What is a welded steel pipe?

What is a welded steel pipe?

Welded steel pipe refers to welded steel strip or plate by means of bent round, square and other shapes and then welded into the surface of the steel pipe seam. 
Welded steel blanks is used in steel sheet or strip, according to the different welding processes, it can be divided into the furnace pipe, ERW pipes and automatic arc welded pipe. Because of the different forms of welded pipe, which is divided into two and spiral welded pipe. Its shape is divided into round ends welded and shaped (square, flat, etc.) pipe. 

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Welded steel pipe, welded steel common market in general are: double submerged arc welded pipe(DSAW pipe), high-frequency welded pipe (HFW pipe), spiral submerged arc welded pipe(SSAW pipe). Double submerged arc welded pipe is made using the molding machine has a tube opening, re-use double-sided submerged arc welding. Pipe is the use of high-frequency skin effect of high frequency current production of steel. The use of a narrow spiral submerged arc welded pipe is made of steel pipe-shaped roll, and use submerged arc welding, spiral weld, the weld compared Longitudinal length, compared with Longitudinal, it can use a specification made with a variety of large-diameter steel pipe.
Longitudinal spiral steel strength is generally higher than that, which can narrow the billet production of large diameter pipe, you can also use the same width of the billet production of different pipe diameters. However, compared with the same length of straight seam pipe, weld length of 30 to 100%, and lower production rate. Therefore, smaller diameter pipe is mostly a straight seam welded, large diameter spiral welded pipe is mostly used.
Common surface defects of welded steel pipe
(1) layered steel 
Layered wall steel tube means the cross section is divided into two floors, exposed steel surface showing the hierarchical vertical cracks. Some exhibits inside and outside surfaces of steel recessed or raised locally, layered rendering suddenly raised, dents or warping in and out of the weld. 
(2) sticky scar 
Pipe sticky scar refers to internal and external surfaces of the massive steel focal adhesion plaques scar. 
(3) holes 
Pipe through the hole refers to a local steel tube bi holes exist. 
(4) open welding 
Welded steel pipe opening means through long cracks in steel welds or partial showing of. 
(5) Local lap welding 
Welded steel pipe refers to a partial outer surface of the steel pipe presents a partial arc weld. 
(6) Welding ditch 
Pipe welding groove is the outer surface of the steel pipe occurs through a long groove weld. 
(7) the collapse of welding 
Welded steel pipe collapse refers to the outer surface of the weld metal skeletons showing through a long trench, where the surface of the corresponding convex edges. 
(8) pipe joints dislocation
Steel tube joint dislocation refers to the steel pipe weld, occurred the phenomenon of staggered up and down.
The advantages and disadvantages of welded steel pipe
The average wall thickness of the tube light table table high (steel exterior appearance of steel grade resolution, brightness) can be arbitrary length. Thus, it showed its economy and aesthetics in accuracy, the use of low-pressure fluid. 
Welded steel pipe production process is simple, high production efficiency, low cost, to carry out faster. Spiral Welded Longitudinal higher than normal, can narrow the billet production of large diameter pipe, you can use the differences diameter pipe billet production of the same width. What, then with the same length of straight pipe joints compared weld length add 30 to 100%, and lower production rate. Thus, most of the smaller diameter pipe straight seam welded, large diameter spiral welded pipe for the most.
Classification of welded steel pipe
1, High frequency welded steel pipe features: tubular good, uniform thickness, internal and external burrs produced by the welding tool calibrating the appropriate online through strict control of non-destructive testing of weld quality, high degree of automation, low production costs. But relatively thin wall thickness, diameter is relatively small, usually no more than a wall thickness of 12mm, diameter generally does not exceed 610mm, particularly suitable for the production of steel tube truss structure. 
2, LSAW pipe features: welding under static conditions, high weld quality, short welds, the probability is very small defects. By expanding full-length steel tubular good, precise size, wide range of pipe wall thickness and diameter range, the diameter range up 406-1829mm, wall thickness range up 6.0-60mm, high degree of automation, Compared to seamless steel, it is lower production costs, for buildings, bridges, dams, offshore platforms and other steel bearing with columns, large span structures as well as wind and seismic requirements for electrical tower mast structure. 

