Characteristics of straight seam and spiral seam welding of large diameter steel pipe

　　Straight seam welding of large diameter steel pipes is produced from steel plates, while spiral welding of large diameter steel pipes is produced by hot rolled coils. The rolling process of the hot strip mill has a series of advantages, and has the metallurgical process capability for producing high quality pipeline steel. For example, a water cooling system is installed on the output gantry to accelerate cooling, which allows the use of low alloying compositions to achieve special strength levels and low temperature toughness, thereby improving the weldability of the steel. However, this system is basically not available in steel plate production plants.

　　The alloy content (carbon equivalent) of the coiled sheet is often lower than that of similar grades, which also improves the weldability of the spiral weld of large diameter steel pipes. Since the rolling direction of the spiral welded coil is not the direction of the vertical steel pipe axis (the pinching depends on the helix angle of the steel pipe), and the rolling direction of the straight seam welded steel plate is perpendicular to the axial direction of the steel pipe, the crack resistance of the spiral welded material Better than straight seam welding.

　　The magnetic flaw detection of straight seam welding can only find the defects of the surface and near surface of the magnetic Q235B straight seam submerged arc welded pipe, and only the quantitative analysis can be done for the defect. The nature and depth of the defect can only be estimated based on experience. The magnetic test is to find the defects by magnetic flux magnetization of the ferromagnetic Q235B straight seam welded pipe. According to the method of measuring magnetic flux leakage, it can be divided into magnetic powder method, magnetic induction method and magnetic recording method, among which magnetic powder method is the most widely used. The penetration test of spiral welding is to discover and display defects by using physical properties such as permeability of certain liquids, including coloring test and fluorescent flaw detection, which can be used to check defects on the surface of ferromagnetic and non-ferromagnetic materials.