Narrow Gap Welding Research Articles

Process stability and parameters optimization of narrow-gap laser vertical welding with hot wire for thick stainless steel in nuclear power plant

Incomplete fusion, pore and melt sagging are common defects for multi-layer narrow-gap laser welding with filler wire in vertical positions for large and heavy structures. This investigation showed that vertical-up laser welding had a lower sensitivity to pores than vertical-down welding, whereas increasing welding speed could improve welding stability and reduce the formation possibility of pores. For laser vertical welding with hot wire, surface tension transfer mode could effectively guarantee pre-heating of filler wire and achieve a stable welding process. The keyhole stability and melt flow varied with welding directions. The melt sagging defect was usually observed and mainly caused by the concentrated laser energy, which easily led to discontinuous weld bead. Laser beam oscillation technology increased laser melting area and promoted wetting behavior of filler wire with base metal. The weld width increased and concave weld metal surface was obtained with suitable process parameters. Narrow-gap welding process could be carried out at a small defocusing position with a lower laser power, further decreasing tendency of melt slagging. The 20-mm defect-free joint was obtained by vertical-up and vertical-down narrow-gap laser oscillation welding with hot wire.

Effect of multiple passes on Lüders/yield plateaus, microstructure and tensile behaviour of narrow-gap thick-section weld plates

During nuclear reactor pressure vessel manufacturing, conventional arc welding processes are often employed involving multiple passes to join thick sections. Such thick-section welding may cause variable through-thickness metallurgical and mechanical properties due to variable heat input, particularly in the heat-affected zone (HAZ) and weld metal. Structural steels for pressure vessel components can be selected based on properties such as the Luders plateau and yield/tensile (YS/UTS) ratio in order to assure extended service lives. In the present work, through-thickness tensile properties of narrow-gap (NG) welds performed on 78-mm-thick SA533 plate welded according to ASME IX with 58 passes of gas tungsten arc welding (NG-GTAW) and 38 passes of narrow-gap submerged arc welding (NG-SAW) are examined and compared. The Luders plateau strength, length and velocity were determined along with yield, YS/UTS ratio, ultimate tensile strength and elongation. These properties were analysed in connection with metallurgical and microstructural properties for both welding approaches and established the correlation among them in both cases with the mechanical properties and Luder behaviour. Multiple thermal cycles have tempered and refined the microstructure and hardness in weld and HAZ, which pretends to be a heat treatment of weldment and promotes the formation of Luders plateau. The YS/UTS ratio and Luders plateau behaviour has been evidenced to be influenced by large numbers of thermal cycles. Additionally, the heat input is also vital concern as it is observed that the Luders plateau stress is more in GTAW process than the SAW. However, Luders plateau length is significant in SAW than GTAW process. The implication of these variations on structural integrity aspects and the correlation of mechanical-microstructural properties have been justified in the present work.

Narrow gap welding of low alloy and austenitic stainless steels using different Inconel alloys: comparison of microstructure and properties

In the present investigation, SA508 low alloy steel and 304LN austenitic stainless steel were joined by hot wire narrow gap gas tungsten arc welding. Inconel 82 and 52 were used as filler alloys. Evaluation of microstructure, micro-hardness, and tensile strength through in situ test in scanning electron microscope were attempted for the welded assemblies. Low alloy steel consisted of a heat affected zone adjacent to interface. A thin layer of martensite and Type-I boundary were revealed near the fusion boundary between low alloy steel and weld metal. Micro-hardness distribution showed a peak near the fusion boundary between low alloy steel and filler alloy owing to the presence of lath martensite, Type-I boundary, and complex alloy carbides. The micro-hardness was further dropped within the filler alloy. During the in situ tensile test, deformation was confined within the filler alloy, as it was the weakest link across the fusion boundary. It was revealed that the joint produced with Inconel 82 filler alloy has a better mechanical strength than the assembly fabricated with Inconel 52 filler alloy. Inferior quality of the latter was attributed to the relatively coarser grain structure and the presence of larger alloy carbides within filler alloy when compared with the former. Improved strength of the weld with Inconel 82 filler alloy was further endorsed by the lower slip distance for dislocation movement and higher strain hardening rate in comparison to the weld fabricated with Inconel 52 filler alloy.

