What is a welded pipe?

What is a welded pipe?

Welded pipe is also called welded steel pipe. It is a steel pipe made by welding and forming a steel plate or strip steel. It is usually 6 meters long. The welded steel pipe has simple production process, high production efficiency, many varieties and specifications, and low equipment investment, but the general strength is lower than that of seamless steel pipe.
Since the 1930s, with the rapid development of high-quality strip continuous rolling production and the advancement of welding and inspection technology, the quality of welded joints has been continuously improved, the variety specifications of welded steel pipes have been increasing, and more and more fields have replaced Seam steel pipe. Welded steel pipes are divided into straight welded pipes and spiral welded pipes in the form of welds. Classified according to production methods: process classification – arc welded pipe, electric resistance welded pipe, (high frequency, low frequency) gas welded pipe, furnace welded pipe.

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Small-diameter welded pipe adopts straight seam welding, and large-diameter welded pipe adopts spiral welding; according to the shape of the end of steel pipe, it is divided into round welded pipe and special-shaped (square, rectangular, etc.) welded pipe; it is divided into mining fluid according to material and use. Conveying welded steel pipes, galvanized welded steel pipes for low-pressure fluid transportation, and belt-welded steel pipes for belt conveyors. According to the specification size table in the current national standard, the outer diameter* wall thickness is sorted from small to large.
Product Standards
Commonly used materials for welded pipes are: Q235A, Q235C, Q235B, 16Mn, 20#, Q345, L245, L290, X42, X46, X60, X80, 0Cr13, 1Cr17, 00Cr19Ni11, 1Cr18Ni9, 0Cr18Ni11Nb, etc.
The blanks used for welded steel pipes are steel plates or strip steels, which are divided into furnace welded pipes, electric welding (resistance welded) pipes and automatic arc welded pipes due to different welding processes. Due to the different welding forms, it is divided into straight seam welded pipe and spiral welded pipe. Because of its shape, it is divided into round welded pipes and special-shaped (square, flat, etc.) welded pipes. Welded pipes are divided into the following varieties due to their different materials and uses:
GB/T3091-2008 (welded steel pipe for low-pressure fluid transportation): It is mainly used for conveying general lower pressure fluids and other uses such as water, gas, air, oil and heating hot water or steam. Its representative material is: Q235A grade steel.
GB/T14291-2006 (mineral fluid conveying welded steel pipe): It is mainly used for straight seam welded steel pipe for mine pressure, drainage and shaft discharge gas. It stands for material Q235A, B grade steel.
GB/T12770-2002 (stainless steel welded steel pipe for mechanical structure): mainly used for machinery, automobiles, bicycles, furniture, hotel and restaurant decoration and other mechanical parts and structural parts. It represents materials 0Cr13, 1Cr17, 00Cr19Ni11, 1Cr18Ni9, 0Cr18Ni11Nb, and the like.
GB/T12771-1991 (stainless steel welded steel pipe for fluid transportation): mainly used for conveying low pressure corrosive media. Representative materials are 0Cr13, 0Cr19Ni9, 00Cr19Ni11, 00Cr17, 0Cr18Ni11Nb, 0017Cr17Ni14Mo2, and the like.
In addition, decorative welded stainless steel pipe (GB/T 18705-2002), stainless steel welded pipe for building decoration (JG/T 3030-1995), and welded steel pipe for heat exchanger (YB4103-2000).
Production Process
Straight seam welded pipe has simple production process, high production efficiency, low cost and rapid development. The strength of the spiral welded pipe is generally higher than that of the straight welded pipe. It is possible to produce a welded pipe with a large diameter by using a narrow blank, and it is also possible to produce a welded pipe having a different pipe diameter by using a blank of the same width. However, compared with the straight pipe of the same length, the weld length is increased by 30 to 100%, and the production speed is low.
Large diameter or thick welded pipes are generally made directly from steel billets, while small welded pipe thin-wall welded pipes need only be welded directly by steel strips. Then after a simple polishing, the wire can be used.
Supplement: The welded pipe is welded with strip steel, so it has no seamless pipe height in its original position.
Welded pipe process
Raw material unwinding – leveling – end shearing and welding – looper – forming – welding – internal and external bead removal – pre-correction – induction heat treatment – sizing and straightening – eddy current testing – cutting – hydraulic inspection – pickling – final inspection (strictly checked) – packaging – shipping.
Characteristics
Straight seam welded pipe has simple production process, high production efficiency, low cost and rapid development. The strength of the spiral welded pipe is generally higher than that of the straight welded pipe. It is possible to produce a welded pipe with a large diameter by using a narrow blank, and it is also possible to produce a welded pipe having a different pipe diameter by using a blank of the same width. However, compared with the straight pipe of the same length, the weld length is increased by 30 to 100%, and the production speed is low.
Application range
The products are widely used in water supply engineering, petrochemical industry, chemical industry, electric power industry, agricultural irrigation, urban construction, and are one of the 20 key products developed in China.
For liquid transportation: water supply and drainage. For gas transportation: gas, steam, liquefied petroleum gas.
For structural use: for piling pipes, for bridges; for piers, roads, building structures, etc.
Types of welded pipes

