdendritic region of ERNiCrMo-3 filler WZ, whereas the skeletal ferrite matrix of ER2594 filler WZ revealed the existence of the Laves phase (Fe2Nb form). Electron beam scattered diffraction (EBSD) investigation showed austenitic solidification for ERNiCrMo-3 and fully ferritic solidification for ER2594 with distinct dendrite formations. Inhomogeneity has been observed in microhardness maps, with ER2594 and ERNiCrMo-3 WZ averaging 310 ± 7 Hv0.5 and 270 ± 10 Hv0.5, respectively. ERNiCrMo-3 (165 ± 5 J and 180 ± 3 J) and ER2594 (100 ± 3 J and 110 ± 6 J) fillers exhibited distinct effects on toughness in the weld cap and root due to laves phase precipitation. Additionally, ER2594 (750 ± 6 MPa) and ERNiCrMo-3 (790 ± 7 MPa) fillers exhibited lower tensile strength than base metals. The structural integrity of WZ can be better understood by residual stress analysis, which showed compressive cap pass and tensile root pass residual stresses for both fillers. In a 3.5 wt% NaCl solution, both fillers exhibited outstanding corrosion resistance in marine conditions. These findings improve subsea manifold structure integrity and performance in corrosive marine environments.

In-process phased array ultrasonic weld pool monitoring

In recent years, there have been increasing economic and industrial drivers for the development of real-time non-destructive evaluation directly at the point of manufacture. Real-time inspection and monitoring of welding processes can help to reduce fabrication costs by detecting defects as they occur, enabling more efficient and cost-effective builds. This paper shows, for the first time, the use of phased array ultrasonics to monitor and analyse the molten weld pool during deposition of multi-pass gas tungsten arc welds. The received ultrasonic signals are shown to contain information related to key physical transitions occurring within the welding process, namely the melting and solidification of the weldment. Furthermore, the technique used here is shown to be effective for determining weld quality in real-time with significant signal changes occurring when defects such as Lack of Root Penetration are present. The accurate focusing and steering capabilities offered by phased arrays are used to successfully isolate the molten weld pool from the surrounding solidified weldment during deposition of multiple layers of a multi-pass weld.

Hot Corrosion Behavior of Fe–Cr–Ni-Based Austenitic Heat-Resistant Steel Weld Metal in Na2SO4–NaCl Molten Salts at Different Temperatures

The hot corrosion behavior of the austenitic heat-resistant steel weld metal produced by multi-pass gas tungsten arc welding was investigated. A bilayer oxide scale formed on the surface of the weld metal by the selective oxidation of the element Cr and Fe, and the dissolution of the oxide film occurred at elevated temperatures due to the existence of the molten salt. Morphologies and structures of the oxide scale were characterized by electron microscopy and X-ray diffraction techniques, and then the underlying reason for the hot corrosion of the nonequilibrium weld metal was identified. The interdendritic region could act as the shortcut path for the oxygen, which promotes the internal oxide distribution in chains, and a large internal oxidation zone appears under the bilayer oxide scale.

Enhancing mechanical properties and creep performance of 304H and inconel 617 superalloy dissimilar welds for Advanced Ultra Super Critical power plants

This investigation is conducted to find improvements in the mechanical properties of the dissimilar welds between austenitic stainless steel and Nickel superalloy used in the manufacturing of Advanced Ultra Super Critical (AUSC) power plants boiler components. Nickel superalloys (IN617) have excellent creep rupture properties, so they are employed in higher temperature regions (above 700 °C) of the boiler. In contrast, austenitic stainless steel (304H) is used in moderate temperature regions (up to 650 °C) part of the boiler, also striking off the economic balance need. The welding is carried out with manual multipass Gas Tungsten Arc Welding using an IN617 filler to compare 304H and IN617 weld without and with a buttering layer. Multiple attenuated buttering material layers are deposited at the 304H side, further machining the butter weld layer to the weld groove and closure weld to the IN617 part. The buttering technique improves mechanical properties and high-temperature creep rupture life through microstructure transformation. The dissimilar weld characterization shows a supplementary dendritic growth layer for the buttering deposition. The tensile strength is found to be 728.214 MPa with buttering higher than without buttering weld (620.25 MPa). The V-notch Charpy impact toughness value of the buttered weld sample is 66 J, greater than the non-buttered sample, i.e., 63.25 J. The creep rupture duration significantly increased with buttering. Strain hardening at higher applied stress helps to resist creep failure even at higher temperatures.