Study on the weldability, microstructure and mechanical properties of thick Inconel 617 plate using narrow gap laser welding method

Multi-pass narrow-gap welding of thick Inconel 617 plate was conducted with YLS-10000 fiber laser. The influence of different welding parameters on the joint geometry and defects was investigated. The dendrite morphology, grain distribution, crystal texture and stress distribution were analyzed. The microstructure evolution such as element segregation and precipitate behavior was also studied. The micro-hardness, tensile strength at room and high temperature, and toughness impact were measured to evaluate the mechanical properties. The results revealed that the U-shaped or V-shaped filling pass could help the formation of good layer without defects. The maximum grain size of base metal and heat affected zone was less than 200 μm. The grain size of weld zone was large, and the maximum grain size reached 500 μm. Serious stress concentration occurred in the WZ and HAZ. The elements Ti and Mo segregated seriously in the dendrites. The carbides including M6C, Ti(C, N), (Cr, Ni)(C, N) and M23C6 were precipitated in the weld metal. The microhardness, tensile strength and impact toughness of WZ were higher than that of BM at room temperature. The tensile strength decreased with increasing temperature. The fracture mode at high temperature was ductile rupture, which was caused by the accumulation of tiny cavities.

Narrow gap applications of swing TIG-MIG hybrid weldings

To solve the lack of sidewalls penetration in gas metal arc welding (GMAW), a swing TIG-MIG hybrid welding method was proposed and applied in the narrow gap welding. The arc shapes and voltage changes in MIG, TIG-MIG hybrid welding and swing TIG-MIG hybrid welding were compared. The result reveals that the swing TIG-MIG hybrid welding can be used in narrow gap welding to attain good sidewall penetration. The assisted TIG arc can widen the MIG weld width and obtain good sidewall penetration, and stabilize the MIG welding voltage thereby improving welding quality. The melted welding wire in the MIG welding molten pool will spread to the TIG welding molten pool, which makes the weld width increased and the welding spread improved.

Research on the seam tracking of narrow gap P-GMAW based on arc sound sensing

Arc sensor is important in the automation of narrow gap welding process because only the arc signal can reflect the position of the arc in the groove in real time, as well as the heat input to the side wall. Except the current and voltage of the arc, the deviation state of the welding arc can be identified by experienced welders by the associated arc sound, therefore, the arc sound signal presents a potential welding seam tracking signal. In this study, arc sounds were acquired and processed to examine the correlation with weld deviation during narrow gap pulsed gas metal arc welding (P-GMAW). The influences of groove sidewall on the arc sound signal and the sensitive of arc sound features to welding torch deviation were studied, and the short-time energy was determined as the best parameter to reflect the arc deviation in the narrow gap grooves. Furthermore, a system was established for seam tracking using the arc sound short-time energy and the results show that the arc sound can be used as the control signal to realize seam tracking to a certain degree of precision, thus, providing a new control signal for narrow gap P-GMAW welding seam tracking based on multi-information fusion.

Application of Cold Wire Gas Metal Arc Welding for Narrow Gap Welding (Ngw) of High Strength Low Alloy Steel.

Narrow gap welding is a prevalent technique used to decrease the volume of molten metal and heat required to fill a joint. Consequently, deleterious effects such as distortion and residual stresses may be reduced. One of the fields where narrow groove welding is most employed is pipeline welding where misalignment, productivity and mechanical properties are critical to a successful final assemblage of pipes. This work reports the feasibility of joining pipe sections with 4 mm-wide narrow gaps machined from API X80 linepipe using cold wire gas metal arc welding. Joints were manufactured using the standard gas metal arc welding and the cold wire gas metal arc welding processes, where high speed imaging, and voltage and current monitoring were used to study the arc dynamic features. Standard metallographic procedures were used to study sidewall penetration, and the evolution of the heat affected zone during welding. It was found that cold wire injection stabilizes the arc wandering, decreasing sidewall penetration while almost doubling deposition. However, this also decreases penetration, and incomplete penetration was found in the cold wire specimens as a drawback. However, adjusting the groove geometry or changing the welding parameters would resolve this penetration issue.

