Views: 1 Author: Site Editor Publish Time: 2020-07-21 Origin: Site
The production process of longitudinally welded pipe is relatively simple. The main production processes are high-frequency welded longitudinally welded pipe and submerged arc welded longitudinally welded pipe. The longitudinally welded pipe has high production efficiency, low cost, and rapid development. The strength of spiral welded pipe is generally higher than that of straight seam welded pipe. The main production process is submerged arc welding. Spiral steel pipe can produce welded pipes with different pipe diameters from the same width of blanks, and can also use narrower blanks to produce welded pipes with larger pipe diameters. However, compared with the straight seam pipe of the same length, the weld length is increased by 30-100%, and the production speed is lower. Therefore, most of the smaller diameter welded pipes adopt straight seam welding, and the large diameter welded pipes mostly adopt spiral welding. T-welding technology is used in the production of larger diameter longitudinal seam steel pipes in the industry, which means that short sections of longitudinal seam steel pipes are butt-joined to a length that meets the needs of the project. The probability of defects in T-welded longitudinal seam steel pipes is greatly increased, and T-welding The welding residual stress at the seam is relatively large, and the weld metal is often in a three-dimensional stress state, which increases the possibility of cracks.
In terms of welding process, the welding method of spiral welded pipe is the same as that of straight seam steel pipe, but the straight seam welded pipe will inevitably have many T-shaped welds, so the probability of welding defects is greatly increased, and the welding residue at the T-shaped weld The stress is large, and the weld metal is often in a three-dimensional stress state, which increases the possibility of cracks.
Moreover, according to the submerged arc welding process regulations, each weld should have an arc ignition point and an arc extinction point, but each longitudinal welded pipe cannot meet this condition when welding the circumferential seam, so there may be an arc extinction point. More welding defects.
When the pipe is subjected to internal pressure, two main stresses are usually generated on the pipe wall, namely radial stress δ and axial stress δ. The resultant stress δ at the weld, where α is the helix angle of the weld of the spiral welded pipe.
The helix angle of the spiral welded pipe weld is generally degrees, so the resultant stress at the spiral weld is the principal stress of the longitudinal welded pipe. Under the same working pressure, the spiral welded pipe of the same pipe diameter can be reduced in wall thickness than the straight seam welded pipe.
According to the above characteristics:
When the spiral welded pipe is blasted, because the normal stress and the composite stress on the weld are relatively small, the blasting opening generally does not originate at the spiral weld, and its safety is higher than that of the straight welded pipe.
When there are defects parallel to the spiral weld near the spiral weld, because the spiral weld is less stressed, the risk of its expansion is not as great as the straight weld.
Since the radial stress is the maximum stress that exists on the steel pipe, the weld is subjected to the maximum load when it is in the direction of the vertical stress. That is, the load borne by the straight seam is the largest, the load borne by the circumferential weld is the smallest, and the spiral seam is between the two.
Static pressure blasting strength
Related comparative tests have verified that the yield pressure of spiral welded pipe and longitudinally welded pipe is basically consistent with the actual and theoretical values of burst pressure, and the deviation is close. But whether it is yield pressure or burst pressure, spiral welded pipe is lower than that of straight seam welded pipe. The blasting test also showed that the circumferential deformation rate of the spiral welded pipe blasting port was significantly greater than that of the straight welded pipe. This proves that the plastic deformation ability of spiral welded pipe is better than that of straight seam welded pipe, and the blast opening is generally limited to one pitch, which is caused by the strong restraint effect of spiral weld on the expansion of the crack.
Toughness and fatigue strength
The trend of pipeline development is large diameter and high strength. As the diameter of the steel pipe increases and the grade of steel used increases, the tendency of the ductile fracture tip to grow steadily becomes greater. According to the test conducted by relevant research institutions in the United States, although spiral welded pipe and longitudinal welded pipe are of the same level, spiral welded pipe has higher impact toughness.
Due to changes in the transportation volume, the steel pipes are subjected to random alternating loads during actual operation. Understanding the low cycle fatigue strength of steel pipes is of great significance for judging the service life of pipelines.
According to the measurement results, the fatigue strength of spiral welded pipes is the same as that of seamless pipes and resistance welded pipes, and the test data are distributed in the same area as seamless pipes and resistance pipes, and are higher than general submerged arc longitudinal welded pipes.