A low temperature reheater specifications is φ50.8 ×4.0 mm and material trademark is SA213-T91.Continuous fracture of steel tube occurs when the cold bending plastic deformation exceeds 90°. Through the chemical composition analysis,mechanical property test,metallographic exa-mination,microhardness and fracture morphology analysis of steel tube,the bending fracture causes are determined. The high carbon content on the internal surface of SA213-T91 cases the high hardness and a sharp decline in plasticity of the internal surface of steel tube,resulting in steel pipe bending angle cracks by brittle cleavage when cold bending. Based on the analysis of manufacturing technology,casting segregation exists during the manufacturing of die casting billet,and the head is not cut clearly,resulting in the heart carburization in the tube blank and the carburization phenomenon on the internal surface of steel tube.