Production process of welded steel pipes

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2020-03-25

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Steel Pipe Knowledge

The development of steel pipe manufacturing technology began with the rise of the bicycle industry, the early‑19th‑century oil boom, shipbuilding, boiler and aircraft production during the two world wars, and, after World War II, the manufacture of thermal power plant boilers. Additionally, advances in the chemical industry, as well as the drilling, extraction, and transportation of oil and natural gas, have all strongly propelled the steel pipe industry forward in terms of product variety, output, and quality. Steel pipes are typically classified into two main types based on their manufacturing process: seamless steel pipes and welded steel pipes. This discussion will focus primarily on welded steel pipes. Welded steel pipes, also known as seam‑welded pipes, are produced by bending and rolling tube blanks—made from steel plates or strips—into the desired cross‑sectional shape and dimensions using various forming methods, followed by welding along the seam to create the finished pipe. Compared with seamless steel pipes, welded pipes offer higher dimensional accuracy, especially in wall thickness; simpler primary equipment; a smaller footprint; the ability to operate continuously; greater production flexibility; and a broader range of products that can be manufactured on a single production line. I. The general production process for spiral welded steel pipes is as follows: 1. Raw materials include steel strip coils, welding wire, and flux. 2. Prior to forming, the steel strip undergoes leveling, edge trimming, edge planing, surface cleaning, conveying, and pre‑bending operations. 3. A weld gap control device is used to ensure that the weld gap meets welding requirements, with strict control over pipe diameter, misalignment, and weld gap dimensions. 4. After cutting into individual pipe lengths, the first three pipes of each batch undergo rigorous initial inspection, assessing weld mechanical properties, chemical composition, fusion quality, and surface finish. Nondestructive testing is also performed to confirm compliance with manufacturing standards before the pipes are officially put into production. II. Longitudinal Submerged Arc Welded Pipes (LSAW): Longitudinal submerged arc welded pipes are generally manufactured from steel plates. Through various forming processes, including double‑sided submerged arc welding and post‑weld expanding, these pipes are produced. Key equipment includes edge milling machines, pre‑bending machines, forming machines, pre‑welding units, and expanding machines. There are several distinct forming methods for LSAW pipes, such as UOE, RBE, and JCO (JCOE). In the UOE process, the steel plate is first pressed into a U‑shape, then further formed into an O‑shape, followed by internal and external submerged arc welding. Post‑weld expansion is typically applied either at the ends or along the entire length, resulting in what is called a UOE pipe; if no expansion is performed, it is referred to as a UO pipe. For RBE pipes, the steel plate is rolled and bent into shape, then subjected to internal and external submerged arc welding, with subsequent expansion. If no expansion occurs, it is designated as an RB pipe. In the JCO process, the steel plate is sequentially formed into J‑, C‑, and O‑shapes, followed by post‑weld expansion, yielding a JCOE pipe; without expansion, it becomes a JCO pipe. UOE longitudinal submerged arc welded pipe forming process: The three primary forming steps in UOE pipe production involve pre‑bending the edges of the steel plate, forming a U‑shape, and finally forming an O‑shape. Each step employs a dedicated forming press, sequentially completing edge pre‑bending, U‑shaping, and O‑shaping to transform the steel plate into a circular tubular form. As shown in the figure: JCOE longitudinal submerged arc welded pipe forming process: During forming, the JC0 machine performs multiple incremental stamping operations—first pressing half of the steel plate into a “J” shape, then shaping the other half into a “C” shape, and finally applying pressure at the center to produce an open “0”‑shaped blank. Comparison between JCO and UO forming methods: JCO forming uses progressive pressure, transforming the pipe‑forming process from the two steps of UO formation into multiple stages. Throughout this process, the steel plate deforms uniformly, residual stresses remain minimal, and the surface remains free of scratches. The resulting pipes exhibit greater flexibility in diameter and wall‑thickness specifications, allowing for both large‑scale and small‑batch production. They can manufacture high‑strength, thick‑walled pipes of large diameters as well as small‑diameter, thick‑walled pipes. Particularly when producing high‑grade, thick‑walled pipes—especially medium and small diameters—JCO offers advantages unmatched by other processes, satisfying users’ diverse requirements for pipe specifications. Investment is relatively low, but production efficiency is comparatively modest, with annual output typically ranging from 100,000 to 250,000 tons. UO forming, which employs both U‑ and O‑shaped pressure steps, is characterized by its high capacity and substantial output, often reaching 300,000 to 1 million tons per year, making it suitable for large‑volume production of a single specification. However, the capital investment required is considerable, often beyond the reach of developing countries. III. Longitudinal Electric Resistance Welded Pipes (ERW): Longitudinal electric resistance welded pipes are produced by taking hot‑rolled coil stock, forming it through specialized machinery, and then utilizing the skin effect and proximity effect of high‑frequency currents to heat and melt the edges of the pipe blank. Under pressure from the rolling mill, the heated edges are fused together, completing the weld. Welded steel pipes, also known as weld pipes, are manufactured by rolling steel plates or strips into shape and then welding them together. Their production process is simple, highly efficient, and offers a wide variety of grades and specifications, requiring relatively little equipment. However, their overall strength is generally lower than that of seamless steel pipes. Since the 1930s, with the rapid advancement of continuous rolling technology for high‑quality steel strips and improvements in welding and inspection techniques, weld quality has steadily improved, leading to an increasing diversity of welded pipe grades and specifications. Consequently, welded pipes have increasingly replaced seamless pipes across many applications. Welded pipes are categorized according to weld type into longitudinal welded pipes and spiral welded pipes. By manufacturing method, they are further divided into arc‑welded pipes, resistance‑welded pipes, gas‑welded pipes (high‑frequency and low‑frequency), and furnace‑welded pipes. Longitudinal welded pipes feature a straightforward production process, high efficiency, low cost, and rapid development. Spiral welded pipes generally exhibit higher strength than longitudinal welded pipes, enabling the use of narrower blanks to produce larger‑diameter pipes, and even allowing the same‑width blanks to yield pipes of different diameters. Nevertheless, compared with longitudinally welded pipes of equal length, spiral welded pipes typically have weld seams that are 30%–100% longer, while their production speed tends to be slower. Product Standards: Common materials used for welded pipes include Q235A, Q235C, Q235B, 16Mn, 20#, Q345, L245, L290, X42, X46, X60, X80, 0Cr13, 1Cr17, 00Cr19Ni11, 1Cr18Ni9, 0Cr18Ni11Nb, among others. The raw material for welded pipes is steel plate or steel strip, which, depending on the welding process, is classified into furnace‑welded pipes, electric‑welded pipes (resistance‑welded), and automatic arc‑welded pipes. Based on the welding configuration, they are further divided into longitudinal welded pipes and spiral welded pipes. Depending on the end‑shape, they may also be categorized as round‑ended welded pipes or irregular‑shaped welded pipes (square, flat, etc.). According to material and intended application, welded pipes can be grouped into the following categories: GB/T 3091‑2001 (galvanized steel for low‑pressure fluid conveyance).

