Carbon steel round pipes and their types

Carbon steel round pipes are circular cross-section pipes mainly made of carbon steel and may contain small amounts of elements such as manganese and silicon.
Due to the fact that the higher the carbon content, the higher the hardenability and strength of the steel, but the plasticity and weldability decrease, carbon steel is usually classified into four major categories: low carbon steel, medium carbon steel, high carbon steel and ultra-high carbon steel, to meet the mechanical property requirements of different applications.

The main classification of carbon steel round pipes

There are five types of carbon steel round pipes, each with its own distinct features. They are widely used in various fields, including construction, furniture, and mechanical parts.

1. Electric Resistance Welding (ERW) :
Electric Resistance Welding (ERW) requires the carbon steel raw materials to be rolled and formed into a circular or cylindrical shape. The skin effect and proximity effect are generated at the joint of the tube blank by using high-frequency current. The edge of the plate is heated to a molten state, and the pressure welding forming is achieved under the action of the extrusion roller.
Depending on the welding frequency and equipment, it can be classified into low-frequency resistance welding, medium-frequency resistance welding and high-frequency resistance welding (HFW), among which high-frequency resistance welding is suitable for applications that require higher weld strength.

Characteristics and Applications

High production efficiency and low cost: The process is simple, with less investment in unit equipment, making it suitable for large-scale production of standard-sized pipes.
Wide size range: Common outer diameters range from 1/2" to 24" (approximately 21.3mm - 609.6mm), and wall thicknesses range from 1.65mm to 20mm. Customized lengths, flat ends, inclined ends, threaded ends, etc. can be provided according to requirements.
Mechanical properties: Generally, its strength is slightly lower than that of seamless pipes. It is not suitable for ultra-high pressure or harsh corrosive environments, but is widely used in fields such as petrochemicals, structural pipes, and mechanical manufacturing.

2. Cold-drawn seamless tubes

Cold-drawn seamless tubes first involve acid washing and lubrication of the "blank tubes" formed through piercing and hot rolling or extrusion. Then, at room temperature, they undergo multiple drawing (cold rolling) and annealing processes to gradually reduce the tube diameter, increase the wall thickness and refine the grains. Finally, they are straightened, refined and inspected to obtain precision seamless tubes.

Performance advantage

High strength and dimensional accuracy: The cold drawing process causes strain hardening, significantly enhancing tensile strength and yield strength. The dimensional accuracy can reach the ± 0.01mm level, and the surface finish is excellent.
Microstructure refinement: Multiple cold deformations and annealing refine the grains, enhancing the fatigue performance and pressure resistance of the pipe.
Scope of application: It is commonly used in hydraulic systems, precision machinery, automotive parts and other occasions that require high dimensional accuracy and surface quality.

3. Stretch the mandrel tube (DOM)

DOM (Drawn Over Mandrel) pipes are based on resistance welded pipes or hot-rolled seamless pipes. They are equipped with conical or cylindrical mandrels inside and processed by cold drawing at room temperature through the cooperation of a drawing machine and mandrels. This enables the inner wall of the pipe to withstand tensile and compressive stresses, achieving higher-precision pipe diameter control and inner surface quality.

Main features

Excellent size and concentricity: After mandrel correction, the outer diameter, inner diameter, wall thickness and concentricity of the pipe are all more precise than those of ordinary ERW pipes or hot-rolled seamless pipes.
The inner surface is smooth and no deburring is required: The inner wall of the weld seam is smoothed during the mandrel drawing process, and the subsequent processing volume is small.
Full utilization of materials: Commonly made of low-carbon steel (such as SAE 1020, 1026), it is suitable for components with high requirements for precision and inner wall smoothness, such as mechanical structures and automotive bearings.

4. Hydroforming tubes

Tube Hydroforming is a process where the ends of prefabricated carbon steel tubes are sealed and placed in open-close molds. High-pressure liquid (often water) is applied to the tubes to apply pressure, causing the tube walls to expand outward and fit the mold cavity, thereby forming complex cross-sections or special-shaped pipe fittings at one time.

Advantages and Applications

Complex cross-section one-time forming: It can manufacture T-shaped, Y-shaped, box-shaped and other complex pipe fittings, reducing welding and assembly processes.
Lightweight and high rigidity: The uniform wall thickness distribution and seamless structure endow the formed parts with an excellent stiffness-to-weight ratio.
Widely applied in fields such as automotive frames, aerospace brackets, bicycle frames, and petrochemical heat exchangers.
Lower mold cost: In hydraulic forming, the internal pressure liquid is used as the upper mold, and the mold only needs to be set in shape, reducing the number and cost of hard molds.

5. Hot-processed seamless tubes

There are usually two types of hot-processed seamless tubes:

Hot rolling piercing process: Solid tube billets (or steel ingots) are heated above the recrystallization temperature, and then the tube billets are pierced into blank tubes with the help of rotary piercing machines or extruders. After multiple passes of three-roll cross-rolling, continuous rolling or extrusion sizing, the finished products are obtained through heat treatment, straightening and finishing.
Extrusion seamless process: Under high-temperature conditions, the tube blank is forcibly extruded through the die hole by hydraulic or mechanical means to form the pipe. It is suitable for the production of large-diameter and thick-walled pipes.

In conclusion, from resistance welding to hot rolling extrusion, various carbon steel round pipe processes each have their own characteristics in terms of production efficiency, equipment investment, finished product specifications and performance. When selecting the type, users should comprehensively consider the usage environment, mechanical indicators, dimensional accuracy and economy, and choose the optimal pipe type and manufacturing process.