What is Stainless Steel ERW Pipe?
Stainless steel erw pipe, also known as Electric Resistance Welding, is abbreviated as ERW. It is made of hot-rolled coil as raw material. After a series of processes such as uncoiling, leveling, and forming, the edges of the steel strip are welded together through the resistance welding process to form a straight seam steel pipe.
Resistance welding uses the resistance heat generated by electric current passing through the weldment and the contact point to locally heat the weldment to a plastic or molten state, while applying pressure to connect the weldments together. This welding method has the characteristics of high production efficiency, low cost, material saving, and easy automation, and is widely used in various industrial sectors. In the field of transportation pipes, stainless steel erw pipes occupy an important position. They can be used to transport gas and liquid objects such as oil and natural gas, and can meet various high and low pressure requirements.
Advantages of stainless steel erw pipe
1. Significant cost advantage
The production process of stainless steel erw pipe is relatively simple, with fast forming and welding speeds, which can achieve high production efficiency and greatly reduce time costs. At the same time, during the processing of the raw material steel strip, there is less waste of scraps, high utilization rate, and reduced material costs. Overall, its cost is lower than some seamless steel pipes and other similar pipes welded by complex processes, which saves money for engineering projects and has obvious advantages when used on a large scale.
2. Excellent processing performance
Because of its uniform material and stable internal structure, the incision is smooth during cutting, and there will be no problems such as edge collapse and deformation, and it can be accurately cut into the required length. During the bending process, cracks and wrinkles are not likely to occur, and it can be bent into various arcs to meet the needs of complex pipeline layouts. In terms of welding, it has good compatibility with various welding processes, high weld quality and strong sealing.
3. High dimensional accuracy
During the production process, with the help of advanced forming equipment and precise welding process control, the dimensional deviations of the pipe's outer diameter, wall thickness and other dimensions can be controlled within a very small range. For example, the outer diameter tolerance can be controlled at ±0.5mm, and the wall thickness tolerance at ±0.2mm. This high precision allows the pipes to fit more closely during installation. In petrochemical pipeline installation, high-precision stainless steel erw pipes can ensure accurate pipeline connections, reduce the difficulty of sealing processing, and improve the overall stability and safety of the pipeline system.
Disadvantages of stainless steel erw pipe
1. Slightly weaker corrosion resistance
Although the stainless steel erw pipe has a certain corrosion resistance, its surface treatment process is relatively simple, and only conventional passivation treatment is used to form a passivation film. In some highly corrosive environments, such as high-concentration acid and alkali solutions and coastal high-salt fog areas, this passivation film is difficult to resist erosion for a long time. Compared with pipes that have undergone special surface treatments, such as hot-dip galvanizing and nickel plating, it is more susceptible to corrosion and has problems such as rust and perforation, thus affecting the service life and the quality of the conveying medium.
2. Limited pressure tolerance
Stainless steel erw pipe has certain limitations in withstanding high pressure. Due to its welding process characteristics, the strength at the weld may be slightly different from that of the parent material, which limits its application in ultra-high pressure environments. Generally speaking, when the working pressure exceeds a certain value, such as 10MPa, there may be risks of leakage, rupture, etc. at the weld.
3. There are hidden dangers in welding
During the resistance welding process, the metallographic structure of the welding area is different from that of the base material, and the grain morphology and structure of the weld change. If the welding process parameters are not properly controlled, such as the welding current is too large or too small, the welding speed is unstable, etc., defects such as pores, slag inclusions, and incomplete penetration will occur in the weld. These defects will become stress concentration points, which may cause crack propagation when the pipe is subjected to pressure, vibration, and temperature changes, affecting the overall performance and safety.
Manufacturing Process of Stainless Steel ERW Pipes
(1) Material selection
Cold-rolled stainless steel strip is used as raw material, and the surface condition is BA grade. Before the steel belt is used in the factory, the steel belt must be inspected in detail. Inspection items include: thickness, width, edge quality status, appearance and physical performance inspection. When necessary, the grain size of the raw materials must be inspected, and they can only be used for production after passing the inspection.
