The short answer
High-frequency (HF) welding is the welding step of an ERW tube mill. After the forming rollers have bent the strip into an open round tube, high-frequency current is fed into it, either by an induction coil around the tube or by sliding contacts on the strip edges. The current flows along the two edges to the point where they meet and heats them to welding temperature in a fraction of a second. Squeeze rollers then press the edges together: molten metal and oxides are pushed out as beads, and the edges bond in the solid state. No filler metal or shielding gas is used. Most ERW pipe made today is welded this way, which is why it is also called HFW pipe; see What Is ERW Pipe? ERW vs Seamless, Uses and Sizes.
The machine that supplies the current is the HF welder, also called a high frequency welding machine: a solid-state power source with its output to the coil or contacts, an impeder inside the tube and water cooling.
InGen MillTek does not manufacture HF welders. On our tube mill lines, the HF welder is made by specialist HF welder manufacturers; our engineers select it for your tube sizes, wall thickness and line speed and configure it as part of the line.
Not the same as plastic "HF welding"
"High frequency welding machine" is also the name of radio-frequency (RF) welders that seal PVC and other plastics by heating the material between two electrodes. They are a different machine for a different material. This guide is about HF welding of steel strip into tube on an ERW tube mill.
Where HF welding sits in the line
HF welding comes straight after the fin passes, the last forming stands, which bring the two strip edges together in a controlled shape. In the direction of strip travel, the welding area has a seam guide, the induction coil (or the contacts), the squeeze roller stand, an outside bead scarfer and ironing rollers, followed by cooling. The impeder sits inside the open tube, under the coil. The whole line, from coil to finished tube, is described in our guide ERW Pipe Making Process: From Steel Coil to Finished Tube.
How the current heats only the edges
Two electrical effects make HF welding work. The skin effect makes high-frequency current flow near the surface of a conductor, and the proximity effect pulls opposite currents in nearby conductors toward each other. Together they make the current run along the two facing strip edges, which approach each other in a narrow "V" ahead of the squeeze rollers. The current is most concentrated near the point where the edges meet, the apex of the vee, so the edges heat fastest there and reach welding temperature just as the squeeze rollers press them together.
The edges should meet parallel to each other, so that the whole wall thickness heats evenly from the inside surface to the outside. The fin passes prepare this edge shape.
Most HF tube welders work at a few hundred kilohertz. In the API 5L line pipe standard, HFW means pipe welded with a current frequency of at least 70 kHz.
Induction welding or contact welding
There are two ways to feed the current into the tube:
- Induction welding: a water-cooled copper coil surrounds the tube just ahead of the squeeze rollers and induces the current without touching the strip. Nothing rubs on the tube surface, so there are no contact marks and no contact wear. It is the usual choice for most tube and pipe sizes.
- Contact welding: two sliding contacts ride on the strip edges just ahead of the weld point and feed the current directly. Less power is lost, so it is used mainly for larger diameters and heavier walls. The contacts wear and need regular replacement, and they can mark the strip.
Our tube mill lines are configured for induction welding.
The impeder
Current induced in the open tube can flow two ways: along the edges to the vee, which does the welding, or around the inside surface of the tube, which only wastes power. The impeder blocks the second path. It is a ferrite core, usually ferrite rods in a nonmagnetic casing, held inside the tube under the induction coil and cooled by water. It raises the impedance of the inside path, so that more of the current flows along the edges.
Ferrite loses its effect when it overheats, so the impeder needs a steady flow of cooling water. It is a wear part: the ferrite rods and casing are replaced from time to time. Its position relative to the coil and the weld point is set when the mill is set up for each size.
Squeeze rollers and the weld bead
The squeeze roller stand forges the hot edges together. Depending on the tube size, it has two, three or four rollers around the tube. The rollers press the tube slightly smaller than the open shape before them, so the edges are pushed into each other. This squeeze drives the molten metal and oxides out of the joint, where they form a bead on the outside and the inside of the tube, and leaves clean metal bonded across the wall. How much squeeze is used is set for each size during setup.
The outside bead is cut off in line by the outside scarfer, and ironing rollers smooth the weld area. The inside bead is normally left in place; an inside scarfer can be added if your product needs it.