Improving mechanical properties of P91 and IN617 dissimilar weld joints for advanced ultra super critical power plants by buttering technique

• Enhancement of mechanical properties using buttering layer for dissimilar weld. • Buttering helps in achieving favorable ductility for dissimilar welds. • With buttering creep rupture life of dissimilar welds has increased. • Buttered dissimilar welds can be implemented for very high-temperature boilers. To improve the mechanical properties of ferritic steel and nickel superalloy, while maintaining economic symmetry, this investigation is performed on fabricating dissimilar metal welds for advanced ultra-supercritical (AUSC) boilers, with a buttering technique. The present work compares a dissimilar weld of IN617 alloy and P91 steel with and without buttering by multipass gas tungsten arc welding for mechanical properties variations, creep strength at elevated temperature, and microstructural evolution. Thin layers of buttering material are applied at the ferritic end, followed by the closure weld consisting of welding a second member to a butter layered component. Further, significant improvement in mechanical properties (tensile strength, toughness, and hardness) and creep rupture life (though the failure remains ductile characterized by dimples variations) 2 to 3 times longer has been found with buttering in contrast with no buttering.

Development of Multipass Gas Tungsten Arc Welding Technique for Aerospace Grade 18% Ni-Co-Mo Based Maraging Steel to Improve the Metallurgical and Mechanical Properties by Reducing Austenite Pools

Abstract The maraging steel is an impeccable choice for aerospace applications due to its high strength and excellent toughness. Welding is importantly involved in fabricating various structures out of maraging steel. Gas tungsten arc welding (GTAW) is the most commonly adopted technique for joining maraging steel. However, the major concern regarding the welding of maraging steel is the formation of the reverted austenite phase in the fusion zone and the heat affected zone. This reverted austenite deteriorates the mechanical property performance of welded joints. The present study focuses on GTAW of 12 mm thick MDN 250 maraging steel. An attempt was made to suppress the reverted austenite by employing suitable post-weld heat treatments. Three different types of post-weld heat treatments were adopted, i.e, (i) direct aging (W-DA), (ii) solutionizing + aging (W-SA), and (iii) homogenizing + solutionizing + aging (W-HSA). The micrograph and XRD analysis of the fusion zone with W-DA and W-SA conditions reveal the presence of reverted austenite. The SEM/EDAX examination of the fusion zone of as-welded, W-DA, and W-SA conditions revealed micro-segregation of Ni, Mo, and Ti elements from the matrix to the grain boundaries. On the other hand, the W-HSA condition was free from micro-segregation and austenite reversion. Electron backscatter diffraction (EBSD) analysis was used to estimate the percentage of reverted austenite in the fusion zone. The tensile test shows the highest strength of 1721 MPa (UTS) in W-HSA conditions. Similarly, the microhardness of the W-HSA conditions depicts a higher hardness with an even distribution in the microhardness values across the weldment. Hence, considering both metallurgical and mechanical examination of the weldment, the W-HSA heat treatments give superior properties in the case of GTA welded MDN 250 grade maraging steel.

Influence of PWHT Parameters on the Mechanical Properties and Microstructural Behavior of Multi-Pass GTAW Joints of P92 Steel.

The 9% Cr steels were developed for ultra-supercritical (USC) power plants to meet the requirements of high operating temperature and pressure. These steels are produced to operate at high temperatures where impact toughness is not a concern; however, it becomes important for the welded joints to have good impact toughness at room temperature for manufacturing. The present work investigates the effect of the post-weld heat treatment (PWHT) parameters, i.e., temperature and time, on the impact toughness of multi-pass gas tungsten arc welded (GTAW) joints of ferritic/martensitic grade P92 steel. The microstructural evolution in welded joints given varying post-weld temperatures and times was studied. The lath martensitic structure of the weld metal for the as-welded joints resulted in high hardness and low impact toughness. The weld fusion zone toughness was 12 J, which was lower than the minimum specified values of 41 J (ASME standards) and 47 J (EN ISO 3580:2017). The PWHT temperature and time were found to have a significant effect on the impact toughness of the weld metal. A drastic increase in the impact toughness of the weld metal was noticed, which was attributed to lath break-up, reduction in dislocation density and reduction in solid solution hardening. The maximum impact toughness of 124 J was measured for PWHT temperature and time of 760 °C and 120 min, respectively. The effect of PWHT parameters on tensile strength was also investigated, and test results showed that the joint was safe for USC boiler application as it failed from the region of the P92 base metal. The variation in microstructural evolution along the weldments resulted in hardness variation. PWHT led to homogeneity in microstructure and, ultimately, reduction in hardness value. According to the study, the optimum temperature and time for PWHT of a GTAW joint of P92 steel were found to be 760 °C and 120 min, respectively.

