The Characteristics and Use of Precision Welded Pipe

Precision welded pipe is a broad concept, according to the different use requirement, there are many different personalized quality requirements. Precision welded pipe is the further improved result that based on the general welded steel pipe, is the fine products in the welded steel pipe products. Prime Steel Pipe as the comprehensive steel pipe manufacturer and exporter from China, has been committed to provides high-quality steel pipe for sale, if you have any requirement of precision welded pipe, please contact us.

Key Features of Precision Welded Pipe

Precision welded pipe has a fine geometric size, thickness of the transverse wall is uniform, shape of steel pipe is good, oval is small, the quality of welding line is good and burrs are small, the outer surface is smooth. Steel pipe after welding line heat treatment or protective atmosphere heat treatment, can achieve the effect of surface without oxygen, steady mechanical properties and metallurgical uniform. Its production costs are much lower than similar seamless steel pipe, and it can be directly or after a small amount of processing used for some equipment, which will reduce equipment processing costs.

Use of Precision Welded Pipe

Precision welded pipe is mainly used in chemical, textile, machinery, instrumentation, aviation, automotive and other fields. Such as cars, motorcycles on the buffer, reducer, condenser, tank pipe, drive shaft, silencer, control shaft, thrust brake tube, hydraulic cylinder, cylinder, jack, all kinds of conveyor rollers, belt roller, Textile printing and dyeing roller, textile fiber machine with draft pipe, hydraulic support tube, deep well pump pipe, all kinds of mechanical light pipe, electric heater outer tube and household appliances, etc..

API 5L Welded Steel Pipe

API early in 1926 released by the American Petroleum Institute (API) API-5L standard, initially including three steel grade A25, A, B, and later released several times. API 5L welded steel pipe is mainly used for the delivery of the onshore and offshore oil and gas, coal slurry, the slurry media and offshore platforms, power plants, chemical industry and urban construction, structural tubes.

China is currently used in the Petroleum Pipeline tube spiral welded pipe (SSAW steel pipe), LSAW steel pipe, electric resistance welded pipe (ERW steel pipe). Diameter less than 152mm selection of seamless steel tubes. Of the late 1960s to the 1970s, spiral pipe plant rapid development of crude oil pipelines almost all spiral welded steel pipe. The spiral welded steel pipe of the disadvantage is the internal stress is large, the dimensional accuracy is poor, the probability of generation of defects. According to expert analysis, should adopt the policy of "walking on two legs", and actively carry out technological transformation now spiral welded pipe plant, also in promising; vigorously develop straight seam submerged arc control tube industry in China. ERW steel pipe with a smooth appearance, high dimensional accuracy, lower prices, and has been widely used at home and abroad.

Straight seam high frequency welded steel molding process that used in the production of LSAW steel pipe by Prime Steel Pipe is with advanced, reliable quality, welding position, stable welding parameters, welding speed, high yield, and the entire production line equipment and technology are world-class level.

(1) straight seam high frequency welded steel pipe forming process spring back small residual stress.
(2) the strain hardening effect is small, precise geometric dimensions full circle, will help ensure the quality of construction and welding.
(3) straight seam high frequency welded steel bead shape is good, not prone to biting edge, inside and outside the weld misalignment defects.
(4) straight-seam pipe weld distributed in a point on the circumference and, therefore, according to the demand to weld on a favorable position.

The Forming Speed of The Welded Steel Pipe Unit

The speed of the welded steel pipe unit is limited by the high-frequency welding speed. Generally speaking, the unit speed can be opened at a speed of 100 m/s, and the speed of the unit in the world is even 400 m/s Welding, especially in the induction welding can only be 60 meters/second or less, more than 10mm of steel plate molding, the domestic unit production molding speed can only reach 8 to 12 m/s. The welding speed affects the welding quality of welded steel pipe. When the welding speed is high, it will help to shorten the heat affected zone and facilitate the extrusion of the oxide layer from the molten groove. On the other hand, when the welding speed is very low, the heat affected zone becomes wider, resulting in a larger welding burr, , Weld quality deteriorated. Of course, the welding speed of welded steel pipe unit by the output power limit, it is impossible to mention very high. Domestic unit operating experience shows that 2 ~ 3 mm steel pipe welding speed can reach 40 m/s, 4 ~ 6mm steel pipe welding speed can reach 25 m/s, 6 ~ 8 mm welded steel pipe welding speed can reach 12 m/10 ~ 16 mm welded steel pipe welding speed of 12 m/s or less. Contact welding speed can be higher, induction welding will be lower.