What is inside a solid-state HF welder
Older HF welders used vacuum-tube oscillators. Modern tube mills use solid-state HF welders, which are more efficient and easier to maintain. A solid-state HF welder usually has:
- A rectifier section that converts the mains supply to direct current
- An inverter built with power transistors, such as MOSFET or silicon carbide (SiC) devices, that turns it into high-frequency current
- A resonant output circuit, of series or parallel type, with an output transformer that feeds the induction coil or the contacts
- A control panel for power setting and protection, linked to the mill's main control desk
- A cooling system, usually a closed internal water circuit with a water-to-water or air-to-water heat exchanger, which takes heat to the plant's cooling water
How the HF welder is sized
The HF welder's power is chosen from the largest tube outside diameter, the heaviest wall thickness, the target line speed and the material. Larger OD, thicker wall and higher speed all need more power. Because maximum OD, maximum wall and maximum speed are not reached at the same time, the power is checked size by size in the technical proposal; see Step 6 of 10 Steps to Specify an ERW Tube Mill: Tube Size to Full Line.
Frequency is matched to the wall thickness as well. Current at a lower frequency penetrates deeper into the steel, so heavy walls tend to use a lower frequency and thin walls a higher one.
An HF welder's rated welding output is not the same as its electrical input from the plant supply. Keep the two apart when you plan transformers and cables; see our guide Motor power, HF power and incoming electrical capacity.
What the plant must provide
- Electrical supply: sized from the input requirements of the HF welder and the other loads, not by adding nameplate figures; see Motor power, HF power and incoming electrical capacity.
- Cooling water: the HF welder, the induction coil and the impeder are water-cooled. The customer's plant provides HF cooling water from a cooling pool or cooling tower with pumps, usually in addition to the mill cooling water. How to state cooling capacity is explained in Tube mill cooling: flow rate is not heat-rejection capacity.
- Space for the HF welder cabinets beside the mill, as shown on the project layout drawing.
Weld quality and what affects it
ERW tube and pipe are tested to the standard the product must meet. Common tests include flattening and flaring tests, hydrostatic testing and eddy current testing.
Typical weld defects are:
- Cold weld, or lack of fusion: the edges did not get hot enough, or were not squeezed enough, to bond across the whole wall.
- Oxide inclusions, also called penetrators: oxides trapped in the weld line instead of being squeezed out.
- Overheating: too much heat melts away edge metal, causes spatter and leaves an uneven weld.
A stable weld depends less on any single setting than on steady conditions: consistent strip width and clean strip edges, stable forming so that the edges meet the same way every second, the coil and impeder in the right place, and a steady line speed. Good strip edges start at slitting; see What Is a Slitting Line? How a Coil Slitter Works.
Who builds the HF welder on an InGen MillTek line
InGen MillTek does not manufacture HF welders: they come from specialist HF welder manufacturers. YinJiang (InGen MillTek) has focused on strip processing equipment since 1997. We specialize in the entry section of ERW tube mills: uncoilers, leveling machines, shear & butt welders and spiral accumulators are built in-house, and the spiral accumulator is our flagship product. We also supply complete HF ERW tube mill lines with forming & sizing units and flying saws, and HF welders, cold saws and automatic stacking configured to suit the project. If you prefer a particular HF welder brand, tell us in your inquiry.
Every line is configured for your tube sizes, material, line speed and local power supply, and prices are quoted for each configuration.
Frequently asked questions
Prepared by the InGen Engineering Team. Values are reference values and are confirmed in the technical proposal and technical agreement.
- What is high-frequency welding? A welding process in which high-frequency current heats the two edges of a formed strip, and squeeze rollers press them together into a weld without filler metal. On a tube mill it makes the straight seam of ERW tube and pipe.
- Is HF welding the same as ERW? HF welding is the modern form of electric resistance welding (ERW). Most ERW pipe today is welded with HF current and is also called HFW pipe.
- What frequency does an HF tube welder use? Usually a few hundred kilohertz. In API 5L, HFW means a welding current frequency of at least 70 kHz.
- Induction or contact welding: which do I need? Induction welding suits most tube and pipe sizes and leaves no contact marks; contact welding is mainly used for larger diameters and heavier walls. Our lines are configured for induction welding.
- Does HF welding use filler wire or shielding gas? No. The edges are heated by the current and forged together by the squeeze rollers.
- What is an impeder? A water-cooled ferrite core inside the tube under the coil. It makes more of the current flow along the strip edges instead of around the inside of the tube.
- Does InGen MillTek make HF welders? No. HF welders are made by specialist manufacturers; we select and configure the welder for each line.
- How much HF power do I need? It depends on the largest OD, the heaviest wall, the line speed and the material, and is set size by size in the technical proposal.
About this article
Prepared by the InGen Engineering Team. This is a general technical guide, not a specification or performance guarantee for any project. Welding settings are made during commissioning for each size; final parameters are set in the technical proposal and technical agreement.