The Practice of Laying Medium-Carbon Steel Pipes in Field

According to the experience of laying medium-carbon steel pipeline in field, the problems encountered during substituting low-carbon steel pipe by medium-carbon steel pipe are introduced and the solutions to these problems discussed. The medium-carbon steel pipe (J-55) has a poor weldability due to its chemical compositions: carbon content in between that of 40^# and 45^# steel, sulphur content much less than that in low-carbon steel, manganese content changing within a large range, and a calculated carbon equivalent of over 0.6%. Comparing with low-carbon steel, it has a higher tensile strength and yield strength, lower impact flexibility, and larger rate of elongation, and has the characteristics of more than enough strength but not plasticity and flexibility, and a high risk of br ittle fracture. In order to achieve enough mechanical strength without cracking in the welded pipeline, unequivocal regulations are made for the processing, installation and testing of pipeline and related facilities, and detailed requirements specified for pipe welding procedures. It is necessary to carry out safety eva luation for the pipeline system, so as to achieve reasonable process design and qualified construction and to remove timely any hidden danger occured during pipeline operation. It is proved feasable to lay medium-carbon steel pipe in field for oil transportation provided that reasonable welding procedures be adopted, and strict qualification and safety evaluation followed.