Automatic welding imperfections detection in a smart factory via 2-D laser scanner

Welding defect detection is crucial to determine welding quality and assure correct mechanical performance of the welded joint during operation. Most of common detection techniques are performed offline, are time consuming, and strongly dependent on the operator skill. In the present work, two novel automatic online welding imperfections detection approaches are presented for classifying qualified and defective profiles acquired with a 2-D laser scanner on square grooves in a multi-pass Gas Tungsten Arc Welding process. In particular, a Heuristic Algorithm has been developed to analyze different geometrical characteristics of the profile for its classification. The Heuristic Algorithm has been validated on five welded coupons, created in Baker Hughes welding laboratory in Massa, Italy. Coupons have been scanned and resulting profiles have been manually labeled. The Heuristic Algorithm showed good performance, with a F1 score of 94.8%. Additionally, a multi-layer feed forward Neural Network has been trained and tested on the profiles acquired from the five welded coupons, with an overall F1 score of 91.0%. The promising results of the two proposed approaches will lead to a wider test campaign, before implementing a mixed solution in a multi-pass robotic Gas Tungsten Arc Welding industrial process.

Experimental investigation on microstructure, mechanical properties, and residual stresses of dissimilar welded joint of martensitic P92 and AISI 304L austenitic stainless steel

In this study, microstructural characteristics and mechanical properties of martensitic steel P92 and AISI 304L austenitic stainless steel (ASS) dissimilar metal weld (DMW) have been examined. The welding was performed using the multipass gas tungsten arc welding (GTAW) process. The ERNiCrMo-3 (Inconel 625) filler metal was used due to its excellent compatibility with the P92 and AISI 304L ASS metals. The microstructure characterization performed with an optical microscope and scanning electron microscope revealed the presence of an unmixed zone in the form of beach, peninsula, and island at the base metal and weld fusion zone interface. Secondary phases enriched with Nb and Mo, and migrated grain boundaries (MGBs) were also observed at the weld fusion zone. The microstructural characterization at the P92 side revealed the presence of a soft ferrite zone and delta ferrite at the weld and P92 interface. The different peak temperature experienced by the P92 metal during the welding process results into the formation of three distinct zones with diverse mechanical and microstructural properties namely coarse grain heat affected zone (CGHAZ), fine grain heat affected zone (FGHAZ), and inter-critical heat affected zone (ICHAZ). The post weld heat treatment (PWHT) at 760 °C for 2 h followed by air cooling was performed to homogenize this heterogeneous microstructure formed toward the P92 side. It was observed that this PWHT does not have any significant effect on weld fusion zone and 304L SS heat affected zone (HAZ) microstructure. But, PWHT modifies the microstructure of P92 side HAZ. The tensile test, Vickers micro-hardness test, and charpy impact test were carried out to determine the mechanical properties of the DMW. The tensile strength was found as 587.709 MPa in the as-welded condition and 594.515 MPa in PWHT condition. The tensile test results indicated that specimens were failed from 304L stainless steel (SS) base metal (BM) in as-welded and PWHT condition. The charpy impact toughness test results showed that the toughness of the weld joint was very low in as-welded and PWHT condition (47J) compared to the P92 (190J) and SS 304L (285J) BM. From the microhardness examination, it was observed that the ICHAZ was the weakest zone among all the other regions due to its lower micro-hardness value compared to the other regions. After welding process, residual stresses were measured in as-welded and after PWHT condition at the center of the weld and at the HAZ of P92 and SS 304L (through the thickness of the plate) using experimental-based deep hole-drilling (DHD) strain-gauge method. The results showed that both circumferential (hoop stress) and axial (longitudinal stress) welding residual stresses were decreased after PWHT.

Microstructures and mechanical properties of the fusion zone of 316L-316LN stainless steel multi-pass gas tungsten arc welded joint

Microstructures and mechanical properties of the fusion zone of 316L-316LN stainless steel multi-pass gas tungsten arc welded joint

Electrochemical corrosion behavior and microstructural characteristics of electron beam welded UNS S32205 duplex stainless steel

AbstractDifferent electrochemical techniques were used to study the corrosion behavior of UNS S32205 duplex stainless steel (DSS) welded autogenously using a single‐pass by electron beam welding process, supplemented by microstructural characterization. Furthermore, a comparative study was also performed between multipass gas tungsten arc (GTA)‐welded and EB‐welded DSS for their microstructure and corrosion behavior. The differences in weld thermal cycle and chemical composition influenced the fusion zone microstructure of both the welds and eventually their corrosion properties. The general corrosion resistance of the EB weld was lower than the base metal and higher than the GTA weld despite its weld zone being characterized by a relatively unbalanced phase ratio (α/γ) in comparison to the GTA weld. However, the EB weld showed relatively higher susceptibility to pitting corrosion than the base metal and GTA weld due to its poor repassivation characteristics and poor resistance to pit growth.

