Root Pass Research Articles

The Effect of Switchback Parameters on Root Pass Formation of Butt Welds with Variable Gap

Root pass manufacturing in automated welding is still a challenge when the backing plate is not feasible. Using the concept of bead formation in an original way, the GMAW (Gas Metal Arc Welding) switchback technique was assessed against linear movement as a means of facing this challenge. Experimental work was applied, keeping the process parametrization and joint configuration, so that only the switchback parameters were modified, i.e., the stroke lengths and speeds. Thermography was used to estimate the effect of the switchback parameters on bead formation. The results showed the potential of the switchback technique as a means of favoring weld pool control. Surprisingly, the operational gap range is not necessarily larger when switchback is applied. The strong influence of stroke lengths and speeds on the process performance was characterized. In general, the results showed that linear movement leads to larger pools and deeper penetrations, more adequate for gaps with no clearances. Shorter stroke lengths and slower stroke speeds (intermediate pool size) better suit root gaps that are not too wide, while longer stroke lengths and faster stroke speeds (smaller pool size, more easily sustainable) are applicable to larger root gaps.

Investigation on three different weldments on performance of SA516 Gr70 steel material

In the present investigation, 10 mm thick SA516Gr70 steel plate welded using three different consumables includes solid wire (ER70S-6), flux cored wire (E71T-1C) and metal cored wire (E70C-6M) of 1.2 mm in size.Carbon steel plate SA516Gr70 welded using MM shows that both root pass and filler pass filled with metal-cored wire. MF indicates Hybrid welds in which root pass filled with metal-cored wire and filler pass with flux cored wire and MS indicate Hybrid welds shows that root pass filled with metal-cored wire and filler pass with solid wired.The angular distortion is higher with MF and MS hybrid welded compare to MM weld. Average yield strength and tensile strength values are higher for with MM weld. Percent elongation is higher for MM and MF welds as compared to hybrid welds MS. During the tensile test all specimens failed from parent material; means welded joints are stronger than the parent metal. The slightly higher impact toughness of the weld metal reported at −49 °C for the MM and MS welds compared to MF weld. Under the transverse direction, weld metal VHN is higher for hybrid weld (MS, MF) compared to MM weld.

Effect of Pulsed Current on the Microstructure, Mechanical Properties and Corrosion Behavior of Ni-Based Alloy/Super Duplex Stainless Steel Dissimilar Welds

This work addressed the effect of pulsed current (PC) and pass number on the microstructure, mechanical properties and corrosion behavior of Incoloy 825/UNS S32750 welds. The alloys were welded using PC and continuous current techniques. Mechanical properties of the weldments were examined using hardness, Charpy and transverse tensile tests. General corrosion and pitting corrosion behavior of the weld metals were studied using potentiodynamic polarization and electrochemical impedance spectroscopy tests. The weld metals had a dual-phase microstructure including ferrite and austenite phases. Employing the PC mode resulted in an increase in the ferrite content of the weld metals, which led to the improvement in impact resistance. Based on the results of tensile tests, the weakest part of the weldments was Incoloy 825 base. The weld metals at the root pass showed the weakest corrosion behavior. Overall, use of the PC mode improved the properties of the weldments.

Root Pass Microstructure in Super Duplex Stainless Steel Multipass Welds

Root Pass Microstructure in Super Duplex Stainless Steel Multipass Welds

Keyhole GTAW with Dynamic Wire Feeding Applied to Orbital Welding of 304L SS Pipes

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.

Contributions of the High Frequency Dynamic Wire Feeding in the GTAW Process for Increased Robustness

Abstract: The GTAW process is recognized as a process of high metallurgical quality, being widely used in the pipelines welding, mainly in the root pass. The relative independence between the heat delivered to the melting pool and the amount of material added helps to control the melting of the material and to avoid the appearance of welding discontinuities such as lack of fusion and lack ok penetration. When orbital welding is carried out, mainly in the overhead position, there is a tendency of electrode contamination by the melted wire, reducing process productivity and quality. In this scenario, there are some strands of the GTAW process that aim for increased productivity, reconciled with the already present high metallurgical quality. These includes low and high frequency dynamic wire feeding, preheating wire and tangential wire insertion. The present work deals with the contribution of the high frequency dynamic wire feeding for the increase of robustness in the GTAW process, evidencing through voltage monitoring and high speed filming the formation of regular and uniform drops for the process stability.

