Introduction

The use of High Frequency Inductive (HFI) welding is a highly productive process for the manufacture of longitudinally welded pipes from hot‑rolled strip. Longitudinally welded HFI pipes are nowadays used in the most diverse range of applications. Typical of such uses are, for example, pipelines for the conveyance of liquid and gaseous hydrocarbons, potable and utility water, brine, district heating systems, hollow sections and Oil Country Tubular Goods (OCTG). Mannesmann Fuchs Rohr (Siegen and Hamm, Germany) produces HFI pipes in O.D. 4”-20” (114.3-508.0 mm) with wall thicknesses up to t = 0.81” (20.6 mm). Special requirements originating from the offshore sector are thus also fulfilled via the use of high‑quality grades of steel with large pipe‑wall thicknesses [1]. In this sector, HFI pipes are increasingly coming into use in the offshore sector, in competition with SAW and seamless pipes. Not least important of all in this context is the trend toward the production of ever thicker‑walled HFI pipes. The technological preconditions necessary for this purpose, in terms of HFI welding technology, are discussed, with attention to normative aspects, in the following essay, with prime emphasis attended to the topic of "welding frequency".


The welding process

The welding process is the central operation in the chain of production processes involved in the manufacture of pipes from hot‑rolled wide strip. One can differentiate essentially between fusion welding and pressure welding, with a number of diverse process variants [2]. Fusion welding methods take the form of the submerged‑arc and gas metal-arc welding processes for production of longitudinally welded and spiral‑welded pipes. The pressure welding processes include among others the low-frequency and the high‑frequency welding, the latter being the subject of this essay. There are in the relevant literature no generally applicable differentiations based, in particular, on physical principles and/or phenomena for definition of the high‑frequency technology frequency range, particularly in the case of welding. Minimum frequency values for HF welding can be found in international product standards and specifications for pipes, such as EN 10208-2, ISO 3183-2, ISO 3183-3 and API Specification 5L [3],[4],[5],[6]. Here, a frequency of f ≥ 100 kHz is required for HF‑welded pipes, irrespective of all other boundary conditions (API Spec. 5L, PSL 2 only). No upper frequency limit is stated, however. Elsewhere, the term "high‑frequency" is defined, for example, as the "f = 50 to 10,000 kHz" or "f = 10 to 5,000 kHz" frequency range [7],[8].

It is possible, in principle, to differentiate in the context of HF welding processes between conductive and inductive transfer of energy. In the case of conductive technology, input of energy is accomplished with copper contacts via the surface of the pipe, with the resultant possibility of strikes and burns on this surface. In inductive systems energy transfer is accomplished without contact. In both cases, it must essentially be assured that the necessary "skin effect" is generated. Utilizable minimum frequencies were in the past the result primarily of the thermionic valve‑based generator technology and the power thus available. The advance of transistor technology permits the use of lower frequencies combined with higher power, particularly in the case of the HFI process. This is useful, in particular, for welding of thick‑walled HFI pipes which is one factor permitting the increased use of such longitudinally welded pipes in offshore application. Anyway, the minimum frequency in this context is 100 kHz, in order to fulfill the requirements of the various standards and specifications.


Physical effects

In both conductive and inductive HF welding, the principle is based on the fact that a current flows on the pipe surface and the band edges (Fig. 1). This high‑frequency current flows on the surface of the pipe via the edges of the strip to the welding point at which the melted strip edges are forced together and welded.
 

Fig. 1: Current flow on the surface of the pipe

The precondition for the achievement of the flow of current on the pipe surface and on the edges of the strip is the so‑called "skin effect" (Fig. 2). A further increase in current density on the strip edges results in the "proximity effect". The current's penetration depth is determined essentially by the temperature of the material relative to the Curie temperature.
 

