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Water Robot Repair Welding: a Technical Note Leilei WANG

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Water Robot Repair Welding: a Technical Note Leilei WANG ISIJ International, Vol. 57 (2017),ISIJ International,No. 1 Vol. 57 (2017), No. 1, pp. 203–205 underwater wet welds are inherently inferior to welds made Note in air.6) There are three major drawbacks of underwater wet welding. Drawback I. Defects caused by the rapid cooling rate in the water environment: bubbles cannot be released from the welding pool, so they are trapped in the solidifying metal, Innovative Methodology and Database for Under- resulting in porosity, low ductility7) and a brittle martensite water Robot Repair Welding: A Technical Note phase.8) Drawback II. Defects caused by the high oxygen and hydrogen content in the water environment: hydrogen Leilei WANG, Fangxiang XIE, Yunliang FENG and embrittlement,9) hydrogen cracking,10) porosity and oxide Zhenmin WANG* inclusions.11) Drawback III. Defects caused by the unstable School of Mechanical and Automotive Engineering, South China arc and metal transfer in the water environment: bead University of Technology, 381 Wushan Road, Guangzhou, irregularity, craters and spatter. These issues are seldom Guangdong, 510641 P. R. China. mentioned, but the authors believe them to be more severe (Received on July 19, 2016; accepted on September 12, 2016; than Drawback I and Drawback II. J-STAGE Advance published date: November 30, 2016) Underwater wet welds are appropriate for temporary repair and salvage applications, but it is difficult to satisfy the requirements of nuclear power plant repair welding. Underwater local dry welding is a modified wet welding The current research status and main drawbacks of underwater welding are briefly summarized. A novel welding torch is proposed process, in which the arc combusts in a small movable to overcome the problems and issues. The design details and work- chamber. It is a combination of dry welding and wet ing principle are introduced. A series of welding experiments are welding. It is an attractive method and has attracted large conducted, obtaining nearly defect-free joints. The experimental amounts of investigations. Underwater local dry welding results indicate that the innovative methodology achieves a higher is a theoretically ideal technique because it overcomes welding speed and better welding stability than conventional Drawback II and Drawback III. Unfortunately, it is not ideal underwater welding. The feasibility and superiority of the innova- in practice. The hyperbaric gas flow generates an isolation tive methodology are verified. An optimal database and corre- gas block and causes turbulence that makes the arc unstable. sponding weld bead diagrams are presented for direct employment Therefore, Drawback III still occurs in the actual welding in subsequent research. process. KEY WORDS: local dry robot welding; rotary gas block; weld An innovative methodology is proposed based on above defect; welding process stability. research status and issues. The new methodology overcomes Drawback II and Drawback III using a special designed welding torch. Technical details instead of theoretical analysis are presented in the following paper. A well-tested database is also presented for worldwide technical exchange 1. Introduction in underwater welding. Subsequent research can thus utilize As a safe, clean, economical and efficient energy source, these data directly instead of approaching them by trial and nuclear energy will be applied worldwide in the future. error. One concern is nuclear reactor leakage, which can cause irreparable harm. Underwater robot repair welding is a fea- 2. Methodology sible solution for the repair of nuclear power reactors and offshore platforms.1) Schematic view, 3D mechanical drawing and sectional Studies regarding underwater gas tungsten arc welding2) view of a specially designed welding torch are shown in and laser beam welding3) are reported in the literature. Fig. 1. This novel welding torch differs from a conventional However, the poor gap bridging ability caused by the lack welding torch in its specially designed inlets, outer nozzle of deposit metal restricts the popularization of this approach. and composites made waterproof. Two features of the novel Underwater dry welding isolates the weld from water using welding torch should be highlighted. First, the four inlets a welding chamber.4,5) The obvious characteristic is that the are tangentially oriented with respect to the outer nozzle so effect of water on the welding process is restrained. When a that the gas flow in the outer nozzle is rotating. Second, the non-hyperbaric welding chamber is employed, the welding cross-section area of the outlet is much smaller than the area environment is identical to welding in air, and thus the