PURPOSE: To finish the bent tubes of an elbow tube and an U-shaped tube, etc., without generating a crack or an oxidized damage by bending with high frequency the raw tube of a centrifugal casting tube of double layered laminated tube of the inner layer of a high nickel alloy steel and the outer layer of a high strength carbon steel.

CONSTITUTION: The bending is executed in the condition of an inside surface temperature 750-800°C, an outside surface temperature ≤1,000°C, the feeding speed 0.5-1.5mm/sec, and a bending radius (R) ≥3D (D is the outer diameter of the tube). A high nickel alloy steel forming the inside layer of the tube consists of ≤0.05% C, ≤1% Si, ≤1% Mn, 38-46% Ni, 19.5-23.5% Cr, 2.5-3.5% Mo, 1.5-3% Cu, 0.6-1.2% Nb, and the balance of Fe essentially.