Nickel-Based Alloy Surfacing Welding for Tube Sheet of Steam Generator in Nuclear Power Plant

Quality Control of Nickel-Based Alloy Surfacing Welding for Tube Sheet of Steam Generator in Nuclear Power Plant

(Tech Center of Shanghai HY Industry Co., Ltd)

1 Preface

Nickel-Based Alloy Surfacing Welding for Tube Sheet of Steam Generator in Nuclear Power PlantSteam generator is the core equipment of a pressurized water reactor nuclear power plant, and it is also one of the equipment that has the most failures during the operation of the nuclear power plant. Most of the failures are caused by various corrosion. The tube sheet is the most critical component of the steam generator, and it plays a role in isolating the primary and secondary circuits of the nuclear island. Because one side of the tube sheet (hereinafter referred to as the “primary side”) has long-term exposure to radioactive and corrosive high-temperature coolants, during the manufacturing process of the tube sheet, Nickel-Based Alloy Surfacing Welding must be performed on the primary side to meet its resistance. The need for high temperature and corrosion resistance.

Ni-based alloy surfacing of tube sheet is a key process in the manufacturing process of steam generators. The quality of surfacing directly affects the manufacturing progress of equipment, product quality and the difficulty of subsequent processes. This equipment adopts electrode electroslag welding for surfacing nickel-based alloys. This welding method is much more difficult than ordinary welding, and it is mainly reflected in the following aspects:

  1. Nickel-based alloy electroslag welding has large welding current and high heat output. In the nickel-based alloy surfacing welding of tube sheet, due to the large difference in properties of the base material and the surfacing layer material, it is easy to cause coarse grains in the heat-affected zone of the base material.

  2. Nickel-based alloy ribbons (width 60mm) are used during the primary side nickel-based surfacing welding of the tube sheet, so that the electromagnetic force has a greater influence on the shape of the molten pool during the welding process, and it is easy to cause the liquid metal in the molten pool to shrink and undercut Defects affect the quality of welding.

  3. Primary side surfacing area of the tube sheet is large, and each round of nickel-based alloy weld bead constitutes a closed ring, which is not conducive to the release of thermal stress of the material during the welding process, and it is easy to cause the deformation of the base material.

In this paper, through the analysis of the difficulties in the nickel-based alloy electroslag surfacing welding of the reactor steam generator tube sheet, the relevant measures such as pre-welding preparation, welding process control and post-welding inspection are put forward. Through the control of various factors affecting the welding quality, it is ensured that The quality of product surfacing.

2 Welding quality control

  • In order to ensure the quality of the nickel-based alloy surfacing welding on the primary side of the steam generator tube sheet, the entire welding process should be effectively controlled to avoid unnecessary defects in the final product, resulting in quality degradation.

2.1 Preparation before welding

Preparatory work should be done before the tube sheet surfacing, which mainly includes: welding equipment, welders and non-destructive testing personnel qualifications, welding procedure qualification and other inspections.

  1. Welding equipment and tools. Before surfacing welding, firstly, it is necessary to debug the electric heaters, rotors, and welding machines used in welding to ensure that the equipment is in good condition and within the validity period of the verification; secondly, the welder should prepare special wood for walking on the nickel-based surfacing layer. Shoes, welding insulation carpets, vacuum cleaners and other auxiliary equipment; finally, prepare the aluminum-based iron-free grinding wheels, portable thermometers, stopwatches, steel rulers and thickness gauges that need to be used in the welding process.

  2. Welding documents and personnel qualifications. Before surfacing, the completion documents and process transfer cards of the tube sheet should be reviewed as required to ensure that the actual quality of the components meets the requirements of the upstream design documents. Similarly, the qualifications of welders and non-destructive personnel must also be reviewed before welding. The surfacing of the steam generator tube sheet should be welded by welders with relevant welding qualifications. The qualifications of the welders must meet the HAF603 “Civil Nuclear Safety Equipment Welding Personnel Qualification Management Regulations” “The qualifications of non-destructive testing personnel must also comply with the relevant provisions of HAF602 “Regulations on the Management of Civil Nuclear Power Safety Equipment Non-destructive Testing Personnel Qualifications”.

