Michisuke Nayama

10-kW-class YAG laser application for heavy components

The authors have put the YAG laser of the kW class to practical use for repair welding of nuclear power plant steam generator heat exchanger tubes, all-position welding of pipings, etc. This paper describes following developed methods and systems of high power YAG laser processing. First, we apply the 6 kW to 10 kW YAG lasers for welding and cutting in heavy components. The beam guide systems we have used are optical fibers which core diameter is 0.6 mm to 0.8 mm and its length is 200 m as standard one. Using these system, we can get the 1 pass penetration of 15 mm to 20 mm and multi pass welding for more thick plates. Cutting of 100 mm thickness plate data also described for dismantling of nuclear power plants. In these systems we carried out the in-process monitoring by using CCD camera image processing and monitoring fiber which placed coaxial to the YAG optical lens system. In- process monitoring by the monitoring fiber, we measured the light intensity from welding area. Further, we have developed new hybrid welding with the TIG electrode at the center of lens for high power. The hybrid welding with TIG-YAG system aims lightening of welding groove allowances and welding of high quality. Through these techniques we have applied 7 kW class YAG laser for welding in the components of nuclear power plants.

Measurement of Welding Residual Stresses of Reactor Vessel by Inherent Strain Method : Measurement of Residual Stresses of Pipe-Plate Penetration Joint

This study aims to ensure the safety of nuclear power plants. The accidents involving leaks from the welded zones at the pipe penetration part of a reactor vessel or at a coolant pipe are reported at home and abroad. One of the main causes is the welding residual stress. So, it is important to know the welding residual stress for maintaining high safety of the plants, the estimation of plants life cycle and the plan of maintenance. The welded joints of the nuclear power plants have complex shapes, and the welding residual stresses also have complex distributions three-dimensionally. In this study, the inherent strain method combined with finite element method is used to measure the welding residual stresses accurately. The mock-up is idealized for the welded joint at the pipe penetration part of the actual reactor vessel. The inherent strain method is applied to measure the residual stresses. In this method, the inherent strains are unknowns. When the residual stresses are distributed complexly in a three-dimensional stress-state, the number of unknowns becomes very large. So, the inherent strains are expressed with some functions to decrease the number largely. The theory, the experiment process and the analysed results are explained. The characteristics of the distributions of residual stresses and their production mechanisms are discussed. The inherent strain method gives the most probable values and the deviations of the residual stresses. The deviations are small enough for the most probable values. It assures the high reliability of the estimated results.

Coaxial TIG‐YAG & MIG‐YAG welding methods

The practical use of laser welding has been promoted as a low strain and highly efficient welding process; in comparison with the CO2 laser, the YAG laser facilitates beam transmission and has a stable beam quality, so its practical use in the following areas has been advanced: welding of nuclear power related apparatus using a 7 kW class system, application to decommissioning cutting techniques using a 10 kW class laser and welding of gas turbine components using a 10 kW class YAG laser. However, the application of these techniques is restricted to certain products with high work accuracy for welds, high groove work accuracy and high butting accuracy. In cases where components have no high product work accuracy it is necessary to prepare the environment at the preceding stage of the welding process so that laser can be applied; this is one of the factors which inhibits the practical use of laser welding. Accordingly, the authors have facilitated the application of lasers to products, to which welding was conventionally carried out by arc welding, by improving the hybridisation of the arc and laser to a practical level with the aim of the expansion of a further application of laser welding capable of achieving high quality and highly efficient welding. In particular, attempts were made, by means of hybridisation, of relaxing target groove tolerance, of achieving higher quality by controlling porosity, of stabilising the MIG arc under a pure Ar atmosphere and also of facilitating high speed welding. In addition, with the aim of applying this technique to three dimensional shaped sections and to develop a compact welding head, an investigation was carried out into coaxial TIG-YAG and MIG-YAG welding systems where the arc and laser beam are arranged to be coaxial. In this report, the characteristics of the systems developed are described and the advantages of such processes are demonstrated.

