Abstract: In a certain domestic company, a batch of 30CrMnMo steel pipes as outside-supplied blanks and having undergone quenching& tempering treatment are tested with the ultrasonic detection process, As a result most of the pipes tested are bearing heat treatment-related defects.Test samples are taken from some defective pipe bodies, and from bodies of some blank pipes as raw materials.The defective samples are analyzed for their chemical composition, metallographic structure, and macro and micro morphologies by means of direct-reading spectrometer, metalloscope, and scanning electron microscope, etc.The results reveal that original defects like folding, uneven inner surface, and P segregation stay with the batch of outside-supplied blank pipes.And when the pipes are quenched, it is these original defects that cause stress concentration which leads to development of lots of quenching cracks, and in turn cracking of the pipe body.Accordingly, it is suggested for the pipe-supplying manufacturer to take such corrective actions as improving the rolling process for eliminating defects like rolled-folding and pit, etc., and further improving the metallurgical quality of the steel, and strictly controlling the contents of hazardous chemical elements like P, and so on.