Narrow Gap Submerged Arc Welding Research Articles

Comparison of Microstructure and Mechanical Properties of Ultra-Narrow Gap Metal Active Gas Arc Welded and Narrow Gap Submerged Arc Welded Q235A Low Carbon Steel.

The 18 mm thick Q235A low carbon steel plates were welded via the ultra-narrow gap metal active gas arc welding (ultra-NGMAGW) and narrow gap submerged arc welding (NGSAW), and the microstructure and mechanical properties of the welded joints' area were characterized. The results showed that there is acicular ferrite (AF) in the weld zone of the joint obtained via the ultra-NGMAGW. The AF grains are fine and have a great difference in growth direction, resulting in high local dislocation density. However, there is no AF in the welded joint obtained via the NGSAW. Using numerical simulation analysis of the temperature field distribution and the thermal cycle curve in the welding process of the ultra-NGMAGW, it was found that the mechanism of microstructure evolution is that during the welding process of the ultra-NGMAGW, the heat input is low, the cooling rate is quick, and the residence time in the high temperature region is short. Therefore, pearlite with coarse grains is basically not formed. AF nucleates in different directions with inclusions as the core. The tensile strength of the weld joint obtained via the ultra-NGMAGW is 643 MPa, which corresponds to 139% of that of the base metal, and 132% of that obtained via the NGSAW. The ultra-NGMAGW joints exhibited better tensile strength and higher microhardness than the NGSAW joints, which is mainly due to the existence of AF.

High cycle fatigue behavior of a dissimilar metal welded joint in ultra-supercritical steam turbine rotor

The high cycle fatigue (HCF) property and microstructure of a dissimilar metal welded joint (DMWJ) for ultra-supercritical steam turbine rotor were systematically investigated in this paper. The DMWJ was fabricated using narrow gap submerged arc welding (NG-SAW) technique with buttering layer. Conditional fatigue strength of 30Cr1Mo1V (BM-1), buttering layer (BL), weld metal (WM) and 30Cr2Ni4MoV (BM-2) based on S-N curve was obtained by HCF tests at room temperature. The microstructure and second-phase carbide as well as fracture appearance of BM-1, BL, WM and BM-2 were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The results show that the BL has lower fatigue strength, which are related to more content of the soft ferrite and less content of the hard carbide. The acicular ferrite represents better fatigue properties than massive ferrite due to barrier effect. Additionally, dislocation density sharply increases near the carbides, grain boundaries, and lath structures resulting in dislocation tangle after HCF test, which helps to improve the fatigue strength of the welded joint. The carbide precipitated, aggregated, and grown in the grain boundaries promoted crack nucleation, reduced the crack propagation rate and affected the crack propagation path.

Pitting behavior of welded joint and the role of carbon ring in improving corrosion resistance

Pitting behavior and its transition process to cracking were investigated after immersion at 180 °C in pure water and 3.5% NaCl solution for NiCrMoV welded joint manufactured by narrow gap submerged arc welding technology. Dislocation outcrops in the interior of the grains formed small anodes, which mainly attributed to the pit growth for weld metal (WM) and heat-affected zone (HAZ). The serious pitting in WM was induced by strong strain concentration generating from welding thermal stress. Competition behavior between pitting and cracking for WM in pure water and chloride solution was also proposed. Chloride accelerating anodic dissolution caused serious pitting that predominated over cracking in chloride solution, while cracking preferentially formed and replaced mild pitting in pure water. In addition, carbon rings around pits decreased pitting susceptibility for HAZ. The formation of carbon rings generated from carbon diffusion was driven by strain aging and selective corrosion. Moreover, micro-Raman manifested the carbon ring was covered by protective oxide layer that inhibited pit growth and improved pitting resistance for HAZ. This work contributes to the understanding of pit growth and crack initiation for welded joint used in nuclear turbine.

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.

Prevention of carbon migration in 9% Cr/CrMoV dissimilar welded joint by adding tungsten inert gas overlaying layer

High chromium (9–12% Cr) steels with excellent heat resistance and CrMoV steels with good toughness were potential candidates for combined rotor for steam turbine operated over 620 °C. Two welding techniques were used to fabricate 9% Cr and CrMoV dissimilar welded joint. The results show that the carbon migration only appears in the specimen using narrow gap submerged arc welding (NG-SAW) technique, yet it can be effectively prevented by adding tungsten inert gas (TIG) overlaying process before the NG-SAW. The carbon migration occurred in NG-SAW resulting from the sharp transition of the strong carbide-forming element Cr between the weld (~ 2.7 wt%) and the base metal (~ 9 wt%). On the contrary, the application of TIG overlaying layers can promote the diffusion of Cr element, and therefore result in its much smaller concentration gradient. That is to say, a gentle transition zone of Cr element can be created among the SAW weld, TIG overlaying layers and the base metal, which effectively prevents the carbon migration and therefore produces a decreased carbon concentration adjacent to the fusion line.

