Electric Resistance Welded Tubing High Frequency Induction Welding (Low Frequency Resistance Welding) High Frequency Induction Welding We will now turn our attention to two different but related processes used particularly for tubing and other rapid linear welding applications. The first is electric resistance welded tubing which utilizes standard low frequency 50-60 cycles per second in the resistance welding applications. This is often called ERW (electric resistance welds). The second uses very high frequency (I.e. near radio frequencies) to consummate the weld.

ERW & High Frequency Welding Learning Activities View Slides; Read Notes, Listen to lecture Do on-line workbook Do Homework Lesson Objectives When you finish this lesson you will understand: The difference between low frequency Electric Resistance Welding and High Frequency Welding Applications of each Keywords Electric Resistance Welding, High Frequency Welding, Tube Welding, Proximity Conductor, Induction Coil, Induction Current, Impeder, Seam Annealing

Resistance Tube Welding (ERW) In the ERW process,metal strip from a coil is brought into a series of rollers which progressively bent or form it into a tube shape and the longitudinal seam of this tube is resistance welded. W. Stanley, Resistance Welding McGraw-Hill, 1950

The rolled tube is then brought together by two pinch rollers (pressure Rolls) located right at the point where two welding electrodes, separated by a very small gap, contact the tube. One electrode contacts on one side of the joint and the other on the second side of the joint such that they are placing weld current across the weld joint. This indeed is the current path which performs the work of making the weld, however there is a second shunt current path going around the back side of the tube. This path adds heat but does not contribute to the weld formation. The joint heats under the influence of the weld current to melting and a longitudinal seam weld is made and the welded tube exits continuously or semi-continuously out the exit end of this welding apparatus. Because of the shunt current, this is not the most efficient tube welding process available. The Making Shaping & Treating of Steel, USS Corp, 1964

Here is a photograph of the electrode wheels and the final forming and pressure rolls making the weld. Cooling fluid is sprayed over the joint to extract heat. The Making Shaping & Treating of Steel, USS Corp, 1964

Current Flow in a conductor as a function of Frequency KiloHertz AC High Frequency Before we start to consider high frequency welding, let us review some physics of current conduction in metallic materials. If we were to examine the electron flow within a conductor at various frequencies, we would not the following. With DC current (0 frequency oscillation, current flowing only in one direction) the electrons flow uniformly across the entire conductor cross section. As we start increasing the frequency, the constantly expanding and contracting magnetic field around the conductor interacts with the conduction electrons and the current begins to move to the outside surface of the conductor until at very high frequencies, only the very surface of the conductor passes all of the electron flow. We can use th fact to establish a high frequency welding process which improves efficiency over that of the ERW process. DC 60 HZ AC

High Frequency Induction Welding Observing the figure in the upper left, if we contact the tube with sliding contacts upstream from the final “v”, then high frequency current will enter the tub close to the “v” edge and run down the “v” to the point of welding and back up the other side. Actually this counter current in both edges will pull the electrons even closer to the edge along the “v”. The all the heating will occur along the “v” and the weld will be consummated here. Because of the induction nature of high frequency current, a second variation of high frequency welding is possible. We don’t actually even need sliding contacts if we can couple the current in a coil surrounding the tube with the electrons already present within all metals and induces a current within the tube itself. This induced current will also follow along the surfaces. Thus we have the high frequency induction welding process. Appreciating Hig-Frequency Welding Welding Journal, July 1996

Metals Handbook, Vol 6 ASM International, 1983

Impeder Inside the Core Promotes Path ADC Linnert, Welding Metallurgy AWS, 1994

High Frequency Welding Applications Induction Coil HF HF HF High frequency welding includes those processes in which the coalescence of metals is produced by the heat generated from the electrical resistance of the work to high frequency current, usually with the application of an upsetting force to produce a forged weld. There are two processes that utilize high frequency current to produce the heat for welding: high frequency resistance welding (HFRW), and high frequency induction welding (HFIW), sometimes called induction resistance welding. The heating of the work in the weld area and the resulting weld are essentially identical with both processes. With HFRW, the current is conducted into the work through electrical contacts that physically tough the work. With HFIW, the current is induced in the work by coupling with an external induction coil. There is no physical electrical contact with the work. The above and the following two slides show some basic applications of high frequency welding. Tube Butt Seam Tube Butt Seam Tube Mash Seam [Reference: Welding Handbook, Volume 2, p.653, AWS]

