Autogenous Gas Tungsten Arc Welding Research Articles

The role of the microstructural changes during induction preheating on the HAZ liquation cracking susceptibility of Ni-based superalloy

This work presents the influence of high-frequency induction preheating (900, 1000, 1100 °C) on liquation crack formation in the René 108 Ni-based superalloy. The investigation was divided into two parts: (1) characterization of the material's microstructure after preheating and (2) determining the influence of preheating on liquation cracking during autogenous gas tungsten arc welding. During preheating, the dissolution of γ′ precipitates showed accelerated progress with increase in temperatures. This dissolution involved the continuous thinning of each precipitate, as well as more intricate mechanisms, such as splitting. The mean size of the secondary γ′ decreased from 0.32 to 0.26 μm. In the heat-affected zone (HAZ) induced by welding, constitutional liquation of mainly γ' precipitates, with a contribution of M23C6 carbides and M5B3 borides, was observed. The formation of a thin non-equilibrium liquid film along high-angle grain boundaries led to the crack initiation and their further propagation during cooling. The eutectic γ–γ' re-solidification products were visible on the crack edges independently of preheat temperature. Preheating at 900 °C decreased the length and amount of liquation cracks, while preheating at 1100 °C allowed to prevent them due to the liquid-healing effect.

Metallurgical and mechanical properties evolution in cusp-magnetic field-assisted GTAW of low carbon steel

In this work, magnetic field-assisted autogenous gas tungsten arc welding (GTAW) of low carbon steel has been studied. The external magnetic field was generated by custom made rectangular shaped samarium cobalt permanent magnets which were arranged in a manner to generate a cusp-type magnetic field (CMF). The impact of CMF on the morphology and microstructural evaluation of the weld joints were examined prudently using optical microscope, Field-emission scanning electron microscope and X-Ray Diffraction technique. The experimental results exhibited that the CMF assisted GTAW produce joints with better mechanical and metallurgical properties compared to conventional GTAW. In CMF assisted welds, width of weld beads was reduced by an average of 4% and depth of penetration was increased by 13%. Temperature measurement on the work piece showed a significant amount of temperature decrease for the same amount of heat input under the influence of the CMF. An improvement in grain refinement was observed in the heat-affected zone (HAZ) adjacent to fusion boundary by 12% and HAZ adjacent to base metal by 15% compared to conventional GTAW, which in turn promoted a considerable increase in hardness. The tensile properties in CMF assisted welds were enhanced by 3% and 8%, respectively, for an increase of heat inputs from 0.34 to 0.45 kJ mm−1.

Microstructural evolution in CoCrFeNi and CoCrCuFeNi alloys processed by autogenous fusion welding

In this work, CoCrFeNi and CoCrCuFeNi multi-principal element alloys were welded using an autogenous gas tungsten arc welding process. Tensile testing, microhardness, X-ray diffraction, scanning electron microscopy, electron back scattered diffraction techniques were used to study the mechanical properties, crystal structure and microstructures of the base metals (BMs) and welds. It was found that CoCrFeNi alloy was easily weldable while CoCrCuFeNi alloy produced transverse cracks. Only 13.5% decrease in ultimate tensile strength as compared to its BM was observed in CoCrFeNi alloy. Whereas poor tensile properties were observed in CoCrCuFeNi alloy due to Cu segregation and micro-cracks. Fractography analysis of the BM and weld specimen shows ductile failure in both the alloys.

Metallographic, Structural, and Mechanical Characterization of Weld Nuggets in Fe–Mn–Al–C Low‐Density Steels Microalloyed with Ti/B and Ce/La by Gas Tungsten Arc Welding Process

