Views: 7110 Author: Site Editor Publish Time: 2023-06-13 Origin: Site
The primary aim for induction bending is that the end results of integrity (material properties and defects) 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 important start 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 forward at 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 end applications where the effort expended before manufacture of any of the production bends can be very extensive. For a typical X grade linepipe the process would involve careful evaluation of all factors 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; required metallurgical 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 results the 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 consolidated manufacture data report detailing all aspects of manufacture, testing and inspections.
Each project represents a unique set of circumstances which must be defined and a suitable Manufacture Procedure Specification (MPS) developed. Experience plays an important role in the assessment of bending proposals and informing the client at the earliest possible opportunity of any risks or issues to be considered. Historical data is valuable in saving time and reducing costs in determining 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 DN50 through 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 Australia, the current available induction bending capacity is based on Inductabend’s induction bending machine with a rated maximum pipe diameter and wall thickness limit of DN900 and 100mm respectively (this should not be interpreted as capacity to bend DN900 pipe with a wall thickness of 100mm).The bend radii available from Inductabend’s machine, depending on pipe size, varies from 100mm to 12,500mm; and can be as tight as 1.5D. Longer radii are possible using non conventional techniques.
Caution is recommended in the interpretation of induction bending capacity charts as they give no clue 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’s machines have been specifically configured for enhanced process control necessary to manufacture high quality pipeline bends from high X grade carbon steel pipes for the pipeline industry.
The beauty of induction heating is that it is controllable non-contact focussed heating. Induction heating as applied to the induction bending process is configured as a single induction coil to heat a relatively narrow circumferential band of pipe. The induction coil generates an intense localised magnetic flux and “induces” an electric current to circulate within the pipe wall directly beneath the induction coil but leaves no residual magnetism. It is the induced circulating current and the pipe material resistivity which efficiently generates the heat necessary for hot bending. The induction coil can be designed to give various heating affects such as a narrow or wide heat band to take account of heat conduction into thick pipe walls; and with various configurations of cooling water spray or forced air depending on particular requirements.
The induction coil and cooling water spray system as shown in the diagram is based on water sprayed from the induction coil directly onto the outside surface of the pipe bend as it emerges from the induction coil. The difference in peak temperature and rate of cooling between the outside (O), mid-wall (M) and inside (I) would be greatest for thick wall pipe.
Distortion of the pipe in the bend area due to induction bending includes ovality and wall thinning at the bend extrados and a corresponding increase in wall thickness at the bend intrados. Expected distortions for general bending can be estimated from tables. Actual distortions may vary frompredicted values due to the particular induction bending process requirements such as speed, temperature, cooling method, coil design and material type.
Induction bends for pipe lines have typical bend radii between 10D and 5D, but may be as tight as 3D. For these radii, the expected wall thinning as a function of the actual starting wall thickness would be 7%, 11% and 15% respectively.
To meet particular project requirements it may be necessary to use thicker pipe or select larger bend radii. In many projects it will be possible to allocate heavier wall pipe for the induction bends by a planned allowance for additional heavy walled pipe ordered for the special class locations such as crossings etc.
There are three principal process parameters for induction bending which affect the material properties – these are: speed, peak temperature and rate of cooling. Secondary process parameters, which are very specific from machine to machine and depend on the sophistication of the control process for each machine, are the start and stop procedures. Once qualified, these parameters must be set as the target parameters for all subsequent production bends.
Modern HFW line pipe steels are relatively low carbon micro-alloyed steels. Induction bending is generally carried out in the temperature range 875C to 1075C which is above the austenitizing temperature where re-crystallisation takes place. Over this temperature range the dissolution of micro-alloyed elements increases with temperature. For a given starting chemistry, the peak temperature achieved during induction heating and the rate of cooling determine the resulting material properties. The established relationship of increasing strength and hardness with increasing temperature and/or cooling rate is complex and is not the point of detailed discussion here – suffice to say that the strengthening mechanism is a combination of grain size effects, the solution and re-precipitation of micro-alloying constituents and the formation of low temperature transformation products.
To confidently achieve high strength and toughness directly off the induction bending machine, the peak temperature and cooling rate needs to be carefully controlled and this process must be determined and supported by physical testing.
