### Introduction to Common Types of Seamless Steel Pipes

1. **Structural Pipes**:
These normally refer to seamless steel pipes used for general structural and mechanical structures. The standard for fluid seamless steel pipes applies to seamless pipes used for conveying fluids. The main difference between fluid seamless pipes and structural seamless pipes is that the former undergoes individual hydraulic testing or ultrasonic, eddy current, and magnetic flux leakage inspections. Therefore, in the selection of standards for pressure pipeline steel pipes, it is not advisable to use fluid seamless steel pipes. Seamless steel pipes are denoted by outer diameter and wall thickness. Thick-walled seamless pipes are mainly used in machining, coal mining, hydraulic steel, and other fields. The materials for thick-walled seamless pipes include 10#, 20#, 35#, 45#, 16Mn, 27SiMn, 12Cr1MoV, 10CrMo910, 15CrMo, 35CrMo, etc.

2. **Fluid Pipes**:
Fluid pipes are specifically used for conveying media with fluid properties. Besides liquid media such as water, oil, and solutions, solid media like cement, grain, and coal powder can also flow under certain conditions. Fluid pipes can be made of steel, copper, titanium, or other non-ferrous metals, and even plastic or other non-metallic materials.
Fluid pipes must have a hollow cross-section, but they can also be square, triangular, or any other shape. Some equipment requires rectangular pipes due to space constraints, but the majority still uses round pipes. Round pipes offer the smallest perimeter-to-area ratio of all geometric cross-sections, meaning the maximum internal cross-section can be achieved with the least material.
Steel pipes are widely used in modern fluid transportation due to their low cost and high strength. According to their production process, steel pipes are divided into seamless pipes and welded pipes. Welded pipes include high-frequency straight seam welded pipes (ERW), spiral welded pipes (SSAW), and submerged arc welded pipes (UOE). In the past, fluid pipes were traditionally seamless, but with advances in material science, forming technologies, and equipment, welded pipes have seen significant development. Welded pipes offer advantages such as better uniformity in wall thickness, higher precision, lower energy consumption, and higher production efficiency. In the past, almost all transmission pipes for petroleum and natural gas (API standard) used seamless pipes, but today, over 95% in developed countries like the US, Japan, and Europe have been replaced by welded pipes.

3. **Low and Medium Pressure Boiler Pipes**:
These pipes are made from steel ingots or solid billets that are pierced to form hollow pipes, then hot-rolled, cold-rolled, or cold-drawn.
**Mechanical properties**: tensile strength σb (MPa) ≥ 410(42), yield strength σs (MPa) ≥ 245(25), elongation δ5 (%) ≥ 25, reduction of area ψ (%) ≥ 5, hardness ≤ 156HB (untreated), sample size: 25mm.
**Heat treatment specifications and microstructure**: normalized at 910°C, air-cooled. Microstructure: ferrite + pearlite.
Seamless steel pipes hold an important position in China’s steel pipe industry. According to incomplete statistics, there are currently over 240 seamless pipe production enterprises in China, with more than 250 production units, and an annual production capacity of over 4.5 million tons. Pipes with diameters smaller than φ76 account for 35%, while those between φ159-650 account for 25%. In terms of varieties, general-purpose pipes account for 1.9 million tons (54%), oil pipes 760,000 tons (5.7%), hydraulic support and precision pipes 150,000 tons (4.3%), and stainless steel, bearing, and automotive pipes 50,000 tons (1.4%).

4. **Petroleum Cracking Pipes**:
Petroleum cracking pipes are long steel pipes with a hollow cross-section and no seams along the periphery, making them a type of economic cross-sectional steel.
Seamless steel pipes for petroleum cracking (GB9948-2006, non-equivalent to ISO 9329/2:1997) are used for boiler pipes, heat exchanger pipes, and pressure pipes in petrochemical industries.
**Weight calculation formula** for common carbon pipes:
\[(outer diameter – wall thickness) \times wall thickness \times 0.02466 = KG/M\]
Using petroleum cracking pipes to manufacture ring parts can improve material utilization, simplify manufacturing processes, and save materials and machining hours. These pipes are widely used to make rolling bearing rings, jacks, and are essential materials for conventional weapons like gun barrels and cannon tubes. Petroleum cracking pipes can be round or shaped. Since round pipes have the largest cross-sectional area for a given perimeter, they allow the transportation of more fluid.
These pipes are widely used for structural and mechanical parts, such as oil drill pipes, automotive drive shafts, bicycle frames, and steel scaffolding used in construction. They are also used for seamless steel pipes in oil refineries, furnaces, heat exchangers, and pipelines.

5. **High-Pressure Boiler Pipes**:
High-pressure boiler pipes are a type of seamless steel pipe used in boiler applications. While their manufacturing process is similar to that of seamless pipes, the steel grades used for high-pressure boiler pipes have stricter requirements. These pipes often operate under high temperatures and pressures, where they are exposed to oxidizing and corrosive environments. Therefore, the pipes must have high long-term strength, excellent anti-oxidation and anti-corrosion properties, and good structural stability.
They are primarily used for making superheater pipes, reheater pipes, air guides, and main steam pipes in high and ultra-high pressure boilers.
**Specifications**: 6mm-1240mm × 1mm-200mm
**Production process**: precision drawing, bright finish, hot rolling, cold drawing, hot expansion.

6. **Hydraulic Support Pipes**:
Hydraulic support pipes are made by adding one or more alloying elements to high-quality carbon structural steel to enhance the steel’s strength, toughness, and hardenability. After forming, these steels typically undergo heat treatments such as tempering, chemical heat treatment, and surface hardening. Compared to high-quality carbon structural steel, hydraulic support pipes have better overall mechanical properties. They are often rolled into round, square, and flat shapes, used as key structural components in machinery or machines.
Materials for hydraulic support pipes include 20#, 45#, 27SiMn, 30CrMoSiA, etc.
**Heat treatment**: 20# and 45# are normalized; 27SiMn is quenched at (920 ± 20)°C and tempered at (450 ± 50)°C. The cooling medium used is oil or water.
The rust resistance of hydraulic support pipes is not as good as stainless steel, but they offer better wear resistance and machinability.
**Weight calculation**:
\[(outer diameter – wall thickness) \times wall thickness \times 0.02466 = KG/M (weight per meter)\]
These pipes are used in hydraulic cylinders and pillars in coal mine hydraulic supports, as well as other hydraulic systems.

7. **Alloy Pipes**:
Alloy pipes are a type of seamless steel pipe. They can be categorized as structural seamless pipes or high-pressure heat-resistant alloy pipes. The main difference lies in the production standards and applications. Alloy pipes undergo annealing and tempering to adjust their mechanical properties, tailoring them to meet required processing conditions.
The performance of alloy pipes is more variable and valuable compared to general seamless steel pipes, primarily due to the higher Cr content in their chemical composition, which provides enhanced high-temperature resistance, low-temperature resistance, and corrosion resistance. Unlike carbon seamless pipes, which contain little or no alloy elements, alloy pipes are widely used in industries such as petroleum, aerospace, chemical, power, boilers, and military sectors due to their highly adjustable mechanical properties.