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.

The Hardness Test of Seamless Steel Pipe

In general, seamless steel pipe uses Brinell, Rockwell, Vickers three hardness indicators to measure the hardness.

1. Brinell hardness test

In the standard of seamless steel pipe, Brinell hardness is used most, it often use indentation diameter to represent the hardness of the material, which is both intuitive and convenient. But it is not suitable for harder or thinner steel pipes.

2. Rockwell hardness test

The Rockwell hardness test of seamless steel tubes is the same as the Brinell hardness test, they are both indentation test method. The difference is that Rockwell hardness measures the depth of indentation. Rockwell hardness test is a widely-used method currently, HRC is next only to after Brinell hardness HB in the steel pipe standard. Rockwell hardness can be applied to measure metal material from very soft to very hard, which makes up for the weakness of Brinell method. And it is simpler than the Brinell method, it can read out the hardness value directly from the hardness of the dial. However, its hardness value is not as good as the Brinell method because its indentation is small.

3. Vickers hardness test

Vickers hardness test of seamless steel tube is also an indentation test method, it can be used to measure the thin metal materials and surface layer hardness. It has the main advantages of the Brinell and Rockwell methods, and overcomes their basic shortcomings. But it is not as easy as the Rockwell method, so Vickers method is rarely used in the steel pipe standard.

Differences between LSAW Steel Pipe and Spiral Welded Pipe

Welded carbon steel pipe includes LSAW (longitudinal submerged arc welded) steel pipe and spiral welded pipe. Some people those do not very understand this industry may not know differences between the former and the latter very clearly. Don’t worry, today we will talk about this topic.

Spiral welded pipe is made by using low carbon construction steel strip or low alloy structural steel strip to roll into a tube according to a certain helix angle (called the forming angle), and then welding the seam together. It can use the narrow band steel to produce steel pipe with large diameter. Spiral welded pipe is mainly used for oil and natural gas pipeline, its specifications are expressed by outer diameter * wall thickness. Spiral welded pipe with also includes single-sided welding and double-sided welding.

LSAW steel pipe is made by using strip steel as raw materials and doing longitudinal welded in the high-frequency welding equipment(hence the name due to the weld forms a straight line). In accordance with the use of different, there are different following production process (can be divided into scaffolding tube, fluid tube, wire casing, bracket tube, fence tube, etc.).

From the production process, the production process of straight seam welded pipe is simple, the production efficiency is high, the cost is low, and it develops rapidly. But the strength of the spiral welded pipe is generally higher than that of the LSAW steel pipe. Spiral welded 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. However, compared with the same length of the LSAW steel pipe, the weld length of spiral welded pipe increased by 30 to 100%, and the production rate of it is low.

From the welding process, the welding method of spiral welded pipe and straight seam welded pipe is consistent. But the straight seam welded pipe will inevitably have a lot of T-shaped welds, so the probability of appearing welding defects are greatly improved. And the welding residual stress at TIG welding is large, the weld metal is often in three-dimensional stress condition, which increases the possibility of producing crack. Moreover, according to the submerged arc welding process, each weld should have starting arc place and arc blowout place, but each LSAW steel pipe can not achieve the conditions in the welding ring, which may cause there are more welding defects in the arc blowout place.

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.

Anti-corrosion Technology of Spiral Welded Pipe

We all know that the individual quality of spiral welded pipe is relatively large, so it must be stacked outdoors. But it is difficult to avoid being exposed to the sun and rain. So the rust problem has been puzzled the storage time and conditions of spiral welded pipe. Today, we need to carry out a comprehensive solution to the knowledge of anti-corrosion technology of spiral welded pipe.

The toolrust-removal of spiral welded pipe mainly uses steel wire brush and other tools to polish the steel surface, and then cleans and preheats the spiral welded pipe, which can remove loose or turnup mill scale, rust, welding slag and so on. Using manual tool to remove rust can reach Sa 2 level, using power tools can reach Sa 3 level. If there is steady oxide-scale adhering to the steel pipe surface, the effect of toolrust-removal is not ideal, it can not up to the depth of abrasion pattern that required by anti-corrosion construction.

Pickling refers to use solvents, emulsions to clean the surface of the spiral welded pipe (SSAW steel pipe) delivered by low pressure fluid, to achieve removing of oil, grease, dirt, lubricants, and similar organic matter, but it can not remove the rust, mill scale, welding flux, etc. on the surface of spiral welded pipe. So Pickling can only as an auxiliary mean in the anti-corrosion production. Manufacturer general use chemical and electrolytic two methods for pickling treatment, pipeline anti-corrosion technology only adopts chemical pickling, that can remove mill scale, rust, the old coating. And sometimes it can be used as re-treatment after derusting by sandblast. Although Chemical cleaning can make the surface to achieve a certain degree of cleanliness and roughness, but its anchor is shallow, and it is easy to cause stacking environment pollution of spiral pipe welding.

Reasonable using the means to clean the rusty pipe can ensure that spiral welded pipe serves the production for a long time and create more production efficiency.

Surface Treatment of High-frequency Welded Steel Pipe

long-distance oil/gas pipeline is an important way of energy security, in the oil (gas) pipeline anti-corrosion construction process, the surface treatment of high-frequency welded steel pipe is one of the key factors to determine the useful life of the pipeline. It is the premise that the erosion resistant coating can be firmly bonded to the steel pipe. According to the research institutions, the life of the erosion resistant coating depends on the coating type, coating quality, construction environment and other factors. The surface treatment of steel pipe accounted for about 50% on the life of the erosion resistant coating. therefore, we should be strictly in accordance with the requirements specification of erosion resistant coating to steel pipe surface, continue to explore and summarize, and constantly improve the surface treatment of steel.

1. Cleaning

Using solvents and emulsions to clean the surface of steel to achieve the removal of oil, grease, dust, lubricants and similar things, but it can not remove the rust, oxide, welding, etc. on steel surface, so it only can be an auxiliary means in anti-corrosion production.

2. Toolrust-removal

We mainly use steel wire brush and other tools to polish the steel surface, which can remove loose or tilt oxide, rust, welding slag and so on. Manual tool derusting can reach Sa2 level, power tools derusting can reach Sa3 level. If the steel surface attached to a solid iron oxide skin, the effect of toolrust-removal is not ideal, and can not up to the depth of the anchor required by anti-corrosion construction.

3. Pickling

Generally, we use chemical and electrolytic two methods for pickling treatment. Pipeline corrosion only uses chemical pickling, which can remove the oxide, rust and the old coating. And sometimes it can be used as a re-treatment after derusting by sandblast. Although using chemical to clean the surface can achieve a certain degree of cleanliness and roughness, but the anchor is shallow, and it easy to cause pollution to the environment.

4. Abrasive blasting

Abrasive blasting is through high-power motor rotating to drive jet vane to rotate on high speed to make steel sand, steel balls, wire segments, minerals and other abrasives doing spray treatment on the surface of the steel pipe in the centrifugal force. This method not only can completely remove the rust, oxides and dirt, but the steel pipe under the action of the abrasive impact and friction also can achieve the required uniform roughness.

After the spray treatment, we not only can expand the physical adsorption of the surface of the tube, but also enhance the coating and the pipe surface of the mechanical adhesion. Therefore, the spray (throw) treatment is the ideal way to rust rust pipe repair. In general, shot peening (sand) derusting is mainly used for pipe internal surface treatment, shot blasting (sand) derusting is mainly used for pipe external surface treatment.