  3.  Welding procedure qualification. It is one of the key factors to ensure the welding quality to implement the surfacing welding of the tube sheet according to the effective process qualification. Before the surfacing welding of the steam generator tube sheet, the corresponding welding procedure qualification must be carried out, and the welding procedure specification of the product shall be formulated according to the qualified and effective welding procedure qualification report. The actual welding product process specification should comply with the corresponding welding process qualification report, especially the allowable variation range of the parameters.

2.2 Welding process control

  • During the surfacing welding process of the steam generator tube sheet, suitable welding consumables (to meet RCC-M standards), excellent welding parameters and strict testing methods should be selected to better control the welding quality of the product.

  1. Selection of welding consumables The welding consumables used for the primary surface electroslag welding of the tube sheet are E Q N i C r-3 nickel-based alloy ribbon (0.5mm×60mm), the brand is SANICRO72HP. Electrode arc welding adopts ϕ4.0mm, ENi Cr Fe-3 nickel-based alloy electrode, brand name SANICRO71Q. Before surfacing, all welding materials need to be subjected to chemical composition analysis and mechanical performance test of deposited metal. In addition to the quality of welding consumables meeting the requirements of the design documents, attention should also be paid to the storage conditions of the welding consumables, the procedures for the requisition of the welding consumables, the baking of the welding consumables, and the requirements for heat preservation. Ensure that each batch of welding consumables in use is qualified.

  2. Welding process The material of the tube sheet used in the steam generator of the CPR1000 reactor is 18MND5, the thickness is 557mm, the diameter is 3485mm, and the surface needs to be deposited with a nickel-based alloy surfacing layer (thickness ≥8mm). The welding is dissimilar metal welding. The surface of the tube sheet should be inspected before surfacing to ensure that there is no water, oxide particles, oil stains and other impurities that affect the welding quality, and the area to be surfacing should be subjected to magnetic particle testing.

  • Before surfacing on the primary side of the tube sheet, the base material of the tube sheet is preheated as a whole (preheating temperature ≥160℃). The preheating can effectively control the cooling rate of the weld and reduce the hardening of the weld and the heat-affected zone. To improve the crack resistance of the welded joint; at the same time, it also reduces the temperature gradient of the welded workpiece in the surfacing area, reduces the welding stress and strain rate, and helps avoid welding cracks. Secondly, during the entire welding process, it is necessary to ensure that the temperature of the tube sheet surface to be surfacing exceeds the preheating minimum temperature (160°C), and the operator needs to monitor the temperature of the tube sheet surface to be surfacing every 30 minutes during the welding process.

  • In the manufacturing process, the tube sheet surfacing welding adopts the method of combining nickel-based welding tape with electrode electroslag surfacing welding and electrode arc welding. , The primary side surfacing area of ​​the steam generator tube sheet is divided into the center manual surfacing area, the edge R zone manual surfacing area, and the electroslag welding belt pole surfacing area. In order to meet the technical conditions in the RCC-M standard, that is, the thickness of the surfacing layer must be more than 8mm, the nickel-based alloy surfacing on the surface of the tube sheet is generally divided into three layers, and each layer of surfacing weld bead is back to the previous layer. The fire effect releases the residual internal stress of the upper layer of surfacing layer, alleviates the tendency of joint cracks, and promotes the grain refinement of the heat-affected zone of the base material of electroslag surfacing. In the surfacing process, the welding direction of the electroslag surfacing of each layer of nickel-based ribbon is opposite to that of the previous layer, which is used to offset the deformation of the tube sheet after the surfacing of each layer.