The use of low‐density (LD) steels has become increasingly important due to the advantages they offer, such as high strength, high ductility, and principally weight reduction. However, there are few studies regarding their weldability by conventional processes due to the metallurgical complexity of these alloys, including kappa carbides precipitation and hot‐cracking associated with the high contents of Mn, Al, and C. Herein, the main objective is the characterization of the fusion zone and different heat‐affected zones of austenitic Fe–Mn–Al–C LD steels microalloyed with Ti/B and Ce/La. For this purpose, weld nuggets were prepared by autogenous gas tungsten arc welding process using heat inputs of 660 and 975 J. The weld nuggets were characterized metallographic, structural, and mechanically by light optical microscopy, scanning electron microscopy, X‐ray diffraction, and Vickers microhardness. In general, weld nuggets shows three well‐defined zones where γ‐austenite is the main phase and AlN and MnS particles act as nuclei of complex precipitated particles of microalloying elements such as TiC and (Ce, La)O. A considerable decrease in hot‐cracking was noticed in the microalloyed grades. A modulated structure of kappa‐carbide particles was responsible of the largest hardness values (≈450 Vickers microhardness) in the second heat‐affected zone.

Study on effect of grain refinement of P92 steel base plate on mechanical and microstructural features of the welded joint

The work described the effect of the grain refinement in virgin P92 steel plate on the microstructural and mechanical behavior of the P92 welded joint. The as-received P92 steel plate was subjected to varying heat-treatment, i.e., conventional heat treatment (CNT) and double normalized heat treatment (DNT) to refine the grain structure. The DNT is mainly done for the grain refinement in the as-received P92 steel plate. The autogenous gas tungsten arc welding (GTAW) process was utilized to prepare the P92 steel welds joint. Four set of the welds joint were prepared, two by using the CNT plate, termed as AW1 and two by using the DNT plate, termed as AW2. The welded plate has been subjected to heat treatment at 760 °C for 3 h (T1, T2) for both the conditions to stabilize the microstructure of weldments, i.e., tempering of the fresh martensite. The grain growth, particle size, and particle distribution in P92 weldments have been studied. For mechanical characterization, tensile tests, hardness tests and Charpy toughness tests (CTT) have been carried out. Soft δ ferrite formation was observed in weld fusion zone (WFZ) and coarse-grained heat-affected zone (CGHAZ) for AW and T condition and formation is attributed to high heat input and presence of the ferrite stabilizer like W and Mo. Heterogeny in microstructure and mechanical properties was measured across the weldments for AW condition and get minimized as a result of the PWHT. The average hardness for WFZ was measured 363 and 358 HV for AW1 and AW2 conditions, respectively. The difference in the hardness of WFZ can be neglect. The peak hardness of CGHAZ was measured at 358 and 397 HV for AW1 and AW2, respectively and it is attributed to the solid solution strengthening mechanism. The variation in hardness along the welded joint was reduced to a minimum level as a result of the heat treatment. The AW 2 welds joint showed maximum tensile strength of 943 MPa and get reduced drastically after the heat-treatment (T2) (700 MPa) but still showed a higher value as compared to T1 (602 MPa). A significant improvement in tensile strength without any loss in ductility was obtained for the welds joint prepared using the DNT plate as compared to CNT plate.

Effect of autogenous GTAW on the reciprocating sliding wear behavior of a carbon martensitic steel

Martensitic steels have been successfully employed in resource-based industries where components must endure aggressive conditions. In industrial practice, many parts of these components are joined by welding techniques. The aim of this work was to understand the influence of welding on the wear resistance of quenched and tempered carbon martensitic steel subjected to dry linear reciprocating sliding micro-wear tests. Weld-joints were produced using autogenous Gas Tungsten Arc Welding process (GTAW). Micro-wear tests were performed at base metal (BM), weld metal (WM), coarse grained heat affected zone (CG-HAZ) and lowest hardness region of heat affected zone (LHR-HAZ). LHR-HAZ was softened during welding process so plastic deformation was facilitated, and consequently adhesion, material displacement and micro-ploughing. WM and CG-HAZ presented a similar martensitic structure, which explain the similarities found on wear behavior. These regions presented the lowest worn volume average values (w). It was interesting to note that despite its highest microhardness value, the highest w was observed for BM. For some BM samples, debris had a key role promoting material loss by micro-cutting which causes great extent of material removal compared to other micro-wear mechanisms as micro-ploughing and adhesion. Due to debris action BM also presented a great dispersion in w results. The results suggest that material loss of welded joint and BM was strongly controlled by micro-wear mechanisms.

