Welded Pipe/Spiral Pipe/ HF-ERW Pipe/Seamless Pipe

Welded Pipe

Welded can mean longitudinal seam welded tubing manufactured by an autogenous (without filler metal) fusion welding process, as opposed to tubing manufactured by other welding processes, such as solid-state processes.

Welded tubing is made by forming flat products (strip, sheet or plate) into the desired shape, in this case, normally round. Once the desired shape has been achieved, a high energy source is used to melt the metal locally at the weld joint. It is squeezed together and allowed to solidify, forming a weld bead. The high energy source may be an electric arc, a plasma arc, a laser beam, or even an electron beam.  The as-welded weld bead is typically somewhat thicker than the adjacent base metal and needs to be modified to match the base metal thickness, and to correct the undesirable physical, chemical and corrosion resistance attributes of the weld.

-  Manufacturing welded tube

Depending on the outer dimension, wall thickness and final application, there are different ways of manufacturing welded tubes and pipes. Strip welded tubes are typically made in accordance with the production route. The material is decoiled, preferably into a strip accumulator which then enables continuous tube production.

Typical production route for inline strip welded tubes
Source: http://www.outokumpu.com/SiteCollectionDocuments/Welded-Stainless-Steel-Tubes-and-Pipes-vs-Seamless-Acom.pdf

The strip edges may be precision-trimmed in order to create perfect joint geometry before welding. Cold forming is performed step by step from flat strip into a round profile and the edges are welded together as they approach the welding rolls at the welding station. Typical welding methods for strip welded tubes are traditionally autogenous tungsten inert gas (TIG) and plasma arc welding (PAW) or combinations of these. The tendency is that more and more tube manufacturers now are using the more productive laser welding method. Welding can be carried out with filler metal when this is a requirement in the product standard. Outside grinding of the weld seam follows welding. Solution annealing or stress relieving may be necessary by code or application requirements. Today, most of the modern welding lines are equipped with inline induction annealing. Such heat treatment homogenises the weld to be structurally indiscernible from the rest of the tube and improves properties and reduces residual stresses of the cold formed tubular product. After annealing the tube is calibrated to nominal diameter and straightened. The tube is normally 100% eddy current tested (ET), marked, cut to standard or special lengths, de-burred, visually inspected and finally packed. The cleaning with acid pickling as part of the process can be performed either inline or offline.

Heavy-walled pipe, typically thicker than 6 mm, and pipes with large OD, typically larger than 500 – 600 mm are made piece by piece from plate or sheet. Depending on thickness, the forming is done in a roller bending machine or in a hydraulic press. Welding is commonly performed in I, Y or X shaped joints with PAW and (tandem) submerged arc welding (SAW) or combinations of these in specially designed welding machines. TIG welding is frequently used for dressing (remelting) of the root.

All welding is normally done with filler metal, except when welding under codes that do not allow use of filler metal. After welding, the weld seam can be ground and the pipes can be annealed and water quenched as specified in the applicable standard or customer specification. The pipes are calibrated to fulfil specified dimension tolerances, followed by possibly sandblasting and acid pickling. The pipes are finally tested with hydrostatic (HT), radiography (RT) or other NDT methods like dye penetrant testing (PT), but also DT in accordance with actual product standards.

Typical production route for heavy-walled pipes
Source: http://www.outokumpu.com/SiteCollectionDocuments/Welded-Stainless-Steel-Tubes-and-Pipes-vs-Seamless-Acom.pdf
 -  Advantages of welded pipe
  • Welded pipes are typically more cost effective than their seamless equivalents.
  • Welded pipes are usually more readily available than seamless. The longer lead time required for seamless pipes can not only make timing problematic, but it also allows more time for the price of the materials to fluctuate.
  • The wall thickness of welded pipes is generally more consistent than that of seamless pipes.
  • The internal surface of welded tubes can be checked before manufacturing, which isn’t possible with seamless.

Seamless Pipe

Seamless pipe is produced domestically in sizes NPS 'Ie through NPS 26 00. Seamless pipe is produced without a seam or weld in the circumference. Seamless pipe is produced by a variety of methods. To put it in its most simple terms, seamless pipe is produced by piercing a solid billet of deoxidized and conditioned steel, which has been properly prepared and heated to the proper temperature. It is then processed through a series of mills where the pipe is finished to its prescribed dimensions. Seamless sizes over 14 inch 00 are usually rotary rolled from 14 inch seamless shells which expand the diameter and reduce the wall thickness to the approximate dimensions required. Small sizes of seamless pipe are generally obtained through the use of a stretch reduced mill. In this process the outside diameter and the wall thickness of the pipe is reduced through a series of rolls. Seamless pipe goes through various finishing operations including straightening, inspection, testing, and end finishing. Seamless pipe is widely used in construction, oil refining, chemical and petro-chemical industries.