The Welded Steel Pipe Used for Structure

Structural welded steel pipe is used in steel structure, bridge support, building structural pipe, pile foundation tube, etc.. As the structural tube needs great egret capacity to pressure-bearing and withstand voltage, generally, we use the pipe that the wall thickness is greater than 8mm. The main material of the structural welded steel pipe is carbon steel, and mainly includes LSAW steel pipe, ERW steel pipe, and spiral welded pipe. The main parameters of the structural welded steel pipe include diameter, wall thickness, length, welding technology, whether the need for surface corrosion insulation treatment. The welded steel pipe for exporting needs to be grooved, painted, added the cap. Structural welded steel pipe is made with double-sided submerged arc welding or high-frequency welding.

Structural welded steel pipe is manufactured by continuous on-line production, the thicker the wall thickness is, the greater the unit and welding equipment investment is, the structural welded steel pipe has more no economical and practical. The wall thickness is thinner, its input-output ratio will decrease accordingly. Secondly, the process of the product determines its advantages and disadvantages. General welded steel pipe has high precision and uniform wall thickness, and the surface brightness of the pipe is high (surface grade of steel pipe determines the brightness of pipes), and it can be sized any length. Therefore, it reflects its economy and aesthetics in the high-precision, low-pressure fluid applications. According to the welding technology section structural welded steel pipe is divided into automatic welding, manual welding, automatic welding commonly used submerged arc welding, plasma welding; manual welding general use of argon arc welding. According to the weld section is divided into straight LSAW steel pipe and SSAW steel pipe.

Prime Steel Pipe has many year experience to manufacture and export all kinds of welded steel pipe with high-quality, if you have any requirements of that, please inquiry us without any hesitation.

The Application of Large Diameter LSAW Steel Pipe

Large diameter LSAW steel pipe, also known as welded steel pipe, is made of steel plate or steel strip after bending (electric model 0317 and check 6012755 standard) and then is made of by high-frequency welding and submerged arc welding. According to the form of seam welded steel pipe is divided into LSAW steel pipe and spiral welded pipe. According to the use it is divided into general welded pipe, galvanized steel pipe, blowing oxygen pipe, wire casing, metric pipe, roller tube, deep well pump, automotive tube, transformer tube, welded thin-walled tube, welded pipe and spiral welded pipe.

General pipe (manufactured bu Prime Steel Pipe) is used to transport low pressure fluid. Implementation of the standard is GB/T0317 No. 6012889. It is manufactured with Q195A, Q215A, Q235A steel. It can also be made of other mild steel that is easy to weld. Steel pipe must be tested by water pressure, bending, flattening and other experiments, the surface quality has certain requirements, the delivery length usually is 4-10m, and it often requires delivery by length (or double length). The specifications of the welded pipe are denoted by nominal caliber (mm or inch). Nominal caliber is different from the actual one. The welded pipe has two different kinds of steel pipe according to the specified wall thickness, including ordinary steel pipe and thickened steel pipe.

The Groove of The Welded Steel Pipe

In the production of welded steel pipe, the groove of the tube is the shape of the cross-section, the general manufacturers in the slitting directly into the high-frequency welding, the groove is a "I" shape. When the thickness of the welding material is greater than 8 ~ 10mm above the pipe, if the use of this "I" shaped groove, because the relationship between the curved arc, you need to melt off the tube first contact with the inner layer, forming a high Burr, and easy to cause the center of the plate and the outer layer of insufficient heating, affecting the welding strength of high-frequency weld. Therefore, in the production of thick-walled tube, the tube is best through the planing or milling edge treatment, so that the groove was "X" shape, the practice proved that this groove for the uniform heating to ensure the quality of the weld seam of welded steel pipe has a great relationship. The selection of the groove shape also affects the size of the adjustment angle. The design of the welded joint is a weak link in the design of the welding project of welded steel pipe. It is not uncommon for the construction of many steel structures. The design of the groove is not from the hand of the welding engineering technician, the hard sleeve standard and the poor performance. The groove form plays an important role in controlling the internal quality of the weld and the quality of the welded structure. The groove design must test the fusion ratio of the base metal, the welding space, the welding position and the comprehensive economic benefits. The lateral shrinkage value ΔB should be calculated by the following equation.

