When communicating with customers, customers often ask us that both straight seam steel pipes and spiral steel pipes are welded pipes. Is there any difference between these two steel pipes? Sometimes when we explain the situation of the two steel pipes to the customer at the same time, we often do not have much time to let the customer understand the difference between the two steel pipes in more detail. Next, we will better from the process and physical properties of the two steel pipes Understand the difference between straight seam steel pipe and spiral steel pipe.
One: welding process. In terms of the welding process, the welding method of spiral steel pipe and straight seam steel pipe is the same, but the straight seam steel pipe will inevitably have many T-shaped welds, so the probability of welding defects is also greatly increased, and the welding residue at the T-shaped welding seam The stress is large, and the weld metal is often in a three-way stress state, which increases the possibility of cracks. Spiral steel pipe is welded with a 16mn steel plate. 16Mn is the old national brand. Now it is classified as low-alloy high-strength structural steel. The current brand is called Q345, but Q345 steel is the old brand of 12MnV, 14MnNb, 18Nb, 16MnRE, 16Mn, and other steel grades instead of only 16Mn steel. In chemical composition, 16Mn and Q345 are not the same. More importantly, there is a big difference in the thickness grouping size of the two sheets of steel according to the difference in yield strength, which will inevitably cause changes in the allowable stress of materials with certain thicknesses. Therefore, it is not appropriate to simply apply the allowable stress of 16Mn steel to Q345 steel, but according to the new submerged arc welding process regulations, each weld should have arc initiation and arc extinction, but When a straight seam steel pipe is welded with a circular seam, this condition cannot be achieved, so there may be more welding defects at the arc extinguishing point.
Two: under pressure. The allowable stress is re-determined by the material thickness group size. The proportion of main constituent elements of Q345 steel is basically the same as that of 16Mn steel, the difference is that V, Ti, Nb trace alloying elements are added. A small amount of V, Ti, Nb alloying elements can refine the grains, improve the toughness of the steel, and greatly improve the comprehensive mechanical properties of the steel. Because of this, the thickness of the steel plate can be made larger. Therefore, the comprehensive mechanical properties of Q345 steel should be better than that of 16Mn steel, especially since its low-temperature properties are 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 kind of steel guarantees its mechanical strength when it leaves the factory, and does not require alloy composition. That is to say when the 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 degrees, so the resultant 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 of the same diameter can be reduced compared with that of the straight seam steel pipe.