Straight seam welded pipe
Straight-welded pipes, pipes made of hot-rolled or cold-rolled steel sheets or steel strips are directly welded to the welding equipment. (Because the welded portion of the steel pipe is in a straight line, it is named).
Large diameter straight seam welded pipe production process:
1. Panel inspection: After the steel plate used to manufacture the large-diameter submerged arc welded straight seam steel pipe enters the production line, the whole board ultrasonic inspection is first carried out;
2. Milling edge: double-sided milling the two edges of the steel plate by the edge milling machine to achieve the required plate width, edge parallelism and groove shape;
3. Pre-bend: use the pre-bending machine to pre-bend the edge of the board so that the edge of the board has the required curvature;
4. Molding: On the JCO molding machine, half of the pre-bent steel plate is first stepped and pressed into a “J” shape, and the other half of the steel plate is also bent and pressed into a “C” shape, and finally an opening is formed. “O” shape
5. Pre-welding: joints of straight seam welded steel pipes after forming and continuous welding by gas shielded welding (MAG);
6. Internal welding: welding by longitudinal multi-wire submerged arc welding (up to four wires) on the inside of a straight seam steel pipe;
7. External welding: welding by longitudinal multi-wire submerged arc welding on the outside of straight seam submerged arc welded steel pipe;
8. Ultrasonic inspection I: 100% inspection of the inner and outer welds of straight seam welded steel pipes and the base metal on both sides of the weld;
9. X-ray inspection I: 100% X-ray industrial TV inspection of internal and external welds, using image processing system to ensure the sensitivity of flaw detection;
10. Expanding diameter: Expanding the full length of the submerged arc welded straight seam steel pipe to improve the dimensional accuracy of the steel pipe and improve the distribution of stress in the steel pipe;
11. Hydraulic test: The steel pipe after diameter expansion is tested on the hydraulic testing machine to ensure that the steel pipe meets the test pressure required by the standard. The machine has automatic recording and storage functions;
12. Chamfering: The steel pipe after passing the inspection shall be processed at the end of the pipe to achieve the required pipe end groove size;
13. Ultrasonic inspection II: Ultrasonic inspection is carried out again and again to check for defects that may occur after straightening and water pressure of straight seam welded steel pipes;
14. X-ray inspection II: X-ray industrial TV inspection and tube end weld filming of the steel pipe after the expansion and hydrostatic test;
15. Tube End Magnetic Particle Inspection: Perform this inspection to find tube end defects;
16. Anti-corrosion and coating: The qualified steel pipe is preserved and coated according to user requirements.
Stainless steel welded pipe
Stainless steel welded pipe, commonly used stainless steel or stainless steel strips are welded through the unit and the mold after crimp forming. The welded steel pipe has simple production process, high production efficiency, many varieties and specifications, and low equipment investment, but the general strength is lower than that of seamless steel pipe.
Stainless steel welded pipe features
First, the small-diameter stainless steel welded pipe is continuously produced online. The thicker the wall thickness, the greater the investment of the unit and the welding equipment, and the less economical and practical it is. The thinner the wall thickness, the lower the input-output ratio will be. Secondly, the process of the product determines its advantages and disadvantages. Generally, the welded steel pipe has high precision, uniform wall thickness, and high brightness inside and outside the pipe (the steel pipe is determined by the surface grade of the steel plate). Surface brightness), can be arbitrarily fixed. Therefore, it demonstrates its economy and aesthetics in high-precision, medium- and low-pressure fluid applications.
Calculation formula for the weight of stainless steel welded pipe
Commonly used: material 304/304L (diameter – wall thickness) x wall thickness x0.02491x length = weight (kg)
Material 316/316L (diameter – wall thickness) x wall thickness x0.02513x length = weight (kg)
Unit: kg/m
Furnace welded pipe
The furnace-welded pipe is a steel pipe which is welded and formed by a steel strip or a steel plate and deformed into a circular shape or a square shape, and has a joint on the surface. When manufacturing a continuous furnace welded pipe, the heating temperature in the width direction of the strip is required to be different, that is, the heating temperature of the strip edge is higher than the temperature in the middle of the strip. The higher heating temperature of the strip edge is to ensure the quality of the weld, while the lower heating temperature in the middle of the strip is to maintain the strength of the strip when the strip is pulled from the furnace by a forming welder.
Production characteristics of furnace welded pipe
The steel pipe produced by the furnace welding method is to heat the strip to a welding temperature of 1350 to 1400 ° C, and then is pressed and welded into a steel pipe by a boring welding machine as shown in the following figure.
The continuous furnace welded pipe unit is a high-productivity production welded pipe equipment. According to the product specifications, the unit can be divided into three types: large (425~100mm), medium (Φ15-75mm) and small (Φ5~40mm). The unit has high production efficiency, low cost, high mechanization and high degree of automation, and the speed is as high as 420~680m/min. The productivity of one unit is about 6-7 times higher than that of the same size welded tube unit. Furnace welded pipe costs are about 20% lower than welded pipes and 30% lower than seamless pipes. However, the weld strength is lower than that of the electric welded pipe, and is generally limited to the welding of low-carbon boiling steel pipes, and is mainly used as a water gas pipe, a cable protection pipe, and a structural pipe. However, due to the high energy consumption of the furnace welded pipe, its further development is limited.
Electric welded pipe
The electric welded pipe utilizes the skin effect and the proximity effect of the high-frequency current to cause the current concentrated on the edge of the blank to heat the joint surface to the welding temperature, and then to the welded pipe by extrusion and rolling. There are two types of high frequency welding: induction welding and contact resistance welding. When the steel pipe is welded, the frequency of the current is 350~450kHZ, and the induction welding can be selected lower, but the minimum is not less than 150kH: when the non-ferrous metal pipe is welded, the current frequency is not less than 450kH:. The working principle of high frequency welding is a schematic diagram of the principle of high frequency induction welded pipe. The inductor and the tube blank correspond to the primary and secondary coils of the transformer. When the inductor passes the high-frequency current, the outer surface of the tube blank generates an induced current, a V-shaped loop I along the edge of the blank, called the welding current, for useful work; and a shunt I: along the surface of the blank, called the circulating current Only the temperature rise of the tube blank causes heat loss and is useless. In order to reduce the loss of circulating current and improve the efficiency of electromagnetic induction, a resistor is placed in the tube to increase the inductive value of the inner surface of the tube.

Electrical resistance welding

Electrical resistance welding - What is a welded pipe?
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Electrical resistance welding will be welders parts formed pressed between two electrodes, and subjected to current, the resistor thermal effects generated by the current flowing through contact with the workpiece surface and the neighboring regions to be heated to melt the plastic state, so thatmetal-binding method. Resistance welding methods are mainly four kinds, namely, spot welding, seam welding, projection welding, butt welding.

