Discussion on Quality Improvement of Class D Welded Joints in Pressure Vessels

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1: Definition of Class D joints: Joints that connect to the shell, such as nozzles (including manway cylinders), flanges, and reinforcement rings, all fall under Class D welded joints, except for those that have been classified as Class A, Class B, or Class C welded joints. The main types include the following: 1. Connection joints between the take over and the shell; 2. Connection joints between the manhole cylinder and the shell; 3. Connection joints between the flange and the shell; 4. Connection joints between the reinforcement ring and the shell. II: Existing problems: 1. At present, there is no ideal non-destructive testing method that can accurately assess the internal quality of these components, so due attention is not given during the manufacturing process, resulting in some human-induced defects that could have been avoided. 2. Due to limitations in the welding space, certain difficulties arise during the welding process, which can easily lead to internal welding defects. 3. Due to its structural characteristics, the unevenness in the microstructure of the joint, its chemical composition, and the distribution of welding stresses becomes more pronounced. 4. The welding procedure qualification and welder skill training assessments lack sufficient guidance and support for welding Class D weld joints in actual products. 5. During use, due to uneven structural deformation, stress concentrations several times greater than those of the basic film stress occur, allowing defects to propagate easily and leading to failure. Improving the quality of Class D welded joints is crucial for ensuring the overall safety of pressure vessels. III: Improvement measures 1. To ensure the strength of the weld, in design and manufacturing, a welding groove shape that facilitates achieving full penetration should be selected first. The root pass and tack welds of X-shaped grooves are difficult to remove; therefore, for structures where thorough root cleaning is possible, single V or single U-shaped grooves should be preferred. The gap between the grooves should be increased as much as possible, the amount of root edge should be reduced, and the groove angle should be increased appropriately, so that thorough cleaning of the back side of the groove can be achieved at a shallower depth ； For structures where thorough root cleaning is difficult (such as cylinders with a small diameter), the gap between the grooves can be increased appropriately, and a design using metal shims placed on the back side can be adopted. After pressing a gasket 2–3 mm thick tightly against the cylinder and the pipe fittings, the gaps where the gasket comes into contact with the cylinder and pipe fittings are first sealed using a welding technique suitable for small areas, and then the remaining welds are completed. The size of the connection hole in the thin metal gasket allows for an easy machining process to ensure a tight fit with the connection pipe, while also enabling it to deform and conform to the surface of the opening in the curved cylinder. This not only reduces the dimensional accuracy requirements for manually cut grooves but also ensures full penetration in Class D weld joints. At the same time, the increased cross-sectional area of the groove reduces the difficulty of welding, which helps to minimize defects. 2. When selecting the specifications of welding materials, efforts should be made to ensure that the weld is formed through multiple layers and passes. Multi-layer, multi-pass welding with small parameters can effectively reduce the overall heat input to the welded joint, decrease welding stress, result in finer grains, and improve the uniformity of the microstructure and chemical composition distribution. At the same time, by utilizing the tempering effect of each subsequent weld pass on the preceding pass, secondary crystallization is improved, the weld microstructure is further enhanced, and the overall plasticity and toughness of the joint are increased. 3. Based on the specific structural form of Class D welded joints in the products, establish enterprise standards for the welding procedure qualification of such joints as well as for welder skill training, and incorporate them into the enterprise’s quality assurance system documents. This ensures that the welding procedure qualifications and welders’ operational skills provide guidance and support for the actual welding of Class D welded joints. 4. Refine the inspection process and clarify the responsibilities of personnel assigned to carry out these tasks. By strengthening inspection efforts during the manufacturing process, the sense of responsibility among welders and inspectors regarding the internal quality of Class D welded joints can be improved. While providing relevant personnel with enhanced quality training, the product welding inspection procedure sheets specify corresponding stop-check items and points for issues that tend to occur in Class D weld joints, requiring the responsible personnel to sign off upon implementation. For example, set inspection checkpoints for the groove preparation and assembly dimensions before welding, as well as checkpoints for the root pass weld and the quality of root cleaning during welding; at the same time, strengthen the monitoring of the compliance with welding parameters throughout the welding process. 5. Improve the dimensional shape of the weld. If necessary, the higher parts of the weld shall be ground to ensure a smooth transition between the weld and the pipe fittings as well as the cylinder body, thereby reducing the formation of surface cracks.