Production Process of Seamless Steel Pipe

What is Seamless Pipe?

Seamless pipe is a steel pipe variant manufactured without any welded joints, hence the name. Seamless pipe has no seams, reducing the risk of fracture under high pressure and stress. Unlike welded pipe, seamless pipe has a smoother surface and more uniform diameter.

These characteristics give it exceptional durability and corrosion resistance. All of these exceptional qualities make it an ideal choice for the oil and gas industry and the construction industry.

Core Processes of Seamless Steel Pipe

Seamless pipe production begins with a solid round billet. After being heated to a high temperature, the billet is rolled into a hollow shell through a piercing mill. This is then rolled through multiple passes, sizing, and heat treatment to achieve seamless pipes that meet dimensional and performance requirements.

Core Processes Include:

1. Piercing

Rolling the hot billet into a hollow shell is the key process that distinguishes seamless pipe from seam pipe.

2. Rolling and Elongation

The shell is extended through a continuous rolling mill or a pipe jacking mill to control the outer diameter and wall thickness.

3. Sizing/Reducing

Further adjusts the pipe diameter to ensure precise geometric dimensions.

4. Heat Treatment

Improve microstructure and properties, enhancing toughness, ductility, and corrosion resistance.

Detailed Production Process of Seamless Pipe

1. Raw Material Selection

Seamless pipe is a durable option for a wide range of applications. However, to maximize its versatility, it must offer a variety of processing materials. Raw material selection is key to achieving the desired pipe properties, including tensile strength, yield strength, and hardness.

The following are the most commonly used materials for seamless pipe manufacturing:

Carbon Steel

Commonly used for general-purpose piping, it offers good strength and weldability. It is suitable for low-pressure applications such as water, gas, and cryogenic systems.

Alloy Steel

Composed of alloying elements such as chromium, nickel, and molybdenum. This composition provides increased strength, toughness, heat resistance, and corrosion resistance.

Stainless Steel

Ensures excellent corrosion resistance and toughness. It is commonly used in corrosive environments such as marine and chemical processing.

2. Heating

The steel billet is heated to a high temperature of 1150°C to 1250°C to achieve a ductile state. Temperature control is crucial; too low a temperature can make piercing difficult, while too high a temperature can cause coarse grains.

3. Extrusion

Industrial rotating tools penetrate the hot billet, forming a hollow tube shell. The drawing process passes the shell through a series of heavy rollers, ultimately forming the desired initial shape and dimensions for the pipe.

4. Expanding

During the expansion process, workers continuously press a mandrel into the tube. This step is crucial for ensuring a smooth inner diameter and uniform wall thickness. Mandrel milling involves continuous rolling and drawing until the desired dimensions are reached.

5. Cooling

The rolled tube billet is cooled in a controlled environment to prevent cracking caused by thermal stress. Air cooling or controlled cooling is often used to maintain uniform microstructure.

6. Straightening

Even after cooling, the pipe will still deform and bend. Therefore, the pipe still needs to pass through rollers or straighteners to apply controlled pressure to align the pipe. This ensures the geometric accuracy and usability of the final seamless pipe.

7. Post-Processing

Post-processing may include surface polishing, threading, or heat treatment to enhance pipe performance. Workers may also add surface coatings to prevent wear and corrosion.

8. Quality Control

The quality control department inspects pipes for dimensional accuracy, strength, and surface defects. Safety inspection mechanisms, including X-ray and ultrasonic testing, ensure the pipes are free of defects. Pipes undergo final inspection before shipment.

Seamless Steel Pipe Production Craftsmanship

Generally, seamless steel pipe production processes are divided into cold rolling and hot rolling.

1. Hot-Rolled Seamless Steel Pipe

Main production processes (main inspection processes) for hot-rolled seamless steel pipe:

Pipe Preparation and Inspection → Pipe Heating → Pipe Piercing → Pipe Rolling → Rough Pipe Reheating → Sizing (Reducing) → Heat Treatment → Finished Pipe Straightening → Finishing → Inspection (Non-destructive, Physical and Chemical, Benchtop Inspection) → Warehousing

Hot-rolled seamless steel pipes are generally delivered in the hot-rolled state after heat treatment. Hot-rolled seamless steel pipes undergo rigorous manual selection after quality inspection. The surface is then oiled and subjected to multiple cold-drawing tests. After hot-rolling, a perforation test is performed. If the perforation is too large, the pipes are straightened. After straightening, they are conveyed by conveyor to a flaw detector for testing. Finally, they are labeled, sorted according to specifications, and stored.

2. Cold-rolled Seamless Steel Pipe

The main production process for cold-rolled (drawn) seamless steel pipes is as follows:

Blank preparation → Pickling and lubrication → Cold rolling (drawing) → Heat treatment → Straightening → Finishing → Inspection

The production process for cold-rolled seamless steel pipes is generally more complex than that for hot-rolled pipes. If no reaction cracks are observed on the surface during the sizing test, the round pipe is cut using a cutting machine into blanks approximately one meter in length. Annealing is then performed, followed by pickling with acid. During pickling, pay attention to any significant blistering on the surface. If significant blistering is present, the pipe quality does not meet reaction standards.

In appearance, cold-rolled seamless steel pipes are shorter than hot-rolled seamless steel pipes. The wall thickness of cold-rolled seamless steel pipes is generally thinner than that of hot-rolled seamless steel pipes. However, the surface of cold-rolled seamless steel pipes appears much shinier than thick-walled seamless steel pipes, with less roughness and fewer burrs on the diameter.

Hot-Rolled Seamless Steel Pipe VS. Cold-Rolled Seamless Steel Pipe

Summary

In summary, the production of seamless steel pipes is a highly sophisticated and complex process, requiring the coordinated efforts of multiple steps and quality control measures. By strictly controlling raw material quality, optimizing piercing and rolling processes, rationally selecting heat treatment methods and finishing processes, and implementing rigorous inspection and control measures, we can produce high-quality seamless steel pipes that meet the needs of various industrial sectors.