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ERW pipes means Electric Resistance Welded Pipes

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Teleparty welding of ERW pipe - What is a welded pipe?
Teleparty welding of ERW pipe

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A plate rolled to become a pipe and welded using Electric Resistance Welding process.

Spot welding (Spot Welding)

Spot welding assembly into the lap joint, and pressed between two cylindrical electrodes, the resistance of hot-melt the base metal to form a solder joint resistance welding method. Spot welding is mainly used for sheet metal welding.

Seam welding (Seam Welding)

Seam welding and spot welding, just a rotating disc-shaped wheel electrode instead of the columnar electrode and the weldment assembly into a lap or butt joint, and placed between the electrodes of the two wheel roller pressure welding and rotation, continuous or intermittent transmission to form a continuous weld resistance welding method.

Projection welding

Projection welding is a variant form of spot welding; pre-bump in a workpiece, projection welding, once formed in the joints of one or more of the nugget.

Aeronautics and Astronautics, aerospace, electronics, automotive, household appliances, industrial development, resistance increasingly wide attention. Meanwhile, resistance welding quality also put forward higher requirements. The good news is that the development of microelectronics technology in China and high-power thyristor rectifier development, resistance welding technology to improve the conditions.

At present, China has produced the excellent performance of the secondary rectifier welder. integrated circuits and micro-computer control box for the new welder supporting the transformation of the old welder. Constant current, dynamic resistance, and thermal expansion of advanced closed-loop control technology has been in production application. All help to improve the welding quality, and expand its application areas.

ERW steel pipes and tube are available in various qualities, wall thicknesses, and diameters of the finished pipes.

Technical requirements For oil and gas transport For low pressure fluid conveying 
Material Gr.b Gr.b
Pipe body diameter D<508mm, ±0.75%; D≥508mm, ±0.75% D≤168.3, ±1.0%;  168.3<D≤508,±0.75%;  
Wall thickness D<508mm,+15.0%,-12.5%; D≥508mm, +17.5%,-10% ±12.5%
Bending ≤0.2% ≤0.2%
Ovality D≥508mm,≤±1% ≤±0.75%
Bevel ≤1.59mm ≤5mm
Hydrostatic testing  100%  100% 
Nondestructive testing 100% non-destructive weld inspection Ultrasonic flaw detection is 100%

ERW pipe technical requirements

ERW steel pipes and tubes are used in various engineering purposes, fencing, scaffolding, line pipes etc.

  • Yield strength of the N80 is higher than the J55 up to 173 ~ 206 MPa.
  • On the same area of the sample, the elongation of N80 is higher than J55.
  • Under the same size, the same sample orientation, the same minimum sample size, J55 grade couplings, coupling stock, coupling material, semi-finished and coupling attachment material can absorb lower than N80 steel grade.
  • The nondestructive testing methods of seamless pipe,coupling stock, welded tube with J55 and N80 is differeent.
  • J55 and N80 steel grade is not the same color: length greater than or equal 1.8m, J55 painted a bright green, N80 painted a red;
  • J55 chosen by the manufacturer, or organize according to the order specified length normalized (N), normalizing and tempering (N & T) or quenching and tempering (Q & T). N80 is a whole, full-length heat treatment is mandatory. By the manufacturer selected for normalizing (N) or normalizing and tempering (N & T) + quenching (Q).

Spiral welded pipe
Spiral welded pipe: It is made by rolling a low carbon carbon structural steel or a low alloy structural steel strip into a tube blank at a certain angle of a helix (called a forming angle), and then welding the pipe joint to make it narrower. Strip steel produces large diameter steel pipes. The specification shall be expressed by the outer diameter * wall thickness. The welded pipe shall ensure that the hydraulic pressure test, the tensile strength of the weld and the cold bending performance are in compliance with the regulations.