ΔB = 5.1Aω / t + 1.27d

Where Aω - weld cross-sectional area, mm,
           t - plate thickness, mm,
           d - weld root gap, mm.

After finding the relationship between ΔB and Aω, we can analyze and process the data according to the relationship list, optimize the design, and finally determine the form of butt weld.

Welded Steel Pipe: Control of Welding Slag Inclusion and Stomata Defects

When steel pipe manufacturer (such as Prime Steel Pipe) producing welded steel pipe, it is unavoidable that some welding slag inclusion and stomata defects may appear if workers don't control very well during operation. So we will talk about the causing reason of welding slag inclusion and stomata defects here, and then we will give preventive measures for you. More details are as follows.

Causing reasons of welding slag inclusion:

1) Back chipping after backing welding is halfway, which results in the root slag did not completely overflow when heat welding rapidly.
2) The way to backing weld back chipping may be wrong, so that the groove on both sides of root pass is too deep, and shows "W". In the rapid heat welding, the slag flowing into the deep trough is too late to overflow and form slag.
3) At 6 o'clock position, arc too fast is also easy to produce slag.

Preventive measures:

1) Using grinding wheel to remove slag after the backing weld, back gouging must be thoroughly, each joint point must be flat.
2) When back chipping the root weld bead will be clear into the "U" shaped groove, and avoid to be the "W" shaped slot.
3) When retreating the arc at 6 o'clock we need to fill the molten pool, and then move arc to the forming of the weld to collect arc, and use flat swing method to extinguish the arc.

Causing reasons of stomata defect:

1) The molten pool is overheating when cosmetic welding, and adsorb a lot of ambient air.
2) The amplitude of swing of welding rod is too large when cosmetic welding, the molten pool is be protected badly.
3) The root gap is too small, which is easy to produce root needle bubble.
4) The electrode is not used for a specified period of time or prolonged exposure to air.

Preventive measures:

1) The current should not be too large when capping, we should use the small current, short arc, and welding with fast welding speed to avoid overheating and prevent surface pores.
2) We should use the appropriate transport technology to weld, otherwise it will make the molten pool ahead, and easy to cause long short circuit and the electrode stick on the weld, which is unfavorable to deoxidize and easy to produce pores. But the width of electrode swing should not exceed twice the diameter of the electrode, otherwise it is easy to produce pores.
3) We should prevent the group on the gap is too small, if the group on the gap is too small, it will cause overlarge base metal dilution effect in the welding, and impede the exhaust, which will results in the root of bubbles.
4) Electrode should be stored in the electrode insulation tube in the use of the process. It is strictly prohibited that welding rod is exposed outward, we need to prevent the welding rod damp.

Welded Steel Pipe: Control of Welding Slag Inclusion and Stomata Defects

When steel pipe manufacturer (such as Prime Steel Pipe) producing welded steel pipe, it is unavoidable that some welding slag inclusion and stomata defects may appear if workers don't control very well during operation. So we will talk about the causing reason of welding slag inclusion and stomata defects here, and then we will give preventive measures for you. More details are as follows.

Causing reasons of welding slag inclusion:

1) Back chipping after backing welding is halfway, which results in the root slag did not completely overflow when heat welding rapidly.
2) The way to backing weld back chipping may be wrong, so that the groove on both sides of root pass is too deep, and shows "W". In the rapid heat welding, the slag flowing into the deep trough is too late to overflow and form slag.
3) At 6 o'clock position, arc too fast is also easy to produce slag.

Preventive measures:

1) Using grinding wheel to remove slag after the backing weld, back gouging must be thoroughly, each joint point must be flat.
2) When back chipping the root weld bead will be clear into the "U" shaped groove, and avoid to be the "W" shaped slot.
3) When retreating the arc at 6 o'clock we need to fill the molten pool, and then move arc to the forming of the weld to collect arc, and use flat swing method to extinguish the arc.

Causing reasons of stomata defect:

1) The molten pool is overheating when cosmetic welding, and adsorb a lot of ambient air.
2) The amplitude of swing of welding rod is too large when cosmetic welding, the molten pool is be protected badly.
3) The root gap is too small, which is easy to produce root needle bubble.
4) The electrode is not used for a specified period of time or prolonged exposure to air.

