PURPOSE: To reduce thinning that occurs in back side and tensile residual stress that occurs on outer surface and obtain a bent tube excellent in corrosion resistance and fatigue resistance by forming compressive residual stress on inner and outer surfaces of bent part of a tube.
CONSTITUTION: The tube is heated in longitudinal narrow area, and bending moment is given to the heated area by external force to shift the heated area and the position of action of bending moment. The part just behind the heated area in the direction of shifting is cooled, and compressive force to give compressive residual stress to whole section of the heated area is given to the tube, and bending of the tube is performed. After bending the tube, outer surface of bent part of the tube is heated to high temperature that generates stress exceeding yielding point, and then cooled rapidly. Thus, only compressive residual stress becomes in existence on the outer surface to eliminate tensile residual stress remaining on the outer surface by compressive force.
KUNIYA JIROU
MATSUSHITA SHIZUO
MASAOKA ISAO
YONEMURA HIDEO
