In the welding process of steel pipes, it's better to choose the carbon dioxide gas welding and the following steps should be paid attention to:
① Preheating before welding
Before the welding, preheat the seam of the steel pipe. Set the temperature between 100 and 150℃. Choose an electric heater to heat it, and weld it after half an hour at a constant temperature.
② Welding
To control the deformation of welding, choose the concept of symmetrical construction by six persons. The welding direction should be from bottom to top for the inner welding. A little welding should be done for the first and second layers. This welding becomes a concentrated factor that restricts the welding deformation, and after the welding of the second layer is completed, clean the backside in time. After the cleaning by carbon arc gouging, grind the welding seam by machines and remove the carburization of the welding seam's surface; make the metallic luster show, and reduce the probability of cracks caused by carbonization on the surface. The outer part is welded in sequence by two layers. Finally, the remaining part of the inner mouth is welded. For the welding of the second layer, the specific welding direction should be opposite to that of the first layer, and the welded joints of each layer should be staggered by about 15 to 20mm.
③ Heat treatment and inspection after welding
The heat treatment should be conducted within one day after the welding is performed. The ultrasonic detection should be carried out for the welding joint and the inspection ratio should be 100%.
④ Key points of welding
First of all, the wind speed of carbon dioxide gas shielded welding should exceed 2m/s. On rainy and snowy days, safe protection measures should be taken to promote sufficient temperature at the welding part, so that the operation of the welder won't be affected and the welding process can be promoted. Secondly, remove oil stains, slag and water marks at the groove positions and 10-20mm on both sides before construction. Finally, store the welding rod and flux in a ventilated and dry environment, and record the distribution of the welding rod.
⑤ Inspecting the welding seam
Ultrasonic and radiological inspection methods are selected for the internal inspection of the welding seam. Some welding seams are subject to ultrasonic inspection for the first time, followed by radiological re-inspection. 100% radiographic inspection and ultrasonic re-examination should be performed for the T welding seams in the intersection of welding seams and circumferential welding seams.
 
(3) Repair welding and treatment for defects
If the inspection determines the cracks and defects on the surface of the steel pipe, repair welding should be performed immediately. Generally speaking, the lack of internal welding seams should be treated by gouging and grinding wheel. Before the formal repair welding, the operator has to check the welding seam's position and welding groove many times, and then carry out the repair welding. It should be noted that during the repair welding, the best way of the grinding wheel is to grind the abraded parts of the steel pipe, especially the arc scratches on the surface of the steel pipe, to achieve a better repair welding treatment effect.
 
(4) Anticorrosive coatings of steel pipes
The anti-corrosion coating is convenient to control the corrosion of the water supply steel pipe, so it is necessary to pay great attention to the treatment of the anti-corrosion coating. The operator should completely clean the outer wall of the steel pipe, that is, clean the soil, grease and debris on the outer surface of the steel pipe, and polish and remove welding spatter, ensuring that the outer wall and welding seam's surface is smooth enough. Introduce manual rust removal and electric steel brush to remove rust marks. Next, evenly apply the corrosion remover to the corresponding parts of the steel pipe. In the actual operation process, always ensure the dry and clean characteristics of the steel pipe itself, and at the same time complete the anti-corrosion coating of the steel pipe in combination with a unique sequence to eliminate the possibility of clots and bubbles in the anti-corrosion coating.
 
3. Conclusion
In summary, the research on the installation and construction technology of large-diameter water supply steel pipes is of great significance and value. Under the implementation of the integration of construction methods, internalization of construction processes and efficient measures for construction management, the construction quality of the installation of large-diameter steel pipes is fully guaranteed. In order to perform the installation and construction of large-diameter water supply steel pipes with high quality, the specific process should be regulated, including hoisting, riveting and welding, repair welding and defect treatment, anti-corrosion coatings of steel pipes. Shorten the construction time as much as possible, and improve the economic benefits of construction, providing technical support for the follow-up operation of the steel pipe project.