-  Manufacturing seamless tube

Seamless tubes are generally made in multiple steps starting with hot extrusion to form the tube hollows from billets, Figure 3a. The extrusion billet has typically a drilled hole and is pushed through a die and over a mandrel followed by air cooling or water quenching. The ID of the extrusion die determines the OD of the extruded tube and the OD of the piercing ram determines the ID. The gap between the piercing ram and the extrusion die defines the tube wall thickness. The wall thickness uniformity (concentricity) is difficult to control and it is difficult to achieve good surface quality. The surface of the extrusion billet is initially machined in order to remove any surface defects that otherwise would have a negative effect on the surface quality of the finished product. Further reduction of the tube hollows down to the required tubing sizes is done cold using pilgering to final finish and size of the tubes. Cold work improves the mechanical properties and tolerances. Cold pilgering is the preferred production process since this technique provides a high forming rate, narrow tolerances and good productivity yields, but cold drawing gives narrower tolerance ranges and better surface quality. In general both seamless and welded tubes receive some type of finishing treatment and heat treatment after completion of the manufacturing process. For example, ASTM A269, Standard Specification for Seamless and Welded Austenitic Stainless Steel for General Service, states the tubing may be hot finished or cold finished and that all tubing shall be supplied in the heat treated condition.

Seamless pipe process
Source: http://report2011en.tmk-group.com/tmk/annual/2011/gb//English/20/70/10/Picture_1.png

 -  Advantages of seamless pipe
  • The main perceived advantage of seamless pipes is that they don’t have a weld seam. Traditionally, the seam of welded pipes has been viewed as a weak spot, vulnerable to failure and corrosion. For many years, this fear was probably justified. However, in recent years improvements in the manufacturing process for welded steel pipes and other welded pipes have boosted the strength and performance of the weld seam to levels indistinguishable from that of the rest of the pipe.
  • Seamless pipes provide peace of mind. Although there should be no issues with the seams of welded pipes supplied by reputable manufacturers, seamless pipes prevent any possibility of a weak seam.
  • Seamless pipes have better ovality, or roundness, than welded pipes.

Spiral Pipe

Spiral Weld Pipe, as the name implies, is a steel pipe which has a seam running its entire length in a spiral form. In the past, due to the method of manufacture, Spiral Welded pipe was relegated to low pressure and structural applications. With the development of the Submerged Arc Welding process, the production of large hot rolled coils of sufficient width and the development of dependable non-destructive testing methods, it is now possible to produce Spiral Weld pipe for high pressure service.

Present Spiral Weld mills consist of a de-coiling device (in the case of strip base material) or a plate preparation table (where the base material is in plate form) a strip connecting welder, straightening rollers, edge preparation tools (shearing and trimming), prebending devices, a three roller bending and cage forming system, an internal welder, an external welder (both Submerged Arc), ultrasonic testing apparatus and cutting devices. The material passes through all these production stages continuously. The angle between the flat strip being I fed into the machine and the finished pipe leaving the machine controls the pipe diameter in ratio to strip width and the angle of the weld in the pipe. 

Because of the method of manufacture, a wide variety of diameters can be produced. The diameter tolerance is small, particularly with regard to ovality; and the pipe, due to its axial symmetry, has an inherent straightness. The length range is infinite and is controlled only by the economics of transportation. Spiral Weld Pipe is used for dredging, slurry, water and other pipelines, as well as piling and structural applications. Spiral Weld Pipe is produced in accordance with the dimensional and tolerance requirements of various ASTM, AWWA, and API Specifications.

Spiral weld pipe process
Source: http://www.hitechpipe.com/images_4/image018.jpg
 -  Advantages of spiral pipe
  • Customize size that meet exact design requirement (Diameter, Thickness, Length).
  • Greater strength, double welded seam has the effect of a spiral band around the pipe.
  • Greater pressure resistant, Stress, and Friction. The spiral band has the effect of adding greater structural strength, 25% higher pressures than longitudinally welded pipes and ERW Pipes.


HF-ERW (High Frequency Electric Resistance Welded)  pipes are made from hot rolled flat steel strip, formed into tubular shape and the longitudinal seam is welded by the application of mechanical squeezing of edges and heating the edges through High Frequency Electric Resistance applied by induction or conduction. The weld joint is achieved without addition of any filler metal.

High frequency electric resistance weld
Source: http://www.impeder.com/Efficiency/contact%20vs%20induction.gif
 -  Advantages of HF-ERW pipe

Weld Quality. Solid-state, high-frequency welding for ESS in general requires minimal heat input, produces a narrow heat-affected zone (HAZ), and results in improved weld properties. The solid-state weld process uses less heat input because high-frequency (150-400 kHz) electromagnetic energy is used in conjunction with high pressure to join two materials. The material is joined under heat and pressure instead of melted together with a filler material. Controlling key welding process parameters, such as frequency, enables you to control the time and temperature, resulting in a narrow HAZ and improved weld properties in the weld zone.

The strength of the weld zone in solid-state welded structures is near that of the parent material, whereas the strength of fusion weld zones is approximately 70 percent that of the parent material for typical steels.

Flexibility. HF ERW has the flexibility to weld low- and high-strength materials, dissimilar metals, and various sizes and shapes.

Efficiency. With this process, producing ESS inline enables you to yield greater operational efficiencies when compared to traditional hot rolling lines.

Production Rates. The process can ­accommodate speeds from 15 to 30 ­meters per minute.

No Restraightening. Through the control of key parameters such as time and temperature, you can produce sections that are straight as produced off the line.



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