For a fixed speed and constant cooling rate, the peak temperature is controlled by the level of induction power applied during the bending process. The cooling is rate determined by the speed of bending and the cooling water spray system comprising pressure, volume and apertures etc.
The above diagrams illustrate the effect of the wall thickness and the inferred rate of cooling, and induction bending peak temperature on the hardness at the outside (heat sink) surface; mid wall, and inside surface.
An important consideration for induction bends is the use of post bend heat treatments including normalise, anneal, temper, and quench and temper.
In some cases there may be a conflict between the bending process parameters required to achieve material properties – for example in heavy wall high strength pipe, the process parameters required to achieve the yield strength and tensile strength may cause the outside surface hardness limits to be exceeded. And the only way of solving that problem may be the application of a post bend heat treatment. Heat treatment may also resolve an impasse where the process parameters required to limit the wall thinning (the bend is formed with very cold extrados) in a critical application, does not achieve the required material strength.
Post bend heat treatment is restricted by the size and availability of suitable furnaces. There are very few furnaces available which are capable of heat treating induction bends made from large diameter pipe. This is especially so for bends which require quench and temper heat treatments.
Incorrect use of post bend temper heat treatments may cause more problems than it solves – in particular a temper heat treatment required for the bend area may adversely affect the unbent straight tangent on each end of the bend.
Because of the size range of HFW pipe (limited diameter and relatively low wall thickness) and that the chemistry is generally well suited to the induction bending process, heat treatment is rarely required for induction bends formed from HFW linepipe.
To understand where the boundaries and risks lie for pipeline induction bending it is important to understand the characteristics of the various types of linepipe and how they relate to the induction bending process.
Most transmission pipeline induction bends in Australia are based on high frequency welded (HFW) linepipe with a range of wall thicknesses and grades such that the necessary material properties can be produced directly from the induction bending machine without any furthe r treatments.
For HFW linepipe in the size range DN100 to DN600, wall thickness up to 14.3mm and grades X42 to X80, the pipeline designer should have every confidence that induction bends can be produced with material properties equivalent to the motherpipe. Linepipe manufactured in modern HFW pipe mills is produced from thermo-mechanically control rolled steel strip with chemistries to meet grade and high speed seam weldability requirements. HFW pipe chemistry is generally well suited to the requirements for the induction bending process. This can partly be explained in that modern HFW linepipe mills utilize in-line induction heating for the weld seam annealing heat treatment process. This annealing treatment - albeit at a different temperature and speed - is not dissimilar to the induction bending process thermal effect on material properties.
Larger diameter and heavier wall SAW pipe may slow the induction bending process and thereby restrict the range for the various process parameters. This is particularly the case for high X grade materials where higher temperatures and faster cooling rates derived from faster process speeds are required. For large diameter and heavy wall pipes, high strength properties may not be achievable without a corresponding increase in pipe chemistry to ensure that the pipe material is sufficiently responsive (hardenable) for the lower peak temperature at the pipe bore and the slower rate of cooling.
Achieving high strength properties directly off the induction bending machine tends to be more problematic for seamless pipe compared to the equivalent size and grade of welded pipe.
High strength seamless carbon steel linepipe is manufactured in a manner quite different than that used to make pipe from rolled plate or strip. Seamless pipe is hot formed to achieve the required pipe diameter and wall thickness; it is then heat treated to achieve the required strength and toughness. Pipe mills naturally design pipe chemistries to suit the rapid internal and external mill quench and heat treatment process. Induction bending is practically limited to external water spray cooling (ie from one side only) at relatively slow speeds and therefore cannot achieve the same quench rate as pipe mills. For lean chemistry high strength seamless pipes with wall thicknesses above 13mm it may be necessary to perform a full body post bend quench and temper heat treatment otherwise only downgraded material properties may be achieved off the bending process.
As has been demonstrated, chemistry plays and important role in achieving the required pipeline properties – this is particularly the case for high strength induction bends from heavy wall line pipe.