Transition and fracture shift behavior in LCF test of dissimilar welded joint at elevated temperature

This work focused on the low-cycle fatigue (LCF) behavior of modified 9Cr/CrMoV dissimilar welded joint at elevated temperature. Narrow gap submerged arc welding (NG-SAW) process via multi-pass and multi-layer techniques was employed to fabricate the welded joint. LCF tests at different strain amplitude range from 0.22% to 0.75% were performed at strain ratio R=−1. The two-slope behavior based on fracture location shift was presented both on the cyclic stress-strain (CSS) curve and Manson-Coffin (M-C) curve, which could be applied to predict the fatigue life more precisely especially at relatively low strain amplitude. The results indicated that the joint failed in CrMoV-base metal (BM) at relatively low strain amplitude below 0.4% while failure shifted to CrMoV-over tempered zone (OTZ) at higher strain amplitude above 0.4%. Fatigue failure occurred in CrMoV-BM at low strain amplitude could be attributed to temperature softening effect in CrMoV-BM combined with cyclic strengthening in CrMoV-OTZ. While CrMoV-OTZ with a comparable number of grain boundaries and much lower hardness than that of CrMoV-BM was deemed to be the weakest zone across the welded joint at higher strain amplitude. EBSD investigations also revealed that CrMoV-BM experienced more fatigue damage at relatively low strain amplitude, while CrMoV-OTZ accumulated more plastic strain at higher strain amplitude.

Investigation on the weakest zone in toughness of 9Cr/NiCrMoV dissimilar welded joint and its enhancement

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Study on the microstructure and toughness of dissimilarly welded joints of advanced 9Cr/CrMoV

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Narrow gap laser welding for potential nuclear pressure vessel manufacture

High power density electron beam and laser beam welding show their unique advantages in the welding of thick-section steels for potential nuclear pressure vessel manufacture. Compared with traditional submerged arc welding or newly developed narrow-gap submerged arc welding, electron beam welding and laser beam welding techniques offer high welding speeds, low heat inputs, lower levels of residual stresses and distortion, while consuming less filler material and power. As a part of the new nuclear manufacturing EPSRC Program, this study aimed to develop narrow gap laser welding procedures and parameters for joining 30–130 mm thick ferritic steels for potential application to pressure vessel manufacture in civil nuclear power plants. The results showed that high quality welded joints in 30 mm thick steels had been achieved with a combination of an autogenous root pass and multipass narrow gap (4–5 mm parallel grooves) welding with a defocused laser beam welding and filler wire addition, at laser powers of 7.5–8 kW, a welding velocity of 0.4 m/min, a diameter of the laser spot of 6 mm, and wire feed rates of 4–6 m/min. The results also showed that cracking, lack of sidewall fusion, and porosity were the main defects in multiple pass narrow gap laser welds in thick-section ferritic steels if the welding parameters were not optimized. The microhardness of the multipass weld was lower than that of the autogenous weld as tempered martensite was achieved in multipass welding.

Role of stress in the high cycle fatigue behavior of advanced 9Cr/CrMoV dissimilarly welded joint

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Microstructure correlation and fatigue crack growth behavior in dissimilar 9Cr/CrMoV welded joint

The fatigue crack growth behavior and the related microstructure of the dissimilar welded joint made from advanced 9Cr and CrMoV steels were systematically investigated in this paper. The dissimilar materials of 9Cr and CrMoV with heavy section were welded by narrow-gap submerged arc welding (NG-SAW) via multi-layer and multi-pass technology. The fatigue crack growth behavior was comparatively studied for different regions including advanced 9Cr and CrMoV base metals (9Cr-BM, CrMoV-BM), heat affected zones of both sides (9Cr-HAZ, CrMoV-HAZ) and weld metal (WM). The results indicated that a lower fatigue crack growth rate (da/dN) as well as a higher fatigue crack growth threshold (ΔKth) for WM was gained compared to the BMs and HAZs, while a larger da/dN and lower ΔKth for 9Cr-HAZ were obtained under the same stress ratio. The WM showed a better fatigue crack growth resistance than others, while the 9Cr-HAZ with narrow size became the weaker part of the whole welded joint. Microstructure was revealed that the different microstructure character such as tempered martensite and tempered bainite could account for this diversity in properties. In addition, the observation of fatigue crack growth path displayed that the obstacles of coarse columnar grains and the ductile equiaxed grains in WM caused the higher resistance of fatigue crack growth, while the tempered martensite with and without lath structure led to the higher and lower fatigue crack growth resistance for 9Cr-BM and 9Cr-HAZ, respectively. However, the fatigue crack growth behavior of 9Cr-HAZ would not great influence the whole welded joint for its narrow size.