Ferrite content measurement in super duplex stainless steel welds

Approaches to determining ferrite fraction (%) and ferrite number (FN) were examined for super duplex stainless steel (SDSS) welds. A reference sample was produced by bead-on-plate gas–tungsten arc welding of a type-2507 SDSS plate. By comparing different etchants and measurement practices, it was realized that etching with modified Beraha followed by computerized image analysis (IA) was the most accurate and quickest technique to measure ferrite fraction, which determined the same ferrite fraction (68.0 ± 2.6%) as that measured by electron diffraction backscattered analysis (67.6 ± 2.3%). A Round Robin test was performed on a reference sample at University West, Swerea KIMAB, Outokumpu Stainless, and Sandvik Materials Technology to investigate the repeatability of the technique. The ferrite fraction measurements performed at different laboratories showed very small variations, which were in the range of those seen when changing microscope in the same laboratory. After verification of the technique, the relationship between ferrite fraction and ferrite number (measured with FERITSCOPE®) was determined using 14 single (root) pass welds, including butt, corner, and T-, V-, and double V-joint geometries. The best-fit equation found in this study was ferrite number (FN) = 1.1 × ferrite fraction (%). To conclude, the ferrite fraction technique suggested in the present paper was accurate and repeatable, which made it possible to determine a ferrite fraction–ferrite number formula for SDSS single-pass welds.

A New Approach to Simulate HSLA Steel Multipass Welding through Distributed Point Heat Sources Model

Mechanical properties of welded joints depend on the way heat flows through the welding passes. In multipass welding the reheating of the heat affected zone (HAZ) can form local brittle zones that need to be delimited for evaluation. The difficulty lies in the choice of a model that can simulate multipass welding. This study evaluated Rosenthal’s Medium Thick Plate (MTP) and the Distributed heat Sources (DHS) of Mhyr and Gröng models. Two assumptions were considered for both models: constant and temperature-dependent physical properties. It was carried out on a multipass welding of an API 5L X80 tube, with 1016 mm (42″) external diameter, 16 mm thick and half V-groove bevel, in the 3G up position. The root pass was welded with Gas Metal Arc Welding (GMAW) process with controlled short-circuit transfer. The Flux Cored Arc Welding (FCAW) process was used in the filling and finishing passes, using filler metal E111T1-K3M-JH4. The evaluation criteria used were overlapping the simulated isotherms on the marks revealed in the macrographs and the comparison between the experimental thermal cycle and those simulated by the proposed models. The DHS model with the temperature-dependent properties presented the best results and simulated with accuracy the HAZ of root and second welding passes. In this way, it was possible to delimit the HAZ heated sub-regions.

Study of Microstructure 304L Austenitic Stainless Steel Weld Deposited by GTAW for Root Pass and SMAW for Filler Passes

In the present experimentation, a 10mm thick austenitic stainless steel plate type 304L is welded using single V-joint configuration and approaching the joint from one side. Back purging has been employed to protect the rear side of the root pass weld metal against oxidation. The root pass has been deposited by gas tungsten arc welding (GTAW) process. The filler passes are deposited by shielding metal arc welding (SMAW) process at 90A and 120A welding currents giving heat inputs of the order of 0.679 and 0.933 kJ/mm respectively while the speed of weld deposition was kept practically constant. The results of experimentation show that the micro-hardness of weld metal and heat affected zone (HAZ) of weldments produced at lower heat input is higher whereas impact toughness value of weld metal and HAZ is lower than that of joints produced at higher heat input. The microstructure of weld metal and heat affected zone developed at lower weld heat input has been observed finer in comparison to that resulted at higher heat input. This has primarily happened due to a higher rate of cooling at low heat input.

