Kobelco Technology Review

Kobe Steel’s Research and Development in Welding Materials, Robotic Welding Systems, and Welding & Joining Processes

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While conventional short-circuit wire feed control processes are effective in achieving low spatter and low-heat-input welding, they present challenges when it comes to high-current ranges, medium-to-thick plates, and multi-layer multi-pass welding. To address this issue, the world’s first non-short-circuit-type wire feed control process, AXELARC™, has been developed, in which the process leverages the inertia for droplet transfer by alternating the wire feed direction forward and backward. The applicability of the process to the field of medium-to-thick plates has been examined. The results confirm its ability to achieve low spatter and low fume welding in a wide range of conditions, from low to high currents. Additionally, it enables deep penetration, high deposition, and high-speed welding capabilities. The newly developed process is expected to make a significant contribution to improving welding quality and efficiency in the field of medium-to-thick plates.

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SENSARC™ RA500 is an arc-welding machine newly developed with highly efficient processes. It was launched in the summer of 2021 as a higher-end model encompassing the current SENSARC™ AB500. This high-end welding machine is equipped with highly efficient processes and functions suitable for automated welding of medium-to-thick plates for, for example, architectural steel frames, construction machinery, bridges, and vehicles, which are Kobe Steel’s specialty. This paper introduces the highly efficient welding methods realized by SENSARC™RA500, including pulsed MAG welding with expanded efficiency, REGARC™ with even lower spatter, and tandem welding specialized for high-speed welding.

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The rapid development of information and communication technologies, as well as the widespread use of AI technologies, is expected to accelerate automation and labor-saving measures in production sites. Kobe Steel has been developing various functions to automate welding operations and increase the ratio of robots used in welding processes. However, the adoption of welding robot systems has resulted in new manual tasks such as teaching and maintenance. By reducing these manual operations, the introduction of welding robot systems can be more effective than ever before, leading to increased productivity. To overcome these challenges, Kobe Steel has leveraged ICT/AI technology to develop an automatic function that generates welding programs, reducing the need for manual teaching work. This feature has been added to the ARCMAN™ Offline Teaching System. A camera has been mounted on the ARCMAN™ PRODUCTION SUPPORT, expanding remote visualization capabilities, which is expected to improve safety by reducing the number of work at elevated areas.

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A state-of-the-art welding robot system has been developed for architectural steel frames, featuring the latest technology such as the ARCMAN™ A60 manipulator and the SENSARC™ RA500 welding power source. The system is aimed at increasing productivity and has a reduced cycle time compared with the previous model. This paper provides an overview of the system’s components, highlighting the technology that has contributed to the reduction in cycle time. Specifically, the REGARC™ process is covered in detail, which has been improved by the new welding power source, and the welding wire, which now benefits from surface treatment for improved feedability. These advancements have resulted in highly efficient welding conditions, improving the welding current and speed compared with conventional welding techniques, as outlined in this paper. Additionally, the development technology used to create the system equipment is described.

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Japan’s domestic policy aimed to achieve carbon neutrality by 2050 has resulted in a growing interest in power generation methods that employ renewable energy sources with low CO₂ emissions. Among these methods, offshore wind power generation has garnered significant attention in recent years. Wind turbines in offshore facilities rely on foundations such as monopiles for support. With the increasing size of wind turbines, the monopiles have also grown gigantic in diameter and length, making it necessary to use extra-thick steel plates for their construction. Welding these plates requires an appropriate welding process and welding consumables. To meet these requirements, Kobe Steel has developed a new electroslag welding process for extra-thick plates, SESLA™, and welding consumables for narrow-groove submerged-arc welding(SAW), FAMILIARC™ US-29HK and TRUSTARC™ PF-H55LT-N.

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The demand for liquefied natural gas (LNG) is expected to rise, and 9% Ni steel is the material of choice for the tanks (both for above-ground and marine applications) used to store and transport it. However, welding of 9% Ni steel requires high skill levels and is prone to defects like lack of fusion. To address this issue, this paper introduces a portable, specialized robot system designed for the highly efficient and deskilled welding of 9% Ni steel. The system utilizes a cartesian coordinate robot with mounting jigs, sensing methods, welding power supply, and optimal welding conditions tailored to the manufacturing of 9% Ni steel LNG tanks. This robot system allows welders to produce sound-quality welded joints during vertical welding of 9% Ni steel, with reduced defects. Furthermore, by using two robots simultaneously with one operator, significant improvements in efficiency can be expected.