Correlation of microstructure and fracture toughness of advanced 9Cr/CrMoV dissimilarly welded joint

In this paper, the fracture toughness and the related microstructure characteristics of dissimilarly welded joint manufactured by advanced 9Cr and CrMoV steels were systematically investigated. The dissimilarly welded joint was fabricated by narrow gap submerged arc welding (NG-SAW) applying multi-layer and multi-pass technique. Fracture toughness, as one of the most important property to assess the reliability of welded joint, was studied for different regions including CrMoV base metal (CrMoV-BM), heat affected zone (HAZ) of CrMoV side (CrMoV-HAZ), weld metal (WM), heat affected zone of 9Cr side (9Cr-HAZ) and 9Cr base metal (9Cr-BM). It was found that the fracture toughness of CrMoV-BM, CrMoV-HAZ and WM was better than that of 9Cr-HAZ and 9Cr-BM. In order to illustrate these results, the microstructure of the whole welded joint was observed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) detailedly. It was found that the fine high-temperature tempered martensite and bainite in WM, CrMoV-BM and CrMoV-HAZ contribute to the higher fracture toughness, while lower fracture toughness for 9Cr-BM and HAZ was caused by coarse tempered lath-martensite. Furthermore, the fracture morphology showed that ductile fracture occurred in WM and CrMoV side, while brittle fracture appeared in BM and HAZ of 9Cr side.

Soft zone formation by carbon migration and its effect on the high-cycle fatigue in 9% Cr–CrMoV dissimilar welded joint

This paper studies carbon migration in the welds between 9% Cr steel and CrMoV steel during high-cycle fatigue (HCF) test at various temperatures. The dissimilar joints were fabricated using a narrow gap submerged arc welding (NG-SAW) technique. The HCF tests were carried at room temperature (RT), 400°C and 470°C. It was observed that the specimens tested at RT and 400°C fractured at the CrMoV side while the specimens tested at 470°C fractured at the 9% Cr fusion line. The microstructure of the 9% Cr fusion zone showed that the fracture occurred in a carbon-depleted zone. In addition, the micro-hardness test revealed that the hardness in the carbon-depleted zone was lower than at other locations of the welds while the hardness in the carbon-enriched zone was the highest in the whole welded joint. It is concluded that the carbon migration at elevated temperatures is considered as a very important factor in high stress HCF performance.

Microstructure characteristics and temperature-dependent high cycle fatigue behavior of advanced 9% Cr/CrMoV dissimilarly welded joint

Advanced 9% Cr and CrMoV steels chosen as candidate materials are first welded by narrow-gap submerged arc welding (NG-SAW) to fabricate the heavy section rotor. The present work focuses on studying the high-cycle fatigue (HCF) behavior of advanced 9% Cr/CrMoV dissimilarly welded joint at different temperatures. Conditional fatigue strength of this dissimilarly welded joint was obtained by HCF tests at room temperature (RT), 400°C and 470°C. It was observed that the failure occurred at the side of CrMoV base metal (BM), weld metal (WM) and heat affected zone (HAZ) of CrMoV side over 5×107 cycles for the specimens tested at RT, 400°C and 470°C. The detailed microstructures of BMs, WMs and HAZs as well as fracture appearance were observed by optical microscopy (OM) and scanning electron microscopy (SEM). Precipitation and aggregation of carbides along the grain boundaries were clearly detected with the increase of temperature, which brought a negative effect on the fatigue properties. It is interesting to note that the inclusion size leading to crack initiation became smaller for the HCF test at higher temperature. Therefore, reduction in the inclusion size in a welded joint helps to improve the HCF performance at high temperature.

Design on Cleaning Device for Slag of Narrow Gap Submerged Arc Welding

At present, the Narrow Gap Submerged Arc Welding is an advanced welding technology. As the weld is very deep and narrow, the slag is difficult to clean and will have impact on the quality of welding seam. Therefore, a new kind of narrow gap submerged arc welding slag cleaning device is designed to clean the welding slag. It is consisted of welding walking tractor, height adjuster, slag masher, slag cleaner and slag collector. The narrow gap submerged arc welding slag removal device used for the welding process can realize the cleaning that manpower cannot finish. It is a kind of time-saving, energy-saving and high efficient welding cleaning method.