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Advantages and disadvantages of spiral welded pipe
Advantages of spiral welded pipe:
(1) Steel pipes of the same width can be used to produce steel pipes of different diameters, especially for producing large diameter steel pipes with narrow strip steel.
(2) Under the same pressure conditions, the spiral weld is subjected to less stress than the straight seam, which is 75% to 90% of the straight welded pipe, so it can withstand a large pressure. Compared with the straight seam welded pipe of the same outer diameter, the wall thickness can be reduced by 10% to 25% under the same pressure.
(3) The size is accurate, the general diameter tolerance is not more than 0.12%, the deflection is less than 1/2000, and the ellipticity is less than 1%. Generally, the sizing and straightening process can be omitted.
(4) It can be continuously produced. In theory, it can produce infinitely long steel pipes, with small loss of cutting head and tail cutting, and can improve metal utilization rate by 6% to 8%.
(5) Compared with straight seam welded pipe, it is flexible in operation and convenient in changing varieties.
(6) The equipment is light in weight and has low initial investment. It can be made into a trailer-type mobile unit, and the welded pipe can be directly produced at the construction site where the pipeline is laid.
(7) It is easy to realize mechanization and automation.
The disadvantage of the spiral welded pipe is that due to the use of the coiled steel strip as the raw material, there is a certain crescent bend, and the welding point is in the elastic strip edge region, so it is difficult to align the welding torch and affect the welding quality. To do this, set up complex weld tracking and quality inspection equipment.
Spiral welded pipe production process
Spiral welded pipe is also one of the welded pipe equipments. Its strength is generally higher than that of straight welded pipes. It can produce welded pipes with larger diameters with narrower blanks, and can also produce welded pipes with different pipe diameters with blanks of the same width. However, compared with the straight pipe of the same length, the weld length is increased by 30 to 100%, and the production speed is low. Therefore, most of the smaller diameter welded pipes are straight seam welded, and the large diameter welded pipes are mostly spiral welded.
Spiral steel pipe production process: It is a spiral seam steel pipe which is made of steel coiled coil as raw material, which is often heated and extruded, and welded by automatic double-wire double-sided submerged arc welding process.
1. Raw materials are steel coils, welding wires and fluxes. Strict physical and chemical testing is required before input.
2. The steel head and tail are butt jointed, and the single wire or double wire submerged arc welding is used. After the steel pipe is rolled into a steel pipe, automatic submerged arc welding is used.
3. Before forming, the strip is leveled, trimmed, planed, surface cleaned and treated.
4. The electric contact pressure gauge is used to control the pressure of the cylinder on both sides of the conveyor to ensure the smooth conveying of the strip.
5, using external control or internal control roll molding.
6. The weld gap control device is used to ensure that the weld gap meets the welding requirements, and the pipe diameter, the amount of misalignment and the weld gap are strictly controlled.
7. Both internal and external welding are performed by single-wire or double-wire submerged arc welding using a Lincoln electric welder in the United States to obtain stable welding specifications.
8. The welded welds are inspected by online continuous ultrasonic automatic injury inspection to ensure the non-destructive testing coverage of 100% spiral welds. If there is a defect, the alarm is automatically alarmed and sprayed, and the production worker adjusts the process parameters at any time to eliminate defects in time.
9. Cut the steel tube into a single root using an air plasma cutting machine.
10. After cutting into a single steel pipe, the first three inspections of each batch of steel pipe shall be subjected to a strict first inspection system to check the mechanical properties, chemical composition, fusion condition, surface quality of the steel pipe and non-destructive testing to ensure the pipe production process. After that, it can be officially put into production.
11. The part of the weld with continuous acoustic flaw detection mark, after manual ultrasonic and X-ray review, if it is defective, after repairing, it will pass the non-destructive test again until it is confirmed that the defect has been eliminated.
12. The tube of the butt weld of the strip and the joint of the butt joint intersecting the spiral weld are all inspected by X-ray TV or film.
13. Each steel pipe is subjected to hydrostatic pressure test and the pressure is sealed by radial. The test pressure and time are strictly controlled by the steel pipe water pressure microcomputer detection device. The test parameters are automatically printed and recorded.
14. The pipe end is machined to make the end face verticality, groove angle and blunt edge accurately controlled.
Anti-corrosion spiral welded pipe technology
Since the individual quality of the spiral welded pipe is relatively large, it must be stacked outdoors, but it is difficult to avoid being exposed to the sun and rain, so the problem of rust has been plagued by the storage time and conditions of the spiral welded pipe. Therefore, it is necessary for us to provide a comprehensive answer to the rust prevention knowledge of spiral welded pipes.
The surface of the steel is mainly polished by using a wire brush or the like, and the cleaning and preheating of the spiral welded pipe can remove the loose or raised scale, rust, welding slag and the like. The rust removal of the hand tool can reach the Sa2 level, and the rust removal of the power tool can reach the Sa3 level. If the surface of the steel is adhered to the iron oxide scale, the rust removal effect of the tool is not ideal, and the anchor depth required for the anti-corrosion construction cannot be achieved.
