First, the cooling form of large-diameter steel pipe:
Large-diameter steel pipe is a steel pipe with an outer diameter of more than 1000MM. Large-diameter seamless steel pipe is made of steel ingot or solid tube billet by perforation into the rough tube, and then hot-rolled, cold-rolled, or cold-drawn.
1. Chain cooling bed. In the past, chain cooling beds with simple structures were mostly used, which had simple structure and low cost. However, it is rarely used now because it is easy to cause chain dislocation, which causes large-diameter steel pipe to bend, and large-diameter steel pipe cannot be collected freely from the input roller to the cooling bed entrance.
2. Stepping cooling bed. This cooling bed is composed of a stepping beam and a fixed beam. The large-diameter steel pipe to be cooled is supported by the stepping beam, moved forward a distance, and then placed in the tooth groove of the fixed beam. By properly adjusting the stroke of the rack, the large-diameter steel pipe can roll twice for each step, to achieve the effect of straightening the large-diameter steel pipe. At present, almost all newly built tube rolling mills use step-by-step rack cooling beds.
3. Spiral cooling bed. This cooling bed relies on the spiral line on the spiral rod to push the large-diameter steel pipe on the cooling bed forward for cooling. As the spiral rod rotates, the large-diameter steel pipe is subjected to a lateral thrust in addition to the forward thrust. Therefore, it moves sideways while moving forward, and only moves forward.
When the cooling speed is faster, the carbon content and alloy composition are higher, the uneven plastic deformation produced under the action of thermal stress during the cooling process is greater, and the residual stress formed later is greater. On the other hand, when the steel changes in structure during the heat treatment process, that is, when austenite transforms to martensite, the increase in specific volume will be accompanied by the expansion of the workpiece volume, and the various parts of the workpiece will undergo phase changes one after another, resulting in inconsistent volume growth and tissue stress. The final result of the change in tissue stress is tensile stress on the surface and compressive stress on the core, which is exactly the opposite of thermal stress.

Second, construction preparation before welding of large-diameter steel pipes:
1. Organize relevant operators, familiarize themselves with construction drawings and regulations, formulate welding processes, and prepare welding operation instructions.
2. Welders should be trained for the corresponding projects and have the corresponding operation qualification certificates.
3. Check whether the roundness and misalignment of the steel pipes received meet the relevant technical requirements.
4. Check whether there are defects such as scars, cracks, and severe rust on the surface of the metal pipe joints.
5. Before installing large-diameter steel pipes, the pipe joints should be measured and numbered one by one, and it is advisable to use pipe joints with small diameter differences for welding.
6. There should be windproof, rainproof, and snowproof sheds for welding in windy, rainy, and snowy weather.
7. Before welding, make the welder clear about the requirements and welding measures.