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Geometric variations and surface residual stresses in U-bending processes of an UNS N06690 row-1 heat exchanger tubes

UNS N06690 제1열 시제전열관의 U-굽힘성형에서 형상변화와 표면잔류응력

Kim, Woo-Gon;Jang, Jin-Sung;Kuk, Il-Hiun;Joo, Jin-Won;Kim, Sung-Chung
김우곤;장진성;국일현;주진원;김성청

  • Published : 1998.01.01

Abstract

Surface residual stresses as well as wall thickness and ovality changes after U-bending process on UNS N06690 row-1 heat exchanger tubes, were estimated. Surface residual stresses were measured by Hole Drilling Method(HDM), calculating the stresses from relieved strains of 3 rosette strain gages. After bending of the tubes, dimensional tolerances for wall thickness and ovality were satisfied with ASTM requirements. Residual stresses at the extrados were introduced with compressive stress(-) by bending operations, and its maximum value reached-319 MPa in axial direction at ${\phi}=0^{\circ}$ in position. Tensile residual stresses(+) of ${\sigma}_zz=45$ MPa,${\sigma}_zz=25$ MPa were introduced in the intrados surface at position of ${\phi}=0^{\circ}$ Maximum tensile residual stress of 170 MPa was detected on the flank side at position of ,${\phi}=95^{\circ}$i.e., at apex region. It appeared that higher stress gradients were generated at the irregular transition regions. In the trend of residual stress changes with U-bend position, the extrados is related with the changes of ovality and the intrados is related with the changes of wall thickness.

Keywords

U-Bending Process;Residual Stress;Hole Drilling Method;Relieved Strain;Rosette Strain Gages;Tube Wall Thickness;Ovality