Abstract: Aiming at the inner folding defects of a domestic 34CrMo4 seamless steel tube in the production process, optical microscope (OM), scanning electron microscope (SEM) and energy spectrometer (EDS) were used to carry out systematic inspection and analysis on microstructure and component of the defects. And the cause of formation of inner folding defects in 34CrMo4 seamless steel tube were discussed. The results show that there are a large number of obvious cracks in the inner surface of the steel tube, and Cu enrichment is found in the inner folding area, when the tube blank after hot processing in the production line and the tube blank after heat treatment under simulated production line. During the perforation and biting process of the tube blank in the production line, high Cu-containing inclusions are crushed, squeezed to the grain boundary, and infiltrate along the grain boundary, finally, destroy the continuity of the grain boundary. Due to the axial tensile stress generated by the perforation and the presence of high Cu content inclusions, the matrix discontinuity is destroyed during the axial deformation of the tube blank. Therefore, micro-cracks appear during the continuous extension of the tube blank, and the inner folding defects are formed on the inner wall of the seamless steel tube during the continuous rolling and sizing process.

Key words: 34CrMo4 steel, seamless steel tube, inner folding, element enrichment