(2) Open the book
Before the steel strip enters the welded pipe unit, it is necessary to use a steel strip uncoiler to unroll the steel strip. For the final steel strip, a leveling mechanism needs to be added to level the steel strip to ensure that the steel strip enters the welded pipe unit in a straight state. The forming part successfully completes the bending and forming of the steel strip.
(3) Tube blank forming
Using continuous roll bending forming equipment, the steel strip is continuously rolled and deformed to make an open tube blank. The steel strip needs to pass through the joint action of the introduction module, rough rolling module and closing module of the welded pipe unit. Each module uses different types of rollers to bend different parts of the steel strip. Finally, the steel strip is gradually transformed from a flat surface. Bend to form an open tube blank, ready for welding into stainless steel tubes.
(4) Welding
Electric Resistance Welding (ERW) is employed to join the edges of the stainless steel strip or coil longitudinally. A high-frequency electrical current is passed through the edges of the material, causing them to heat up and fuse together under pressure, forming a strong weld.
(5) Weld leveling
Due to the impact of the welding current and the influence of gravity, the internal welds of stainless steel product pipes will protrude and the external welds will also be dented. These problems will not have an impact if used in ordinary low-pressure fluid environments. If used in a high-temperature, high-pressure, high-speed fluid environment, it will cause usage problems. Special weld leveling equipment must be used to eliminate this defect.
(6) Online heat treatment
During the roll bending process of the steel strip, work hardening will occur, which is not conducive to the later processing of the pipe, especially the bending process. During the welding process, there will be welding stress at the weld, which will cause cracking risks during later use, which requires heat treatment.
The commonly used heat treatment process at present is bright solution treatment under a hydrogen protective atmosphere. The 304 stainless steel pipe is heated to above 1050°. After a period of heat preservation, the internal structure morphology changes to form a uniform austenite structure. In the hydrogen atmosphere Under the protection of atmosphere, no oxidation will occur.
(7) Sizing and finishing
The outer diameter of the stainless steel ERW welded pipe will deform during the welding and heat treatment process, causing the outer diameter to fail to meet the usage requirements and requires sizing and finishing. The commonly used sizing and finishing unit consists of 4 sets of flat rollers and 4 sets of side rollers. The flat rollers are designed to be flat oval (or circular), and the side rollers are designed to be oval (or circular). The sizing amount depends on the outer diameter and wall thickness of the pipe, and is generally in the range of 0.20~0.50mm. After sizing and finishing, the outer diameter tolerance of the pipe can be controlled within ±0.05mm, which can fully meet the accuracy requirements.
(8) Online flaw detection
Due to the fact that it is subject to a certain amount of pressure and requires a certain degree of reliability, it is difficult to detect internal defects in stainless steel ERW welded pipe welds by relying only on pressure detection. Online eddy current flaw detection or ultrasonic flaw detection can be used to perform 100% detection online in real time. The welding status can be corrected at any time based on the alarm information to reduce the production of unqualified products.
(9) Cutting to length
Use online fixed-length cutting equipment to perform fixed-length cutting according to the length required by the customer. You can use rotary cutting (chipless) technology. Compared with chip cutting, chipless cutting has the characteristics of no cutting burrs and does not need to be performed in the later stage. Deburring of ports.
(10) Straightening
The straightness of the welded pipes produced by the roll bending unit cannot meet the usage requirements. Generally, the straightness of such welded pipes needs to be controlled at 0.10/100mm. It is necessary to use a straightening machine to straighten the welded pipe to meet the required product requirements.
(11) Stress test
An automatic static hydraulic pressure testing machine is used to perform a pressure test on stainless steel ERW welded pipes. Loading, flushing, pressure testing, and unloading are all completed automatically. After maintaining pressure for a certain period of time, unqualified welded pipes are sorted out by the pressure difference method.
(12) Clean and dry
After the mechanical processing of stainless steel ERW welded pipes, final cleaning and drying are required to remove residual coolant, oil and other pollutants on the inner or outer walls of the pipes, and then hot air drying is used to prevent rust during transportation and storage. Finally, the finished stainless steel welded pipes are inspected and packaged before leaving the factory.
Read more: Galvanized ERW Pipe: the All-rounder of Industrial Pipes