Fig. &2: Basic principles [9],[10],[11]

The skin effect causes on the surface of a conductor an increase in current density dependent on frequency. Current density drops in the form of an e‑function as distance from the conductor edge increases. Reduction of current density to 1/e (approx. 37 %) is defined as current penetration depth. The influencing factors are material‑dependent characteristics and frequency. Frequency thus provides a method of controlling penetration depth and temperature increase in the boundary zone of the band edges. It must be noted that influence is relative to the square root and that a quadrupling of frequency thus results in a halving of penetration depth. In the case of HFI welding, the skin effect results in the current flowing only on the pipe surface and in heating up of the strip edges to welding temperature at the welding point.

The proximity effect also causes a further increase in current density in the strip edge zone. This effect is based on the fact that the charges, with currents flowing in opposite directions, attract each other.

The third aspect with a significant influence on the profile of temperature of the strip edge is the excess of Curie temperature. This occurs, in accordance with the iron‑carbon diagram, at T = 769 °C [2]. Ferritic steels exhibit ferromagnetic behaviour below this temperature, and paramagnetic behaviour above it. This has a significant effect on the current's penetration depth. Penetration depth will rise, for example, around 22‑fold if the temperature at the strip edge rises above the Curie temperature during HF welding.


The dependence of temperature profile on frequency

The heating profile at the edges of the strip can be manipulated, inter alia, via the selection of frequency. It must be technologically assured that the skin effect is achieved. The penetration depth of the heat‑affected zone (HAZ), particularly at the center of the strip, is generally increased as frequency decreases. The irregular configuration of the heat‑affected zone, in the form of an hour‑glass, is lessened. This results in the steel's welding temperature also being reached with processing certainty at the center of the strip (Fig. 3, Fig. 4). The overheated corners of the strip edges possible at higher frequencies are eliminated at lower ones, such as may be used in HFI process.
 

Fig. 3: The effects of frequency on temperature profile in connection with HFI welding

 

Fig. 4   Heating profile at strip edges in production of thick-walled pipe

Only a few systematic investigations of the influence of frequency on the process, on the one hand, and on the technological properties of the pipe, on the other hand, have been published. A number of these are discussed below.

The development of transistor technology has made it possible to use frequencies below f = 200 kHz for the purpose of HFI welding [12]. In the past, due to valve‑based technology, the minimum frequency from a technological and economic standpoint was, on a system‑engineering view, around f = 250 kHz. Extremely fast MOSFET transistors have been in use in HFI resonant circuits since the mid‑1980s; in addition, high‑current IGBT transistors have increasingly become established in the welding‑systems sector in around the last ten years, and thus permit frequencies down to 100 kHz for HFI welding [12],[13]. Both types of transistor are nowadays used, depending on their strengths and the corresponding requirements. The potential for technologically and economically rational use of frequencies down to 100 kHz for HFI welding must, as already mentioned above, also be viewed in close conjunction with the trend toward production of ever thicker HFI pipe‑wall thicknesses.

Finite Element Method (FEM) analysis studies have been performed into the influence of various process parameters, and frequency, in particular, on the HFI welding process [14],[15],[16]. The following influencing factors were examined here: welding angles, distance of the welding point from the inductor, resilient flexing of the strip edge, welding speed and welding frequency. The reasons for performance of these published investigations can be found in welding problems encountered in the production of pipes with a wall thickness of t = 12.7 mm, which were attributed to a cold strip center. It was ascertained that all the factors investigated significantly influence the configuration of the heat profile in the edge of the strip. As far as frequency is concerned, the frequency 100 kHz cause a much more uniform increase in temperature up into the center of the strip edge than the frequency 300 kHz. The percentage of melted material in the vicinity of the corners of the strip edge falls significantly as frequency falls, thus reducing burn‑off of these corners. It becomes apparent in the relevant investigations, however, that the other parameters examined also have a significant influence on the heat profile in the edge of the strip material and must therefore also be used for description of the process. A factor of two in the case of the angle of approach of the weld vee, for example, thus has a greater influence than a factor of three for frequency.

