Effect of dynamic wire in the GTAW process Gonçalves e Silva, Régis Henrique; Correa Riffel, Kauê; Pompermaier Okuyama, Marcelo ...
Journal of materials processing technology,
July 2019, 2019-07-00, 20190701, Letnik:
269
Journal Article
Recenzirano
Odprti dostop
Experimental tests were carried out to evaluate and compare metallic transfer aspects and welding process behavior, in GTAW with dynamic wire feeding in relation to conventional mode (wire insertion ...in a continuous way with constant speed / steady state). Inconel 625 cladded steel plates were laid out at four main welding positions (overhead, vertical up, vertical down and flat). The metallic transfer analysis was based on wire’s tip monitoring, by means of high speed videography and monitoring of the difference of electric potential between tungsten electrode, wire and plate. Continuous wire feeding resulted in rupture of the contact between wire and puddle, which generated large droplets on the wire’s tip, which transferred in irregular intervals. This fact is a common occurrence mainly out of flat position and incurs in irregularity of metal transfer and of oscillographic behavior. For dynamic wire feeding the amount of molten metal in the wire’ tip (droplet) was minimized and the transfer frequency became regular, rendering a more stable and robust process. Also, the risk of electrode contamination was reduced. The benefits of dynamic feeding were observed for both constant and pulsed current.
Experimental tests were carried out to evaluate and compare welding bead characteristics depending on wire’s oscillation frequency in the GTAW with dynamic wire feeding. Two constant currents (200 A ...and 250 A) and three frequencies (0 Hz, 5 Hz and 18 Hz) were used to lay an Inconel 625 filler metal over ASTM A36 steel substrates. The weld metal microstructure analysis was based on Scanning Electronic Microscopy (SEM) and the corrosion performance was assessed by means of the Cyclic Potentiodynamic Polarization method. As main results, the increase in wire’s frequency tends to promote more convex welding beads and a 22 % reduction of dilution, in addition to a higher spreading (and higher isotropy) of the precipitates in the material’s structure. For the high energy condition (250 A) the highest frequency also increased significantly the welding bead corrosion resistance. However, for 200 A cases the effect of the wire’s oscillation in the dilution and bead’s reinforcement was not so pronounced, although with higher frequency a greater spreading in phases precipitated was verified.
This study approaches conflicting aspects of high-performance MIG/MAG associated with penetration. It demonstrates that a simpler version with voltage control enables the obtainment of good results ...as long as some premises, related to the power source, are maintained. The molten pool in this case is a cavity in the piece, which is caused by a condition referred to as “buried arc.” In this technique, a short circuit is imminent, and dynamic characteristics are required of the power source employed, so that the arc stability is maintained through a change in the value of the current at different rise and fall rates. The use of different gas mixtures was also analyzed, including the version referred to as T.I.M.E. Even though it influenced the geometry of the melted zone, this gas mixture does not provide a significant improvement in the buried arc in relation to other mixtures. A comparison, in terms of the dynamic morphology of the pool, between the process with voltage control and the variant Rapid Arc (pulsed current) also shows the efficiency of the simpler version for a deep penetration. The employment of less expensive mixtures, such as 50% of CO
2
in argon, technically achieves the same objectives in high-performance welding.
Most of pipeline welding still applies manual procedures, which increase production time and is stressful to the welding operator. This happens mainly due to the accurate melt pool control that hand ...operation enables. It yields high flexibility between material addition and heat source and is therefore adaptable to the welding condition and situation of each moment. This feature is not fully found when mechanized welding with automatic feeding is performed, despite every benefit of welding automation. This renders an optimized parameterization of a complex task. Automatic orbital welding is already a reality, though only applied in large scale in developed countries and/or by few expert companies from developed countries, due to such controllability, repeatability, and robustness difficulties. In this paper, a concept for dynamic wire feeding and respective implementation and analysis are presented. It consists of a low-frequency wire speed oscillation, aiming to decouple wire speed and arc power to a larger extent, which approaches to manual procedure as it guarantees user flexibility, but still keeping the benefits of welding automation. ASTM 139 Grade D tubes were welded under stable processing conditions. The macrographs did not indicate discontinuities such as porosity or lack of fusion, resulting in complete joint penetration. The average welding speed reached was 27.8 cm/min (10.9 in/min), much higher than that found by other authors.
