Classifications of Seamless Carbon Steel Pipes

Seamless steel pipes (SMLS pipe) come in a wide variety of materials, each with its own unique properties and uses. Seamless carbon steel pipes are the most common type of seamless steel pipes, and are primarily composed of elements such as carbon, silicon, manganese, phosphorus, and sulfur. This type of steel pipe has high strength and toughness, especially at room and low temperatures. 

Seamless carbon steel pipes are widely used in industrial pipelines, transmission pipelines, and structural pipes. Their low cost and good processing properties make them the preferred material for many engineering and machinery manufacturing. However, seamless carbon steel pipes are relatively poor in corrosion resistance and are not suitable for use in corrosive media. This article will explore different types of seamless carbon steel pipes, their advantages and disadvantages, and the best practices for using them.

There are mainly the following types of seamless carbon steel pipes:

1. Classification by carbon content
Depending on the carbon content, seamless carbon steel pipes can be divided into low carbon steel, medium carbon steel, and high carbon steel. Low carbon steel has good toughness and weldability, and is suitable for manufacturing general structures and fluid delivery pipelines; medium carbon steel has high strength and wear resistance, and is often used to manufacture mechanical parts and bearings; high carbon steel is often used to manufacture knives and cutting tools due to its high hardness and wear resistance.

2. Classification by use

Seamless carbon steel pipe for fluid transportation: mainly used to transport various fluid media, such as oil, natural gas, water, etc. This type of steel pipe requires good pressure resistance, sealing and corrosion resistance.

Seamless carbon steel pipe for structure: used for structural support in the fields of construction, machinery manufacturing (mechanical tubing), etc. It has high strength and rigidity and can withstand large loads.

Seamless carbon steel pipe for boiler tube: used in power station boilers, industrial boilers and other equipment, and withstands the action of high temperature and high pressure steam. It is required to have good heat resistance, pressure resistance and oxidation resistance.

Seamless carbon steel pipe for petroleum cracking: used in cracking units in the petrochemical industry, and withstands high temperature, high pressure and corrosive media. It has good corrosion resistance and high temperature strength.

3. Classification by material

Ordinary carbon structural steel seamless steel pipe: Made of ordinary carbon structural steel such as Q235, Q275. It has certain strength and toughness, relatively low price, and is widely used in general engineering structures and fluid transportation fields.

High-quality carbon structural steel seamless steel pipe: Made of high-quality carbon structural steel, such as 20#, 45#, etc. It has high strength, hardness and toughness, and can be used to manufacture mechanical structural parts, transmission shafts, etc.

Low-alloy high-strength structural steel seamless steel pipe: Contains a small amount of alloying elements such as Mn, Si, V, etc., has high strength and toughness, and is suitable for structures and engineering with large loads.

4. Classification by manufacturing process

Seamless carbon steel pipes can be divided into three categories according to the manufacturing process: hot-rolled seamless steel pipe (HRS), cold-rolled seamless steel pipe (CRS) and cold-drawn seamless steel pipe (CDS). In each category, there are several variants dedicated to specific applications (such as oil or gas transportation) or specific strengths.

1) Hot-rolled seamless pipe
Hot-rolled seamless pipes are made by heating steel billets to high temperatures, then piercing and rolling them into a cylindrical shape, which is suitable for manufacturing large diameter and thick-walled steel pipes. They are often used in engineering applications that require higher strength than cold-rolled or cold-drawn pipes. The heat treatment process makes them exceptionally hard and strong, able to withstand high temperatures and high pressures. The advantages of hot-rolled seamless pipes include cost-effectiveness, strong structure, and the ability to customize shapes. The disadvantage is their weight, which can make them difficult to transport and install. Hot-rolled seamless pipes are often used in large projects involving higher pressures and temperatures.

2) Cold-rolled seamless pipe
Cold-rolled seamless pipes are cold-rolled on the basis of hot-rolled steel pipes, which can achieve higher dimensional accuracy and surface quality, and are suitable for manufacturing high-precision, small-diameter steel pipes. Unlike hot-rolled steel, cold-rolled steel is harder and more brittle. It can also be processed and formed more accurately, making it ideal for equipment that requires precision. The advantages of cold-rolled pipes include stronger corrosion and wear resistance, better machinability, and lower cost. Disadvantages of cold-rolled tube include lack of strength compared to hot-rolled steel, greater susceptibility to cracking, and limited customizability. Cold-rolled tube is often used for smaller applications that don’t require as much strength or precision.

3) Cold drawn seamless pipe
Cold drawn seamless pipes are made by forcing hot rolled pipes through one or more dies, which can further improve the dimensional accuracy and surface quality of the steel pipes, while also improving the strength of the steel pipes. They are suitable for making high-precision, high-strength steel pipes. This forces the material to become denser, making it stronger. They are ideal for applications where strength and pressure resistance are critical, such as automotive hydraulic lines and pipes. The advantages of cold drawn seamless pipes include high strength, low cost, and the ability to make precise shapes. Disadvantages include that they may crack if tempered improperly, lack flexibility, and limited customizability. Cold drawn seamless pipes are most commonly used in automotive and industrial applications.

General seamless steel pipes: They are made by hot or cold rolling of high-quality carbon steels such as 10, 20, 30, 35, 45, low alloy structural steels such as 16Mn, 5MnV, or alloy steels such as 40Cr, 30CrMnSi, 45Mn2, 40MnB. Seamless pipes made of low carbon steels such as 10 and 20 are mainly used for fluid delivery pipelines. Seamless pipes made of medium carbon steel such as 45 and 40Cr are used to manufacture mechanical parts, such as load-bearing parts of automobiles and tractors. Generally, seamless steel pipes must ensure strength and flattening tests. Hot-rolled steel pipes are delivered in hot-rolled or heat-treated states; cold-rolled pipes are delivered in heat-treated states.

Seamless steel pipes for low and medium pressure boilers: used to manufacture superheated steam pipes, boiling water pipes, water-cooled wall pipes and superheated steam pipes, large smoke pipes, small smoke pipes and arch brick pipes for various low and medium pressure boilers and locomotive boilers. Hot-rolled or cold-rolled (drawn) seamless steel pipes are made of high-quality carbon structural steel, mainly made of No. 10 steel and No. 20 steel. In addition to ensuring chemical composition and mechanical properties, water pressure tests, curling, flaring and flattening tests are also required. Hot rolling is delivered in hot-rolled state, and cold rolling (drawing) is delivered in heat-treated state.

High-pressure boiler steel pipes: mainly used to manufacture high-quality carbon structural steel, alloy structural steel and stainless heat-resistant steel seamless steel pipes for steam boiler pipes with high pressure and above. These boiler pipes often work under high temperature and high pressure. The pipes will also suffer from oxidation corrosion under the action of high temperature flue gas and water vapor, so the steel pipes are required to have high endurance strength, high anti-oxidation performance and good corrosion resistance.

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