II. Key Points of Welding Technology

Selection of welding materials: Choose welding wires that match the chemical composition and mechanical properties of the base material, strictly control the impurity content, and ensure that the performance of the welded joint meets the requirements of nuclear-grade equipment.

Determination of welding process parameters: Determine appropriate parameters through process qualification tests. At the same time, adjust the parameters according to the pipe diameter and wall thickness to ensure good weld fusion.

Design and processing of bevels: Use appropriate bevel forms to ensure root penetration and weld formation. Commonly used are single-sided V or single-sided U bevels. The processing accuracy of the bevels is high, and the surface roughness is low to avoid impurity residues.

Control of welding environment: Welding should be carried out in a clean and dry environment. Before welding, the surfaces of the heat exchange tubes and tube sheets should be cleaned.

III. operation process and quality control Before welding preparation: strictly inspect the pipe and tube plate, clean the oil, rust and other impurities in the groove and nearby area. When the group is matched, the gap is uniform, and the extension or shrinkage of the tube head meets the requirements of the drawings and welding process. Bottom welding of the first layer: bottom welding is the key, the use of small current, short arc operation to ensure that the root of the groove is completely fused to ensure the penetration requirements. In order to obtain good penetration requirements, the method of increasing welding current is often used, but the structure is small in size, the weld is annular, the welding Angle of each point changes very fast, and the groove is unilateral structure. The welding operation with large current often causes excessive heat input, and the cover does not completely melt the root area of the groove when the liquid metal flows, resulting in local poor fusion and substandard penetration. Reduce the effective bearing thickness of the weld. Therefore, in the welding process of the first layer, a small standard current welding operation should be used to keep the distance between the tungsten pole and the molten pool constant, adjust the Angle of the welding torch according to the position of each point, so that the tungsten extreme head always points to the root of the groove, the welding speed is uniform, and the welding torch does a small horizontal swing to ensure that the welding arc is swept to all positions of the root of the groove and good fusion. The entire operation process can be self-melting or a small amount of welding wire. Prevent the formation of narrow and deep weld forms.

Filler welding: After the bottom welding is completed, clean the weld surface and the argon-coated groove with a wire brush for filler welding. The welding current increases relative to the first layer welding. The filling layer welding should pay attention to interlayer fusion, follow the principle of melting and filling first, and cannot be over-filled because of improving production efficiency, resulting in poor fusion or even non-melting between layers. Ensure the thickness of the weld, reserve the appropriate depth for the cover welding, (about 0.5 mm) to ensure the surface of the weld is smooth and the width is uniform and beautiful. Cover welding: cover layer as a direct contact surface with the equipment operating medium, good weld corrosion resistance is the main direction of welding operation should be controlled, smooth weld surface is conducive to reducing the media in the operation of the equipment for a long time on the weld surface, and increase the corrosion time. At the same time, it should have a good surface forming to meet the subsequent detection requirements.

During operation, the interlayer temperature and welding heat input should be strictly controlled. In order to obtain a smooth surface, wide and narrow weld. In the welding process, it can be segmented welding and only welding the easiest operation area, which can effectively ensure the welding quality of tube plate joint, reduce the difficulty of operation, and improve production efficiency.

Quality inspection: After welding, the appearance inspection shall be carried out, and the weld surface shall not have cracks, pores, slag inclusion and other defects, and the residual height and width shall meet the standard. Use non-destructive testing methods, such as surface PT, weld RT, etc., to ensure weld quality. For unqualified welds, analyze the reasons, make repair plans, and repair strictly according to the requirements.

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