(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.