Quality issues of welded joints of stainless steel pipes and plates

Leakage in the welded joints of stainless steel tubes and plates during use is the main reason why stainless steel heat exchangers cannot work properly. The technical rules in the industry have strict regulations on parameter changes and quality evaluation of stainless steel pipe-tube sheet welded joints. In order to ensure that product quality meets the requirements, appropriate welding processes and parameters must be determined through experimental methods.
The auxiliary feed water deaerator is a heat exchanger product used in some domestic nuclear power plants. It is produced in a single piece and is used to replace the original auxiliary feed water deoxygenation equipment. This kind of heat exchanger uses a square tube plate with a large number of heat exchange tubes. The heat exchange tubes and stainless steel tube plates are both austenitic stainless steel, and the wall thickness of the heat exchange stainless steel tubes is very thin. The welding of thin-walled stainless steel tube heat exchange tubes and tube sheets is relatively difficult.
In power station auxiliary machines and chemical containers, the wall thickness of the heat exchange tubes of the heat exchanger is generally thin. The tube end can extend or retract relative to the stainless steel tube plate, or it can be flush with the tube plate, and the wall thickness is small. The heat exchange tubes can only use joint types that are flush with the tube plate or extend from the tube end.
The characteristics of thin-walled stainless steel pipes, especially tube-sheet welding, are obvious. The stainless steel pipe and tube-sheet welded joints require uniform weld formation, good root fusion, and the thickness of the weld must meet the design requirements. The shrinkage of the pipe hole after welding is usually not greater than Pipe hole inner diameter. For the welded joints of heat exchanger tubes and tube plates of thin-walled stainless steel tubes, the size of the molten pool can be small, otherwise the shrinkage of the tube holes will exceed the standard, affecting the further tube expansion process and the heat exchange efficiency during operation.
Therefore, the welding heat input and molten pool shape must be strictly controlled during welding, so that a complete molten pool is generated at the welding joint without excessive shrinkage of the nozzle. For joints between stainless steel pipes and tube sheets that are below the thickness of the pipe wall, the pipe ends are prone to flanging inward during the welding process, and the stainless steel pipes and tube sheets cannot be welded together. The main reason is that there is always a certain gap between the stainless steel pipe and the tube plate after assembly. The tube plate is thick and the tube is thin. During welding, the edge of the tube plate hole has not yet melted but the tube has melted, causing the stainless steel tube to flang inward and unable to generate welding melt. pool. Because austenitic stainless steel has a higher thermal conductivity, it is easier to form inward flanging of the pipe end. In order to adapt to the welding of stainless steel thin-walled pipes and tube sheets, the pulse welding process must be used. The welding parameters are appropriately adjusted through experiments, and special tooling and positioning methods are used to ensure the inherent quality of the weld and better weld formation.