(1) Defects on the surface;
(2) There are inclusions at the centre;
(3) There is martensite abnormal structure at the centre;
(4) The pulling stress is over the tensile strength of the materials;
(5) Insufficient lubrication;
(6) Poor welding;
(7) There are some problems in terms of pre-treatment technology such as the pickling time is too long or the concentration and temperature of acidizing fluid are too high.