A seamless steel pipe is a circular pipe with a hollow section and no seams or joints. This type of pipe is made of alloy steel, carbon steel, stainless steel ingot, or solid tube blank. These pipes are usually made by hot rolling, cold rolling, or cold drawing processes. Seamless steel pipes perform more efficiently than welded pipes as they are processed with intrinsic mechanical strength, without seam welds, using monolithic steel billets.
According to the manufacturing method, the seamless pipe can be divided into two sections that are hot-rolled seamless steel tubes and cold-drawn seamless steel pipes. Here to remember that the cold-drawn tube is also divided into the circular pipe and shaped pipe.
Hot-rolled seamless steel pipe:
Hot rolled is relative to the cold-rolled. Cold rolling is processed under the recrystallization temperature of the rolling, and hot-rolled is carried out above the recrystallization temperature of the rolling.
Advantages:
Hot-rolled seamless steel pipe can harm the cast microstructure of the steel ingot, refine the crystal grains of the steel, and eradicate the microstructure flaws so that the steel organization compacting enhances the mechanical properties. This modification is reflected in the rolling direction so that the steel is no longer, to a certain extent, isotropic. Additionally, the formation of cracks, bubbles, and osteoporosis, under high temperature and pressure can also be welded together.
Disadvantages:
- After the hot rolling process, the non-metallic inclusions in the interior of a steel (mainly sulfides and oxides, as well as the silicate) were pressed into a sheet. The layering of the steel in the thickness direction caused by the pull performance deteriorated significantly, and it may appear in the weld shrinkage interlayer tear. Weld shrinkage-induced local strain often reaches several times the yield point strain. The strain is much more immense than the causing load.
- Residual stress caused by uneven cooling marks a significant disadvantage here. The residual stress is addressed to the stress of internal self-phase equilibrium in the absence of external force. The hot-rolled steel of various cross-sectional has such residual stress. Usually, steel sectional dimension is becoming more significant with the residual stresses.
- Hot-rolled steel products have poor control of thickness and edge width. Thermal expansion and contraction are quite familiar to us. Even if the beginning of the hot-rolled pipes is of standard length and thickness, there will be some negative results after cooling. This negative differential makes the edge width wider, increasing the thickness.
So, no law requires too precise for large steel for steel edge width, thickness, length, angle, and side-line.
Cold-Drawn seamless steel pipes:
Cold-rolled seamless pipes are cold-drawn, or cold rolled precision seamless steel pipes that come with high dimensional accuracy of the dimension and good finish of the surface used for machinery and hydraulic applications. The cold-rolled process is the method to turn the steel plates or strips into various types of steel products by cold working processes like the cold bend and cold drawn processes. Cold-rolled seamless pipes not only include general steel pipes, high-pressure boiler steel pipes, low and medium pressure boiler steel pipes, alloy steel pipes, petroleum cracking steel pipes, stainless steel pipes, but also include carbon steel pipes, alloy steel pipes, stainless steel pipes with thin walls and special-shaped steel pipes. The hot-rolled seamless pipes usually have OD larger than 32mm and wall thickness between 2.5mm to 75mm. The cold-rolled seamless pipes usually have an outer diameter of less than 6mm and wall thickness of less than 0.25 mm. And the thin-wall pipes usually have an outer diameter of less than 5mm and wall thickness of less than 0.25mm. The cold-rolled pipes usually have a higher dimensional accuracy than the hot-rolled pipes.
Advantages and disadvantages of cold-rolled seamless pipes
Advantages:
- The high molding speed and productivity will not damage the coating. The cold-rolled process can make the sections in various shapes to adapt to different conditions. This process mostly increases the yield point of steel pipes.
- Cold rolled processes can eradicate the casting structure of steel ingots, refine the grain of steel products and eliminate the microstructure defects to give the structure of steel products a compact finish and improve the mechanical property of products. This kind of improvement can be witnessed mainly along the rolling direction, making steel products non-isotropic bodies any longer to some extent. In addition, this process repairs the cracks, bubbles, and shrinkage formed in casting under the high-temperatures and pressure.
Disadvantages:
- The cold-rolled steel in the open section generally has relatively low free torsion stiffness. The steel is twisted easily when bent; flexural-torsional buckling shows up quickly when steel is rolled. The torsion resistance is relatively poor.
- The wall thickness of the cold-rolled steel pipe is relatively less. The corners connecting plates are not thickened. As a result, the bearing ability of the partial concentrated load is weak.
A few professional mills in our country have an integrated manufacturing unit where both processes are being executed to produce all kinds of steel pipes and tubes to serve every specific requirement of the customers from different industries. There, mostly expert technocrats having intense experience in steel seamless piping industry supervise and execute the whole manufacturing process with utmost precision. Integration of both hot finishing and cold drawn manufacturing processes in the same unit is always considered as a distinct advantage to the consumers from different spheres of the world.