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Over the past few years, the demand for larger structures like spherical tanks and pressure vessels has risen, leading to a requirement for higher strength steel and welding consumables. However, the use of post-weld heat treatment (PWHT) to relieve welding residual stress has been known to deteriorate the notch toughness of high-strength weld metal. To investigate the factors contributing to this deterioration, an analysis has been conducted on the fracture surface morphology and microstructure after impact testing of weld metal for 780 MPa class steel, with electrodes for flux-cored arc welding as the main focus. The findings suggest that temper embrittlement and precipitation hardening caused by carbide are the main reasons for the deterioration of notch toughness after PWHT. Following several studies, an empirically derived component system of weld metal has been developed to minimize the negative aspects of temper embrittlement or precipitation hardening caused by carbide after PWHT. Utilizing these results, Kobe Steel has launched TRUSTARC™ ^{Note 1)} LB-80LSR as a shield-metal arc-welding consumable for 780 MPa class steel, which boasts excellent notch toughness even after PWHT.

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ASME Gr. 93 steel is a heat-resistant steel, in which creep strength is increased by adding elements such as W and B to the conventional 9% Cr ferritic heat-resistant steel. An investigation has been conducted on the effects of W addition on the creep rupture time and metallographic structure of weld metals for Gr.93 steel. The results of the creep rupture test on weld metals with varying additions of W show that creep rupture time increases with the increasing amount of W. Observations of specimens thermally aged at 650℃ have confirmed the presence of the Laves phase, suggesting that the increase in creep rupture time is attributable to particle dispersion strengthening by the Laves phase. Increasing the additive amount of W increases the number density of the Laves phase and, furthermore, suppresses the coarsening of the Laves phase during thermal aging. The coarsening rate of the Laves phase during thermal aging has been in good agreement with the calculated value obtained from the theoretical equation for Ostwald ripening. Like M₂₃C₆ , the Laves phase may contribute to the creep strengthening by suppressing the coarsening of the lath size.

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To improve the durability and reliability of arc weld joints of automobile chassis parts, a welding consumable has been developed that can improve the electrodeposition coatability of weld bead by adjusting the deoxidizing elements in the welding wire. In this paper, the lap fillet weld joints were prepared using conventional and developed wire, and the surface and cross-section of weld slag were observed to investigate the relationship between the oxide state and electrodeposition coating properties. It has been confirmed that the composition of the major oxides in the weld slag of the developed wire is multiple Mn-based oxides, rather than Si-Mn composite oxides. In addition, it has been confirmed that the constitutional state of the Mn-based oxides in the welding slag has changed due to the dilution effect of the base steel plate, and that these factors also affected the electrodeposition coatability.

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Liquid metal embrittlement (LME) cracking, seen in resistance spot welds of ultra-high strength steel, has become a subject of discussion in the automobile industry. However, only a few articles concerning the effect of this type of crack on joint properties have been reported. Therefore, in this study, LME cracks were classified into 3 types based on location and the influences of these cracks on static and fatigue strengths were investigated. Type A crack, occurring within an indentation of a spot weld, slightly affected the joint strength, while type B crack, generating on the indentation periphery or outside it, significantly lowered both tensile-shear and cross-tension strengths, which decreased by up to 35% and 44%, respectively. Type C crack, seen near the interface of the sheets, deteriorated fatigue strength as well.

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In recent years, adhesive bonding has attracted interest as a key joining technology for lightweight metals and dissimilar materials, and its importance is increasing across various industrial fields.
When bonding metallic materials, surface preparation technology plays a crucial role in maximizing the inherent bonding properties of the adhesive, because its bonding strength is greatly affected by the surface condition. This paper aims to discuss the relationship between the surface condition of metallic materials and adhesion properties. Additionally, it will cover the improvement of adhesion by surface preparation, and efforts to evaluate adhesion strength and durability, while considering the actual environment of use.

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