### Differences Between Hot-Rolled and Cold-Rolled Seamless Steel Pipes

1. **Manufacturing Process**:
– **Hot-rolled seamless steel pipes** are made by heating a round billet, piercing it, and then rolling it using a three-roll skew rolling process, continuous rolling, or extrusion. After rolling, the pipe goes through sizing, cooling, straightening, hydrostatic testing (or inspection), marking, and then storage.
– **Cold-rolled seamless steel pipes** are produced by heating a round billet, piercing it, and performing multiple passes of cold drawing or cold rolling after annealing, acid pickling, and oiling (or copper plating). After the cold rolling process, the pipe undergoes heat treatment, straightening, hydrostatic testing (or inspection), marking, and storage.

2. **Temperature of Processing**:
– **Hot rolling** is performed above the *recrystallization temperature* of steel.
– **Cold rolling** is carried out below the *recrystallization temperature*.

3. **Size and Precision**:
– **Hot-rolled seamless pipes** are generally used for larger diameters, typically with outer diameters greater than 32mm and wall thicknesses between 2.5mm and 75mm.
– **Cold-rolled seamless pipes** are often used for smaller diameters, with outer diameters as small as 6mm and wall thicknesses as thin as 0.25mm. Cold-rolled pipes offer higher precision in size compared to hot-rolled pipes.

4. **Surface Finish and Tolerances**:
– **Cold-rolled pipes** have a smoother surface finish, tighter tolerances, and better dimensional accuracy than hot-rolled pipes.
– **Hot-rolled pipes** are more prone to surface imperfections due to the high temperature of processing, leading to oxidized surfaces and less control over the final dimensions after cooling.

5. **Cost**:
Cold-rolled seamless pipes are generally more expensive than hot-rolled pipes due to their higher precision and more complex manufacturing process.

6. **Applications**:
– **Hot-rolled seamless pipes** are typically used in applications where high precision isn’t critical, such as structural supports, low and medium pressure boiler tubes, and general-purpose pipes.
– **Cold-rolled seamless pipes** are often used where high precision and a smooth surface finish are required, such as in hydraulic systems, precision mechanical parts, and high-pressure boiler tubes.

### Advantages and Disadvantages of Hot-Rolled Seamless Pipes

**Advantages**:
– The hot rolling process can break down the casting structure of the steel ingot, refine the grains, and eliminate microstructural defects. This leads to a denser structure and improved mechanical properties, particularly in the rolling direction.
– Gas bubbles, cracks, and looseness that form during casting can be welded under high temperatures and pressure.

**Disadvantages**:
1. **Layering/Delamination**: Non-metallic inclusions (mainly sulfides, oxides, and silicates) in the steel are often pressed into thin sheets during hot rolling, resulting in delamination. This significantly reduces the tensile strength in the thickness direction and may lead to interlaminar tearing during welding.
2. **Residual Stress**: Uneven cooling can lead to residual stress within the hot-rolled product. Although these stresses are self-balancing, they can negatively affect the performance of steel components under external forces, especially in terms of deformation, stability, and fatigue resistance.
3. **Dimensional Precision**: Due to thermal expansion and contraction, hot-rolled products have less precise control over thickness and width. Even if the dimensions are correct when hot, cooling can result in negative deviations, especially in wider or thicker sections.

### General Overview of Seamless Steel Pipes

Seamless steel pipes are divided into two main categories based on their manufacturing process: **hot-rolled (extruded) seamless pipes** and **cold-drawn (cold-rolled) seamless pipes**.

– **Hot-rolled seamless pipes** are further categorized into types such as general steel pipes, low and medium-pressure boiler pipes, high-pressure boiler pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, geological pipes, and others.
– **Cold-rolled (cold-drawn) seamless pipes** include not only general steel pipes, low and medium-pressure boiler pipes, high-pressure boiler pipes, alloy steel pipes, stainless steel pipes, and petroleum cracking pipes, but also carbon thin-walled pipes, alloy thin-walled pipes, stainless thin-walled pipes, and shaped steel pipes.

### Applications of Seamless Steel Pipes

– **General seamless steel pipes** are commonly made from high-quality carbon structural steels like 10, 20, 30, 35, and 45, or low alloy structural steels like 16Mn, 5MnV, and alloy steels like 40Cr, 30CrMnSi, 45Mn2, and 40MnB.
– Low-carbon steels like 10 and 20 are mainly used for fluid transportation.
– Medium-carbon steels like 45 and 40Cr are used to make mechanical parts, such as load-bearing components of automobiles and tractors.

### Conclusion

– **Hot-rolled seamless pipes** are typically used when high precision is not required, and they are preferred for larger diameter pipes.
– **Cold-rolled seamless pipes** are used when higher precision, better surface finish, and tighter tolerances are needed, especially for smaller diameters.
– When people refer to **seamless steel pipes** in general, they are often referring to **hot-rolled seamless pipes** unless specified otherwise.