The welding method of thick-walled steel pipe should be selected according to the material and wall thickness of thick-walled steel pipe. 
Because different welding methods have different arc heat and arc force, different welding methods have different characteristics.
For example, tungsten arc welding is characterized by low current density, stable arc combustion, and good weld shape, which is especially suitable for thin plate welding, while thick plate welding is not the choice; plasma arc is characterized by high arc column temperature, high energy density, good straightness of plasma arc, the wide adjustment range of rigidity and flexibility, stable operation, but complicated operation.
Submerged arc welding has the characteristics of penetration ability and high deposition rate of welding wire, so the welding speed can be improved and the welding cost is low, but the working conditions and environment are relatively poor.
Thus it can be seen that different welding methods have different capabilities and different operating costs. According to the material and wall thickness of thick-walled steel pipes, a reasonable selection of welding methods is very important to work to improve welding quality, improve productivity and reduce costs.
Pickling of thick-walled steel pipe is a method of using an acid solution to remove oxide scale and rust on the surface of the steel, which is called pickling.
The acids used for pickling are sulfuric acid, hydrochloric acid, phosphoric acid, and mixed acid.
The pickling process is to remove the oxide scale on the surface, followed by lubrication treatment (carbon steel-phosphorus saponification, stainless steel-butter lime, copper-aluminum pipe-oiling), and then drawing deep processing using the old process-copper plating.
If the thick-walled steel pipe is not pickled, there may be oxides and oil stains on the surface, which can not be removed by nuclear power, and the phosphating quality will be reduced.
And thick-walled steel pipe in the manufacturing process, after many processes, if you do not pay attention to it, will leave scars on the surface of thick-walled steel pipe, reduce the corrosion resistance of parts and components, and directly affect the service life.
What details should be dealt with before the use of thick-walled steel pipe?
1. Thick-walled steel pipe cutting: according to the actual pipeline length, a metal saw and a toothless saw should be used to cut the pipe.
When using water welding in the cutting process, the raw materials should be protected accordingly, and the heat-resistant materials should be used as baffles at both ends of the fracture to catch the sparks and hot iron beans that fall during the cutting, so as to protect the original plastic layer of the raw materials.
2. Thick-walled steel pipe connection: after plastic repair, the pipe, and pipe fittings are connected and installed, rubber pads are added between the flanges in the connection process, and the bolts are fastened to a sealed state.
3, thick-walled steel pipe plastic coating treatment: after polishing, use oxygen and C2H2 to heat the pipe mouth outside, heating to the internal plastic layer part of the melting phenomenon, and then the technical workers will use the prepared plastic powder to evenly smear the pipe mouth, should pay attention to the corresponding smear in place, flange plastic should be smeared above the waterline.
This process should strictly control the heating temperature, if the temperature is too high, there will be bubbles in the plastic coating process, such as the temperature is too low, the plastic powder will not melt completely in the plastic coating process, and the plastic layer will fall off after the pipeline is put into use. in the later stage, part of the thick-walled steel pipe was corroded and damaged.
4. Thick-walled steel pipe mouth grinding: after cutting, the angle grinder should be used to polish the plastic layer of the pipe mouth, the purpose of which is to avoid melting or even burning of the plastic layer during flange welding.
Use an angle grinder to grind the plastic layer of the pipe mouth.
In order to improve the corrosion resistance of thick-walled steel pipe and prolong the service life of the product, it is necessary to carry out pickling and passivation surface treatment to form a protective film on the surface of thick-walled steel pipe.
The thick-walled steel pipe has high hardenability, good machinability, moderate cold deformation plasticity, and weldability, and the toughness of the steel does not decrease much during heat treatment, but it has quite high strength and wears resistance, especially during water quenching.
However, this steel is sensitive to white point, tempering brittleness and overheating sensitivity during heat treatment, high strength and hardenability, good toughness, small deformation during quenching, high creep strength, and long-term strength at high temperature.
It is used to manufacture forgings with higher strength and larger quenched and tempered cross-section than 35CrMo steel, such as large gears for locomotive traction, turbocharger drive gears, rear axles, connecting rods, and spring clips under heavy load. it can also be used for drill pipe joints and fishing tools in oil deep wells below 2000m and can be used in the die of bending machine.