In contrast to this, there are also studies concerning the influence of frequency, the essential criteria of which are the spread of the heat‑affected zone and thus necessary power [17],[18]. These investigations are also unanimous in stating that penetration depth in the f = 100 to 600 kHz range becomes greater as frequency falls. The unbalanced conclusion of these authors, failing to take account of other technological boundary conditions, is in this case that higher frequencies are assumed to be more economical, since a greater penetration depth would be a waste of energy, and that higher power is therefore necessary. With respect to the complete melting of the edges of the strip, this must be critically evaluated, however. At lower frequencies in the case of HFI welding, heating up to the center of the strip is assured with processing certainty even in the case of greater wall thicknesses.

Practice also demonstrates that the process as a whole, including the environment in which it takes place, must also be studied, in addition to these physical principles. If the process chain, including upstream and downstream facilities, is thus examined, it will be necessary, for example, to analyze the interaction of possible power input and welding speed. The principal objective, that of producing, cost‑effectively, pipes which meet the specification, must continuously be borne in mind during all such deliberations, however.

The use of adjustable‑frequency HF generators is recommended for large wall‑thickness/diameter ratios [19]. A significant advantage is, accordingly, the use of a frequency tailored to the particular application, in order to minimize the hour glass and therefore the "waist‑like" constriction of the HAZ, and thus create a very homogeneous weld, with a uniform hardness distribution. The author does not state exemplary frequencies for specific applications, however.
All in all, these investigations indicate that the beneficial effects of a lower frequency on the HFI welding process are clear. There are, on the one hand, only few well founded publications on this subject and, on the other hand, diverse conceptual interpretations, however. Practice at Mannesmann Fuchs Rohr indicates that a uniform heat‑affected zone and extremely good welding results are achieved. Uniform melting of the entire edge of the strip is assured, particularly in production of pipes with large wall thicknesses of up to t = 20.6 mm.


Control of heat input to the edges of the strip

As already outlined above, heat input and the heat profile in the edge of the strip in HFI welding depend not only on the frequency used, but also on a large number of other influencing factors. Major influencing factors are shown in Fig. 5. These can be subdivided into six groups, i.e., ambient media, energy conversion at the inductor, strip‑edge condition, impeder state and arrangement, welding speed and welding vee. A total of sixteen different parameters, of which current frequency is one, are shown here. Frequency is, without doubt, an important factor in control of the welding process and thus the welding result, but can only be examined in context, together with the other parameters. A number of results from [14],[15],[16] on this subject have already been discussed. Detailed examination of the influence of each individual factor is not the subject of this essay.
 

 

Fig. 5: Factors influencing heat input

 

Fig. 6: HFI systems: Frequency and power ([21],[22],[23],[24],[25] in 2006)

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12. Leistungsschalter für Schweißinverter – Thyristor, BJT, FET und IGBT. Kemppi GmbH (www.kemppi.com)
13. Asperheim, J. I.; Grande, B.; Markegård, L.; Buser, J. E.; Lombard, P.: Computation and analysis of temperature distribution in the cross‑section of the weld Vee. Tube Int., Nov. 1998
14. Asperheim, J. I.; Grande, B.: Temperature Evaluation of Weld Vee Geometry and Performance. Tube Int., Oct. 2000
15. Grande, B.; Asperheim, J. I.: Factors Influencing Heavy Wall Tube Welding. Tube & Pipe Technology, March/April 2003
16. Scott, P.: The Effects of Frequency in High Frequency Welding. Transactions of Tube 2000, Toronto, ITA Publication (www.tubenet.org.uk)
17. Morin, T.; Scott, P.: Modern Methods of High Frequency Welding Used to Produce Conistent Quality. www.tubenet.org.uk
18. Gardiner, D.: Variable Frequency on Demand – The Ultimate in Flexibility for Today’s Tube and Pipe Producers. Tube & Pipe Technology, July 2003
19. Quaglia, A.: Selecting the right H.F. Generator. www.tubenet.org.uk
20. www.sms‑elotherm.com
21. www.emmedi.it
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24. www.efd‑induction.com

 
 Contact Details

Mannesmann Fuchs Rohr GmbH
Kissinger Weg
D-59067 Hamm
Germany
Email.
Website: www.mannesmann-fuchs.com