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3, Spiral submerged arc welded pipe features: spiral weld line distribution, long welds, especially under dynamic conditions is in the welding, the weld had a chance to cool and left the molding point, easy to produce welding hot cracking. Direction parallel to the weld crack, a certain angle the axis of the steel pipe, typically between  30-70 °. This angle is just consistent with shear failure angle, so the bending, tensile, compressive and torsional performance is far better than LSAW pipe welding position and because restrictions, saddle ridge seam and fish the impact beautiful. In addition, the construction process, intersecting line pipe weld spiral welded parent node of fragmented spiral seam, a greater welding stress, thus greatly weaken safety performance components, and therefore efforts should be strengthened spiral welded non-destructive testing of welds, ensure the welding quality, otherwise important occasion not to use steel spiral submerged arc welded pipe.

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High frequency welded pipe production process
High Frequency Welded pipe banner - What is a welded steel pipe?

The process of high-frequency welded pipe production depends on the product variety, from raw materials to finished products need to go through a series of processes, the completion of these processes require a variety of mechanical equipment and welding, electrical control, detection devices, these devices and devices in accordance with different processes with a variety of reasonable layout of the process, high-frequency welded pipe typical process: open book – strip leveling – head and tail shear – steel butt welding – live storage – forming – welding – remove the burr – sizing – initial inspection – straightening of pipe – pipe section processing – water pressure test – flaw detection – printing and coating – finished product.
High-frequency welding is the use of continuous contact with the workpiece surface of the high-frequency current generated by the heat generated by heating and in the case of the application of forging force, the workpiece metal to achieve a series of welding methods. It is similar to ordinary resistance welding, but there are many important differences.High-frequency welding for carbon steel pipe production has been 40 years of history. High-frequency welding has a large power supply, the different materials, caliber and wall thickness of the steel pipe can achieve a higher welding speed (higher than the maximum welding speed of welding more than l0 times). Therefore, the high-frequency welding production of general-purpose steel pipe has a high productivity because of high-frequency welding speed, to the pipe to remove the burrs difficult, which is the current high-frequency welded steel pipe is not acceptable for the chemical industry, nuclear industry one. From the welding material, high-frequency welding can be welded all types of steel pipe.
Important part of high frequency welded pipe production process
1. In the process of high-frequency welded pipe production, how to ensure that product quality in line with the requirements of technical standards and customer needs, the steel pipe production process should be the impact of product quality factors.

Production range of longitudinal welded pipe
Longitudinal welded pipe on the form of welding is roughly divided into two types: the first is double-sided submerged arc welding, the second is high-frequency welding. But in between the two can make the diameter of the steel pipe is very different. Such as high-frequency welding of the pipe wall thickness of the general is not more than 12mm, this situation is caused by the performance of high-frequency welding, too thick welding machine can not melt the steel can be welded to shape. And double-sided submerged arc welding is different from it, it is not subject to the size of the steel pipe to be processed, diameter and thickness are far more than the size of high-frequency welding. Therefore, double-sided submerged arc welded pipe(dsaw steel pipe) is divided into butt welded pipe (also known as girth pipe) and a seam of the straight seam steel pipe, stealth seam steel pipe. 

The so-called butt weld pipe is produced by the roll tube machine (due to the impact of the machine bed, the general length of 1.5 m -2.5 m) between the shorter pipe, and then in a period of welding together, so that able to achieve the required length, called the butt welded steel pipe. A seam welded pipe is made of a large plate by a large hydraulic press molding (its length can be based on the length of the steel plate, the longest can be done 14 meters), and then in the welding, school circle. The stealth welded pipe, generally by the diameter of 402 below the wall thickness of 12mm or less, in the form of high-frequency welding made out, and then expanded through the expansion of the aperture has reached the required size, large diameter straight seam submerged arc welding steel pipe, through the u-hydraulic molding, double-sided submerged arc welding, the maximum diameter of 1420mm thickest 80mm. 