Pickling Use a solvent or emulsion to clean the surface of a welded steel pipe (welded pipe) for low-pressure fluid transport to remove oil, grease, dust, lubricants and similar organic matter, but it does not remove rust, scale, flux, etc. from the steel surface. Therefore, it is only used as an auxiliary means in anti-corrosion production.
Generally, chemical cleaning and electrolysis are used for pickling treatment. Pipeline anti-corrosion is only chemical pickling, which can remove scale, rust and old coating, and can sometimes be used as re-treatment after sandblasting and rust removal. Although chemical cleaning can achieve a certain degree of cleanliness and roughness, the anchor pattern is shallow and it is easy to cause pollution to the spiral welded pipe stacking environment.
Reasonable use means to ensure the long-term service of the pipe fittings when cleaning the rusty pipe fittings, and create more production benefits.
ERW straight seam welded pipe
ERW straight seam welded pipe, also known as “resistance welded straight seam welded pipe”, ERW straight seam welded pipe is mainly divided into two forms: ERW AC welded steel pipe and ERW DC welded steel pipe. ERW straight seam welded pipes are divided into low frequency welding, medium frequency welding, super intermediate frequency welding and high frequency welding according to the frequency. High-frequency welding is mainly used for the production of thin-walled steel pipes or ordinary wall-thickness steel pipes. High-frequency welding is divided into contact welding and induction welding.
ERW straight seam welded pipe is generally used for wire casing in the power industry. Performance characteristics: 100% ultrasonic inspection of the base metal to ensure the inner quality of the pipe body; no unwinding–disc shearing process, base metal puddle Less scratches; there is basically no residual stress in the finished pipe after stress elimination; the weld bead is short, the probability of defects is small; the wet acid natural gas can be transported conditionally; after the diameter expansion, the geometrical accuracy of the steel pipe is high; After the molding is completed, it is carried out in a straight line at a horizontal position. Therefore, the wrong side, the slit, the pipe diameter circumference are better controlled, and the welding quality is excellent. The main products of Tianjin Shenzhoutong Steel Pipe Co., Ltd. are divided into three categories: two straight seam steel pipe production lines, the product specifications are 4.3114.3mm-Ф508mm, and the wall thickness is 3.58mm-16.3mm. Ten spiral pipe production lines, product specifications Ф219.1mm-Ф3000mm, wall thickness 5mm-30mm. Two oil-specific oil casing production lines, product specifications Ф60.32mm-Ф508mm wall thickness 4.24mm-16.3mm. There are two supporting anti-corrosion insulation production lines. The annual output of steel pipes is 500,000 tons. Product executable standards: API 5L, API 5CT, ASTM, EN10219-2, GB/T9711, 3091-20081, 3793-2008, 14291-2006 and other international latest standards. Product grades include: GRB, X42, X52, X60, X65, X70, J55, K55, N80, L80, P110, and other steel grades. Products are widely used in oil, natural gas, gas, coal, machinery, electricity, piling and other purposes. The company has selected international advanced technology and equipment: such as W-FF molding, solid-state high-frequency induction welding, ultrasonic flaw detection, magnetic flux leakage inspection, and high-end testing equipment: such as metallographic analysis, Vickers hardness tester, impact tester, spectrum Analyzers, universal testing machines and other equipment. Since its establishment, it has obtained various international certifications and honors: such as API 5L, API5CT, ISO9002-2000 pressure pipeline production license, industrial product production license, and China Petroleum and Petrochemical membership card, China Quality Product Certificate, Tianjin Famous Trademarks, Tianjin Exemption Products, Tianjin Top 100 Private Enterprises and other honors. Products are exported to South America, North America, the European Union, Southeast Asia and other places. It has been well received by customers all over the world for many years.
Main use of ERW straight seam welded pipe
ERW straight seam welded pipes are mainly used in domestic water supply engineering, petrochemical industry, chemical industry, electric power industry, agricultural irrigation and urban construction. For liquid transportation: water supply and drainage. For gas transportation: gas, steam, liquefied petroleum gas. For structural use: for piling pipes, for bridges; for piers, roads, building structures, etc.
Quenching treatment of ERW straight seam welded pipe
ERW straight seam welded pipe surface quenching and tempering heat treatment is usually carried out by induction heating or flame heating. The main technical parameters are surface hardness, local hardness and effective hardened layer depth. The hardness test can be carried out using a Vickers hardness tester or a Rockwell or surface Rockwell hardness tester. The choice of test force (scale) is related to the effective hardened layer depth and the surface hardness of the straight seam welded pipe. There are three hardness testers involved here.
The Vickers hardness tester is an important means to test the surface hardness of heat-treated straight seam welded pipes. It can be used to test the surface hardened layer as thin as 0.05mm thick with a test force of 0.5-100kg. The precision is the highest and the surface of the heat-treated workpiece can be distinguished. A small difference in hardness. In addition, the depth of the effective hardened layer is also measured by a Vickers hardness tester. Therefore, it is necessary to equip a unit with a surface heat treatment or a large number of surface heat treatments for the Hunan straight seam welded pipe.