Numerous GMAW processes have appeared for one-pass welding of medium to high thickness. This concept originated in the 1980s with the T.I.M.E. technique and the 1990s and 2000s saw the emergence of ...Rapid Arc, Rapid Melt and Buried Arc techniques. These processes did not include any special control features, but rather parameter combinations to improve bead geometry. A new interpretation of the factors that are critical to achieving high performance GMAW (penetration) is presented herein. Tests with a voltage controlled power source with inductance control and the Rapid Arc process were conducted on bead-on-plate for high speed videography. Later, validation tests were conducted on butt joints, with the technique developed, with and without groove on AISI 1020, 3/8" thick steel plates. The power source may be of voltage control but has to provide a dynamic response able to sustain a metastable equilibrium of the molten pool applying the semi-keyhole technique. The technique developed was able to weld, in a single pass, 3/8" thick butt joint steel plates. The results showed that a power source, to achieve the GMAW buried arc condition, only needs to provide dynamic characteristics able to maintain the molten pool in a metastable equilibrium.
Effect of sigma phase on CVN impact toughness in HDSS weld metal Acuna, Andres; Riffel, Kaue Correa; Ramirez, Antonio
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
October 2024, 2024-10-00, Letnik:
912
Journal Article
Recenzirano
This work uses kinetics calculation and a thermomechanical physical simulator to evaluate the sigma phase precipitation in Hyper Duplex Stainless Steel (HDSS) as-welded microstructure for impact ...toughness evaluation. Precipitation bars were machined out of a HDSS deposited clad mockup and submitted through aging on the thermomechanical physical simulator. Bars with sigma phase volumes of 0 %, 0.16 %, 0.52 %, 0.9 % and 4.3 % were created and machined to sub-size CVN specimens. Through impact CVN testing, complete ductile-to-brittle-transition-temperature (DBTT) curves were developed based on absorbed energy (kV), lateral expansion (LE), and shear fracture appearance (SFA) criteria for each sigma phase volume. It was seen that sigma phase presence provides drastic reduction on toughness the HDSS. The DBTT increased from −52.29oC to 38.32oC while the upper shelf energy (USE) is reduced from 68.85J to 11.66J with the increase sigma phase volume. Both the DBTT and USE presented a behavior well fitted through a sigmoidal curve. While very low sigma phase volumes caused little change on the DBTT and USE values, a saturation effect could be inferred on the USE at 4.3 % vol of sigma phase. CVN samples’ secondary cracks high-resolution images suggest that the ferrite and austenite arrest crack propagation while the brittle sigma grains, depending on size and orientation propagate the cracks.
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•Sigma Phase presence significantly reduces the impact toughness of HDSS, posing challenges for various applications.•Ductile-to-Brittle-Transition-Temperature (DBTT) curves, considering absorbed energy, lateral expansion, and surface fracture appearance as evaluation criteria.•Sigma Phase sigmoidal relationship with impact toughness parameters. Toughness decreases significantly as sigma phase volume increases.•Secondary Crack suggests propagation is influenced by sigma phase grain size and orientation, providing valuable insights for fracture mechanics.
Abstract Currently, in the Brazilian Oil and Gas scenario the execution of welding processes for stainless steel consists mostly of manual applications, rendering hard any increase in productivity ...and repeatability. Therefore, the automation of this step allows several advances such as more reliable results, lower costs and healthier condition for welders. With the aim of developing automation for GTAW welding of these applications, experimental tests were carried out in a 304L stainless steel pipe laid out at 5G position. The samples were orbitally welded by GTAW with dynamic wire feeding process, using a robotic manipulator in two steps of 180º in vertical down progression. The welding parameters and the stable condition obtained are discussed and the weld pool behavior during the root pass was online monitored by HDR (High Dynamic Range) videography. It was possible to reach high welding speed in the root pass, 50 cm/min in a keyhole mode. The wire dynamic movement contributed to reach great process stability and robustness for all joint passes, from root to finishing. The macrographs and x-ray analysis did not indicate discontinuities as porosity nor lack of fusion.
Resumo Atualmente, no cenário brasileiro de óleo e gás a execução dos processos de soldagem para aços inoxidáveis consiste em sua grande maioria de aplicações manuais, dificultando qualquer aumento de produtividade e repetibilidade dos processos. Portanto, a automação desta etapa permite grandes avanços com resultados mais confiáveis, menores custos relacionados à mão-de-obra e condições de saúde mais favoráveis para os soldadores. Com o objetivo de automatizar a soldagem GTAW em aplicações orbitais, testes experimentais foram realizados em tubos de aço inoxidável 304 L, posicionados na posição 5G de soldagem. As amostras foram soldadas pelo processo GTAW com alimentação dinâmica de arame, utilizando um manipulador robótico em 2 etapas de 180º na progressão vertical descendente. Os parâmetros de soldagem e a condição estável de soldagem obtidos foram discutidos e o comportamento da poça durante a soldagem do passe de raiz foi monitorado por videografia em HDR. Foi possível se alcançar alta velocidade de soldagem no passe de raiz, 50 cm/min com o modo keyhole. O movimento dinâmico do arame contribuiu para alcançar-se grande estabilidade do processo e robustez na soldagem de todos os passes, da raiz ao acabamento. As macrografias e ensaios de raio-x realizados na junta não indicaram a presença de descontinuidades como poros e falta de fusão.