The development of steel pipe manufacturing technology began with the rise of the bicycle industry. The early 19th-century exploitation of oil, the construction of ships, boilers, and aircraft during the two World Wars, the production of thermal power plant boilers after World War II, as well as the growth of the chemical industry and the drilling, extraction, and transportation of oil and natural gas, all strongly propelled the steel pipe industry’s advancement in product variety, output, and quality.
Typically, steel pipes are classified into two types—seamless steel pipes and welded steel pipes—based on their manufacturing process. This time, we will focus on welded steel pipes.

Welded steel pipe, also known as seam‑welded steel pipe, is manufactured by bending and rolling tube blanks—made from steel plates or steel strips—into the desired cross‑sectional shape and dimensions using various forming processes, followed by welding the seams together with different welding techniques.
Compared with seamless steel pipes, welded pipes offer higher product precision—particularly in wall‑thickness accuracy—simpler primary equipment, a smaller footprint, the ability to operate continuously, greater production flexibility, and a broader product range.

I. The production process of spiral welded steel pipe is roughly as follows:
1. The raw materials for spiral welded steel pipes include strip steel coils, welding wire, and flux.
2. Prior to forming, the strip steel undergoes leveling, edge trimming, edge planing, surface cleaning and conveying, as well as pre‑bending of the edges.
3. A weld gap control device is employed to ensure that the weld gap meets welding requirements, with strict control over pipe diameter, misalignment, and weld gap.
4. After the steel pipes are cut into individual lengths, the first three pipes of each batch must undergo a rigorous initial inspection, covering weld mechanical properties, chemical composition, fusion quality, and surface finish, as well as nondestructive testing. Only after confirming that the pipe‑making process meets all specifications may production proceed officially.
II. Longitudinal Submerged-Arc Welded Pipe:
Longitudinal Submerged Arc Welded (LSAW) pipes are typically manufactured from steel plates, which undergo various forming processes and are produced through double-sided submerged arc welding followed by post-weld sizing.
The main equipment includes edge milling machines, pre-bending machines, forming machines, pre-welding machines, and diameter‑expanding machines. Meanwhile, the forming processes for longitudinal submerged‑arc welded pipes encompass various methods, such as UO (UOE), RB (RBE), and JCO (JCOE).
The steel plate is first pressed into a U‑shape within the forming die, then further formed into an O‑shape, after which it undergoes internal and external submerged‑arc welding. Following welding, the pipe is typically expanded at the ends or along its entire length; this process yields UOE welded pipe, while pipes that are not expanded are referred to as UO welded pipe.
The steel plate is roll‑bent into shape, followed by internal and external submerged‑arc welding. After welding, the pipe is either expanded to form an RBE‑welded pipe or left unexpanded to produce an RB‑welded pipe. Alternatively, the steel plate can be formed in a J‑, C‑, or O‑shaped sequence; post‑weld expansion yields a JCOE‑welded pipe, while no expansion results in a JCO‑welded pipe.
UOE longitudinal submerged-arc welded pipe forming process:
The three primary forming processes in the UOE submerged-arc welded steel pipe manufacturing process are: pre‑bending of the steel plate edges, U‑forming, and O‑forming. Each step is performed on a dedicated forming press, sequentially executing edge pre‑bending, U‑forming, and O‑forming to transform the steel plate into a circular tubular shell. As shown in the figure:
JCOE longitudinal submerged-arc welded pipe forming process:
Forming: On the JC0 forming machine, multiple progressive stamping operations are performed. First, one half of the steel sheet is stamped into a “J” shape; then, the other half is stamped into an open “J,” resulting in a C‑shaped profile. Finally, pressure is applied at the center to form an open “0”‑shaped tube blank, as shown in the figure.