Effect of backing gas mixtures on corrosion properties of stainless steel grade 304 weld metal by autogenous GTAW

Pitting corrosion would pose difficulty for the cleaning of equipment used in food processing, even when using the Cleaning in Place (CIP) process. It tends to occur at a location requiring welding. In general, Gas Tungsten Arc Welding (GTAW) is the most applicable process for food and drug equipment. To improve the properties of the weldment, filler metals are commonly used. However, the use of thin-sheet stainless steel in the application of filler metals would be difficult in food processing. In this research, the weld metal properties would be improved only by using N2 mixing to the backing gas during welding. Experiments were conducted using various mixtures of N2 on 1.5, 2.0, and 3.0 mm thicknesses of 304 stainless steel. The roots of weld metal taken from the samples were evaluated for corrosion properties in 3.5%NaCl from the polarization curves. The results showed the improvement of pitting corrosion from 379 mV to 757 mV, while the general corrosion rate decreased from 0.00477 mm/y to 0.00099 mm/y with 100%Ar and 100%N2, respectively. Both Nitrogen content and solidification rate together affected the solidification modes and ferrite contents that would affect the corrosion properties. At least 15%N2 mixing would be sufficient for weld metal property improvement. Due to the decrease of ferrite content from N2 addition, solidification cracking susceptibility test by fishbone test was acceptable with no cracking observed, showing the potential to recommend N2 mixtures for actual production.

Microstructure and metallurgical property investigation of welded IRSM 41-97 rail steel joints

IRSM41-97 rail steel is a type of structural steel which is highly resistant to atmospheric corrosion. It is a low carbon steel having less than 2% of carbon and other alloying elements like Cr, Cu, Ni, Si, Mn, S and P less than 2%. It is a HSLA structural steel primarily utilised in the railway industry for the construction of railway coaches, bridges, and frame building constructions, wagons, support structures etc. IRSM41-97 rail steel can be welded using GTAW and GMAW welding technique. The common industrial copper coated ER70S-6 Filler wire is used to weld IRSM41-97 rail steel for railway applications. During welding, the metallurgy of the IRSM41-97 rail steel completely changes due to the high heat dissipated during welding and improper selection of process parameters and not choosing the right filler material resulting in losing its properties. Autogenous gas tungsten arc welding (AGTAW) is a no-filler welding operation which is appropriate for low heat input (Q), longer weld times, and high weld speeds. In this research AGTAW IRSM41-97 rail steel weld joints from 2 mm thick plate is prepared as per ASTM standards and the metallurgical properties are investigated by carrying out macro structural and micro structural investigations. This research leads to widen the application of IRSM41-97 rail steel not only in railway sector industry as well as other industrial sectors.

A Coupled Thermo-Mechanical-Metallurgical Model Capturing Essential Transformations in the Welding of High Strength Low-Alloy Steels

Abstract: A well-known challenge is to predict the transformations occurring during the metal alloys welding aiming to control the weldment properties. Thus, this study presents a Thermo-Mechanical-Metallurgical model to numerically predict the thermal history, the solid-state phase transformations, the solidification microstructure and the hardness distribution during and after the welding of high strength low-alloy steels. The model was numerically implemented in an in-house computational code based on the Finite Volume Method, which allowed to dynamically track and calculate the volume fractions of ferrite, pearlite, bainite and martensite at the heat-affected zone, besides the formation and determination of dendrite arm spacing at the fusion zone, whereas the hardness distribution at the heat-affected zone was calculated by applying the phase mixture rule. For this, single-pass autogenous Gas Tungsten Arc Welding welds were numerically simulated and experimentally carried out on high strength low-alloy AISI 4130 steel samples, including their preheating to evaluate the effectiveness of the proposed model to simulate the workpieces welding in different initial thermal conditions and a close agreement between the calculated and experimental results were obtained.