Preventive measures:

1) The current should not be too large when capping, we should use the small current, short arc, and welding with fast welding speed to avoid overheating and prevent surface pores.
2) We should use the appropriate transport technology to weld, otherwise it will make the molten pool ahead, and easy to cause long short circuit and the electrode stick on the weld, which is unfavorable to deoxidize and easy to produce pores. But the width of electrode swing should not exceed twice the diameter of the electrode, otherwise it is easy to produce pores.
3) We should prevent the group on the gap is too small, if the group on the gap is too small, it will cause overlarge base metal dilution effect in the welding, and impede the exhaust, which will results in the root of bubbles.
4) Electrode should be stored in the electrode insulation tube in the use of the process. It is strictly prohibited that welding rod is exposed outward, we need to prevent the welding rod damp.

Advantages and Disadvantages of Welded Steel Pipe

The technology of welded steel pipe is continuous online production. So the thicker the wall thickness is, the greater the investment in the unit and welding equipment is, and welded steel pipe will have less economical efficiency and practical applicability. The wall thickness is thinner, its input-output ratio will be decreased accordingly. The process of the product determines its advantages and disadvantages. In general, the welded steel pipe with high precision, uniform wall thickness, high brightness of tube inside and outside (surface grade of steel plate determines the surface brightness of steel pipe), can be fixed length wilfully. Therefore, welded steel pipe reflects its economy and aesthetics in the high-precision, low-pressure fluid applications.

The production process of welded steel pipe is simple, the production efficiency is high, the cost is low, and develops rapidly. The strength of the spiral welded pipe is generally higher than that of the straight seam welded pipe. The pipe with larger diameter can be produced with the narrow billet and the pipe with different diameter can also be produced with the same width billets. However, compared with the same length of the straight pipe, the length of weld seam increased by 30% to 100%, and the production rate is low. Therefore, the pipes with smaller diameter are mostly used straight welding, the welded steel pipes with large diameter are mostly used spiral welding.

Prime Steel Pipe is positioned as a steel pipe manufacturer and exporter for all kinds of welded steel pipes in different standard and dimension, including ERW steel pipe, LSAW steel pipe, SSAW steel pipe, etc..

The Welding Quality of Welded Steel Pipe

Welding quality of welded steel pipe refers to the degree to which the welding product meets the design technical requirements. Welding quality not only affects the use performance and life of welding products, more importantly, affects personal and property safety. Welding quality is usually ensured by design quality, processing quality, quality inspection and post-weld treatment and other links of the product.

Design Quality

The type of joint used in the welding product and its calculated strength should meet the actual carrying capacity. Welding method should be suitable for the characteristics of components, and its economical efficiency should be good. The welding process should be able to minimize stress, deformation and stress concentration. Production labor and material consumption should be as small as possible, the convenience of flaw detection should also be taken into consideration when design the joint.

Processing Quality

The performance of adopted the base metal, wire, flux or welding electrodes and other welding materials should meet the design requirements. The performance of welder, auxiliary equipment and testing equipment should be good. Before welding, the welding material should be dried according to the provisions, the welding groove of workpiece should meet the requirements and remove the cutting residue, cracks and dirt.

Quality Inspection

Quality inspection runs through the whole production process from design to finished product, we must ensure that the test method used in the quality inspection process is rationality, the reliability of the testing instrument and inspection staff technical level. The properties of the joints, physical properties, mechanical properties, microstructure, chemical composition, corrosion resistance, appearance dimensions and welding defects of products after welding should be checked by means of various inspection methods.

Welding defects can be divided into external defects and internal defects. External defects include: excess height does not meet the requirements, welding tumor, undercut, crater, arc burns, surface pores, surface cracks, welding deformation and warping. Internal defects include: cracks, not penetration, not fused, slag and porosity. The most harmful in welding defects is the crack, the followed is lack of penetration, incomplete fusion and slag inclusion, pores and structure defect. Individual flaws are allowed to exist. The number and the nature of allowed defects are determined by the using conditions and the quality assessment criteria of the product. If the surplus height of welding line is too high, it is allowed for those products by the static load, but it is not allowed for those products by cyclic fatigue loading in high frequency, even the normal excess weld metal also has to be cut off. The presence of welding defects is related to the groove processing and assembly accuracy, the rigor degree of the welding process, the technology of the welder and other factors.

What is Welded Steel Pipe?