High Frequency Welding Applications (CONT.) Strip Butt T-Joint HF HF With low frequency (50 Hz - 360 Hz), direct current or “square wave” resistance welding, much higher currents are required to heat the metal, and large electrical contacts must be placed very close to the desired weld area. The voltage drop across the weld is very low, and the current flows along the path of least resistance from one electrode to the other. With high-frequency welding, by contrast, the current is concentrated at the surface of the part. The location of this concentrated current path in the part can be controlled by the relative position of the surfaces to be welded and the location of the electrical contacts or induction coil. Heating to welding temperature can be accomplished with a much lower current than with low-frequency or direct current welding. Spiral Tube Fin Spiral Tube [Reference: Welding Handbook, Volume 2, p.653, AWS]

High Frequency Welding Applications (CONT.) Induction Coil Projection Seam HF Although the welding process depends upon the heat generated by the resistance of the metal to high frequency current, other factors must also be considered for successful high-frequency welding. Because the concentrated high-frequency current heats only a small volume of metal just where the weld is to take place, the process is extremely energy efficient, and welding speeds can be very high. Maximum speeds are limited by materials handling, forming and cutting. Minimum speeds are limited by material properties and weld quality requirements. The fit of the surfaces to be joined and the manner in which they are brought together is important if high-quality joints are to be produced. Flux is not usually used but can be introduced to the weld area in an inert gas stream. Inert gas shielding of the welding area is generally needed only for joining reactive metals such as titanium and certain stainless steel products. HF Pipe Butt Bar Butt [Reference: Welding Handbook, Volume 2, p.653, AWS]

AWS Welding Handbook

Typical Tube Welding Conditions for Steels 30 m/min (100 ft/min)at: 600 kW power for 12 mm-wall (1/2 in); diameter of 200 - 1200 mm (8 - 48 in) 60 -240 m/min (200-800 ft/min) 100-400kW power 0.6 - 1.6 mm walls (0.025 - 0.065 in) diameter of 25 - 50 mm (1 - 2 in) Note high speed

Meter Current Penetration Depth, in Frequency KHz

Metals Handbook, Vol 6 ASM International, 1983

Circuitry & Control

Control Devices Input Voltage Regulation SCR’s control input voltage constant Filters used on rectifier output to reduce ripple Variations cause intermittent fusion “stitching” Speed Control Feedback Control on weld power as a function of mill speed Reduces scrap on start and stop Weld Temperature Control Optical Pyrometer aimed at “v” adjusts weld power

Circuit Made of Three Components Filter Tube or SS HF Converter 60 Hz DC Reduce Ripple 50 - 65% Efficient Solid State Circuit Made of Three Components Filter Tube or SS HF Converter Tank Circuit >80% Efficient AWS Welding Handbook

If Efficiency is Below 55% Modifications are needed Ip=Plate Current Ig= Grid Current Ep= Plate Voltage If Efficiency is Below 55% Modifications are needed Nominal Target =75% Ishizaka, HF Resistance Seam Welding, The Fabricator, Nov 1993

Efficiency Improvements Can Come From Two Sources The Power Circuit The Workpiece Arrangement

Proper Matching Relationship between the plate voltage and plate current; and the relationship between plate voltage and grid current are nearly coincident with the rated impedance line. Ishizaka, HF Resistance Seam Welding, The Fabricator, Nov 1993

Overload Matching Occurs when load impedance is too small in comparison with the rated impedance Increase the turns ratio of current transformer Reduce tank capacitance Ishizaka, HF Resistance Seam Welding, The Fabricator, Nov 1993