Application of 7 kW class high power yttrium–aluminum–garnet laser welding to stainless steel tanks

Laser beam welding is used for precise parts such as core internal parts in nuclear power plants that require high quality. To weld large-scale and thick-wall products, the high power laser beam must be transferred and a deep penetration welding procedure must be developed. In this article, therefore, an optical fiber transmission system for 7 kW class high power yttrium–aluminum–garnet (YAG) laser and pulse modulated laser welding techniques were developed to obtain deep penetration. The detailed observation of the weld pool and keyhole dynamics using a high-speed camera and x-ray transmission system was carried out to understand high power YAG laser welding phenomena. It has been clarified that there are proper pulse duty and pulse duration for optimum welding condition to obtain sound and efficient weld. After confirmation of the high power YAG laser welding joint performance, this procedure has been applied to the welding of stainless steel tanks for the nuclear fuel reprocessing plant.

Application of pulse-modulated high-power YAG laser to welding of heavy plates

Lasers operating in air at high energy density are extensively used in a wide variety of industrial fields for thermal processing, such as welding, cutting, etc. The authors have long recognised the superior qualities (low distortion, low heat input) and high efficiency of laser welding and have previously sought to apply a 5 kW CO 2 laser in manufacturing processes in the nuclear power field governed by high precision and rigorous quality standards. For welding of heavy plates targeted by the authors, however, laser powers must be substantially uprated. High power uprating of the CO 2 laser has progressed apace. These advances have, for example, led to application of a 45 kW CO 2 laser in steelmaking. From the perspective of high available power, the conventional CO 2 laser has also seen applications in laser welding of heavy plates. In present circumstances, however, CO 2 laser welding faces some uncertainty in terms of weld quality assurance through the need to incorporate measures for laser-generated plasma control during welding of heavy plates and through measures needed to counteract the changes in beam quality due to the thermal lens effect of transmission-type optical components. Depending on mirror transmission, the CO 2 laser optical system is highly complex, and the beam intensity distribution system in the processing zone is also liable to vary because of the high beam path length. Another drawback of the CO 2 laser is somewhat limited application to heavy industry with its emphasis on numerous large-sized machine components manufactured as one-off items or in very short series. The high-power YAG laser, on the other hand, facilitates optical fibre transmission and is therefore used in the manufacture of large-sized products with complex shapes. This type of laser is also reported to be superior in process terms through being less sensitive to laser-generated plasma and the thermal lens effect of optical components. In recent years, the YAG laser has seen good high power uprating progress, with 3–5 kW class systems being used in the automotive field. While the continuous pulse oscillation type of laser has been focused on heavy plate applications, 7–10 kW class pulse-modulated laser oscillators have been developed for pulse wave oscillation applications with peak powers up to 3-fold of average powers. Through establishment of optical fibre transmission technology and a deep-penetration welding procedure for the high-power YAG laser, it is expected to be possible at a stroke to extend applications of this laser to welding of heavy plates, including areas so far dominated by the conventional CO 2 laser. This article describes application of a ‘world’s biggest’ 7 kW class pulse-modulated YAG laser with a maximum peak power of 21 kW on an actual production line. Through application of optical fibre transmission of optimisation of pulse welding conditions in the high-power range, a technique for deep-penetration welding of heavy plates is established. The keyhole behaviour during welding is examined with a high-speed video camera. An X-ray transmission system is also used to investigate the deeppenetration mechanism and the mechanisms of weld defect generation and prevention during welding of heavy plates. Full-sized application of the newly developed high-power pulse-modulated YAG laser to welding of tanks for a nuclear fuel reprocessing plant is finally reported.

Study on crack generation at root of socket welds

Abstract Because a power generation facility is made up of structures that contain a large number of welds, the reliability of weld joints is important to ensure the safety of power plants. Surveys and research are conducted on special weld joints to verify the reliability of power plants. The results of the investigation to verify the relation between the welding conditions and quality of small-diameter socket joints are described. Some defects are observed in the roots of socket-pipe joints of carbon steel in this project. The authors investigate experimentally the effect of the welding parameters on the generation of defects. The defects of the root are found experimentally to be solidification cracking (hot cracking). It is also revealed that a higher heat input and lower wire feed rate generate more hot cracking at the root of a weld. The authors also give a hypothesis that explains the generation mechanism of hot cracks at the root of a socket-pipe joint, based on finite element modelling analysis and other information.