This study investigates and compares the kinetics of sigma phase formation in established Super Duplex Stainless Steel (SDSS) and recently developed Hyper Duplex Stainless Steel (HDSS) filler metal ...wires. Experimental sigma phase time-temperature-transformation (TTT) maps were developed, revealing nearly equivalent interface area/volume, resulting in similar sigma phase kinetics for 1% volume despite the higher Cr and Mo content in HDSS. However, the growth rate to 5% and 10% sigma phase volumes was slightly higher in HDSS. The sigma phase kinetics in both SDSS and HDSS were analyzed using the exponential Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach based on experimental TTT data. Linearized plots from the JMAK calculations showed a transition in kinetics mechanism from eutectoid decomposition and interface-controlled growth to a diffusion-controlled growth stage in both materials. The higher volumes and morphologies of the sigma phase were found to be diffusion-dependent, mainly occurring during the second kinetics stage. The influence of HDSS's higher Cr and Mo content on the growth rate was observed for these higher volumes. For applications within the 1% sigma phase limit, both wires exhibited equivalent susceptibility to sigma phase formation, regardless of the higher alloying in HDSS.
•A comprehensive sigma phase kinetics comparison between the (27Cr 7Ni 5 Mo) hyper duplex stainless and the well-established industry-standard super duplex stainless steel (25Cr 9Ni 4 Mo) as-welded microstructure.•Stringent control over welding conditions and precipitation experiments ensured comparable initial microstructures, making the observed kinetics differences primarily attributable to the distinct chemical compositions of the alloys.•The study comprehensively examines sigma phase transformation and morphology above, at, and below the maximum kinetics temperature.•For each material, we present sigma phase kinetics formation as a single equation, dependent on temperature and time. This approach lends itself particularly well to ICME implementation.•Up to 1% of sigma phase volume both materials present equivalent kinetics. Only on higher volume fractions that the Cr richer alloy (HDSS) presents a marginal higher sigma phase kinetics.
The Hyper Duplex Stainless Steel HDSS enhanced corrosion resistance and toughness relies upon high alloying to obtain a balanced ferrite and austenite volume and pitting resistance equivalent number ...PREn. However, during welding, sigma phase precipitates might form, hindering corrosion and mechanical performance. Therefore, a kinetics model is developed to avoid the sigma phase's formation during welding and validated using physical simulation, finite element analysis (FEA), welding, and SEM characterisation. The sigma phase kinetics model produced calculated and validated temperature-time-transformation (TTT) and continuous-cooling-transformation (CCT) curves from which a 4°C/s cooling rate was found as a cooling rate threshold for sigma phase formation in this new material. Three-layered gas tungsten arc welding GTAW cladded mockup with 53 beads produced 24°C/s minimum cooling rate. Moreover, microscopy, mechanical, and corrosion testing attested it as a sigma-free weld.
This study focuses on the kinetic analysis of sigma phase formation in filler metal wires on Super Duplex Stainless Steel (SDSS) and Hyper Duplex Stainless Steel (HDSS). Precipitation data reveal ...that in the solubilized microstructure, sigma phase kinetics are more prominent in SDSS. This increased susceptibility is attributed to the greater number of nucleation sites, which is facilitated by the larger interface area/volume and the higher chromium content in the ferrite. The difference in interface area/volume is significantly more influential in determining kinetics than the composition difference, with nucleation sites playing a central role. The sigma phase transformation in both materials was modeled using the JMAK kinetic law. The JMAK plots exhibit a transition in kinetic mechanisms, evolving from discontinuous precipitation to diffusion-controlled growth. In SDSS, the JMAK values indicate “grain boundary nucleation after saturation,” followed by “thickening of large plates.” In contrast, HDSS values point to “grain edge nucleation after saturation,” followed by “thickening of large needles.” The higher kinetics in SDSS are characterized by a smaller nucleation activation energy of 56.4 kJ/mol, in contrast to HDSS's 490.0 kJ/mol. CALPHAD-based data support the JMAK results, aligning with the maximum kinetics temperature of SDSS (875 °C to 925 °C) and HDSS (900 °C to 925 °C). Therefore, the JMAK sigma phase kinetics effectively describe the experimental data and its dual kinetics behavior, even though CALPHAD-based TTT calculations often overestimate sigma formation.
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