Laser Multi-Pass Narrow-Gap Welding – A Promising Technology for Joining Thick-Walled Components of Future Power Plants

Today, the average worldwide efficiency of coal-fired power plants stands at about 33 percent. The consistent use of state of the art technologies would enable an increase of the average efficiency of up to 47 percent and thus a sharp reduction of greenhouse gas emissions. The importance of improvements in this field becomes apparent when reviewing e.g. plans in Europe in 2017 for new power plants to be built across the continent. About 44 percent of the envisaged 153 gigawatts are still to be generated by fossil-fuel power plants [1]. One technical solution is to increase the steam turbine inlet temperature to 700°C. This, however, requires the use of nickel-based superalloys. Only these alloys satisfy all the requirements with regard to high-temperature, corrosion and oxidation resistance and creep behavior [2], [3]. Due to their relatively poor machinability, forgeability and high material costs compared to the steel-based alloys they are to replace, a more effective welding technology is needed to overcome the disadvantages of conventional welding technologies, i.e. large quantities of filler metal required and high energy input per unit length resulting in distortion and the potential reduction of high-temperature properties.

Multi-Pass Laser and Hybrid Laser-Arc Narrow-Gap Welding of Steel Butt Joints

Multi-pass laser and laser-arc narrow-gap welding make it possible to obtain high-quality butt joints of large-thickness pipe steels with hardness up to 2600...2800 MPa and toughness close to the base metal toughness at a temperature of 20°C. When multi-pass welding methods are used, the subsequent pass makes heat treatment of the previous ones, and reduces the rate of their cooling yet. This allows excluding heat treatment of the joint after welding, which significantly reduces the time and costs for manufacturing large-sized structures.

Study on the Welding Process of the Vacuum Vessel Mock-Up for CFETR

China Fusion Engineering Testing Reactor (CFETR) is a superconducting magnet tokamak. The research and development of the key technologies to the vacuum vessel (VV) manufacture have been carried out a few years ago by the Institute of Plasma Physics, Chinese Academy of Science, Hefei, China, including narrow-gap welding, cutting, and nondestructive testing. The manufacture of the poloidal segment (PS) for first 1/32 sector VV mock-up has been completed in 2015; the four PS sectors have been welded into a whole in 2016. This paper will describe the study of the welding process of the 1/32 VV mock-up for CFETR.

Modelling of residual stresses in a narrow-gap welding of ultra-thick curved steel mockup

A plane strain model, an axisymmetric model and a lumped-pass three-dimensional (3D) model were employed to simulate the residual stresses in a narrow-gap welding 300 mm-thick curved steel mockup. The lumped-pass 3D model was built with the lumping scheme based on the two-dimensional (2D) temperature field result. The residual stresses on the top surface of the mockup were measured using the hole-drilling method and compared with the simulated stresses. Results show that the plane strain model cannot capture the asymmetric conditions of the curved mockup; the through-thickness transverse stress within the weld zone presents a self-equilibrated distribution, and the through-thickness hoop stress within the weld zone are almost tensile with the peak value occurring at a certain depth (8 mm) beneath the top and bottom surfaces.