Special features of the formation of welded joints in vertical laser-arc welding of low-alloy steels

ABSTRACTInvestigations were carried out into the special features of the technology of vertical laser-arc welding of large structures made of high-strength and low-alloy steels with a variable welding gap and a large root face.The parameters of the conditions of vertical laser-arc welding of root passes in the low-alloy steel 30–40 mm thick with a root face of 10 mm (welding speed, the amplitude and frequency of transverse oscillations, the intensity of welding current, arc voltage, laser radiation power) resulting in the formation of high-quality welded joints are determined. The experimental results show that the laser radiation power, required for producing continuous penetration of the root face, depends on the thickness of the parent metal: for the metal thickness of 14–30 mm the sufficient laser radiation power is 3.8 kW, for a thickness of 40 mm it is no less than 4.6 kW to penetrate a depth of 5 mm.The welded joints have a fine-dispersed acicular structure of the martensitic – bainitic type. The experimental results were used to develop a technology of welding of low-alloy steels of different thickness using robotic equipment.

Failure Analysis of Weld Metal Cracking in Weld Joint of API 5L X 75 Pipelines Steel

The development of steel alloy which result thin walled, high strength API 5L X75 grade, a restricting parameter controlling widespread use of X75 is the susceptibility to weld metal tracks. The excellent weld ability of this grade of the pipe steel has enhanced the potential for the use of high strength cellulose consumable like E6010 in root pass welding, but the risk of hydrogen assisted cold cracking (HACC) is also increased because of the high strength weld metal. This investigation outlines , the use of grade (E6010) of commercial cellulose consumable to assess conditions leading to hydrogen assisted cold cracking in the diluted weld metal. The research contained clarification of the link among microstructure; preheat temperature and hardness amounts for the weld consumable and its effect on cracking susceptibility. The cracking morphology studies indicated that there are many ways in which the crack can propagate in the weld metal and HAZ region. The mode of cracking observed was microvoide coalescence.

Influencia de la soldadura de raíz con ERNiCrMo-3 en las propiedades de las uniones soldadas de fundición nodular

Este artículo estudia el efecto de la soldadura de raíz (GTAW) con ERNiCrMo-3 sobre la microestructura y propiedades mecánicas de una unión de fundición nodular cuando las pasadas de la cubierta se realizan con la técnica SMAW con electrodos de Ni. Para tener una mejor comprensión de los resultados, se comparan con las características de una soldadura completa de Ni SMAW. Esto se hizo antes y después de un recocido a 900 °C con el fin de evaluar las influencias y los posibles beneficios de este tratamiento térmico. Los resultados muestran que la presencia de una soldadura de raíz de GTAW ERNiCrMo-3, a pesar de la mejora de la penetración, conduce a una pérdida de propiedades mecánicas en la soldadura de raíz que, incluso con un recocido, no es posible eliminar.

Effects of welding on mechanical and microstructural characteristics of high-strength low-alloy steel joints

In this paper effects of welding on mechanical and microstructural characteristics of high-strength low-alloy steel joints is presented. Testing was done at models prepared with V-groove butt joints that are done by two different welding processes. Considered welding processes were done by MMA or MIG for root pass and MAG for other passes with related consumables. Experimentally determined basic mechanical characteristics were put in correlation to relate ones that were numerically calculated. The experimentally obtained yield strength and tensile strength showed that welding process MMA/MAG provide better mechanical properties then MIG/MAG. It is implicated that selection of welding process at high-strength low-alloy steels is multi criterion analysis. If only proper welding is obtained, applications of high-strength low-alloy steel are adequate. Those applications caused specific problems related to its specific characteristics and properties especially weldability. As welding processes can be analysed from different aspects, selection of specific welding process at high-strength low-alloy steel are more complex.