Kinds of Welding Process of Chinese Welded Steel Pipe
There are 9 kinds of weling process of Chinese welded steel pipe:
1.High frequency resistance welding
Using the skin effect of high frequency current and the proximity effect, the edge of the strip of steel billet is quickly heated to reach the molten state, and the molten metal is squeezed and pressed by the squeezing roller to achieve welding. 
2.Submerged arc welding
Arc welding arc welding method in the coverage of the flux. The weld metal pool is solidified into a weld under the protection of the flux coating, and the flux melt layer cools to the slag shell over the outer surface of the weld. 
3.Tungsten inert gas welding (TIG)
The use of pure tungsten or activated tungsten (thorium tungsten, cerium tungsten, etc.) as the electrode of the inert gas welding tungsten inert gas shielded welding, the English referred to as TIG welding. It is under inert gas protection, the use of tungsten arc welding electrodes generated between the hot-melt parent metal and filler wire welding method. 
4.Melting inert gas welding (GMAW)
The use of welding wire for the inert gas welding electrodes. 
5.CO2 gas shielded welding
With a purity of> 99.8% of CO2 as a protective gas welding gas arc welding.
6.Mixed gas welding
By two or more gases, according to a certain percentage of mixed gas as a protective gas gas shielded welding. TIG welding: gas shielded welding using argon as the shielding gas. 
7.Pulse TIG
The use of base current to maintain the main arc of the ionization channel, and periodically add a pulse of the same polarity of high peak pulse current pulse to melt the molten metal and control the transition of argon arc welding.
8.Plasma arc welding
With the help of water-cooling nozzle’s restraining action on the arc, a plasma arc with higher energy density can be welded. 
9.Hot brazing
In the welding process, a metal material with a lower melting point than that of the base metal is used as a brazing filler metal, the weldment and the brazing filler metal are heated to a temperature higher than the melting point of the brazing filler metal and lower than the melting point of the base metal, wetting the base metal with the brazing filler metal, gap and mutual diffusion with the parent material to achieve the method of connecting the weldment is called brazing. Commonly used composite brazing method for induction brazing.
Seamless Steel Pipe VS Welded Steel Pipe
The main difference between seamless steel pipe and welded steel pipe are molding process. General welded steel pipes such as water pipes, is generally flat plate by bending and welded together after that you can find a seam in the above; coarse diameter is usually spiral weld. The molten seamless steel pipe is generally through an annular slit molten state and then by stretching out the backlog of processing molding process, so there is no seam. In performance, especially compared to welded steel has greatly improved the bearing capacity, it is often being used for high-voltage equipment. Hydraulic equipment such as piping connections. Welded steel pipe weld area is its weak link, weld quality is also a major factor in its overall performance impact. Welded steel pipe lived in the north generally have had water pipes or heating pipes burst in the winter cold experience explosive place are generally weld.
Seamless steel pipe is piercing round holes to produce steel. Weld steel pipe is rolled into a bucket and then weld. According to their method of manufacturing seamless steel pipe and welded steel pipe in two. Seamless steel pipe with high-quality carbon steel or alloy steel, hot-rolled, cold-rolled (pull) of the points. Welded steel is rolled into a tube-shaped plate with the seam or spiral seam welded, in the manufacturing method, is divided into low pressure fluid welded pipe, spiral welded steel pipe, directly welded steel pipe, welded pipe. Seamless steel pipe can be used for a variety of liquids, gas pipelines. Welded pipes can be used for water pipes, gas pipes, heating pipes. Seamless steel tubes are used in important applications, uniform, high pressure tolerance, such as the barrel, the barrel and so on. Generally with high-quality steel, alloy steel and even. Weld steel are used in general purpose compression generally well tolerated, such as water pipes, gas pipes, etc., with ordinary steel.

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

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