Light Load Matching Reduce the turns ratio of current transformer Increase tank capacitance Ishizaka, HF Resistance Seam Welding, The Fabricator, Nov 1993

Current flows more to edge when Edges are closer “v” length is shorter Caution: Can get Premature Arcs

Insert Impeder Impeder Mass Closer to Tube Cool Impeder

Effect of Weld Speed on Power and Performance U=The relative power B:A B has less of an effect at higher travel speeds Power = E*I B = Fixed Power (losses etc) A*Sp = Weld Power

Induction Coils Cu Tubing or Bar Normally water cooled Surround = efficiency Mag. Strength reduces with distance = 1/8 - 1 inch between coil and work AWS Welding Handbook

Contacts Cu or Hard Cermets 0.25 - 1 in2 500 - 5000 Amps Cooling required 5 - 50 lbs force Life = 1K - 300K feet AWS Welding Handbook

Cooled: keep below Curie Temp Impeders (Current Flow Around inside Surface of Tubes can cause reduced efficiency. The impeder increases the inductive reactance around inside wall of tube.) Ferritic Material Cooled: keep below Curie Temp Extend from “v” to 1 1/2 tube diameters upstream of “v” Mandrels Used to treat inside weld bead shape or scarfing Nonmagnetic Material like Austenitic SS (Impeders also needed)

Seam Annealing Robotron Web Site

Advantages of High-Frequency Welding Produce welds with very narrow heat-affected zones High welding speed and low-power consumption Able to weld very thin wall tubes Adaptable to many metals Minimize oxidation and discoloration as well as distortion High efficiency High-frequency welding processes offer several advantages over low frequency and direct current resistance welding processes. One characteristic of the high-frequency processes is that they can produce welds with very narrow heat-affected zones. The high-frequency welding current tends to flow only near the surface of the metal because of the “skin effect” and along a narrow controlled path because of the “proximity effect”. The heat for welding, therefore, is developed in a small volume of metal along the surfaces to be joined. A narrow heat-affected zones is generally desirable because it tends to give a stronger welded joint than with the welder zone produced by many other welding processes. With some alloys the narrow heat-affected zone and absence of cast structure may eliminate the need for postweld heat treatment to improve the metallurgical characteristics of the welded joint. The shallow and narrow current flow path results in extremely high heating rates and therefore high welding speeds and low-power consumption. A major advantage of the continuous high-frequency welding process is its ability to weld at very high speeds. High-frequency welding can also be used to weld very thin wall tubes. Wall thickness down to less than 0.005 in. are presently being welded on continuous production mills. The process is adaptable to many metals including low carbon and alloy steels, ferric and austenitic stainless steels, and many aluminum, copper, titanium, and nickel alloys. Because the time at welding temperature is very short and the heat is localized, oxidation and discoloration of the metal as well as distortion of the part are minimal. High-frequency welding power sources have a balanced three-phase input power system. Using conventional vacuum tube welding power sources, as much as 60 percent of the energy is converted into useful heat in the work.

Limitations of High-Frequency Welding Special care must be taken to avoid radiation interference in the plant’s vicinity Uneconomical for products required in small quantities Need the proper fit-up Hazards of high-frequency current As with any process, there are also limitations. Because the equipment operates in the radio frequency range, special care must be taken in its installation, operation, and maintenance to avoid radiation interference in the plant’s vicinity. As a general rule, the minimum speed in carbon steel is about 25 feet/min. For products which are only required in small quantities, the process may be uneconomical unless the technical advantages justify the application. Because the process utilizes localized heating in the joint area, proper fit-up is important. Equipment is usually incorporated into mill or line operation and must be fully automated. The process is limited to the use of coil, flat, or tubular stock with a constant joint symmetry throughout the length of the part. Any disruption in the current path or change in the shape of the vee can cause significant problems. Special precautions must be taken to protect the operations and plant personnel from the hazards of high-frequency current.

Some Products of High-Frequency Welding Examples of a few products that can be fabricated by high-frequency welding are shown in the above slide. [Reference: Welding Handbook, Volume 2, p.665, AWS]

Homework HF Welding