The reason cited in GB151 for not using local annealing to reduce the hardness of heat exchange tubes in the presence of stress corrosion
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Why is there such a provision mentioned in 5.8.2.2, General requirements for the connection of heat exchange tubes to tube sheets, on page 68 of GB151?
An annealing process after tube expansion releases the expansion stress. A gap appears between the pipe and its wall. There is another interpretation I seem to have read about in a book: if local annealing is performed on the tube end, the microstructure of the entire tube changes. An internal galvanic cell will form.
Reply to 2# penryn: If tube-end annealing is used, it should be done before the tube is inserted. Eliminating welding residual stress at the pipe end should be carried out after welding the pipe end to the tube sheet.