The Offshore Pipeline Standard - DNV OS F101 gives maximum allowable chemistries for various grades of line pipe (seamless and welded, tables 6.1 & 6.2) and motherpipe for induction bending (table 7.5). The trend of allowing higher chemistries for higher grades is clearly evident. The allowable maximum percentage of the principal constituents of carbon and manganese, as well as the micro-alloying elements of niobium, titanium and vanadium, all increase with strength grade.
In addition, it can be seen that for induction bends a higher chemistry is allowable over and above that for the equivalent grade seamless pipe; and even more so over that for welded pipe. These trends are most apparent in the consequential increase in the maximum allowable carbon equivalent (CEQ) for each grade and type. The footnote for each table indicates that the maximum allowable chemistry is applicable to quite heavy wall thicknesses.
The actual wall thickness compared to the “nominal” wall thickness, and the variations in wall thickness, can be quite different between welded pipe and seamless pipe.
Welded pipe is made from plate and as such will have a very even wall thickness along the pipe and around the pipe circumference with some thickening in the weld zone. Since pipe mills like to economise, it can be expected that the actual wall thickness for welded pipe will almost invariably be at or slightly under the nominal value.
Seamless pipe wall thickness is dependant on the quality of the pipe mill and can be much more variable that for welded pipe. Wall thickness may vary greatly around the pipe circumference and along the length of the pipe; and between pipe joints from the same heat. The bore may be eccentric to the outer diameter and give thicker and thinner sides to the pipe; and ridges in the bore may give immediately adjacent thick and thin areas of pipe wall.
On top of all of this of course any mark or blemish is going to further detract from the wall thickness. Expectations of the actual motherpipe wall thickness compared to the nominal value should generally be pessimistic – not optimistic!
Things that can go wrong are basically divided into two groups: those relating to the motherpipe; and those relating to the bending process – either the process parameters or those arising from faults and incorrect set-up or defects detected in the bends.
Inspections provide a vital role in the manufacture of induction bends. The section dimensions can be measured through the use of calipers and pigs for ovality and roundness; and ultrasonic techniques for wall thickness. The integrity of the bend can be checked by non destructive techniques including visual inspection; magnetic particle, ultrasonic, radiographic and dye penetrant inspection; surface hardness testing and hydrostatic testing. Whilst bend material properties can be inferred by the relationship between the principal manufacture parameters between the qualification test bend and the production bends.
Defects
Defects in the motherpipe can be exacerbated by the induction bending process. Induction bending cannot turn a sow’s ear into a silk purse - what you start with is largely going to determine what you end up with.
The most common defect in pipe is due to poor handling causing gouges and dents. Obviously thin wall pipe will be more susceptible to damage than thick wall pipe. For HFW pipe, rolled-in inclusions and lack of fusion or cracks in the weld region are possible but generally very rare.
Seamless pipe may have surface laminations and slivers that are revealed during grit blast preparation and hot bending. These defects are rare but can affect whole lengths – and even multiple lengths from the same heat – and are very much associated with the quality of the pipe mill.
Hot induction bending effectively heat treats the pipe material in the bend area. The chemistry of the pipe for induction bending is most critical in high strength requirements for thick wall pipes where slower bending and consequentially slower rates of cooling are experienced. If the chemistry is insufficient the hardenability of the pipe will be low and the required pipe strength may not be achievable directly off the induction bending machine.
Due to mill tolerances for end and mid pipe diameter, large diameter SAWL and particularly SAWH pipe may have a significant numerical diameter difference from the end of the pipe to the middle of the pipe. Where bends are cut mid-joint from these pipes, transition pieces may be required for weld preparation line-up.
Surface contamination by low melting point metals such as copper, zinc or lead can cause “liquid metal embrittlement” and result in surface cracks in the bend extrados. Pre-bend surface treatments, such as inert grit blasting, minimise this risk.
During initial or qualification testing, difficulties in achieving minimum material properties may be identified despite all the best efforts of the bender. Most commonly, the two principal protagonists are: yield strength – which sets the lower bound of the processing parameters; and hardness - which sets the upper bound. For thick wall pipe in sour service - a conflict can arise in that the process parameters required to achieve the necessary strength cause the surface hardness to exceed the specified limit. In this case the bending process window has “closed” and post bend immersion quench and temper heat treatment may be required.