Welded steel pipe, also known as welded pipe, is the steel pipe that made of steel plate or steel strip after crimped and welded. The production process of welded steel pipe is simple, production efficiency is high, variety specifications are many, the investment of equipment is little, but its general strength is lower than the seamless steel pipe. Since the 1930s, with the rapid development of high-quality strip continuous rolling line and the improvement of welding and inspection technology, the quality of welding line has been increasing continuously, and the specifications of welded steel pipes have been increasing. And welded pipe have replaced the seamless steel pipe in more and more field. Sewing steel pipe. The welded steel pipe is divided into straight seam welded pipe(also called LSAW steel pipe) and spiral welded pipe(also called SSAW steel pipe) according to the form of welding line.

Production process of LSAW steel pipe is simple, production efficiency is high, cost is low, and it develops rapidly. The strength of the SSAW steel pipe is generally higher than that of the LSAW steel pipe, and the SSAW steel pipe with larger diameter can be produced with the narrow billet. And it also can use the billet with the same width to produce t SSAW steel pipe with different diameter. However, compared with the straight seam welded pipe in identical length, the length of welding line increased by 30 to 100%, and the production rate was low.

Therefore, pipe manufacturers usually use straight seam welding to produce the smaller diameter of the pipe, use spiral welding to produce large diameter welded pipe.

What type of tube is used in Oil pipeline?

The tube that used in Oil pipeline is mainly carbon steel pipe, according to its manufacturing process, it can be divided into seamless steel pipe and welded steel pipe.

Seamless steel pipe has the characteristics of high strength, specifications and more features, so it is applicable to transport strong corrosive oil or transport in the high temperature conditions. The technology of seamless steel pipe is divided into hot rolling and cold drawn. As the cold drawing process will cause the material hardening, it also needs to do the corresponding heat treatment based on the specific use of pipe.

Welded steel pipe can be divided into two kinds, straight seam steel pipes (LSAW steel pipe) and spiral welded pipe (SSAW steel pipe). The process characteristics of carbon steel pipe leds the steel pipe to tend to become brittle at low temperatures, so it is mainly applied to the normal temperature pipeline, the use of pipe temperature should not exceed 300 degrees Celsius. In general, the temperature of ordinary carbon steel pipe is between 0 to 300 degrees Celsius. If using the high-quality carbon steel pipe, the temperature range can be broadened from -40 to 450 degrees Celsius. The experimental pressure of the steel pipe is generally 1.5 times of the nominal pressure. When the temperature of the material delivered does not exceed 200 degrees Celsius, the working pressure is the nominal pressure. When the temperature exceeds 200 degrees Celsius, the working pressure should be correspondingly lower than the nominal pressure.

Welded Steel Pipe Production: The Role of Welding Preheating

For welded steel pipe production, the technique means that heating the whole of the weldment or local welding area before welding is known as preheating. For those welds with higher weld strength, hardening tendency, particularly good heat-conducting property and big thickness, and when the environment temperature of welding zone is too low, it is necessary to preheating the weldment before welding. The main purpose of preheating is reducing the cooling rate and preheat temperature of welded joint. Preheating can reduce cooling rate but basically has impact on time of staying in high temperature, that is so prefect. So when welding the steel with hardening tendency, the main processing measure to reduce cooling rate and decrease hardening tendency is preheating, rather than increase the heat input.

Types of Welded Steel Pipe

1. Welded steel pipe (GB/T3092-1993) used in Low-pressure fluid delivery, also known as the general welded pipe, commonly known as black tube. It is the welded steel pipe that used to transport of water, gas, air, oil and heating steam, and other generally fluids with lower pressure. Steel pipe is divided into ordinary steel pipe and thickened pipe according wall thickness; is divided into non-threaded steel pipe (light pipe) and threaded steel pipe according to the form of pipe end. The specifications of the steel pipe are denoted by nominal caliber (mm), and the nominal caliber is the approximate value of the inner diameter. It traditionally uses inches to express, such as 11/2 and so on. Welded steel pipe used in Low-pressure fluid delivery in addition to directly use for the transport of fluid, but also is used as the original tube of galvanized welded steel pipe used in Low-pressure fluid delivery.

2. Galvanized welded steel pipe (GB/T3091-1993) used in Low-pressure fluid transmission, also known as galvanized welded steel pipe, commonly known as white tube. It is the hot dipping galvanized welded (butt weld or electrowelding) steel pipe used to transport water, gas, air oil and heating steam, warm water and other generally fluids in lower pressure. Steel pipe is divided into ordinary galvanized steel pipe and thick galvanized steel pipe according to wall thickness; is divided into non-threaded galvanized steel pipe and threaded galvanized steel pipe according to the form of pipe end. It traditionally uses inches to express, such as 11/2 and so on.