Remedies for hydrogen-embrittlement on Grade-91 steel weld joint during long delay in PWHT

This paper discusses the effect of different combinations of preheating and post-heating on the corrosion and hydrogen embrittlement of Grade-91 steel weld joint in the as-welded condition. Though these welds are subjected to post-weld heat treatment (PWHT), there could be long delay in executing this, especially in the case of fabrication of large components, and during this delay, the welds in as-welded conditions are prone to environmental degradations like hydrogen embrittlement and different forms of corrosion. Though recommended minimum preheating and post-heating conditions are already known for welding of this class of steels, marginally increasing the temperature of preheating and post-heating or increasing the duration of post-heating can permit longer delay in the post-weld heat treatment (PWHT), without increasing the risk of environmental degradation. With this objective, a set of thirteen single-V multi-pass gas tungsten arc weld joints of Grade-91 steel fabricated with different preheating and post-heating combinations. Results indicated a significant reduction in residual tensile stresses in the weld metal and reversing into compressive stresses upon preheating and post-heating at 300 °C. Moreover, diffusible hydrogen content and corrosion rate decreased significantly with the preheating and post-heating. However, there are no significant variations in the microstructure, mechanical properties, and hydrogen embrittlement with preheating and post-heating. This study proved that the risk of embrittlement in Grade-91 steel welds due to an indefinite delay in PWHT could be minimized by combined preheating and post-heating.

Analysis of thermal stresses and its effect in the multipass welding process of SS316L

In this work, simulation has been carried out for multipass Gas tungsten Arc Welding of SS316L and their effects were studied for thermal, and residual stresses and compared with experimental results. The sequentially coupled thermo-mechanical, transient thermal and static structural analysis was modelled using ANSYS with relevant Ansys Parametric Design Language coding. Infrared thermography camera was used to obtain the real time temperatures during the welding. The effect of temperatures and the induced residual stresses of the weldment is measured with X-Ray diffraction technique. Tensile residual stresses have been identified at fusion zone and compressive stresses at heat affected zone. Pass wise temperature measurements were taken from analysis and experiment the agreement with 7.24%. The agreement of measured and analysed residual stress had an agreement with 16.66%.

Effects of dilution on the hardness and residual stresses in multipass steel weldments

A thermal-metallurgical-mechanical model was developed to study the effects of dilution in each weld pass for multipass gas tungsten arc and submerged arc welding in low alloy steel (i.e. SA508) plates. Hardness distributions and residual stresses were measured on the transverse sections perpendicular to the welding direction of the manufactured weldments. The predicted hardness and residual stresses were compared with the measurement data and shown to be reasonably accurate. The results showed that dilution can significantly affect both the hardness and the residual stress field in the weld metal. It was found that, for the base and filler materials used, increased dilution led to greater weld-metal hardness and reduced the magnitude of tensile stress or promoted compressive stress in the as-deposited and reheated weld metals. This mechanical behaviour is associated with the tendency for diluted weld metal to experience delayed austenite decomposition, owing to the high hardenability of SA508 steel relative to the filler materials used. Although dilution is irrelevant for the hardness of the base material and its transformation products adjacent to the weld metal, it affected the full-field residual stresses via the equilibrium interaction between the stresses in the base and weld metals.

Effect of inclusions on microstructure and mechanical behavior of multi-pass welded naval grade steel

This paper assesses the metallurgical characteristics and mechanical properties of multi-pass gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) of micro-alloyed steel DMR 249-A. The prime objective was to carry out a comparative study of the microstructure and mechanical properties of the joints produced by the two types of welding. A high volume of larger sized inclusions in GMAW contributed to inferior mechanical properties. The coarse-grained part of the heat-affected zone (CGHAZ) showed a lower microhardness. Fracture always occurred in the heat-affected zone, and it is believed that it is associated with CGHAZ. GTAW joints showed low tensile residual stress, higher hardness, and tensile strength. GTA weldment also showed superior impact toughness at sub-zero temperature (–60 °C). Mn-containing inclusions were seen in GTA weldments; it is believed that they promote the formation of acicular ferrite. This is believed to be responsible for the superior mechanical properties of GTA weldments. The microstructural analysis of the two weldments revealed the presence of a higher volume fraction of acicular ferrite in the GTAW. All in all, GTAW joint was found to perform better than GMAW joint.