Process parameters should not vary from manufacture of the qualification test bend to manufacture of the production bends. Principal process parameters include: speed, temperature, cooling and the start/stop procedures.
It is critical that the speed does not vary during the bending process. The thermal cycle experienced by each elemental piece of pipe which passes through the induction process must be restricted to a narrow range. Slippage in the pipe clamp on the radius arm or an elastic or spongy drive mechanism will cause speed variations during bending. Pipe which “lurches” through the bending process will produce variable properties along the arc length. Some bend regions which have “stalled” in the machine will have higher peak temperatures and slower rates of cooling: whilst others will have lower peak temperature and rapid cooling caused by sudden rapid forward progress of the pipe in the machine.
As has been shown, the bending temperature will have a significant effect on the final bend properties.
Optical pyrometers are the eyes for the induction bending process – they record the temperature of the bending process and support the basis of manufacture.
Aiming the pyrometers is critical in that the peak temperature within the heat band must be within the field of view. Recorded temperatures must practically represent the entire circumference of the pipe. For smaller pipes it may be acceptable to have two pyrometers – one at the intrados and one at the extrados to monitor and record the peak temperature; for larger pipe say >DN300 it may be necessary to have four pyrometers covering the four quadrants of the circumference of the pipe. In addition the bend machine operator must visually monitor the temperature of the heat band circumference for consistency between the pyrometer aim locations. A hand held “roaming” pyrometer can be very useful in this regard.
Some processes are more temperature sensitive than others and identification of the level of temperature control required is an important phase of the preliminary testing process.
Cooling of the pipe bend as it emerges from the induction coil is critical in achieving high strength for linepipe bends. The coil used for production must be the same coil used to manufacture the qualification test bend; and at the same cooling water pressure and temperature.
Probably the least known and described aspect of induction bending, and is generally highly guarded proprietary information.
For critical applications such as high X grade bends with properties derived directly off the induction bending machine, the start and stop process must be programmable - not operator driven – and set as part of the qualification process.
The start and stop procedures must give consistent reproducible results for the thermal transitions at each end of the bend. Note here that the thermal transition (as opposed to the dimensional transition) may actually lie some distance along the straight tangent on each end of the bend. It may not actually be at the tangent point where the bend curvature transitions into the straight tangent.
Bend angles achieved by induction bending are generally very accurate – particularly after the first bend of a batch. Measurement of the bend angle should be made for each bend immediately after forming. Estimates of the likely bend spring-back can be made and adjusted as bends progress.
Any bends outside the agreed angle tolerance can be isolated for discussion. Various angle measuring techniques are required to measure the correct angle – particularly for pipe with short tangent ends where significant ovality in the straight tangent on each end of the bend may complicate measurement of the actual angle.
Actual bend radii are generally within a tolerance of 1% of the target radius. Unless a serious set- up mistake has been made, it would be very unlikely the radius for pipeline bends is an issue.
Bends for pipe lines are generally made at fairly generous radii. If wrinkles or bumps are detected a manufacturing problem may have occurred. A slight bump may be evident at the bend start intrados where bending compression “up-sets” the pipe wall. This “up-set” is associated with pipe wall thickening, where the change in wall thickness tends to exhibit on the outside surface of the pipe. Unless obviously severe the “up-set: is not detrimental to the pipe but can be controlled by good start-up procedures, thicker walled pipe and larger bend radii.
A wrinkle in the middle of the bend may indicate slippage in the clamp, power outage or excessive coil movement.
Loss of electrical power, even if only momentary, will cause the bending process to shut down and will almost always lead to rejection of the bend – particularly if induction bending high strength pipe to achieve high strength material properties.
During hot induction bending using water spray cooling (necessary for high X grade pipes) air is blown from behind the induction coil to draught the cooling water spray away from the heat band. The use of air draught must be kept to a minimum and must be consistent throughout the bending process as the air draught can affect the surface temperature recorded by the pyrometers. Excessive air may suppress the outside surface temperature giving an artificially low reading. The operator may adjust for this apparent drop in temperature by increasing induction power - thereby inadvertently increasing the subsurface temperature of the pipe and adversely affecting the material properties.
