General Information About Induction Bending

Steel products are used in almost every engineering applications since steel products have extreme mechanical properties. Moreover, the earth has abundant iron mines and also there are a lot of factories which produce raw steel. This results in a high amount of steel products which require to get shaped in different forms. Moreover,  the utilization of various processes produces steel parts in different shapes. The induction bending process is the most common method to fabricate the long steel bents. This process uses coils where the electricity flows on the wires and creates induction at the center of the coils. These high amounts of electricity on the coils also results in extreme heats at the center of the coils. The induction bending process essentially utilizes inductors to heat the steel parts locally. The desired shape and bending angle of the steel parts are maintained by a fixed radius arm. When the steel part reaches the desired temperature (between 900 and 1100 degrees Celcius) the steel parts move out the coil while the fixed arm bends the hot location of the steel. Induction bending processes are usable for various applications. The bend products are generally used in petrochemical, chemical, shipbuilding, construction and power generation applications. Almost every type of steel can be shaped by using induction bending machines. The most popular steels that are used in the induction bending applications can be classified as; the carbon steels (low alloyed, high alloyed), stainless steels (austenitic, martensitic, ferritic, duplex) and the special alloyed steels. Please click below the link for further information about the hot induction bends

Why Induction Bending

When comparing with the other shaping processes, induction bending offers some benefits which are briefly;

  • The induction bending process is highly cost-efficient. Bends can be manufactured faster than the standard components that require extra welding for joining.
  • Larger radius bends can be used instead of elbows. This reduces the wear and the friction also dramatically decreases the pump capacity.
  • Usage of the induction bent components decreases the requirement of welding applications
  • Bend parts do not have weld beams at the tangents. These highly increase the homogeneity of the steel which extremely enhances the mechanical properties.
  • The need for the non-destructive tests is lowered by the utilization of the induction bending. This also reduces the total cost
  • Bend components are generally stronger than the elbows since the wall thicknesses of the bend components are homogeneous and almost equal in every point.
  • There is no need for lubricants since the induction bending is a clean process
  • No extra or filler material needed during the process. The induction bending process uses only the straight pipe.

Expected Properties Of A High-Quality Bend

  • During the induction bending, the designation of an accurate bending angle and radius are highly substantial
  • Wall thinning and ovality of the raw steel must be kept at minimum values
  • The temperature during the process must be properly controlled since the soaking temperature directly affects the mechanical properties
  • The transition between the bent areas and straight zones must be smooth
  • The process conditions must be arranged according to the proper standards of induction bending
induction bending ile ilgili görsel sonucu

Fig1. Induction bending of a pipe. Retrieved from: Induction bending. (2020). 18 March 2020, https://www.rohrbiegewerk.de/en/production-process/induction-bending.html

The Induction Bending Process

The induction bending process is a manufacturing method that almost fully automated. The straight raw steel (pipe or slab) is heated particularly and forced by an arm to get into the desired shape. An induction coil heats the local regions of the steel. The heating and cooling sequences of the particular zones are extremely important just because these regions act as heat treated parts and mechanical properties of these regions may alter. Around the induction coil, alternating current flows. This alternating current induces a potential around the steel region and creates an eddy-current in this region. Faraday’s induction law states that when the magnetic field on any conductor material changes, a circular electrical current occurs and this current is called the eddy-current. The eddy-current creates a secondary magnetic field that is perpendicular to the initial magnetic field. When the eddy current passes through a conductor material, it creates heat because of the resistance of the conductor. Therefore, the local regions of the steel pipes are heated according to this principle.

eddy current heating ile ilgili görsel sonucu

Fig2. The principle of induction heating. Retrieved from: Principle of High-frequency Induction Heater Unit | Micro Joining Equipment | NIPPON AVIONICS CO., LTD. (2020). 18 March 2020, http://www.avio.co.jp/english/products/assem/principle/highfrequency/index.html

During the induction bending process, the width of the bending region must be properly controlled otherwise the bent body may excessively deform and this can cause cracks or flaws. The induced temperature on the local region is controlled continuously and kept at a constant value (Above the A-Cementite 3 line). This constant value provides an austenitic structure in the bent body. A pivoted arm forces the raw steel (pipe or slab) and this mechanical forcing results in bending in the raw steel. The pivoted arm is clamped to the steel body and the bending force acts on the body since the steel part (pipe or slab) is pushed through the induction coil by a hydraulic press.

The bending of the steel part causes extreme plastic deformation on the material. The pivoting arm forces the steel part and bends it. Therefore the inner wall thickness and the outer wall thickness of the steel changes. The inner wall thickness increases because of the compressive forces. In contrast to the inner wall thickness, the outer wall thickness decreases because of the excessive strain. The bend radius of the pivoting arm decides the rate of the increase and the decrease of the wall thickness. Thus, the alteration of the wall thickness must be properly considered before the fabrication.

Post Bend Heat Treatment

The induction bending process utilizes local heating of the starting raw steel. The local heating of the steel part may alter the mechanical properties of the steel since the microstructure of the steel changes. In order to yield a homogeneous microstructure in the steel parts, post bend heat treatment is commonly used. The heat treatment process of the bends generally covers the heating soaking and cooling periods. The heating period maintains an equal phase distribution on the steel while the cooling period determines the final microstructure of the steel.

heat treatment of induction bends ile ilgili görsel sonucu

Fig3. Heat treatment of bend pipes. Retrieved from: Heat treatment. (2020). 19 March 2020, https://www.rohrbiegewerk.de/en/production-process/heat-treatment.html

            The common heat treatment methods can be classified as normalizing, normalizing and tempering, quenching and tempering and solution annealing.