XRD investigation of GTAW, GMAW, LBW and AGTAW Corten A588 grade steel weldments

Corten A588 grade steel is a weathering with low carbon content less than 0.2%. It is a high strength low alloy steel with exceptional corrosion resistant property. Hence Corten A588 grade steel is used in various applications like construction of bridges, roof and side walls of buildings, shipping containers, rail wagons etc. During fabrication work, when the Corten A588 steel is welded, it gets exposed to very high temperatures. Due to these few elements having low melting point in the Corten A588 steel base metal gets vaporized, thereby changing the crystalline nature and phases i.e. metallurgy of the steel. The effects of GTAW (Gas Tungsten Arc Welding), GMAW (Gas Metal Arc Welding), LBW (Laser Beam Welding) and AGTAW (Autogenous Gas Tungsten Arc Welding) based on the heat input and various other welding parameters are investigated analysed and compared. In this research GTAW, GMAW, LBW and AGTAW Corten A588 grade steel weldments were prepared and XRD test is performed to investigate the crystalline structure of the weldments. The d-spacing, phase identification, full width at half maximum (FWHM), crystalline size is studied and a comparative analysis of all the four different weld which meliorates the mechanical, corrosion and metallurgical nature of the Corten A588 is made.

Analysis of the residual stresses and deformations during the autogenous GTAW process

PurposeThe gas tungsten arc welding (GTAW) process is a widely used process that produces quality weldments. But the high heat generation from the GTAW arc can cause extreme temperatures as high as 20,000°C. The residual stresses and deformations are high accordingly. One of the methods for decreasing residual stresses and deformations is to change the welding pattern. In the literature, there are not so many examples of modeling dealing with welding patterns. This paper aims to investigate the influence of welding patterns on the deformations.Design/methodology/approachIn this work, back-stepping patterns and partitioning of the weld line were investigated and the distortions and residual stresses were calculated. By doing this, temperature-dependent thermophysical and thermo-mechanical material properties were used. The temperature distribution and deformation from experiments with the same welding conditions were used for validation purposes.FindingsSeven different welding patterns were analyzed. There is only one pattern with a single partition. There are three patterns investigated for both two and three partitioned weldings. The minimum deformation and the optimum residual stress combination is obtained for the last pattern, which is a three partitioned and diverging pattern.Originality/valueThe most important aspect of this paper is that it deals with welding patterns, which is not much studied beforehand. The other important thing is that the structural part and the thermal part of the simulation were coupled mutually and validated according to experiments.

Effect of Heat Input and Shielding Gas on the Performance of 316 Stainless Steel Gas Tungsten Arc Welding

The effect of nitrogen addition and heat input on weld metal microstructure and mechanical properties of alloy 316 is studied. Autogenous gas tungsten arc welding (GTAW) is employed by adding up to 2 vol‌.% N2 in Ar. Welding speed and heat input rate are measured as functions of gas composition. Weld defects are examined by radiographic testing, and weld metal microstructure is studied by optical microscopy. Mechanical properties of welds are determined by uniaxial testing, hardness measurements, and bending test. Weld dendritic structure is refined by increasing N2 content in Ar. The mechanical properties and cooling rate are lower with increasing heat input. Besides, adding nitrogen to argon shielding gas leads to higher values of the ultimate tensile strength and hardness. The tensile strength, yield stress and elongation percent of welds depends strongly on the heat input and nitrogen content of shielding gas. This is discussed on the basis of microstructural characterization. Moreover, the weld nugget area, cooling time and solidification time increase with increasing heat input and nitrogen content. Finally, after applying the bending test up to 180o no cracks, tearing or surface defects could be observed on welded samples.

Dissolution of δ-ferrite and its effect on mechanical properties of P92 steel welds

This paper reports on the dissolution of δ-ferrite in the weld fusion zone using post-weld normalizing and tempering (PWNT) and its effect on mechanical properties. The autogenous gas tungsten arc welding (GTAW) was used to prepare double-sided welded joints of P92 steel. After welding, post-weld heat treatment (PWHT) at 760 °C for 2 h and post-weld normalizing at 1050 °C for 1 h followed by tempering at 760 °C for 2 h were performed. The morphology, composition, structure, and hardness of δ-ferrite are verified. The effect of PWHT and PWNT on mechanical properties is also observed. In as-welded and PWHT conditions, the retention of δ-ferrite was observed while the dissolution of δ-ferrite was confirmed after PWNT. In the PWNT condition, uniform microhardness across the welded joint, increased ductility, and increased Charpy toughness of weld fusion zone (almost equal to as received material) were achieved. Dissolution kinetics of δ-ferrite is quantified using Thermo-Calc for Scheil's solidification calculation and DICTRA for calculating diffusion coefficients of Cr and W at 1050 °C in austenite.