3. Ordinary carbon steel electric wire conduit (GB3640-88) is used in electrical installation ( such as the industrial and civil construction and installation of electrical equipment and other works) to protect the wire of the steel pipe.

4. LSAW steel pipe (YB242-63) is the pipe that the weld longitudinally parallel to steel pipe. It is usually divided into metric welded steel pipe, welded thin-walled pipe, transformer cooling tubing and so on.

5. SSAW steel pipe (SY5036-83) for pressurized fluid delivery is made of hot-rolled steel coil as tube blanks, spirally forming in normal temperature and welded by double-sided submerged arc welding, used for pressurized fluid delivery. The loading capacity of steel pipe is strong, welding performance is good, after a variety of strict scientific inspection and testing, it can be used safely and reliably. The diameter of steel pipe is big, transport efficiency is high, and it can save the laying of pipeline investment. So it is mainly used for conveying oil and gas pipelines.

6. ERW steel pipe used for pressurized fluid transmission (SY5038-83) is made of hot-rolled steel coil as tube blanks, spirally forming in normal temperature, welded by high-frequency lap welding, used for pressurized fluid transportation. The loading capacity of steel pipe is strong, plasticity is good, and it is easy to weld and machine shaping. After a variety of rigorous and scientific inspection and testing, the pipe is safe and reliable to use, the diameter of steel pipe is large, transport efficiency is high, and it can save the laying of pipeline investment.

7. Spiral welded steel pipe (SY5037-83) used in general low-pressure fluid transmission is made of hot-rolled steel coil as tube blanks, spirally forming in normal temperature, use double-sided automatic submerged arc welding or single-sided welding method to weld. It is used for water, gas, air and steam and other general low-pressure fluid transmission.

8. ERW steel pipe (SY5039-83) used in general low-pressure fluid transmission is made of hot-rolled steel coil as tube blanks, spirally forming in normal temperature, use high-frequency lap welding to weld,. It is used for general low-pressure fluid delivery.

9. Spiral welded steel pipe (SY5040-83) used as pile is made of hot-rolled steel coil as tube blanks, spirally forming in normal temperature, use double-sided submerged arc welding or high-frequency welding to make. It is used for civil construction, wharf, bridge and other foundation pile steel pipe.

Detailed Notes to Welding Inspection of Welded Steel Pipe

The contents of the welded steel pipe welding inspection include the inspection of the materials, tools, equipment used in the whole production process, process and finished product from drawing design to product completed. It is divided into three stages: pre-weld inspection, inspection in welding process, finished product inspection after welding. According to whether damage the welded carbon steel pipe or not, test methods can be divided into destructive testing and non-destructive testing two categories. Next, Prime Steel Pipe will provide all for everyone.

Pre-weld inspection

Pre-weld inspection includes the inspection of raw materials (such as base metal, electrodes, flux, etc.) and the inspection of welding structure design.

Inspection in welding process

Inspection in welding process includes the inspection of welding process specifications, the inspection of weld size, the inspection of fixture conditions and structural assembly quality.

Finished product inspection after welding

There are many methods of finished product inspection after welding, commonly used are the following:

No 1. Appearance inspection

The appearance inspection of the welded joint is a inspection method with simple procedure, but it is widely used, and it is an important part of the finished product inspection. It is mainly used to find the defect on the surface of welded steel pipe weld seam and the size of the deviation. We generally observe through the naked eye, or testing by virtue of the standard model, gauge and magnifying glass and other tools. If there is a defect on the surface of the weld seam, it is possible that there is a defect in the weld.

No 2. Confidentiality testing

The defects of brazing joints of welded seam of welded vessel for the temporarystorage of a gas orliquid, such as perfoliate cracks, porosity, slag inclusion, incomplete fusion and loose organization, etc. can be found by compactness test. Compactness test includes kerosene test, water-bearing test, hose test, etc.

No 3. Strength test of the pressure vessel

Except carrying out encapsulation test, we also should carry out strength test to pressure vessel. Two kinds of methods are commonly, hydraulic pressure test and air pressure test. Each of them can test the welded seam compactness of vessel and pipe working under pressure. Pneumatic tests are more sensitive and rapid than hydrostatic tests, the tested products need not to drain off water,it is particularly suitable for difficult drainage products. But the risk of testing is bigger than the hydraulic test. During the test, the corresponding safety and technical measures must be observed to prevent accidents during the test.

