Heat exchange tube treatment method: U-shaped tube heat treatment

^{One. GB/T151-2014 stipulates:}

^{When there is a requirement for stress corrosion resistance or residual stress elimination, the carbon steel and low alloy steel U-shaped heat exchange tube bend section and at least 150 mm straight tube section should be heat treated. The heat treatment of other materials U-shaped heat exchange tube bend section shall be negotiated by the supply and demand parties.}

^{Two. NB/T47019.1-2021 stipulates:}

^{When the contract stipulates, after the U-shaped heat exchange tube is bent, the U-shaped bend section and at least 150 mm straight tube section shall be heat treated, and the heat treatment specifications shall be specified in the design documents or order contract.}

^{Three. Regulations from some technical data:}

^{1. Carbon steel and low alloy steel cold-bent U-shaped tube:}

^{a) If the medium has a tendency to stress corrosion, the U-shaped tube bend section and at least 150 mm straight tube section shall be subjected to stress elimination heat treatment.}

^{b) The cold forming deformation rate (100r/R% - where r is the outer radius of the steel pipe and R is the bending radius of the center line of the steel pipe) is greater than 50% of the lower limit of the elongation after fracture (As) specified in the steel pipe standard, and the steel pipe with a residual elongation after fracture (As) less than 10% after forming should be annealed to restore the mechanical properties.}

^{c) If there is an impact test requirement, the steel pipe with a deformation rate greater than 5% should be annealed to restore the mechanical properties.}

^{2. Austenitic stainless steel cold-bent U-shaped tubes are generally not heat treated. If used in stress corrosion media or the bending radius is less than 5d (steel pipe outer diameter), solid solution treatment should be performed.}

^{3. Ferrite/austenite duplex stainless steel cold-bent U-shaped tubes should be solid solution treated. G is used for low-temperature heat exchangers. If the bending radius is less than 10d (steel pipe outer diameter) when cold bending is used, stress relief heat treatment should be performed.}

^{4. Non-ferrous metal tubes are generally not subjected to stress relief heat treatment. If necessary, stress relief heat treatment can be carried out according to the methods and requirements agreed upon by the supply and demand parties.}

^{Four. HG/T20584-2020 stipulates:}

^{After cold bending of steel pipes, if the deformation rate (the deformation rate can be calculated by the following method) exceeds the following range, heat treatment should be carried out after forming:}

^{1) The deformation rate of the outer fiber of carbon steel and low alloy steel pipes after bending should not be greater than 1/2 of the elongation specified in the steel pipe standard, or the residual elongation of the outer material should be less than 10%;}

^{2) The deformation rate of the outer fiber of stainless steel pipes after bending should not be greater than 1/2 of the elongation specified in the steel pipe standard, or the residual elongation of the outer material should be less than 15%;}

^{3) For steel pipes with impact toughness requirements, the maximum deformation rate of the outer fiber should be less than 5%.}

^{The cold bending fiber deformation rate of steel pipes can be calculated according to formula (F.1), formula (F.2), and formula (F.3):}

^{ε=max(ε₁，ε₂)............................(F.1)}

^{ε₁=100(S-S´)/S%............................(F.2)}

^{ε₂=100r/R%..................................(F.3)}

^{ε——deformation rate (percentage);}

^{S——nominal thickness of steel pipe, mm;}

^{S´——minimum thickness of outer wall of steel pipe after cold bending, mm;}

^{r——radius of steel pipe, mm;}

^{R——radius of bending pipe, mm.}

^{Five:ε——deformation rate (percentage);}

^{S——nominal thickness of steel pipe, mm;}

^{S´——minimum thickness of outer wall of steel pipe after cold bending, mm;}

^{r——radius of steel pipe, mm;}

^{R——radius of bending pipe, mm.}