            Normalizing: Normalizing method is commonly used for carbon steels such as ASTM A106 Gr.B. or ASTM A333 Gr.6. The normalizing heat treatment covers the heating of the steel part at a determined temperature and cooling the steel part to the room temperature by using air cooling.

            Normalizing and Tempering: This method is conveniently used for steel that comprises a high amount of chromium such as ASTM A335 P11 and P22. A similar normalizing method is utilized for the steel part. Following that, the steel part heated at a lower temperature than the normalizing temperature and cooled again to room temperature.

            Quenching and Tempering: To obtain high hardness and high yield strength, the steel part is first cooled rapidly at a determined temperature and tempered. This heat treatment method is typically used for API 5L X65 grade steels. The steel part is rapidly cooled (quenched) in different types of media. This rapid cooling ensures high hardness property. To obtain optimum strength, steel is tempered at a determined temperature. The tempering process softens the steel but increases the toughness of the steel. After tempering, the steel part is cooled to room temperature in the air medium.

            Solution Annealing: This method is most typically used for stainless and duplex steels. The steel part is heated to elevated temperatures. Then, the steel part is rapidly cooled

Test Methods for Induction Bends

            The induction bending technique involves excessive plastic deformation. Hence, inner and outer wall thicknesses may change. Proper observation of the fabrication and a detailed examination of the final product is necessary for high-quality products. In brief, test methods can be grouped as below;

  • Visual inspection of the extrados and intrados must be done by experts
  • Surface cracks or flaws must be controlled via using dye penetrant test or magnetic particle test
  • Homogeneous wall thickness and ovality must be checked
  • Hardness tests must be done after the post bend heat treatment
  • For further examinations of the steel structure, metallographic tests may be used

Economical Considerations On Induction Bending

The induction bending method generally allows fewer production costs for the fabricator. The induction bending is most typically used for the fabrication of the steel pipes. The conventional pipe spools are the most common parts that are used in pipelines. However, the usage of induction bends may be beneficial for both the vendor and demander.

Table1. Cost compare of induction bends and conventional pipe spools

Method Costs Costs for Induction Bends Costs for Pipe Spools
NDT costs for welds Low High
Weld costs(the number of butt-welds) Low High
Handling costs Moderate Moderate
Material costs Low Moderate
Post heat treatment costs Moderate High
Bending costs High Very low

Standards for Induction Bending

The induction bending process involves different steps and these steps must be observed and controlled carefully. The experts may use visual inspection methods but other control parameters must be following the proper standards. The most conventional standards for the induction bends are the ASME B16.49 and ISO 15590-1(en).  Moreover ASME B16.49 standard covers ASME B31.4, B31.8, and B31.11 standards

High Quality Engineering & Procurement

YENA Engineering offers in different range of hot induction bended carbon steel, alloy steel, stainless steel, low and high temperature steel pipes for Power, Oil&Gas, Construction, Chemical Industries. We are able to bend pipes according to EN 15590-1 and customer technical specifications.
We supply precision manufacturing and narrow dimensional tolerances in wall thickness reduction, ovality, bending radius and angles. YENA facilities can bend both seamless and welded pipes between the range of 3D-10D bending radius.

 Our experienced QC Team do dimensional checks, visual control and other non destructive testing methods on welds and raw materials. All welding methods are done according to approved procedures and are applied by certified welders. All steps of procurement/manufacturing/testing/shipment are documented by our QA team. For more information, check out https://yenaengineering.nl/hot-induction-bends/

or feel free to contact us.

https://yenaengineering.nl/contact/

References

  1. Induction Bends for Transportation and Distribution Systems – ASME B16.49. (2020). Retrieved 19 March 2020, from http://www.wermac.org/fittings/hot_induction_bends.html
  2. Barnshaw, C. (2020). Induction bending: the state of the art. Retrieved 19 March 2020, from https://www.thefabricator.com/thefabricator/article/tubepipefabrication/induction-bending-the-state-of-the-art
  3. Induction bending. (2020). Retrieved 19 March 2020, from https://www.rohrbiegewerk.de/en/production-process/induction-bending.html
  4. Inductive bends – Kraftanlagen. (2020). Retrieved 19 March 2020, from https://www.kraftanlagen.com/en/about-us/companies/finow-rohrsysteme-gmbh/scope-of-services/fabrication-processes/inductive-bends/
  5. (2020). Retrieved 19 March 2020, from https://www.iso.org/obp/ui/fr/#iso:std:iso:15590:-1:ed-3:v1:en
  6. Heat treatment. (2020). Retrieved 19 March 2020, from https://www.rohrbiegewerk.de/en/production-process/heat-treatment.html
  7. Collie, G. J., Higgins, R. J., & Black, I. (2010). Modelling and Predicting the Deformed Geometry of Thick-Walled Pipes Subjected to Induction Bending. Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, 224(4), 177–189. doi:10.1243/14644207jmda314 
  8. Barnshaws Induction Bending – an Advanced Metal Bending Process. (2020). Retrieved 19 March 2020, from https://www.barnshaws.com/services/induction-bending/detail
  9. Muthmann, E., & Grimpe, F. (2006). FABRICATION OF HOT INDUCTION BENDS FROM LSAW LARGE DIAMETER PIPES MANUFACTURED FROM TMCP PLATE. Microalloyed Steels For The Oil & Gas Industry International Symposium, pp,2,4
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