Induction formed pipe bends are manufactured from straight motherpipe using specialised bending machines and electric induction heating equipment. It is an inevitable consequence of the process that the material and dimensional properties of the pipe are altered during hot bending. The challenge is to design and control the process such that acceptable material properties are achievedacross the spectrum of grades and sizes that need to be bent. Each material is different, and this means the process variables need to be defined for each situation to get the right results.
The induction heating process has the unique capability to generate a narrow circumferential heat band within the full wall thickness of the pipe. The pipe bending process utilizes the generated heat band as a “plastic hinge”. Large sections can be bent with relatively little force due to the low hotyield strength of the pipe as it moves through the coil. An additional advantage is that all the plastic strain due to bending is confined to the narrow heat band and thereby minimising the tendency for ovality and lowering the threshold for wrinkling.
Aim of induction bending
The primary aim for induction bending is that the end results of integrity (material properties anddefects) and dimensions are achieved as agreed. This requires advanced process control over the principal manufacturing parameters of temperature, speed and cooling rate, as well as the importantstart and stop procedures, in order to achieve consistent and acceptable results.
Simplistically, the induction bending process can be described as: commencing with the straight pipe loaded into the bending machine and clamped to the bending arm at the required bend radius;induction power is applied and when the required temperature is achieved the pipe is driven forwardat controlled speed to initiate bending. The bending arm provides the bending moment to curve the pipe at the clamped radius; and bending progresses in a continuous even process until the required bend angle is achieved.
In reality, the induction bending process is of course much more complex – especially for high endapplications where the effort expended before manufacture of any of the production bends can bevery extensive. For a typical X grade linepipe the process would involve careful evaluation of allfactors which affect the bending process; including: the pipe size and grade, pipe type (seamless or welded), chemistry, the estimation of likely manufacture parameters; service condition; requiredmetallurgical and dimensional properties and therefore critical examination of the necessary starting properties. The pipe for bending would have the surface prepared by grit blasting, visually examined and inspected for wall thickness and defects. The induction coil would be designed for optimum performance and a systematic approach to induction testing would be undertaken followed by fully controlled qualification test bend manufacture with auto start and stop procedure programming; inspections and mechanical testing. On approval of the qualification test bend resultsthe production motherpipe would be prepared and inspected and then induction bent as “clones” of the approved procedure. The completed bends would be machined with bevel ends, tested and inspected, coated as specified and labeled. Documentation would be assembled into a consolidatedmanufacture data report detailing all aspects of manufacture, testing and inspections.
Each project represents a unique set of circumstances which must be defined and a suitableManufacture Procedure Specification (MPS) developed. Experience plays an important role in theassessment of bending proposals and informing the client at the earliest possible opportunity of anyrisks or issues to be considered. Historical data is valuable in saving time and reducing costs indetermining suitable process parameters.
The size and availability of induction bending machines governs the size and availability of induction bends. Internationally, induction bending capacity covers the pipe size range DN50through to over DN1600, and wall thicknesses from 3mm through to 150mm. A wide range of machine types exist – many are one-off designs of varying capability and process control. The bending capacity and capability for any given machine is a complex combination of pipe diameter,wall thickness, material type, bend radius; and the appropriate processing parameters of temperature, speed and cooling; and dimensional requirements.
In China, the current available induction bending capacity is based on induction bending machine with a rated maximum pipe diameter and wall thickness limit of DN2000 and100mm respectively. The bend radii available from Induction bending machine, depending on pipesize, varies from 100mm to 12,500mm; and can be as tight as 1.5D. Longer radii are possible usingnon conventional techniques.
Caution is recommended in the interpretation of induction bending capacity charts as they give noclue to the levels of process controls which may be required to achieve the necessary material properties and consistent dimensions throughout the arc length of the bend. Inductabend’smachines have been specifically configured for enhanced process control necessary to manufacturehigh quality pipeline bends from high X grade carbon steel pipes for the pipeline industry.
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