Study on fatigue property and microstructure characteristics of welded nuclear power rotor with heavy section

Abstract The fatigue property and microstructure characteristics of the welded joint for nuclear power rotor with heavy section, were systematically reported in this paper. The welded joint microstructure is inhomogeneous for NiCrMoV rotor made by narrow gap submerged arc welding (NG-SAW), which could affect the properties in different zones of welded joint. As one of the important indicator to evaluate the running performance of welded rotor, the fatigue crack propagation behavior of the base metal (BM), weld metal (WM) and heat affected zone (HAZ) was comparatively studied. It was found that the fatigue crack propagation threshold (ΔKth) of BM was higher than that of WM and HAZ as stress ratio (R) was 0.1, but ΔKth was very close to each other as R increased. The microstructure, revealed by an optimized corrosive process, was granular bainite in WM and tempered martensite in HAZ, leading to their approximately equivalent resistance of fatigue crack propagation with BM. The experimental results showed that fatigue properties of welded joint for NiCrMoV rotor with heavy section could meet the design requirement, and also push NG-SAW into manufacturing large size rotor.

Studies on fracture toughness of cold wire addition in narrow groove submerged arc welding process

Due to the faster joint completion rates coupled with good mechanical properties, narrow gap submerged arc welding (NGSAW) is widely used for fabrication of thick-walled pressure vessels. Several researchers are working on further increase in productivity in NGSAW. In this paper, we propose to increase the quality and productivity in NGSAW through cold wire addition without addition of heat input. Further toughness at sub-zero temperature is also enhanced. The improvement in toughness in cold wire NGSAW is demonstrated through different tests such as impact energy test, fracture toughness tests, plane strain fracture toughness test K Ic, and crack tip opening displacement test.

Review on Design and Development of Narrow Gap Welding Torches

Narrow gap welding is an effective process for joining thick plates (2 to 12) of carbon steel, low alloys steel and high strength low alloy steels. But it requires some development in torch design, wire feeding mechanism and welding procedure for preventing all sorts of weld defects before practical application. Narrow gap welding technology is associated with various conventional arc welding technology like GMAW, GTAW, SAW, FCAW&SMAW. Several fabricators world over, especially in Japan have developed their own equipment working on different principles [1] Standard torches for NGW are not available routinely in the market. Torches for NGW are specially designed (custom made) for various NGW applications. Design of torch will differ from processes to process and application to application. Paper highlights the different torch designs used by researcher for different narrow gap welding technologies like I) Narrow Gap-Gas Metal Arc Welding (NG-GMAW) Torch Design II) Narrow Gap-Gas Tungsten Arc Welding (NG-GTAW) Torch Design III) Narrow Gap- Submerged Arc Welding (NG-SAW) Torch Design.

Studies on Mechanism of Zigzag Crack Forming Process in Weld Metal of Narrow Gap Submerged Arc Welding of Thick Plate

For improving the toughness of 2.25Cr-1Mo-0.25V heat resisting steel welding joint,the impact test and the three point bending test are done in order to locate the weakest part of the joint,and much more micro analysis is necessary to find out the reason of that.The toughness of weld metal is lower than the other parts of the joint according to the result of the impact test,so the crack growth process in weld metal is further researched with three point bending test.A zigzag crack is found in weld metal after three point bending test.Dimple and quasi cleavage are both exist on the surface of the fracture through scanning electron microscope(SEM) technic.Zigzag crack appears in the columnar grain zone of weld bead with metallographic observation,and its direction is parallel or vertical to the direction of columnar grains.After dealing with picric acid,it appears that the parts of the zigzag crack which are parallel to the columnar grains are the original austenite grain boundaries,the result of electron backscattered diffraction(EBSD) test also proves that conclusion,and the parts which are vertical to them are layer lines.Because of that,the original austenite grain boundaries and layer lines of submerged arc welding(SAW) weld bead should be paid especially attention.The toughness of the joint can be obviously improved by adjusting the composition of welding wire or changing welding parameters,the number of layer lines is decreased due to it.

Analysis of CTOD Test for Proving Rings of Large-Scale Fully-Welded Ball Valves

Fully-welded ball valves are widely used in the crude oil/natural gas transmission pipelines, and their quality has a direct bearing on the transportation safety of national energy. Two paralleled and symmetrical circular welds are used in the main welds of valve body, and the narrow-gap submerged arc welding (NG/SAW) is used in the welding groove. Because of conditional restrictions, it is impossible for the value to be post-weld heat treatment (PWHT). Therefore, the welds must have high impact toughness and strength. In order to study the welding performance of the base metal, SAW is used in the welding of proving rings of steel LF2 fully-welded ball valves. CTOD tests are applied to welding joints, and the result suggests that the minimum CTOD value of the valve’s welds and heat affected zone (HAZ) is more than 0.15mm. According to the criterion DNV-OS-C401, the welds in the fully-welded ball valve are safe enough without PWHT.