  • Each layer on the primary side of the tube sheet is welded with electrode electroslag, and the weld bead is a circular weld bead. Each ring-shaped surfacing layer is welded and formed at one time, and the width is about 65mm. The surface of the surfacing layer is well formed, and the overlap between each ring must be 5-9mm. Point-like pits and slight undercuts appear in the overlap area of ​​the weld bead, and manual polishing is required to remove these defects after each welding circle. When the electroslag welding machine is working, a magnetic control device is installed on the machine head. During the welding process, the operator needs to adjust the size of the magnetic field according to the weld bead shape, so as to suppress the adverse effect of electromagnetic force during the welding process and improve the weld shape.

  • After electrode electroslag surfacing, the central area and the edge R area are welded by electrode arc welding. During the welding process, the completed nickel-based surfacing surface must be protected to prevent spatter during manual welding and arc damage to the base material. When starting the arc, the arc cannot be ignited at the previous end of the arc, and the weld bead joints need to be inspected, and lightly polished and polished as needed to ensure a smooth transition in the overlap area. During manual surfacing of the R zone, the arc surface should be measured regularly to ensure that it meets the drawing requirements. After the manual welding is completed, check the thickness and shape of the weld seam, and pay attention to welding a certain amount of thickness to have enough grinding allowance.

  • After each layer of surfacing is completed, post heat treatment should be carried out. Its function is to eliminate the thermal stress generated by welding, uniform the structure of the weld and heat affected zone, refine the grain of the weld and heat affected zone, and make the welding material and the base metal Better fusion. During the post-heating process, the operator should monitor and measure the post-heat temperature. In order to better control the welding quality, stress relief heat treatment (IRS) should be carried out after the tube sheet surfacing is completed, the holding temperature is 595-620℃, and the holding time is more than 1h.

2.3 Welding inspection

  1. Chemical analysis After the nickel-based surfacing of the tube sheet is completed, 0.5mm should be ground off the surface of the tube sheet surfacing layer, and then a sample within a depth of 2mm should be sampled for chemical composition analysis.

  2. Visual inspection and size inspection After the completion of tube sheet surfacing welding, firstly, visual inspection is to check whether the surface of the surfacing layer meets the requirements of the drawing regulations and the subsequent non-destructive testing requirements. At the same time, visual inspection also includes surface roughness and weld smoothness, and there must be no defects such as surface pores, slag inclusions, cracks, lack of fusion, undercuts, weld bumps, and recesses. The slag and surface spatter around the weld must be removed. . Secondly, the size inspection mainly measures the size of the tube sheet and the size of the arc surface of the manual R zone to ensure that the size meets the requirements of the drawing.

  3. After the ultrasonic inspection and ultrasonic thickness measurement are completed, ultrasonic inspection is required to detect whether the surfacing layer has defects such as volume, continuous unfused, and underlayer cracks. At the same time, the ultrasonic detector is used to measure the thickness of the tube sheet surfacing layer, and the surfacing thickness meets the design drawing requirements (≥8mm).

  4. Liquid penetration detection After the surface of the tube sheet surfacing layer is machined, PT inspection is required at the end to show whether there is a size> 1.5mm on the surface of the surfacing layer, and a display with a size <1.5mm but the spacing <3mm, All should be treated as unqualified, and repaired by hand polishing to meet the follow-up drilling requirements.

3 concluding remarks

  • Nickel-based alloy surfacing welding on the primary side of the steam generator tube sheet is a large-area dissimilar metal electroslag welding, which is difficult to weld, and the welding performance of the nickel-based alloy material is poor, which is one of the key and difficult points in the manufacturing process of the steam generator. In order to obtain a high-quality nickel-based alloy surfacing layer, the optimization of welding parameters, the rationalization of the welding bead layout, the guarantee of the cleanliness of the welding station, the good condition of the welding equipment, and the professional skills and sense of responsibility of the welding workers are indispensable of.

  • Based on the process characteristics of nickel-based surfacing on the tube sheet surface, this paper analyzes the important factors affecting welding quality based on years of manufacturing experience of Shanghai HY Industry Co., Ltd, and proposes measures to control the welding quality, which effectively guarantees the welding quality of the product. It reduces the probability of defects and has a certain guiding effect on improving the welding quality of nuclear island main equipment products.

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