Comparison of JCO and UO forming methods:
JCO forming is a progressive pressure-forming process that transforms the steel pipe‑forming operation from the two‑step UO method into a multi‑step procedure. During forming, the steel plate deforms uniformly, residual stresses are minimized, and no surface scratches occur.
Processed steel pipes offer greater flexibility in terms of diameter and wall‑thickness specifications, enabling the production of both large‑volume and small‑batch quantities. They can manufacture high‑strength, thick‑walled pipes with large diameters as well as small‑diameter, thick‑walled pipes. In particular, they provide unparalleled advantages over other processes when producing high‑grade, thick‑walled pipes, especially those of medium and small diameters.
It can meet users’ broader requirements for steel pipe specifications. Capital investment is low, but production efficiency is relatively low, with typical annual output ranging from 100,000 to 250,000 tons.
UO molding employs two-stage pressure forming using U‑ and O‑shaped dies. It is characterized by high capacity and throughput, with annual production typically ranging from 300,000 to 1 million tonnes, making it well suited for large‑volume production of a single product specification. However, the capital investment required is substantial, often beyond the financial reach of most developing countries.
III. Longitudinal High-Frequency Welded Pipe:
Electric-resistance-welded (ERW) straight-seam welded pipe is produced by first forming hot-rolled coil steel into a tube shape using a forming mill, then heating and melting the tube edge through the skin effect and proximity effect of high-frequency current, and finally joining the edges under pressure with the aid of squeeze rolls.
Welded steel pipe, also known as welded pipe, is manufactured by rolling steel plates or strips into a cylindrical shape and then welding the seams. The production process for welded steel pipe is straightforward, with high efficiency, a wide range of grades and specifications, and relatively low capital investment; however, its strength is generally lower than that of seamless steel pipe.
Since the 1930s, with the rapid development of high‑quality continuous strip‑rolling production and advances in welding and inspection technologies, weld quality has steadily improved, the range of welded steel pipe grades and specifications has expanded, and welded pipes have increasingly replaced seamless steel pipes in a growing number of applications. Welded steel pipes are classified according to weld configuration into longitudinal welded pipes and spiral welded pipes.
Classification by manufacturing method: process-based categories include arc-welded pipes, resistance-welded pipes (high-frequency and low-frequency), gas-welded pipes, and furnace-welded pipes.
The production process for straight-seam welded pipe is simple, with high productivity and low costs, leading to rapid development. Spiral-welded pipes generally have higher strength than straight-seam welded pipes; they can be manufactured from narrower blanks to produce pipes with larger diameters, and the same‑width blanks can also be used to produce pipes of varying diameters.
However, compared with straight-seam pipes of the same length, the weld length increases by 30% to 100%, and the production speed is lower.
Product Standard
Common materials used for welded pipes include: Q235A, Q235C, Q235B, 16Mn, 20#, Q345, L245, L290, X42, X46, X60, X80, 0Cr13, 1Cr17, 00Cr19Ni11, 1Cr18Ni9, and 0Cr18Ni11Nb, among others.
Welded steel pipes are manufactured from steel plates or strip steel and, depending on the welding process, are classified into furnace‑welded pipes, electric‑welded (resistance‑welded) pipes, and automatic arc‑welded pipes. Based on the weld seam configuration, they are further divided into straight‑seam welded pipes and spiral‑welded pipes. According to the end‑shape, they are also categorized as round welded pipes and specially shaped pipes (square, flat, etc.). Depending on material composition and intended application, welded pipes are further subdivided into the following grades:
GB/T 3091-2001 (Galvanized Steel Pipes for Low-Pressure Fluid Transport)