What is the difference between straight seam steel pipe and spiral steel pipe

First: welding process

In terms of the welding process, the welding method of spiral steel pipe and straight seam steel pipe is the same, but straight seam steel pipe will inevitably have a lot of T-shaped welds, so the probability of welding defects is also greatly increased, and the welding residue at the T-shaped welds The stress is large, and the weld metal is often in a three-dimensional stress state, which increases the possibility of cracks. Spiral steel pipe is welded by a 16mn steel plate. 16Mn is the old national standard grade. Now it is classified as low alloy high strength structural steel. The current grade is called Q345, but Q345 steel is the old grade 12MnV, 14MnNb, 18Nb, 16MnRE, 16Mn, and other steel types instead of just replacing 16Mn steel. In terms of chemical composition, 16Mn, and Q345 are also different. More importantly, there is a large difference in the thickness group size of the two plates of steel according to the difference in yield strength, and this will inevitably cause changes in the allowable stress of materials with certain thicknesses. Therefore, it is inappropriate to simply apply the allowable stress of 16Mn steel to Q345 steel, but according to the submerged arc welding process regulations, each weld should have an arc starting point and an arc extinguishing point, but each When the straight seam steel pipe is welded to the circular seam, this condition cannot be achieved, so there may be more welding defects at the arc extinguishing place.

Second: under pressure

The allowable stress is re-determined by the steel thickness group size. The proportion of main constituent elements of Q345 steel is the same as that of 16Mn steel, the difference is that trace alloy elements of V, Ti, and Nb are added. A small amount of V, Ti, and Nb alloying elements can refine the grains, improve the toughness of the steel, and greatly improve the comprehensive mechanical properties of the steel. It is also because of this that the thickness of the steel plate can be made larger. Therefore, the comprehensive mechanical properties of Q345 steel should be better than 16Mn steel, especially since its low-temperature performance is not available in 16Mn steel. The allowable stress of Q345 steel is slightly higher than that of 16Mn steel. The carbon content is less than or equal to 0.2%. This type of steel guarantees its mechanical strength when it leaves the factory, and does not require alloy composition. That is to say when the steel pipe according to the design requirements 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 steel pipe. The helix angle of the spiral steel pipe weld is generally 100°, so the synthetic stress at the spiral weld is the principal stress of the straight seam steel pipe. Under the same working pressure, the wall thickness of the spiral welded pipe with the same pipe diameter can be reduced compared with the straight seam steel pipe. The mechanical strength is selected directly without heat treatment. The meaning of Q345 is that the yield strength of the material can reach 345MPa.