The Influence of Forming Internal Stress on Yield Strength of Oil and Gas Pipeline

API regulates that the yield strength of flatten specimen from pipe body is used to characterized the yield strength of oil pipeline body. A test on the elasticity restoring stress of UOE pipe and spiral weld pipe is made and the influence of forming internal stress of oil and gas pipeline on yield strength of specimen from pipe body and the yield strength of pipe body is analyzed. The test results show that for the UOE pipe, Bauschinger Effect makes the yield strength of flatten specimen lower than that of the yield strength of pipe body duo to the low elasticity restoring stress of UOE pipe, therefore, this characterization is safer. As to the spiral weld pipe, there is a high elasticity restoring stress in the pipe body. When the body specimen is taken, the elasticity restoring stress is reliving. Although Bauschinger Effect of flatten specimen lowers the yield strength of specimen, the elasticity restoring stress lowers greatly the yield strength of pipe body, so, this characterization trends towards unsafe.