A study on assembly technology of the CFETR 1/32 Vacuum Vessel

Chinese Fusion Engineering Testing Reactor (CFETR) is a superconducting magnet Tokamak, which has the equivalent scale with complementary function to International Thermonuclear Experimental Reactor (ITER). The vacuum vessel (VV) which has a double-layer structure, Cooling water circulating through the double-layer structure will remove the heat generated during operation. The VV will provide a high-vacuum environment for the plasma, improve radiation shielding and plasma stability and provide support for in-vessel components. The CFETR VV is composed of 16 sectors, the angle of each sector is 22.5°. The Research and Development (R&D) of the key technologies to the VV manufacture have been carried out a few years ago by Institute of Plasma Physics Chinese Academy of Science (ASIPP), including Narrow-Gap welding, cutting and non-destructive testing, Poloidal Segment (PS) and sector assembly technologies, etc. ASIPP is constructing a 1/8 sector real size VV mock-up now. The manufacture of the PS for first 1/32 sector VV mock-up has been completed in 2015, the PS will be assembled into a whole this year. This paper will describe the study of the assembly technology for the CFETR 1/32 Vacuum Vessel mock-up.

A Review On Rotating Arc Welding Process

Rotating arc welding also known as spin arc welding utilises a unique weld torch in which the welding electrode rotates in a circular motion at a high rate of speed. Centrifugal force propels molten droplets across the arc creating a consistent and sound weld bead. This enables high deposition rate welding for significant increase in productivity. The present paper discuss the effect of welding parameters such as voltage, current, rotating frequency, rotating radius, etc., on weld bead geometry especially in narrow gap welding. Also various output parameters like tensile strength, hardness and microstructures of different weldments were also considered in the study.

Investigation on narrow-gap welding residual stresses in ultra-thick ring-type mockups

The stress distributions within two ring-type mockups manufactured by multi-pass narrow-gap submerged welding were investigated by numerical simulations and experiments. The large mockup consisted of a 300 mm thick ring welded to a large cylinder, and the small one was a butt-welded joint consisted of two 300 mm thick curved plates. The effect of structure form on the stress in the heavy-section component was also investigated. Results show that the stress distribution in the small mockup is similar to that in the large one. The location where the minimum hoop and radial stresses appear at the weld centerline is decided by the weld metal height of the second welding step of the welding procedure used in the present study. In addition, a sudden stress change occurs near the top surface of the weld metal after the first welding step due to the strong stiffness of the mockups. For the 300 mm thick welded mockups investigated, the structure form has no evident effect on the shape of the through-thickness stress distribution at the weld centerline, which is determined by the welding procedure; however, the structure form can affect the value of the minimum stress.

Fatigue crack growth of 316NG austenitic stainless steel welds at 325 °C

316NG austenitic stainless steel is a commonly-used material for primary coolant pipes of pressurized water reactor systems. These pipes are usually joined together by automated narrow gap welding process. In this study, welds were prepared by narrow gap welding on 316NG austenitic stainless steel pipes, and its microstructure of the welds was characterized. Then, fatigue crack growth tests were conducted at 325 °C. Precipitates enriched with Mn and Si were found in the fusion zone. The fatigue crack path was out of plane and secondary cracks initiated from the precipitate/matrix interface. A moderate acceleration of crack growth was also observed at 325°Cair and water (DO = ∼10 ppb) with f = 2 Hz.

Numerical analysis of heat transfer and fluid flow in swing arc narrow gap GMA welding

A three-dimensional model is developed for swing arc narrow gap welding process considering the effect of moving trajectory of arc and joint geometric shape, which can calculate the temperature field and fluid flow in transient state. The arc heat source model also allows for the inclination of arc in the plane perpendicular to weld beam and the variation of the arc moving direction through coordinate system transformation, its distribution parameters being determined based the arc behavior captured by a high speed video system. The droplet heat transfer is assumed as the process of liquid metal with high temperature flowing into the weld pool from the certain area above it. The major forces acing on liquid molten pool, including arc pressure, arc plasma drag force, electromagnetic force, surface tension, Marangoni stress and buoyancy force are taken into account in this model. The characteristics of weld pool dynamic behavior and temperature profile are analyzed and the effect of different welding parameters on them is discussed. In swing arc narrow gap GMA welding, there exists a large vortex in the whole weld pool cross section and fluid flow pattern is beneficial to the growth of sidewall penetration depth. The width of high-temperature region outside weld pool is smaller than that of non-swing arc welding process. The arc swing frequency has minor influence on the cooling rate.