No 4. Physical inspection method

Physical inspection method is a method that using some physical phenomena to measure or test. the inspection of materials or parts of the internal defects, generally use non-destructive testing methods. Currently, non-destructive testing includes ultrasonic testing, ray detection, penetration testing, magnetic detection and so on.

① Ray detection

Ray detection is a detection method that use the characteristics of ray can penetrate the material and has attenuation in the material to find defects. Ray inspection is mainly used to test the cracks inside the weld, incomplete penetration, porosity, slag and other defects.

② Ultrasonic testing

Ultrasound can spread in the metal and other uniform media, due to it will produce reflection in the interface of different media, it can be used for testing internal defects. Ultrasonic can test the defect of any welding material and on any part, and it can be more sensitive to find the location of defects. But the nature of defects, shape and size is more difficult to determine. So ultrasonic testing often used in conjunction with the ray test.

③ Magnaflux inspection

Magnaflux inspection is a detection method that use the leakage of magnetic produced by magnetic field, magnetized iron, magnetic metal parts to find defects. According to different methods of magnetic flux leakage, it can be divided into magnetic powder, magnetic induction and magnetic recording method, magnetic powder is the most widely use method. Magnetic flaw detection can only discover the defect on the surface or the near surface of the magnetic metal.

④ Penetration testing

Permeability testing uses certain liquids such as the permeability of the physical properties to find and display defects, including color inspection and fluorescence detection It can be used to check the defect on the surface of ferromagnetic and non-ferromagnetic material.

Classifications of Welded Steel Pipe

Classifications of Welded Steel Pipe

Welded steel pipe is shaped up by crimping steel plate or steel belt and then it was made after welding. Welded steel pipe can be classified into longitudinally-welded pipe and spiral welded steel pipe according to the ways of welding. The common forming technique of longitudinally-welded pipe is UOE forming process and JCOE steel pipe forming process. According to the application, welded steel pipe is classified into general welded pipe, galvanized welded pipe, oxygen blast welded pipe, electric wire conduit, metric welded pipe, roller tube, deep well pump pipe, automobile tube, transformer tube, electric welding thin walled pipe, electric soldering special pipe and spiral welded pipe.

General welded steel pipe fabricated by Steel Q195A, Q215A, and Q235A is used for conveying low-pressure fluid. It also can be manufactured by other mild rolled steel for easier welding. Steel pipe should be conducted some experiments such as hydraulic pressure, bending, battering and so on and so forth. Production manufacturers can do a more advanced detection for steel pipe in terms of their own conditions. Welded steel pipe usually has certain requirements for surface quality. The delivery length is usually four to ten meters. Moreover, manufacturers can deliver the goods by sizing or ruler-multiplying according to actual production requirements. The specification of welded pipe in nominal diameter express nominal diameter is different from the actual nominal diameter. According to the regulations of the length of wall thickness, welded pipe has two kinds: thin-wall steel tube and thick-wall steel tube. Steel pipe also has two kinds: thread steel pipe and steel pipe without thread according to pipe ends.

For the price of welded steel pipe is lower than that of south seamless pipe which has the same specifications, welded steel pipe is widely used in low-pressure fluid transmission engineering, steel structure engineering, etc.

Defects Of Welded Steel Pipe

Defects Of Welded Steel Pipe

Welded steel pipe production process is a steel sheet,strip,and other various molding methods using the straight press roll helical direction curling into a desired cross-sectional shape,and then by means of heart,prssure,different methods of welding weld weld together to obtain steel.Therefore,defects in the welded steel pipe is divided into two parts:the steel base material defects and weld defects.

1.Steel base material defect
The sheet material defects after rolling and other processes,most of planar,parallel to the surface;their main weakness delamination,cracks,folds,etc.,which is the most common layered internal defects.Stratification will produce a variety of cracks when the tensile stress perpendicular to the surface of the sheet by the hierarchical will seriously affect the strength of the steel pipe,it is not allowed defects.

2.Weld defects
Weld defects refers to defect during welding or after welding resulting in the weld is divided into cracks,proes,slag,incomplete penetration,incomplete fusion,undercut weld defects.Intensive weld porosity,slag,etc.a dense three-dimensional defects,cracks,lack of fusion and other  defects in the case of the flat,great harm.Strip slag.