The surface Rockwell hardness tester is also very suitable for testing the hardness of surface hardened workpieces. The surface Rockwell hardness tester has three scales to choose from. Various surface hardened straight seam welded pipes with an effective hardening depth of more than 0.1 mm can be tested. Although the accuracy of the surface Rockwell hardness tester is not as high as the Vickers hardness tester, it has been able to meet the requirements as a means of quality management and inspection of the heat treatment plant. Moreover, it has the characteristics of simple operation, convenient use, low price, rapid measurement, and direct reading of hardness value. The surface Rockwell hardness tester can be used for batch-type surface heat treatment of Hunan straight seam welded pipe for rapid and non-destructive part-by-piece inspection. . This is of great significance for the production of lake seam welded pipe and machinery manufacturing plants.
When the surface heat-treated hardened layer is thick, a Rockwell hardness tester can also be used. When the thickness of the heat-treated hardened layer is 0.4 to 0.8 mm, the HRA scale can be used, and when the thickness of the hardened layer exceeds 0.8 mm, the HRC scale can be used.
Vickers, Rockwell and Surface Rockwell values can be easily converted to each other and converted to standard, drawing or user-required hardness values. The corresponding conversion tables are given in the international standard ISO, the American standard ASTM and the Chinese standard GB/T.
If the local hardness of the part is high, the local quenching heat treatment can be carried out by means of induction heating. Such a straight seam welded pipe is usually marked with a local quenching heat treatment position and a local hardness value on the drawing. The hardness test of the straight seam welded pipe shall be carried out in the designated area. The hardness testing instrument can use the Rockwell hardness tester to test the HRC hardness value. If the heat treatment hardened layer is shallow, the surface Rockwell hardness tester can be used to test the HRN hardness value.
Welded pipe inspection method
There are many methods for quality inspection of welded pipes, among which physical methods are also the most commonly used inspection methods. Physical inspection is a method of measuring or testing using some physical phenomena. Inspection of internal defects of materials or Q235B straight seam welded pipes is generally carried out by means of non-destructive testing. The current non-destructive testing includes magnetic flaw detection, ultrasonic flaw detection, radiographic inspection, and penetrant inspection.
Magnetic test
Magnetic flaw detection can only find defects on the surface and near surface of the magnetic welded pipe, and only quantitative analysis can be performed on the defects. The nature and depth of the defects can only be estimated based on experience. The magnetic test is to find the defects by magnetic flux magnetization of the ferromagnetic Q235B straight seam welded pipe. According to the method of measuring magnetic flux leakage, it can be divided into magnetic powder method, magnetic induction method and magnetic recording method, among which magnetic powder method is the most widely used.
Penetration test
Penetration testing uses physical properties such as permeability of certain liquids to detect and display defects, including both coloring and fluorescence detection, and can be used to examine defects on the surface of ferromagnetic and non-ferromagnetic materials.
Radiographic inspection
Radiographic inspection is a method of detecting defects by using radiolucent materials and attenuating properties in the material. According to the different rays used in flaw detection, it can be divided into three types: X-ray inspection, γ-ray inspection and high-energy radiation inspection. Because of the different methods of displaying defects, each type of radiographic inspection is divided into ionization method, screen observation method, photographic method and industrial television method. The ray inspection is mainly used to inspect cracks, incomplete penetration, pores, slag inclusions, etc. inside the welded pipe weld.
Ultrasonic flaw detection
Ultrasonic waves can be used for the inspection of internal defects due to reflections at the interface of different media in the propagation of metals and other homogeneous media. Ultrasonic can inspect any weldment material, any part of the defect, and can detect the location of the defect more sensitively, but it is difficult to determine the nature, shape and size of the defect. Therefore, ultrasonic testing of welded pipes is often used in conjunction with radiographic inspection.
Welded steel pipes shall be subjected to mechanical performance tests, flattening tests and flare tests, and shall meet the requirements specified in the standard. The steel pipe should be able to withstand a certain internal pressure, and if necessary, carry out a pressure test of 2.5Mpa to maintain no leakage for one minute. It is allowed to replace the hydrostatic test by means of eddy current testing. Eddy current testing is carried out according to GB7735 “Steel Flow Test Method for Steel Tubes”. The eddy current testing method is to fix the probe on the frame, and the flaw detection and the weld seam are kept at a distance of 3~5mm. The weld is fully scanned by the rapid movement of the steel pipe, and the flaw detection signal is automatically processed and automatically sorted by the eddy current flaw detector. To achieve the purpose of testing. The welded pipe after the flaw detection is cut with a flying saw according to the specified length, and is turned off the frame by the flip frame. Both ends of the steel pipe should be chamfered with flat heads, printed with marks, and the finished pipe is packaged with hexagonal packaging and shipped.
Welded pipe material
Commonly used materials in China are generally Q235A, Q235B, 0Cr13, 1Cr17, 00Cr19Ni11, 1Cr18Ni9, 0Cr18Ni11Nb, 16Mn, 20#, Q345, L245, L290, X42, X46, X70, X80 and so on.
Among them, X42, X46, X56, X80 and other materials are API standard materials, which are not commonly used materials for pipes in China.
Use of welded pipe
Straight seam steel pipes are mainly used in domestic water supply engineering, petrochemical industry, chemical industry, electric power industry, agricultural irrigation, and urban construction.
For liquid transportation: water supply and drainage.
For gas transportation: gas, steam, liquefied petroleum gas.
For structural use: for piling pipes, for bridges; for piers, roads, building structures, etc.