Microstructural evaluation of welded fresh-to-aged reformer tubes used in hydrogen production plants

Abstract Heat resistant reformer tubes comprise a significant fraction of petrochemical reforming plants cost considering their high alloy content (i.e. 25Cr-35Ni-1Nb-0.1Ti). The bottom portion of tubes experiences the highest temperatures in the furnaces leading to microstructural changes, creep damage, and loss of elongation over their service life which in this case is twenty years. There is a cost- and time-driven motivation to only replace this portion of tubes by welding in contrast with replacing entire set of tubes which is the common industrial practice. However, welding new to aged tubes may lead to reliability issues due to difference in mechanical properties as a result of microstructural differences. In the current study, the microstructure and tensile properties of aged and new tubes have been evaluated in an effort to qualify the mechanical integrity of weldments. Welding trials are carried out to investigate the microstructure of the aged-to-new weldments and correlate it with the tensile properties (particularly elongation). Findings reveal that the heat affected zone of aged tubes is prone to micro-cracking of bulky primary carbides and incipient melting particularly at the inner surface where the root pass is applied. Adopting preheating for the root pass is effective in reducing carbide micro-cracking by decreasing cooling rate which assists in the accommodation of stresses generated by thermal contraction. Despite presence of carbide micro-cracks, tensile elongation is not severely affected as aged-to-new welds exhibit comparable and slightly higher elongation than aged base metals (above 4%). It is proposed that this is partially due to the orientation of micro-cracks in carbides. Further microstructural and tensile property results are presented and discussed.

An outlook on comparison of hybrid welds of different root pass and filler pass of FCAW and GMAW with classical welds of similar root pass and filler pass

In the present study, gas metal arc welding and flux cored arc welding were applied on SA516 Gr70 carbon steel material. Two different hybrid passes were applied, wherein flux cored wire and solid wire were applied to root pass and filler pass one by one and vice versa. Besides, two more welds of similar electrode root pass and filler pass of flux cored arc welding and gas metal arc welding were acquired. The comparative analysis was carried out in terms of macrostructure and microstructure examination, tensile testing, hardness variations, and impact testing for these classical welds and hybrid welds. The results reveal that, hybrid welds lead to better impact properties relative to classical welds. Maximum angular distortion of 2.66° was reported with classical weld of gas metal arc welding with solid wire root pass and same filler pass. The maximum impact toughness of 49 J/m3 was reported for flux cored root pass and solid wire filler pass at the weld zone. Maximum tensile strength of 596 MPa was reported for hybrid weld of solid root pass and flux cored filler pass. Microstructures are reported with the presence of different acicular ferrite and grain boundary ferrite. Maximum acicular ferrite of 61% was reported with classical weld of flux cored arc welding.

Modified Short Arc Gas Metal Arc Welding Process for Root Pass Welding Applications

It is well known that, the major limitation of conventional short circuiting gas metal arc welding process (GMAW) for welding of various ferrous and non-ferrous materials is poor bridgeability and generation of large number of spatters. To overcome these difficulties, in recent years, waveform controlled technology has been introduced in GMAW process for specific application to meet required quality and productivity. However, the specific advantage of the process for various welding application primarily depends on the waveform parameters such as peak current, background current, time and voltage. It is often found that, irregular control of waveform leads defects like porosity, undercut and burn through etc. which impairs the weld joint quality. Thus, it is very important to study the effect of different waveform on bead geometry and microstructure. In this regard, in the present investigation aimed to carry out various waveforms on root pass welding of carbon and alloy steels. The bead appearance, soundness, macro and micro structure analysis has been carried out. Based on the results the optimum range of waveform has been derived and appropriately correlated with process parameters.

X‐Ray Diffraction Technique for Residual Stress Measurement in NiCrMo Alloy Weld Metal

In the present work, residual stresses in nickel‐based (Ni 625) superalloy weld metal of a 9%Ni steel‐welded joint were measured by X‐ray diffraction (XRD). This technique presents some difficulties in performing measurements in coarse and preferentially oriented weld metal microstructures. It is proposed a preliminary surface treatment by rotating steel wire brushing to perform the stress analysis through XRD technique possible for this kind of material. Stress measurements with proposed XRD technique showed that the stress state in Ni 625 weld metal on the outside surface of the welded joint is characterised by tensile stresses in the transverse and longitudinal directions, while compressive transverse and tensile longitudinal residual stresses are developed in the root pass region.

