Introduction to the anti-corrosion of the inner wall of large-diameter steel pipes and preparations before welding

First, introduction to the anti-corrosion of the inner wall of large-diameter steel pipes:

1. Before processing, all steel pipes should first be rust-free on the steel plate on one side of the inner wall of the large-diameter steel pipe, until the metal reveals the metallic luster (white). It should meet the sa2.5 standard required in GB8923-88 "Surface rust level and rust removal level of steel before painting".

2. The steel after rust removal should be processed immediately (to prevent secondary rust). After processing, use a clean rag to wipe off the residual rust powder on the inner wall of the pipe (if secondary rust is found, perform secondary rust removal to ensure painting), and promptly use GZ-2 gray new polymer anti-corrosion coating (thick paste type) to brush two coats as primer, and use sky blue paint to brush two coats of topcoat, so that the total thickness of the paint film reaches 250-300μm on both bottoms and sides.

3. Primer should be applied before pipes leave the factory, and topcoat should be applied before pipe installation. No paint should be applied within 200mm at both ends of all pipes (at the installation welds), and paint should be applied after the pipes are installed.

4. When applying primer, only a brush is needed, and spraying is not allowed. The topcoat can be applied by spraying.

5. The painting site should have ventilation conditions to prevent the metal surface from getting wet. The site is prohibited from having bad weather such as strong winds, flying dust, and scorching sun (such as high temperature >70℃, or too low <+5℃ rainy days, etc.). Painting is not allowed. It is best to paint at 10℃-25℃ and relative humidity below 70%.

Second, construction preparation before welding 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 in corresponding projects and have operation qualifications for corresponding projects.

3. Check whether the roundness and misalignment of the steel pipes arriving 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 select pipe joints with small diameter differences for welding.

Third, several stages of the mechanical expansion process of large-diameter steel pipes:

The steel plate is first pressed into a U shape in the forming die, then pressed into an O shape, and then internal and external submerged arc welding is performed. After welding, the diameter is usually expanded at the end of the entire length, which is called UOE welded pipe, and the non-expanded diameter is called UO welded pipe. The steel plate is roll-bent (RollBending), and then internal and external submerged arc welding is performed. After welding, the diameter is expanded to RBE welded pipe or the non-expanded diameter is RB welded pipe. The steel plate is formed in the order of J-type-C-type-O-type, and the diameter is expanded to JCOE welded pipe, or the non-expanded diameter is JCO welded pipe after welding. Among the above-mentioned LSAW welded pipes, UOE is commonly used. The expansion process of large-diameter steel pipes begins with the initial rounding stage. The fan-shaped blocks are opened until all the fan-shaped blocks contact the inner wall of the steel pipe. At this time, the radius of each point in the inner round tube of the steel pipe within the step range is almost the same, and the steel pipe is initially rounded. The fan-shaped blocks start to reduce the movement speed from the front position until they reach the required position, which is the required position of the inner circumference of the finished pipe. The expansion process stage of large-diameter steel pipes is the rebound compensation stage. The fan-shaped blocks start to further reduce the speed at the position of the second stage until they reach the required position, which is the position of the inner circumference of the steel pipe before rebounding required by the process design. The fan-shaped blocks remain stationary at the inner circumference position of the steel pipe before rebounding for a period, which is the pressure-maintaining and stable stage required by the equipment and the expansion process. The last is the unloading and regression stage of the expansion process of large-diameter steel pipes. The fan-shaped blocks begin to shrink rapidly from the inner circumference position of the steel pipe before rebounding until they reach the initial expansion position, which is the minimum shrinkage diameter of the fan-shaped blocks required by the expansion process.