1. Welded steel pipe for low-pressure fluid transportation (GB/T3092-1993) is also called general welded pipe, commonly known as black pipe. It is a welded steel pipe used for conveying general lower pressure fluids such as water, gas, air, oil and heating steam and other purposes. The thickness of the steel pipe is divided into ordinary steel pipe and thick steel pipe; the pipe end form is divided into non-threaded steel pipe (light pipe) and threaded steel pipe. The specification of the steel pipe is expressed by the nominal diameter (mm), and the nominal diameter is an approximate value of the inner diameter. The welded steel pipe for low-pressure fluid transportation is mainly used as a raw pipe for galvanized welded steel pipe for low-pressure fluid transportation, in addition to being directly used for conveying fluid.
2. Galvanized welded steel pipe for low pressure fluid transportation (GB/T3091-1993) is also called galvanized electric welded steel pipe, commonly known as white tube. It is a hot dip galvanizing (furnace welding or electric welding) steel pipe for transporting water, gas, air, oil, heating steam, warm water and other general lower pressure fluids or other purposes. The thickness of the steel pipe is divided into ordinary galvanized steel pipe and thick galvanized steel pipe; the connection end form is divided into non-thread galvanized steel pipe and threaded galvanized steel pipe.
3. Ordinary carbon steel wire casing (GB3640-88) is a steel pipe used to protect electric wires in electrical installation projects such as industrial and civil construction, installation machinery and equipment.
4. Straight seam electric welded steel pipe (YB242-63) is a steel pipe whose weld is parallel to the longitudinal direction of the steel pipe. Usually divided into metric electric welded steel pipe, welded thin-walled pipe, transformer cooling oil pipe and so on.
5. Spiral-slot submerged arc welded steel pipe (SY5036-83) for pressure-bearing fluid transportation is a hot-rolled steel strip coil for tube blank, which is often spiral-formed and welded by double-sided submerged arc welding for pressure fluid transportation. Spiral seam steel pipe. The steel pipe has strong pressure bearing capacity and good welding performance. It is safe and reliable after various strict scientific tests and tests. The steel pipe has a large caliber, high conveying efficiency, and can save investment in laying pipelines. It is mainly used to transport oil and natural gas pipelines.
A simple comparison between the technical characteristics of spiral welded pipe and straight welded pipe:
Metallurgical properties of materials
Straight seam submerged arc welded pipes are produced from steel plates, and spiral welded pipes are produced from hot rolled coils. The hot rolling strip unit rolling process has a series of advantages, and has the metallurgical process capability for producing high quality pipeline steel. For example, a water cooling system is installed on the output gantry to accelerate cooling, which allows the use of low alloying compositions to achieve special strength levels and low temperature toughness, thereby improving the weldability of the steel. But this system is basically not available in steel plate production plants. The alloy content (carbon equivalent) of the coiled sheet is often lower than that of similar grades, which also improves the weldability of the spiral welded pipe.
It should be further noted that since the coil rolling direction of the spiral welded pipe is not the direction of the vertical steel pipe axis (the pinching depends on the helix angle of the steel pipe), the rolling direction of the straight steel pipe is perpendicular to the axial direction of the steel pipe, and thus, the spiral The crack resistance of the welded pipe material is superior to that of the straight seam steel pipe.
Welding process
From the welding process, the welding method of spiral welded pipe and straight seam steel pipe is the same, but the straight seam welded pipe will inevitably have many T-weld joints, so the probability of welding defects is greatly improved, and the welding residual at the T-weld joint The stress is large, and the weld metal tends to be in a three-direction stress state, increasing the possibility of cracking.
Moreover, according to the submerged arc welding process regulations, each weld should have an arc starting point and an arc extinction point, but each straight seam welded pipe cannot meet this condition when welding the circumferential joint, and thus there may be More welding defects.
Strength characteristics
When the tube is subjected to internal pressure, it usually produces two main stresses on the pipe wall, namely radial stress δY and axial stress δX. The synthetic stress at the weld is δ=δY(l/4sin2α+cos2α)1/2, where α is the helix angle of the spiral welded pipe weld.
The spiral angle of the spiral welded pipe weld is generally 50-75 degrees, so the synthetic stress at the spiral weld is 60-85% of the main stress of the straight welded pipe. Under the same working pressure, the thickness of the spiral welded pipe of the same pipe diameter can be reduced compared with the straight welded pipe.
According to the above characteristics, we can know:
A. When the spiral welded pipe is blasted, since the normal stress and the synthetic stress of the welded joint are relatively small, the blasting hole generally does not originate from the spiral welded joint, and its safety is higher than that of the straight welded pipe.
B. When there is a defect parallel to the spiral weld, since the screw weld is less stressed, the risk of expansion is not as great as that of the straight weld.
C. Since the radial stress is the maximum stress present on the steel pipe, the weld is subjected to the maximum load in the direction of the vertical stress. That is, the straight seam is subjected to the largest load, and the hoop weld is subjected to the least load, and the spiral seam is between the two.
Static pressure burst strength
According to the relevant comparison test, it is verified that the yield pressure of the spiral welded pipe and the straight welded pipe is basically consistent with the measured value and theoretical value of the bursting pressure, and the deviation is close. However, whether it is yield pressure or burst pressure, the spiral welded pipe is lower than the straight welded pipe. The blasting test also showed that the circumferential deformation rate of the burrow of the spiral welded pipe was significantly larger than that of the straight welded pipe. It is confirmed that the plastic deformation ability of the spiral welded pipe is better than that of the straight welded pipe. The blasting hole is generally limited to one pitch, which is caused by the strong restraining effect of the spiral weld on the expansion of the crack.
Toughness and fatigue strength
The trend of pipeline development is large diameter and high strength. As the diameter of the steel pipe increases and the steel grade used increases, the tendency of the ductile fracture to expand and expand is greater. According to tests conducted by relevant research institutions in the United States, although the spiral welded pipe and the straight welded pipe are of the same grade, the spiral welded pipe has high impact toughness.
Due to the change of the transmission line, the steel pipe is subjected to random alternating load during the actual operation. Understanding the low cycle fatigue strength of steel pipes is of great significance for judging the service life of pipelines.
According to the measurement results, the fatigue strength of the spiral welded pipe is the same as that of the seamless pipe and the electric resistance welded pipe. The test data is distributed in the same zone as the seamless pipe and the resistance pipe, and is higher than the general buried arc straight seam welded pipe.
Field solderability
The weldability of the site is mainly determined by the material of the steel pipe and the dimensional tolerance of the port.
Considering the requirements for the installation and construction of steel pipes, the consistency of the steel pipe processing and the consistency of the geometrical dimensions are particularly important.
Spiral welded pipe production is a continuous continuous process that is basically stable under the same working conditions: the straight seam welded pipe production process is segmented, including whole plate / indenter / preroll / spot welding / welding / finishing / group pairing Multiple process steps. This is an important feature of spiral welded pipe production that is different from the production of straight welded pipe.
Stable production conditions are very convenient for welding quality control and geometric size assurance. Due to the regularity of the spiral welded pipe and the even distribution of the weld, the spiral steel pipe has a very good ellipticity and end face perpendicularity with respect to the straight welded pipe, which ensures the accuracy of the pair alignment of the on-site steel pipe welding group.
Effect on the flow characteristics of the transport medium
The pressure drop in the transfer line is proportional to the length of the tube, the fluid viscosity coefficient, the fluid velocity, and the fluid drag coefficient, and inversely proportional to the inner diameter of the tube. The fluid drag coefficient is related to both the Reynolds number and the roughness of the inner wall surface of the tube. It has been determined that the roughness of the inner wall surface of the pipe has an effect ten times greater than that of the local raised surface (such as a spiral weld or a longitudinal weld or even an inner annular weld).
Price analysis
Due to the high technical quality and high technical requirements of the hot rolled coil, on the one hand, the domestic manufacturers meet the standards, and the production technology and quality grade determine that the market price is higher than the heat. Rolled steel plate. This is the main reason why the market price of spiral welded pipe is higher than that of straight welded pipe. For the composition of the sales price of steel pipes, material prices are dominant or even decisive factors.
Careful examination of the price difference between spiral welded pipe and straight welded pipe, the price of spiral welded pipe is slightly higher than that of straight welded pipe due to the difference in price of main material. However, steel pipe production is only a part of the project. If considering the overall quality of the project and the overall cost of the project, the spiral welded pipe still has an overall advantage.