Mechanical and Metallurgical Characterization of Dissimilar P92/SS304 L Welded Joints Under Varying Heat Treatment Regimes

This work was performed to characterize the dissimilar austenitic grade SS304 L and martensitic grade creep strength of an enhanced P92 steel welded joint. The welded joint was prepared using the autogenous gas tungsten arc welding process. The welded joint was subjected to conventional heat treatment consisting of tempering at 760 °C for 2 hours, followed by air cooling and unconventional heat treatment (UHT) consisting of normalizing (1040 °C/40 min), further followed by air cooling and, finally, stabilized tempering at 760 °C for 2 hours. The heat-treated welded joint was subjected to the microstructural characterization and mechanical testing. The room temperature Charpy toughness (CT) testing for both the weld fusion zone (WFZ) and heat-affected zone, tensile testing, and hardness testing were performed to characterize the dissimilar welded joint. Stabilized microstructure associated with improved mechanical properties were observed for UHT. The CT and tensile strength were optimum for the UHT.

Elemental Effects on Weld Cracking Susceptibility in AlxCoCrCuyFeNi High-Entropy Alloy

Weld solidification and weldability of AlxCoCrCuyFeNi high-entropy alloys (HEA) were characterized as a function of Al and Cu molar ratio in the alloy composition. Autogenous gas tungsten arc welding (GTAW) was performed on the as-melted ingots. Fusion zone and heat-affected zone microstructures were investigated using optical and electron microscopy, and compared to thermodynamic CALPHAD-based calculations. Weld cracking susceptibility was compared to cast pin tear test (CPTT) results. It was found that strong Cu segregation on solidification promotes hot cracking in fusion welds on AlCoCrCuyFeNi alloys (y > 0.1). Cu-rich liquid forms during the final stages of solidification and facilitates solidification cracking in the fusion zone. The Cu-rich interdendritic readily remelts during reheating and promotes liquation cracking in the heat-affected zone. The cracking mechanisms in the welds change to brittle intergranular cracking in low and no Cu alloy compositions due to a high-hardness BCC microstructure (> 500 HV). Lowering the Al molar ratio in AlxCoCrCu0.1FeNi alloys (x ≤ 0.5) mitigates brittle cracking by promoting FCC solidification structure.

Estimation of Residual Stresses by Nanoindentation in an Experimental High Strength Microalloyed Steel Subjected to Rapid Thermal Cycles

The effect of welding thermal cycles on the microstructure and residual stresses was studied in the welding zone of an experimental high strength microalloyed steel with an acicular ferrite and quasi-polygonal ferrite microstructure. Autogenous gas tungsten arc welding was performed on a plate of microalloyed steel to produce a weld bead neglecting the weld joint design and the selection of filler material. To obtain a high welding quality, the welding speed and distance between the electrode and plate was controlled with a small vehicle. To determine the residual stresses, nanoindentation tests were performed in each one of the welding zones and subzones of the heat-affected zone (HAZ). These results showed that all of the nanoindentations presented pile-up and sink-in, and thus, a modification was proposed to calculate the actual contact area and determine the residual stresses. Tensile residual stresses were presented in the coarse-grained HAZ (grain boundary ferrite, acicular ferrite, and bainite); fine-grained HAZ (polygonal ferrite); subcritical HAZ (acicular ferrite and quasi-polygonal ferrite); and base material (acicular ferrite and quasi-polygonal ferrite). The fusion zone (acicular ferrite and bainite) and intercritical HAZ (quasi-polygonal ferrite) had compressive residual stresses.

