Influence of Welding Frequency on High-frequency Welded Pipe

Welding is critical to the quality of high-frequency welded pipes, and the frequency of high frequency welding has a great impact on welding, because the high-frequency's frequency affects the distribution of current within the steel plate.

The impact of high or low selected frequency of welding mainly acts on affecting the size of the weld heat affected zone. From the welding efficiency, pipe manufacturers should use a higher frequency as far as possible. 100KHz high-frequency current can penetrate 0.1mm ferrite steel, but 400KHz can only penetrate 0.04mm, which means that the current density distribution on the surface of the steel plate of the latter is nearly 2.5 times higher than the former.

In the production practice, pipe manufacturers generally choose 350KHz ~ 450KHz frequency when welding carbon steel material. When welding alloy steel material, and welding more than 10mm thick steel plate, pipe manufacturers can use 50KHz ~ 150KHz low frequency, because the skin effect of chromium, zinc, copper, aluminum and other elements contained by the alloy steel has some differences from steel.

Now, foreign high-frequency equipment manufacturers have mostly used solid-state high-frequency new technology, after setting a frequency range, it will aromatically track and adjust frequency according the thickness of the material, the unit speed, etc when welding.

What is the High-frequency Welding?

High-frequency welding is a welding method that using resistance heat produced by high-frequency current flowing through workpiece serial interface to heat, and in the case of applying forging force, makes the workpiece metal to be interconnected. It is similar to ordinary resistance welding, but there are also many important differences. High-frequency welding for carbon steel pipe production has been 40 years of history. With a larger power supply, high-frequency welding can achieve a higher welding speed (higher than the maximum welding speed of TIG welding more than l0 times) for the different materials, caliber and wall thickness of the steel pipe. So high-frequency welding has a high productivity in the production of general-purpose steel pipe. But high frequency welding speed usually makes removing the burrs difficult. This is also one of the reason why high-frequency welded steel pipe is not accepted yet by the chemical industry and the nuclear industry currently.

The Introduction of Spiral Steel Pipe

Spiral steel pipe, also called SSAW steel pipe or spiral welded spiral pipe, are made by hot rolled strip, it is made by extrusion forming under the normal temperature, and then welded by taking the technology of double wire and double-faced submerged-arc.

During molding process of spiral welded steel pipe, steel plate has uniform deformation and light residual stress, thus the surface has no scratches. Spiral steel pipe has more flexible thicknesses and diameters, especially in making high-grade thick pipes with small or medium aperture, these techniques has incomparable advantages in satisfying your needs.

Spiral steel pipe can be widely used in the production of large diameter spiral tubes for the following reasons:

1) as long as changing the forming angle, we can use the same width of the strip steel to produce a variety of caliber spiral pipe;
2) Since spiral welded steel pipe is formed by the continuous bending, the specified lengths of spiral steel pipe is not limited;
3) The weld in a spiral shape evenly distributed throughout the circumference of spiral steel pipe, so the dimensional accuracy of spiral pipe is high, and its strength is also high;
4) Spiral steel pipe is easy to change the size, so it is suitable for small batch and multi-species production.

Production Process of High Frequency Welded Pipe

The production process of high-frequency welded pipe mainly depends on the variety of products. From raw materials to finished products needs to go through a series of processes, and the completion of these processes requires a variety of mechanical equipment and welding, electrical control, detection devices. These equipment and installation in accordance with the different process requirements a variety of reasonable layout.

Taking ERW steel pipe as an example, typical production process of high frequency welded pipe is shown below:

Uncoiling - Leveling - End cutting - End shearing and butt welding - Spiral loop/accumulator - Edge cutting/Edge milling - Ultrasonic detection - Forming - High frequency welding + Burr removing - Welding seam ultrasonic detection (offline online) + Metallographic examination - Medium frequency heat treatment - Air cooling - Water cooling - Sizing - Fly cutting - Flush out - Cutting/Cropping - Flattening testing - Straightening - End facing and bevelling - Hydrostatic testing- Ultrasonic detection for welded seam and pipe end - Marking and coating - Finished product

ERW Steel Pipe Specifications

Some novices don’t know ERW tubes standards, when they try to buy steel pipe for the first time, they usually feel puzzled. So, there is a brief introduction of Prime’s ERW steel pipes specifications for all of you, including application, specification, standard & grade, end, surface, and packing.

The application of ERW steel pipe:

For low pressure fluid transmission, such as water pipelines, agriculture & irrigation (water mains, sewerage systems,industrial water lines, ;ant piping, deep tube-wells & casing);
For machinery manufacturing, such as engineering purpose, fencing, scaffolding, line pipes etc.;
For Gas pipe lines (pipe lines for natural gas), LPG and other non-toxic gas.

The specification of ERW steel pipe provided by Prime Steel Pipe:

OD (Outside Diameter): 21.3mm ~ 660mm
WT (Wall Thickness): 1mm ~ 17.5mm
Length: 0.5mtr ~ 22mtr (5.8/6/11.8/12 meters, SRL(Single the length, the nominal length is 40FT, 12m), DRL(Double the length, The nominal length is 20FT, 6m))

The standard & grade of ERW steel pipe:

The standard and grade provided by Prime Steel Pipe is ASTM A53 and Grade A/B/C.
ASTM A53 is a carbon steel alloy, used as structural steel or for low-pressure plumbing. The alloy specifications are set by ASTM International, in specification ASTM A53/A53M. A53 pipe comes in three types and two grades, A53 Type F, which is longitudinally furnace butt welded or continuous welded (Grade A only), A53 Type E, which is longitudinally electric resistance welded (Grades A and B), and A53 Type S, which is seamless pipe, produced by hot working, and possibly cold finishing, the steel (Grades A and B).