Classification of welded pipes
Classified by purpose
Also divided into general welded pipe, galvanized welded pipe, blown oxygen welded pipe, wire casing, metric welded pipe, idler pipe, deep well pump pipe, automobile pipe, transformer pipe, electric welding thin wall pipe, electric welding shaped pipe, scaffold pipe and spiral welded pipe .
General welded pipe: Generally welded pipe is used to transport low pressure fluid. Made of Q195A, Q215A, Q235A steel. It can also be made of other mild steel that is easy to weld. The steel pipe is subjected to experiments such as water pressure, bending and flattening, and has certain requirements on the surface quality. Usually, the delivery length is 4-10 m, and the fixed length (or double rule) is often required for delivery. The specification of the welded pipe is expressed by the nominal diameter (mm or inch). The nominal diameter of the welded pipe is different from the actual one. The welded pipe has two kinds of ordinary steel pipe and thick steel pipe according to the specified wall thickness. The steel pipe is divided into two types according to the pipe end form with and without thread.
Galvanized steel pipes: In order to improve the corrosion resistance of steel pipes, general steel pipes (black pipes) are galvanized. Galvanized steel pipes are divided into hot-dip galvanizing and electric steel zinc. The hot-dip galvanizing galvanized layer is thick and the cost of electro-galvanizing is low.
Blowing oxygen welded pipe: used as a steel pipe for oxygen blowing, generally with small diameter welded steel pipe, the specifications are from 8/8 inch to 2 inch. Made of 08, 10, 15, 20 or Q195-Q235 steel strip. For corrosion protection, some are aluminized.
Wire bushing: It is a common carbon steel electric welded steel pipe used in concrete and various structural power distribution projects. The commonly used nominal diameter is from 13-76mm. The sleeve of the wire sleeve is thin, and most of it is coated or galvanized, and a cold bend test is required.
Metric welded pipe: Specification for seamless pipe form, welded steel pipe with outer diameter * wall thickness mm, tropical or cold strip welding with ordinary carbon steel, high quality carbon steel or plain low alloy steel, or tropical welding It is then made by cold dialing. Metric welded pipes are divided into general and thin walls, which are commonly used as structural members, such as transmission shafts, or for conveying fluids. Thin walls are used to produce furniture, lamps, etc., to ensure the strength and bending test of steel pipes.
Roller tube: used for belt conveyor electric welding steel pipe, generally made of Q215, Q235A, B steel and 20 steel, diameter 63.5-219.0mm. The pipe bending degree, the end surface should be perpendicular to the center line, and the ellipticity is required. Generally, the water pressure and the flattening test are performed.
Transformer tube: used to manufacture transformer heat pipe and other heat exchangers. It is made of ordinary carbon steel and requires flattening, flare, bending and hydraulic testing. Steel pipes are delivered in fixed lengths or multiples, which have certain requirements for the bending of steel pipes.
Shaped tube: square tube, rectangular tube, hat tube, empty steel door and window steel tube welded by ordinary carbon knot structural steel and 16Mn steel, mainly used as agricultural machinery components, steel window doors, etc.
Electric welding thin-walled pipe: mainly used for making furniture, toys, lamps, etc. In recent years, thin-walled tubes made of stainless steel strips have been widely used, such as high-grade furniture, decoration, and fences.
Spiral welded pipe: It is made by rolling a low carbon carbon structural steel or a low alloy structural steel strip into a tube blank at a certain angle of a helix (called a forming angle), and then welding the pipe joint to make it narrower. Strip steel produces large diameter steel pipes. Spiral welded pipe is mainly used for oil and natural gas transmission pipelines, and its specifications are expressed by outer diameter * wall thickness. Spiral welded pipes are single-sided welded and double-sided welded. The welded pipe shall ensure that the hydraulic test and the tensile strength and cold bending performance of the weld are in compliance with the regulations.
Classified by production method
(1) According to the process – arc welded pipe, electric resistance welded pipe (high frequency, low frequency), gas welded pipe, furnace welded pipe.
(2) According to the weld seam – straight seam welded pipe, spiral welded pipe.
Classified by section shape
(1) Simple section steel pipe – round steel pipe, square steel pipe, elliptical steel pipe, triangular steel pipe, hexagonal steel pipe, diamond steel pipe, octagonal steel pipe, semicircular steel circle, and others.
(2) Complex section steel pipe—unequal hexagonal steel pipe, five-petal plum-shaped steel pipe, double convex steel pipe, double concave steel pipe, melon-shaped steel pipe, conical steel pipe, corrugated steel pipe, case steel pipe, and others.
Classified by wall thickness
Thin-walled steel pipes and thick-walled steel pipes.
Classified by end shape
Divided into round welded pipe and shaped (square, flat, etc.) welded pipe.
Classification supplement
1. Ordinary carbon steel wire casing (GB/T3640-88) is a steel pipe used to protect electric wires in electrical installation projects such as industrial and civil construction, installation machinery and equipment.
2. Straight seam electric welded steel pipe (YB242-63) is a steel pipe whose weld is parallel to the longitudinal direction of the steel pipe. Usually divided into metric electric welded steel pipe, welded thin-walled pipe, transformer cooling oil pipe and so on.
3. Spiral-slot submerged arc welded steel pipe (SY5036-83) for pressure-bearing fluid transportation is a hot-rolled steel strip coil for tube blank, which is often spiral-formed and welded by double-sided submerged arc welding for pressure fluid transportation. Spiral seam steel pipe. The steel pipe has strong pressure bearing capacity and good welding performance. It is safe and reliable after various strict scientific tests and tests. The steel pipe has a large caliber, high conveying efficiency, and can save investment in laying pipelines. It is mainly used to transport oil and natural gas pipelines.
4. Spiral seam high-frequency welded steel pipe (SY5038-83) for pressurized fluid conveying is a hot-rolled steel strip coil for tube blank, which is often spiral-formed by high-frequency lap welding for pressure fluid transport. Spiral seam high frequency welded steel pipe. The steel pipe has strong pressure bearing capacity and good plasticity, which is convenient for welding and processing. After various strict and scientific inspections and tests, the use is safe and reliable, the steel pipe has large caliber, high conveying efficiency, and can save investment in laying pipelines. It is mainly used for laying pipelines for transporting oil and natural gas.
5. Spiral submerged arc welded steel pipe (SY/T5037-2000) for general low-pressure fluid transportation is made of hot-rolled steel strip coil for tube blank, regular temperature spiral forming, double-sided automatic submerged arc welding or single-sided welding. Submerged arc welded steel pipe for general low pressure fluid transportation such as water, gas, air and steam.
6. Spiral seam high-frequency welded steel pipe (SY5039-83) for general low-pressure fluid transportation is a hot-rolled steel strip coil for tube blank, which is often spiral-formed by high-frequency lap welding and used for general low-pressure fluid conveying spiral. High-frequency welded steel pipe.
7. Spiral welded steel pipe (SY5040-83) for piles is made of hot-rolled steel strip coils, which are often spiral-formed by double-sided submerged arc welding or high-frequency welding for civil engineering structures and wharfs. Foundation piles such as bridges.

Source: China Welded Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@ugsteelmill.com

Please notice that you might be interested in the other technical articles we’ve published:

  • Tips for selecting expansion joints

  • What is the difference between a steel pipe and a steel tube

  • Engineering Specification for Pressure Test of Piping System

  • Stainless Steel Pipe and Tube

  • Manufacturing process of cold rolled steel pipe

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  • What are Nickel Alloys Pipes

References:

  1. https://www.yaang.com

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