Effect of root pass filler metal on microstructure and mechanical properties in the multi-pass welding of duplex stainless steels

This paper is focused on the estimation of the effect of root pass chemical composition, in multi-pass GTA Weldments, on microstructure and mechanical properties of duplex stainless steel welds. We used two different filler metals, the super duplex ER 2594 and duplex ER 2209. Microstructures of different passes of welded joints are investigated using optical microscope and scanning electron microscope. The relationship between mechanical properties, corrosion resistance, and microstructure of welded joints is evaluated. It is found that the tensile and toughness properties of the first weldment, employing the combination of ER 2594 in the root pass and ER 2209 in the remaining, are better than that of the second weldment employing ER 2209 all passes, due to the root pass grains refinement and its alloy elements content as chromium Cr and nitrogen N. The microstructure indicates the presence of austenite in different forms on the weld zone of ER 2209, same in the case of ER 2594, but with higher content and finer grains size, in particular Widmanstatten austenite WA. Potentiodynamic polarization tests of the first weld metal evaluated in 3.5% NaCl solution at room temperature have been demonstrated a corrosion resistance higher than that of the second weld metal. This work addressed the improvement of the corrosion resistance using appropriate filler metal without getting any structural heterogeneity and detrimental changes in the mechanical properties.

Hybrid Approach of Flux-Cored Root Pass with Subsequent Pass of Metal-Cored or Solid Wire in Multifiller Gas Metal Arc Welding

In the present study, gas metal arc welding was applied to SA516 Gr70 carbon steel by hybrid root pass and filler pass, wherein the rest of the process parameters were kept as it is. Flux-cored filler wire is applied at the root pass, while the subsequent filler pass was applied by solid, metal-cored and flux-cored wires in order to form three different welded joints. Macro-graphic examination, microstructures, tensile testing, impact testing, bend testing, and hardness variations were carried out to evaluate the performance of the welds. The results show that the hybrid approach of multipass GMAW of flux-cored root pass–metal-cored filler pass has improved process performances by reducing an overall heat input and output current. Angular distortion was reported minimum at hybrid weld of flux-cored root pass–metal-cored filler pass that was reported as 1.72°. Minor improvement in tensile properties such as tensile strength and yield strength was reported for FS and FM welds relative to FF weld. Minor decrease in fracture to the elongation was reported for FS hybrid weld relative to FM and FF weld. Maximum tensile strength and yield strength of 568 MPa and 385.45 MPa were reported for FS weld, respectively. Impact toughness was observed maximum for the hybrid weld of flux-cored root pass–solid filler pass. Higher macro-hardness was reported at the weld of FF weld relative to FS and FM welds. Hybridization of filler wire of GMAW can enhance overall cost and time reduction, with acceptable properties.

Calculation of the Stable Modes of MIG/MAG Welding of the Root Passes

The article is devoted to development of a technique, which allows calculation of the welding mode of treatment of V-shaped groove for formation of a root seam. The technology of the multipass welding of MIG/MAG (arc welding in a mixture of the shielding gases) of the thick-walled structures is considered. An analysis of the problem statement demonstrated that the main problem is calculation of the penetration technique into the base metal for the V-shaped groove welding. Therefore, the authors proposed a proprietary technique for calculation of penetration using a finite element modeling (FEM). As a result, a heat input regressive model was developed for a particular form of the edge grooving, which is parametrized with respect to the gap and welding speed. For obtaining of an adequate penetration into the base metal, it is suggested to complicate the source model structure of the welding heat and to use the system of two heat sources of a double ellipsoid type. The analysis of penetration was done due to FEM solution of 3D heat equation, taking into account the melting and crystallization processes. The 3D heat equation task for the solid multiphase environment was solved. Further, an algorithm for calculation of the welding stability and feasibility was developed. The n2 dimensionless similarity criterion was used. The proposed method allows us to determine the stable welding conditions for the given range of the welding conditions variation by n2 criteria calculation without numerous welding experiments. Calculations of the sustainable modes of the root pass with 2,0...3,5 mm gap and 2,0...9,0 mm/s welding speed were performed.