Grade A/B/C Chemical Composition

Symbol	Chemical composition
	C max	Si max	Mn max	P max	S max
Grade A	0.14	0.04	0.90	0.035	0.040
Grade B	0.17	0.04	0.90	0.035	0.040
Grade C	0.24	0.04	0.90	0.035	0.040

Grade A/B/C Mechanical Properties

Symbol	Yield point min MPa	Tensile strength MPa	Elongation in 2 in.(50mm)%
Grade A	165	310-450	30 min
Grade B	185	345-450	28 min
Grade C	205	380-515	25 min

The End of ERW steel pipe:

Square Ends/Plain Ends (straight cut, saw cut, torch cut), Beveled/Threaded Ends

The surface of ERW steel pipe:

Bare, Lightly Oiled, Black/Red/Yellow Painting, Zinc/Anti-corrosive Coating

The packing of ERW steel pipe:

Bundled/In Bulk, Plastic Caps Plugged, Waterproof Paper Wrapped

Influences of Steel Strip's Geometric Dimensions on Steel Pipe's Quality

How to ensure the quality of high frequency welded pipe in the process of production? We should take the effects of roll adjustment into consideration, except that, we also need to consider the geometric dimensions of steel strip.

When the width of the steel strip is less than the allowable deviation, the squeezing force of the welded steel pipe will be reduced, which will cause the welding of the steel pipe loose, and cracks or open-end pipe will appear. When the width of the strip is greater than the allowable deviation,the squeezing force will be increased when the steel pipe is welded, which will cause tip, welding or burr welding defects appearing on the weld of the steel pipe. Therefore, the fluctuation of the strip width not only affects the accuracy of the steel pipe's outer diameter, but also seriously affects the surface quality of the steel pipe.

About the steel pipe that requires the same section wall thickness difference does not exceed the specified value (that is to say the uniformity of the steel pipe wall thickness is high), the fluctuation of strip thickness will transfer the allowable value that exceeds the thickness difference of one steel strip to the wall thickness difference of finished steel pipe, which will waste a large number of steel pipe for the thickness of the pipe exceeds the allowable deviation. Thickness fluctuations not only affect the thickness accuracy of the finished steel pipe, at the same time, the difference of steel strip's thickness will cause the extrusion pressure and welding temperature are instability while the steel pipe in the welding, which will result in welded steel pipe weld quality instability.

In addition, the existence of interlayer, impurities, trachoma and other material defects in the interior of steel is also an important factor to affect the quality of steel pipe .

Therefore, before welding the steel strip , we should check the surface quality and geometric size of each steel strip. For the steel strip that the quality does not meet the standard requirements, we shouldn't carry out production to avoid unnecessary losses.

High-frequency Welding Principle of Longitudinal Welded Pipe

The emergence and maturity of high-frequency welding technology, directly promoted the development of Longitudinal Welded Pipe industry, it is the key processes of Longitudinal Welded Pipe(ERW steel pipe) production. High-frequency welding originated in the fifties of last century, it is a new welding process that uses the skin effect and adjacent effects produced by high-frequency current to make the steel plate and other metal materials docked.Good or bad quality of high-frequency welding directly has an impact on the overall strength, quality grades and production speed of welded pipe products. As a manufacturer of welded pipe, you must understand the basic principles of high-frequency welding, understand the structure and working principle of high-frequency welding equipment, and understand the high-frequency welding quality control points.

Relatively the 50Hz AC current frequency speaking, high frequency is generally refers to 50KHz ~ 400KHz high frequency current. When high-frequency current flow through the metal conductor, it will produce two strange effects: skin effect and proximity effect. And high-frequency welding just uses these two effects to weld the steel pipe. So, what are the those two effects?

Skin effect

Skin effect refers to a certain frequency of AC current flow through the same conductor, the current density is not evenly distributed in all sections of the conductor. It will mainly focus on the surface of the conductor, that is, the density of the current in the conductor surface is big, the density of the conductor inside is small. So we vividly called it "skin effect". Skin effect is usually measured by the penetration depth of the current, the smaller the penetration depth, the more obvious the skin effect. And the penetration depth is proportional to the square root of the resistivity of the conductor, and is inversely proportional to the square root of the frequency and permeability. In layman's terms, the higher the frequency, the more concentrated current will be on the surface of the steel plate; the lower the frequency, the more dispersed current will be on the surface. It must be noted that, although the steel is a conductor, its permeability will drop with the temperature rise, that is, when the steel plate temperature increases, the permeability will decline, skin effect will be reduced.

Proximity effect

The proximity effect refers to the fact that when the high frequency current flows counter currently in two adjacent conductors, the current will flow collectively toward the edges of the two conductors. Even if there is a shorter side in two conductors, the current does not flow along the shorter route. We call this effect "proximity effect". Proximity effect is essentially affected by inductive reactance, inductance plays a leading role in the high frequency current. The proximity effect increases as the frequency increases and the spacing of adjacent conductors approaches. If adding a core around the conductor, the high frequency current will be more concentrated on the surface of the workpiece.

These two effects are the basis for achieving high frequency welding of metals. High-frequency welding just used the skin effect so that the energy of high-frequency current concentrated in the workpiece surface, and used the proximity effect to control the location of high-frequency current flow and range. The speed of the current is very fast. It will heating and melting adjacent to the edge of the steel plate in a very short period of time, and achieve docking through the extrusion.