weld of the four inlets, speeding up the gas flow. The composites quality is guaranteed. However, the welding equipment is made waterproof acts as a buffer area, which can effectively complicated, so this approach is limited to deep-sea branch eliminate interference from the convection and waves of pipe welding rather than nuclear power plant repair welding. dynamic water. Underwater wet welding and local dry welding are fre- As a result, the gas flow generates a hyperbaric and rotary quently used currently. Underwater wet welding deposits gas block that prevents water from entering the welding zone. the weld using a flux-cored welding wire. As the base metal The turbulence is also improved. The gas pressure attenuates and some part of the weld pool are submerged in the water, from edge to center in the outer nozzle, so its influence on the arc is minified. Thus, a theoretically stable arc is achieved. * Corresponding author: E-mail: [email protected] DOI: http://dx.doi.org/10.2355/isijinternational.ISIJINT-2016-407 203 © 2017 ISIJ ISIJ International, Vol. 57 (2017), No. 1 Fig. 1. Schematic view, 3D mechanical drawing and sectional view of a novel welding torch. Table 1. Database of underwater welding with corresponding weld beads (in Fig. 3). Welding Gas pressure Mean Peak Base Peak Base Number Speed (mm/s) of inlet (MPa) current (A) current (A) current (A) time (ms) time (ms) 01 6.5 No gas flow 138 320 60 37 02 6.5 0.2 138 320 60 37 03 6.5 0.3 138 320 60 37 04 6.5 0.4 138 320 60 37 05 6 0.25 136.8 300 60 3.2 6.8 06 7 0.25 124 260 60 3.2 6.8 07 7 0.25 140 310 60 3.2 6.8 08 7 0.25 152.8 350 60 3.2 6.8 09 7 0.25 147.5 310 60 3.5 6.5 10 7 0.25 186.5 310 120 3.5 6.5 11 8 0.25 135 310 60 37 12 8 0.25 185 310 60 55 13 8 0.25 136.8 300 60 3.2 6.8 14 10 0.25 136.8 300 60 3.2 6.8 by the innovative methodology is free of bead irregular- 3. Experimental Procedures ity, undercuts, craters, and spatter, and almost no porosity, To verify the feasibility of the innovative methodol- crack, slag inclusions or oxide inclusions appear on the ogy, a series of experiments are conducted. The detailed weld surface. The main weld defects reported in the litera- materials, parameters and procedures are listed below. A ture are bead irregularity, undercuts, porosity, craters, slag self-developed welding power supply with pulsed current inclusions, spatter and oxide inclusions. The welding speed was adopted in this study. A self-developed underwater of the innovative methodology is much higher than for the wire feeder achieved stable wire feeding. A self-developed conventional method, and a nearly defect-free weld joint is novel underwater welding torch was adopted. A LBBBD™ obtained. The innovative methodology is thus superior to robot was adopted to handle the welding torch and track both conventional wet welding and local dry welding. the seam automatically. Before welding, four inlets firstly A weld bead with no gas flow from the outer nozzle is worked to blow the water away, and then the current control presented in Fig. 3(01). Defects such as slag inclusions and mode was adopted to control metal transfer. Some welding oxide inclusions appear on the weld surface, indicating the parameters are presented in Table 1. Other parameters are interference of water vapor with the weld bead. A weld bead as follows. Base metal: 304 stainless steel with a dimension with a 0.4 MPa gas flow from the outer nozzle is presented of 250 × 100 × 4 mm. Welding wire: 304 stainless steel in Fig. 3(04). The arc voltage test result reveals a relatively with a diameter of 1.2 mm. Welding voltage: 25–30 V. high arc voltage due to severe arc compression. As a result, Water depth: 0.2 m. Wire feed rate: 66–88 mm/s. Shielding the weld bead collapses, and the weld formation is inferior. gas: argon. Electrode extension: 15 mm. Joint type: Bead- When the gas flow of the outer nozzle varies from 0.2 to 0.3 on-plate welding. MPa, the weld bead is relatively uniform, and no significant defect is found on the weld surface. Thus, the optimal gas flow from the outer nozzles is approximately 0.25 MPa. Fur- 4. Results and Discussion ther experiments producing the data numbered from 05 to 14 Weld beads for the innovative and conventional underwa- are conducted using a gas flow of outer nozzle 0.25 MPa. ter welding methods are presented in Fig. 2. Weld defects A series of optimal parameters and weld beads are exhib- are also noted in the last column. The weld bead produced ited in Table 1 and Fig. 3. The welding speed varies from 6 © 2017 ISIJ 204 ISIJ International, Vol. 57 (2017), No. 1 Fig. 2. Weld bead characteristics of innovative underwater welding and conventional underwater welding. Weld defects and abbreviations: bead irregularity (BI), undercut (UC), porosity (PO), crater (CT), slag inclusions (SI), spatter (SP) and oxide inclusions (OI).
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