Why Use Welded Steel Pipe

Structural welded pipe (tubing) can be used for a wide variety of mechanical applications. It may be used either statically or dynamically. Its close tolerances, good finish, and dense structure make it ideal for parts such as structural support and parts, frames, racks, trailer beds and trailer components, structural components and supports for buildings, bridges, and highways, conveyors, machine parts, ornamental usages and a wide range of applications in industrial, construction, automotive, appliance and agricultural industries. 

Welded pipes are generally more cost effective than the equivalent seamless alternative. They are significantly more available both from distributor and mill stocks and from a mill production lead time perspective. With longer lead times seamless tubes can be exposed to unknown alloying price extras that can fluctuate substantially, hence it can be difficult to plan and predict the true costs for the material. Therefore in addition to faster delivery times for Erw tubes, there is the added advantage of lower economical risk. Erw welded steel tubes generally have better surface quality as compared with manufacturing seamless tubes. Seamless tubes are generally made in multiple steps starting with hot extrusion to form the tube hollows from billets.

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Select of Straight Seam Submerged Arc Welded Steel Pipe

Select of Straight Seam Submerged Arc Welded Steel Pipe

(1) For high peak load requirements of the pipeline, because the user's gas consumption is very uneven, pipeline pressure fluctuations in the frequent, alternating stress steel to withstand large tubing will expand the existing defects in alternating stress, if more choice weld defects high probability of spiral welded steel pipe, the safe operation of the pipeline will not be guaranteed.

(2) Pipeline crossing earthquake fault or by topical highly seismic region, due to the geological activity of these lots frequent pipeline will have a vertical or axial alternating stresses. Spiral weld much higher than the probability of defect LSAW steel pipe, under long-term stress, spiral welded steel pipe is much higher than the probability of an accident LSAW steel pipe, therefore, in such areas should be used straight submerged arc welded pipe.

(3) Internal and external coating requires a high piping should be used LSAW. Spiral submerged arc welded pipe weld more general than the weld reinforcement LSAW high, when the steel pipe internal and external corrosion, anti-corrosion materials and combination of the bare tube is not as close LSAW, anticorrosive effect is not good LSAW steel pipe.

(4) For important wear across the engineering, use LSAW. Due to difficulties in future maintenance and management than the general line segment, therefore, the use of excellent performance LSAW particularly necessary.

(5) For the pipeline weaknesses, such as heat and simmer elbow pipe should be used LSAW. Heat simmer elbow since the direction of change, their exposure to internal and external forces than the general line of straight pipe should be large, due to the impact of various factors in the simmering process, it is not easy to eliminate stress, is a long-distance pipeline in the weak link , integrated performance good LSAW can make up for these deficiencies.

China's steel exports forcast

Since April 25, the domestic steel market price fall, the ordinary steel price index fell more than 20 points, and part of steel products price has an decline rate over 30%. In the Chinese steel pipe market, both seamless steel pipe and welded steel pipe suffer a large price decrease. In generall, there will be three different prices in one day. The main logic behind the price decline is large profit margins stimulate the resumption of steel products production which led to the expansion conflict between steel supply and demand. When the market price goes down, steel products exporting is one way to relieve the domestic market pressure. Therefore, the market believe that the domestic market steel supply in May 2016 is getting better and better. In fact, themajority of steel prouduction enterprises and large steel products traders ahve such a feeling that the domestic steel export deliveries and new orders  volume performance is good in May. According to the Shanghai Steel Investigation Team Data, Chinese steel export is more than 9.5 million tons with a sequential growth rate in 5%.

In June 2016, the domestic steel exports may be affected. Chinese steel market in Europe and America is suffering from s severe "anti-dumping, countervailing" survey. Chinese steel exports to Europe and America are suppressed. The strong demand for steel consumption in Southeast Asia for the Chinese steel exports contributed a lot of power. Chinese steel export growth is very dependent on the Southeast Asian region. The changes in the Southease Asian market will largely affect the export quantity of  Chinese steel. June is the rainy season in Southeast Asia market, the rainy season will definitely affect the start of the construction site, which would weaken the steel consumption. In addition, the lslamic holy month of Ramadan will begin in June which will also affect the local steel consumtion. Therefore, we believe that in June Chinese steel exports will be affected the Chinese steel export in some degree, but in a limited impact strength.