The comparison of safety between LSAW pipe and SSAW pipe

The residual stress of LSAW pipe is mainly caused by uneven cooling. Residual stress is the internal self phase equilibrium stress without external force. This residual stress exists in hot rolled sections of various sections. The larger the section size of general section steel, the greater the residual stress.

Although the residual stress is self balanced, it still has a certain impact on the performance of steel members under external force. For example, it may have adverse effects on deformation, stability and fatigue resistance. After welding, the non-metallic inclusions in the LSAW pipe are pressed into thin sheets, resulting in lamination. Then the lamination greatly deteriorates the tensile performance of LSAW pipe along the thickness direction, and interlayer tear may occur when the weld shrinks. The local strain induced by weld shrinkage is often several times of the yield point strain, which is much larger than that caused by load. In addition, LSAW pipe will inevitably have a lot of T-welds, so the probability of welding defects is greatly improved. Moreover, the welding residual stress at the T-weld is large, and the weld metal is often in the state of three-dimensional stress, which increases the possibility of cracks.

The welding seam of spiral submerged arc welded pipe are distributed in a spiral line, and the welds are long. Especially when welding under dynamic conditions, the weld leaves the forming point before cooling, which is easy to produce welding hot cracks. The crack direction is parallel to the weld and forms an included angle with the steel pipe axis, generally speaking, the angle is between 30-70 °. This angle is just consistent with the shear failure angle, so its bending, tensile, compressive and anti-twist properties are not as good as LSAW pipe. At the same time, due to the limitation of welding position, the saddle and fish ridge welding seam affect the appearance. Therefore, the NDT of SSAW pipe welds should be strengthened to ensure the welding quality, otherwise SSAW pipe should not be used in important steel structure occasions.