Metallurgical effects of nitrogen on the microstructure and hot corrosion behavior of Alloy 718 weldment

The effects of nitrogen added by autogenous gas tungsten arc (GTA) welding on weld microstructure and hot corrosion resistance of Alloy 718 exposed to Na2SO4–25%wt NaCl salt compound at 650 °C have been studied. The weld metal microstructure was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that nitrogen has low solubility in Alloy 718, leading to precipitation of (Ti,Nb)CN and (Ti,Nb)N precipitates inside the weld. The optimum nitrogen content for hot corrosion resistance was 150 ppm due to formation of compact Cr2O3 and adhesive Nb2O5 layers. Higher concentration of nitrogen aggravates hot corrosion resistance owing to severe microsegregation of Nb and Mo solutes as well as increased NbC and Laves phase in interdenderitic regions.

Solidification cracking susceptibility of ferritic stainless steels using Modified Varestraint Transvarestraint (MVT) method

The Modified Varestraint Transvarestraint (MVT) test was used to investigate the solidification cracking susceptibility of an unstabilised, a Nb-stabilised and two (Ti + Nb) dual-stabilised ferritic stainless steels. Two different welding speeds of 6 and 3 mm/s using autogenous gas tungsten arc welding were employed. At the welding speed of 6 mm/s, the high-content (Ti + Nb) steel was resistant and the Nb-stabilised steel was marginally susceptible to solidification cracking. At the welding speed of 3 mm/s, the Nb and the high (Ti + Nb) steels were found to be marginally susceptible to solidification cracking while the unstabilised and low-content (Ti + Nb) grades were resistant to solidification cracking. The weld metal microstructures transverse to the welding direction revealed columnar grains in all the samples for both welding speeds. The ferritic stainless steels were generally resistant to solidification cracking, except the Nb-stabilised steel, which was marginally susceptible to solidification cracking.

The contribution of impurities to unexpected cold cracks in a thick C-Mn steel plate

The internal cracking observed in a 6″-thick carbon-manganese steel plate after welding is discussed with respect to the presence of material impurities. The morphology and location of the cracks detected during fabrication of a heavy structure are typical of hydrogen-induced cracking (HIC) usually associated to hard untempered martensite. However, typically martensitic microstructures would not be expected for low carbon low alloy steels joints when a suitable welding procedure specification is adopted, which includes preheating and high heat input. This paper illustrates the morphology of the original cracks, which are unrelated to hard microstructures, and presents the results of the welding experiments conducted in samples cut from segregated regions of the 6″-thick plate. Through hydrogenation of thin slices containing impure regions and autogenous gas tungsten arc welding bead, it was possible to reproduce the same cracking characteristics formed during submerged arc welding, indicating that HIC may also be associated to softer phases. Hydrogen entrapment at the weld fusion line can be related to high impurities level due to the expected lower melting points at grain boundaries, explaining the HIC phenomenon. The present paper proposes a model for hydrogen cracking of bainitic structures at the welding heat affected zone that typically is not susceptible to crack.

Improved thermal FE numerical model/DoE based on the Taguchi method to estimate weld penetration/energy and non-metallic inclusions: a case study in Ti-containing TWIP steel butt joints

In this research work, a finite element (FE) numerical analysis was conducted to study the welding thermal field. The finite element model (FEM) was improved by means of the optimal mesh element size using statistical analysis of a design of experiment (DoE) based on the Taguchi method. Through the improved FEM, numerical model were carried out estimations of the penetration depth, heat dissipation phenomenon, and thermal energy input. Then, FE numerical results were linked to Matlab®. The FEM-Matlab® approach proposed the application of thermal energy predictions to estimate welding critical zones extensions and their possible non-metallic inclusions. The FEM-MATLAB® approach was applied to a case study: twinning-induced plasticity steel microalloyed with titanium (TWIP-Ti) weldments. Welding experiments were performed with the autogenous Gas Tungsten Arc Welding (GTAW) process in 6.3-mm thickness plates. The weld samples were analyzed metallographically to measure welding critical regions extensions. Both austenitic grain size and non-metallic inclusions were in good agreement with numerical results. The weld bead obtained by means of the optimal process parameters estimated by the FEM-Matlab® approach achieved full penetration and showed better microstructural characteristics according to the numerical estimation.