First: welding process
In terms of the welding process, the welding methods of spiral steel pipes and straight seam steel pipes are the same, but straight seam steel pipes will inevitably have many T-shaped welds, so the probability of welding defects is also greatly increased, and the welding residues at the T-shaped welds are The stress is large, and the weld metal is often in a three-dimensional stress state, increasing the possibility of cracks.
Spiral steel pipes are welded from 16mn steel plates. 16Mn is an 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, and 18Nb. , 16MnRE, 16Mn, and other steel types, instead of just replacing 16Mn steel. In terms of chemical composition, 16Mn and Q345 are also different. What’s more important is that there are large differences in the thickness grouping sizes of the two steel materials according to their different yield strengths, which 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. Instead, according to the new steel pipe submerged arc welding process regulations, each weld should have an arc starting point and an arc extinguishing point. However, Each straight seam steel pipe cannot meet this condition when welding the circumferential seam, so there may be more welding defects at the arc extinguishing point.
Second: Under pressure
The allowable stress is redetermined based on the material thickness group size. The proportion of the main constituent elements of Q345 steel is the same as that of 16Mn steel. The difference is that trace alloying elements of V, Ti, and Nb are added. A small amount of V, Ti, and Nb alloy elements can refine the grains, improve the toughness of the steel, and greatly improve the comprehensive mechanical properties of the steel. It is precisely 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 its low-temperature properties, which 16Mn steel does not have. 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 is guaranteed to have mechanical strength when it leaves the factory without any requirements on the 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 is, where α is the helix angle of the spiral steel pipe weld. 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 spiral welded pipes with the same diameter can be reduced compared with straight seam steel pipes.
Mechanical strength is selected directly without heat treatment. Q345 means that the yield strength of this material can reach 345MPa.
Post time: Sep-28-2023