Effects of a double sided heating method and its controlling parameters: Development of a residual stress control method for butt-welded pipe joints (1st Report)

Abstract This paper describes a new stress improvement method to reduce tensile residual stress on the inner and outer surfaces near butt-welded pipe joints. Its procedure is as follows. An annular region on both sides of the weld line is heated from the outside at the same time to generate a temperature differential between the weld line and region being heated. The region being heated and the welded joint are not actively cooled after completion of heating. That is to say, the proposed method involves the whole circumference on both sides of the weld line simply being left to cool after heating and includes no water cooling or movement of heating position as required by the low-temperature stress relaxation method. A tensile strain is generated on the inner surface near the welded joint to produce plastic deformation by tensile stress when the annular region is heated. The tensile stress after cooling is reduced by this plastic deformation. The two sided heating method can therefore be expected to exert...

Residual stress relief of welded joints in pipework using ice plugs

Summary This paper describes a residual stress relief method for butt welded joints in pipework made using ice plugs. Named by the authors the Twin Ice Plug (TIP) method, it is basically as follows. Two ice plugs are formed inside the pipework and isolate a section including a welded joint. The internal pressure of the isolated section increases as the ice plugs grow, imparting tensile strain to the pipe wall of the section. When the pressure reaches the target level, it is released, and the residual stress decreases. The paper describes details of this process, measurement of the internal pressure outside the pipework, control of the internal pressure, and the effect of residual stress relief. The internal pressure of the isolated section is estimated by the strain measured by strain‐gauges affixed to the outside surface of the section. Section pressure control is exercised by control of refrigerant (liquid nitrogen) supply and pipe wall heating. The maximum internal pressure attained by this method is 4...

Development of 10kW class YAG laser welding technology

Recently laser power is increased rapidly and the laser welding ability is advanced, then we can use it for thick plate welding in heavy industries. In such a background, we aimed to apply YAG laser welding to manufacture our products, and introduced 10kW and 7kW class YAG laser processing system. Then we have developed the optical fiber transmitting technology for high power beam, and welding technology which could make over 20mmt 1pass weld joint. To achieve such welding ability, we optimized welding parameters and controlled keyhole state, penetration shape and welding efficiency. Then we could get the high quality weld efficiently. After we confirmed the mechanical property of weld joint based on the examination standard for power plants, we applied the YAG laser welding to manufacture stainless vessels for reprocessing plants.Recently laser power is increased rapidly and the laser welding ability is advanced, then we can use it for thick plate welding in heavy industries. In such a background, we aimed to apply YAG laser welding to manufacture our products, and introduced 10kW and 7kW class YAG laser processing system. Then we have developed the optical fiber transmitting technology for high power beam, and welding technology which could make over 20mmt 1pass weld joint. To achieve such welding ability, we optimized welding parameters and controlled keyhole state, penetration shape and welding efficiency. Then we could get the high quality weld efficiently. After we confirmed the mechanical property of weld joint based on the examination standard for power plants, we applied the YAG laser welding to manufacture stainless vessels for reprocessing plants.

Conditions for execution of the two sided heating method and its control procedure : Development of a residual stress control method for pipe butt-welded joints (2nd Report)

Summary The previously reported two sided heating method proposed by the authors is an effective residual stress control method for pipe butt-welded joints. This paper describes the non-dimensional parameters controlling this method and the conditions to be fulfilled to ensure sufficient effectiveness in actual applications. The results obtained may be summarised as follows: The previously reported elastic analysis solution is used to investigate the effect of the two sided heating method as well as the effect of the temperature distribution during two sided heating on the stress generated near the weld line. The results show that there are three parameters - the non-dimensional stress K, non-dimensional heating position βl, and non-dimensional heating width βW - that control the residual stress effect of the two sided heating method. The relationship between βl, βW, and the non-dimensional stress K is defined and plotted graphically. By use of the diagrams presented, it is possible to select the combinat...