Ovality
Ovality caused by bending is mainly confined to the bend area but can extend some distance along the straight tangent on each end of the bend – particularly for thin wall bends formed at tight bend radii. Ovality is generally a function of pipe diameter, wall thickness and bend radius but it is also influenced by the bending temperature, cooling method and material type. Ovality is less likely to occur for heavy wall, large radius bends formed at high temperature giving the lowest bending forces; and using water spray cooling (rather than forced air) to give the narrowest possible heat band. It is generally possible to predict ovality from historical information and simple guidelines.
During induction bending the pipe circumference in the bend area may contract (typically 0.5% for carbon steels, 1% for stainless) due to the coefficient of thermal expansion. Such constriction may impact on very tight internal diameters for pigging etc.
Thinning of the bend wall on the extrados is a feature of all bending processes and, for a given pipe diameter, is largely a result of the specified radius. Uncontrolled wall thinning can result if the extrados becomes hotter than the bend intrados – effectively shifting the bend neutral axis towards the intrados. This highlights the need for good temperature control on the bend intrados and extrados for wall thinning control.
Include the consideration of hot bends in the design (FEED and detail).
Familiarise themselves with the ISO, ASME, DNV standards as necessary.
Give consideration to the pipe material chemistry in relation to the required material strength for the given wall thickness. This is effectively making a risk assessment on the likelihood of achieving the material properties after induction bending.
Give careful consideration to the maximum allowable hardness value. Specifying a value lower than that which is technically required will unduly limit the scope of the bender and may compromise other more critical material characteristics – such as yield strength.
Allow for actual dimensions of the motherpipe - in particular to allow for mill tolerances and some surface marking; take a conservative view of the actual pipe wall thickness.
The material take-off (MTO) for the bends should be determined on the basis of the individual length of pipe required for each bend being nested into the available pipe joint lengths. Do not total the length of pipe required for the bends and divide by the available joint length to determine the number of joints required. The bender can advise a suitable MTO for the pipe joints required for the list of bends. Allow for and expect wastage from trimming and short off-cuts.
Allow for a contingency quantity of motherpipe to cover the need for qualification testing and any reject bends etc. For small quantities of bends this may mean an oversupply of 100% of the pipe actually required for the bends (including the preliminary and qualification bends); on larger jobs it may mean an additional 5% of pipe joints.
Induction bends for pipe lines require that a full qualification test bend be performed per heat. Where possible, select bare uncoated motherpipe all from the same heat - otherwise significant cost impacts will arise due to multiple qualification test bends and a loss of motherpipe consumed in the additional testing.
Allow for suitable straight tangent lengths on each end of each bend to avoid the bend ovality which is greatest closest to the bend. Small diameter thick walled pipe formed to large bend radii shall have the least bend ovality.
Typically, ovality is minimal at least two pipe diameters away from the bend area. Regardless, all pipeline contractors should expect and plan for the use of external line-up clamps when welding hot bends into the pipeline.
Bend angles should be stated as the angle of deflection – not the internal angle. Pipeline routes are often characterised by changes in alignment based on the survey internal angle.
Allow for a suitable lead time and other logistics to manufacture and test the preliminary and qualification test bend before the production bends. For a small project the qualification process of two to three weeks may take longer than the period of time required to manufacture the production bends. Completed bends can be stored at the bender or the coater’s yard and called-up as required, or if remote stored on-site at suitable staging locations.
Transport should be carefully planned. It may be possible to transport only a few bends at a time - especially if the bends are made from large diameter pipe, at large bend radii, with large bend angles and with long straight tangents on each end of each bend. Supporting and padding bends and the use of fabric restraints during transport should be carefully supervised to ensure they can be safely transported and unloaded without damage. Handling of bends requires the use of soft slings from overhead cranes or mobile plant – forklifts are not an acceptable method of handling bends.
Coating systems suitable for buried pipe bends are generally based on spray or roller applied ultrahigh build-up epoxy which must be compatible with the tie-in coating system. Tape wrapped bends have difficulties in wrap adhesion to the three dimensional curved surface of a pipe bend and may be unsuitable. Under special circumstances, fusion bonded epoxy (FBE) coatings may be available on induction bends.
Where possible take advantage of compound formed bends to make compact pipe spools to reduce field welds etc in the piping system.
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