What are the advantages and disadvantages of hot-rolled seamless pipes?

Hot-rolled seamless pipes are compared to cold-drawn seamless pipes. Cold rolling is rolling below the crystallization temperature, and hot rolling is rolling above the recrystallization temperature.

Advantages of hot rolled seamless pipes

It can destroy the casting structure of the steel ingot, refine the grains of the steel, eliminate defects in the microstructure, make the steel structure dense, and improve the mechanical properties. This improvement is mainly along the rolling direction, so that the steel is no longer isotropic to a certain extent; bubbles, cracks, and looseness formed during pouring can also be welded under high temperature and pressure.

Disadvantages of hot rolled seamless pipes

1. After hot rolling, the non-metallic inclusions (mainly sulfides, oxides, and silicates) inside the steel are pressed into thin sheets, resulting in delamination (interlayer). Delamination greatly deteriorates the tensile properties of the steel through the thickness, and as the weld shrinks, interlaminar tears may occur. The local strain caused by the shrinkage of the weld often reaches several times the yield point strain, which is much larger than the strain caused by the load;

2. Residual stress caused by uneven cooling. Residual stress is the internal self-balancing stress without external force. This kind of residual stress exists in hot-rolled steel sections of various cross-sections. In general, the larger the section size of the section steel, the greater the residual stress. Although residual stress is self-balancing, it still has a certain impact on the performance of steel components under the action of external forces. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc.

3. For hot-rolled steel, it is difficult to control the thickness and edge width. We are familiar with thermal expansion and cold contraction. Even if the length and thickness of the hot-rolled product meet the standards at the beginning, there will still be a certain negative difference after cooling. The wider the side width and the thicker the thickness, the more obvious this negative difference will be. Therefore, for large-sized steel, the side width, thickness, length, angle, and edge of the steel cannot be too precise.

Warm reminder: There are two types of seamless pipes: hot-rolled seamless steel pipes and cold-rolled (drawn) seamless steel pipes.

1. Hot rolled steel pipes are divided into general steel pipes, low and medium pressure boiler steel pipes, high pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, geological steel pipes and other steel pipes.

2. Cold-rolled (rolled) seamless steel pipes are divided into general steel pipes, low- and medium-pressure boiler steel pipes, high-pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, and other steel pipes. It also includes carbon thin-walled steel pipes, alloy thin-walled steel pipes, stainless steel thin-walled steel pipes, special-shaped steel pipes, etc.

3. Steel pipe size: The outer diameter of hot-rolled seamless pipe is generally greater than 32mm, and the wall thickness is 2.5-75mm. The outer diameter of cold-rolled seamless steel pipe can reach 6mm and the wall thickness can reach 0.25mm. The outer diameter of thin-walled tubes can reach 5mm, and the wall thickness can be less than 0.25mm. The dimensional accuracy of cold rolling is higher than that of hot rolling.

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