Effect of mechanical mismatch on fracture mechanical behavior of SA 508 – Alloy 52 narrow gap dissimilar metal weld

In modern pressurized water reactor (PWR) designs, dissimilar metal welds (DMW) between the reactor pressure vessel (RPV) nozzles and safe-ends connected to the main coolant pipes are manufactured using a narrow-gap welding (NGW) technique. In addition to the new weld design, the filler metals have been changed from conventional Alloys 82 and 182 to higher Cr containing Alloys 52 and 152 to ensure the structural integrity of the welds during long term operation. The changes in weld design and filler metal have an effect on the mechanical mismatch state of the DMW which has an effect on the fracture behavior of the weld. For the experimental determination of the local strength variations and the fracture mechanical behavior of a modern NG-DMW, a weld mock-up consisting of SA 508 pressure vessel steel with AISI 309L/308L cladding, AISI 304 piping steel, and Alloy 52 weld metal was manufactured. The NG-DMW was characterized in both as-welded and post-weld heat treated (PWHT) conditions. The results showed that the highest strength mismatch existed between the carbon-depleted zone (CDZ) in the SA 508 heat-affected zone (HAZ) and the Alloy 52 weld metal. High fracture toughness values were observed for the different zones of the weld and the crack propagation occurred towards the softer SA 508 HAZ CDZ.

Assembling of thick-section HSLA steel with one seam per layer multi-pass PC-GMA welding producing superior quality

Assembling of 25-mm-thick HSLA steel plate with extra narrow gap between the plates is done with the help of multi-pass one seam per layer pulse current gas metal arc welding process maintaining vertical weld deposition. Assembling of thick plates is also made using conventional groove with commonly used procedure of multi-pass multi-seam per layer weld deposition. Distortion and shrinkage that developed in the weld joints have been suitably determined. The effects of welding parameters in reference to weld thermal behavior on shrinkage stress and bending stress have been studied. The observations show that the use of one seam per layer technique allowing extra narrow gap welding with properly selected parameters can improve quality of weld joint with respect to distortion and stresses. The effect of pulse parameters on weld quality is studied, correlated and discussed. Cost benefits of applying such a welding technique have been justified with respect to consumption of less filler material and time for preparation of weld joint.

Thermal cycle-dependent metallurgical variations and their effects on the through-thickness mechanical properties in thick section narrow-gap welds

The primary components in pressurised water reactors are manufactured by welding thick sections of either SA508 or SA533 pressure vessel steel using processes such as submerged arc welding (SAW) or gas-tungsten arc welding (GTAW). Narrow-groove (NG) variants of these processes have reduced welding times, but thick-section welds still require a large number of passes. In this work, the effects of a large number of welding thermal cycles on the through-thickness variability in microstructure and mechanical properties have been analysed for NG-GTAW and NG-SAW joints made in 78mm thick SA533 steel. Microstructures were characterised using optical and scanning electron microscopy, while mechanical properties were captured in cross-weld tensile tests using digital image correlation and through tests on coupons extracted exclusively from the weld metal and from the heat-affected zone. Charpy impact testing was used to assess toughness. While the toughness was relatively consistent throughout the SAW joint, significant through-thickness variations in toughness were observed in the NG-GTAW joint, which can be attributed to the varying degree to which the weldment was tempered by subsequent welding thermal cycles.

Characteristics of GMAW Narrow Gap Welding on the Armor Steel of Combat Vehicles

The primary purpose of this investigation was to study the characteristics of the armor steel weldment of combat vehicles by using GMAW narrow gap welding (NGW). The results showed that the mechanical properties and residual stress distribution of NGW weldment were improved, compared with conventional X-groove weldment. Additionally, ballistic tests according to MIL-HDBK-1941 were carried out to verify the ballistic ability of NGW weldment and the result showed that the NGW process was qualified for welding the armor steel of combat vehicle.