Microstructure evolution and deformation resistance of heavy-thickness Ti-6Al-4V narrow-gap welded joints

In this work, 60 mm thick Ti-6Al-4V plates were joined by narrow-gap multi-pass gas tungsten arc welding (GTAW) technology to investigate microstructure and deformation resistance of joints. Depending on cooling condition, mixed weld microstructure consists of stable α phase, martensite α′ phase and few retained β phase. Hardness of joined parts is higher than base metal (BM) mainly contributed by martensite strengthening. A simplified model was established to reveal microstructure evolution during welding thermal cycle. It can also demonstrate that different thermal action effect is responsible for distinct microstructure forms especially martensite α′, which varies hardness in different regions of joints.

Microstructure of a domestically fabricated dissimilar metal weld joint (SA508-52M-309L-CF8A) in nuclear power plant

A dissimilar metal weld joint (DMWJ) between low alloy steel and casted duplex stainless steel (CDSS) prepared by automatic multi-pass gas tungsten arc welding (GTAW) with 309L and 52M filler metals is studied by optical microscopy, scanning electron microscopy (with electron back scattering diffraction and energy dispersive X-ray spectroscopy systems), scanning probe microscope techniques and micro-hardness testing. Carbon-depleted zones and microstructure transitions are found in the SA508 heat affected zone (HAZ). Type-I and type-II boundaries exist in 52Mb at some places near the SA508-52Mb interface. The ferrite at the unmixed zone and the zone adjacent to the 309L-CF8A interface (within ~300 μm) changes to a lamellar structure with intermixed ferrite and austenite strips during the buttering process. Epitaxial growth and higher residual strain level are found at the 52Mw-52Mb, 52Mw-309L and 309L-CF8A interface areas. The micro-hardness distribution across the dissimilar metal weld joint is related to the complex chemical composition distribution and the microstructure transition, which in turn made it non-uniform and complicated.

Characterisation of dissimilar P91 and P92 steel welds joint

ABSTRACTIn the present study, dissimilar weld joint was prepared using the P91 and P92 steel plate of 8-mm thickness, using the multi-pass gas tungsten arc (GTA) welding with filler (weld 1) and autogenous tungsten inert gas welding (A-TIG) process (weld 2). Evolution of δ-ferrite patches was studied in weld zone and heat affected zone (HAZ) for both weld 1 and weld 2. Effect of varying post weld heat treatment (PWHT) duration was also studied on δ-ferrite patches and mechanical properties of the dissimilar weld joint. PWHT was carried out at 760°C. For weld 2, weld zone showed poor impact toughness and higher peak hardness as compared to weld 1. After the PWHT, a considerable reduction in hardness was obtained for both weld 1 and weld 2,while impact toughness of weld zone showed a continuous increment with PWHT duration. For weldments characterisation, optical microscope, scanning electron microscope (SEM) and microhardness tester were utilised.

Effect of Low-Temperature Sensitization on the Corrosion Behavior of AISI Type 304L SS Weld Metal in Simulated Groundwater

The manuscript presents the investigations carried out on the effect of low-temperature sensitization (LTS) of 304L SS weld metal on its corrosion behavior in simulated groundwater, for its application as a canister material for long-term storage of nuclear vitrified high-level waste in geological repositories. AISI type 304L SS weld pad was fabricated by multipass gas tungsten arc welding process using 308L SS filler wire. The as-welded specimens were subsequently subjected to carbide nucleation and further to LTS at 500 °C for 11 days to simulate a temperature of 300 °C for 100-year life of the canister in geological repositories. Delta ferrite (δ-ferrite) content of the 304L SS weld metal substantially decreased on carbide nucleation treatment and further only a marginal decrease occurred on LTS treatment. The microstructure of the as-welded consisted of δ-ferrite as a minor phase distributed in austenite matrix. The δ-ferrite appeared fragmented in the carbide-nucleated and LTS-treated weld metal. The degree of sensitization measured by double-loop electrochemical potentokinetic reactivation method indicated an increase in carbide nucleation treatment when compared to the as-welded specimens, and further increase occurred on LTS treatment. Potentiodynamic anodic polarization investigations in simulated groundwater indicated a substantial decrease in the localized corrosion resistance of the carbide-nucleated and LTS 304L SS weld metals, when compared to the as-welded specimens. Post-experimental micrographs indicated pitting as the primary mode of attack in the as-welded, while pitting and intergranular corrosion (IGC) occurred in the carbide-nucleated weld metal. LTS-treated weld metal predominantly underwent IGC attack. The decrease in the localized corrosion resistance of the weld metal after LTS treatment was found to have a direct correlation with the degree of sensitization and the weld microstructure. The results are detailed in the manuscript.