ERW and HFW are the same welding principle, resistance welding of a formed coil with no filler metal. The difference is the current frequency: the older ERW used mains frequency, while HFW uses the high frequency range, which concentrates the heat at the strip edges. In API 5L and ISO 3183 this family is designated HFW.
Which one you are ordering. If the specification names HFW, quote HFW. If it names ERW without stating a frequency, the mill route and the weld preparation are confirmed on the enquiry, and what actually demonstrates the weld is the test regime that travels with the grade – flattening, weld tensile and non-destructive examination of the seam. Put the process designation and the product specification level in the line item and on the mill certificate, because the process and the tests are read together. The geometry matters too: resistance welded pipe is made from coil, so it stops at around 24 inch outside diameter and is not made in the heaviest walls.
One process, two names
A steel coil is unrolled, formed into a cylinder through a series of rollers, and the two edges are pressed together while an alternating current is passed through them. The resistance at the faying surfaces generates the heat, and the pressure roll forges the edges into a solid weld. No filler metal is added. The external flash is scarfed off, and the internal flash is either left or removed depending on the specification.
That description covers both. ERW stands for electric resistance welding, HFW for high frequency welding, and the difference between them is the frequency of the current used to make the weld, not the principle behind it.
What the frequency actually changes
At mains frequency, in the tens of hertz, the current spreads through a larger volume of steel around the strip edge. With high frequency current the skin and proximity effects concentrate the current in a thin layer at the very edge, so the heat is put where the weld is made instead of into the surrounding wall.
The practical results are a narrower heat affected zone, a shorter heating time, heavier wall welded at the same line speed, and less tendency for the heat to be marginal at the faying surfaces. That last point is the one that matters, because insufficient heat at the joint is the origin of the classic resistance weld defects.
Contact or induction
Within high frequency welding there are two ways to get the current into the strip. Contact welding uses sliding contacts that ride on the strip close to the weld point. Induction welding uses a coil that induces the current without touching the strip. Contact is the more efficient and the more common; induction avoids the contact marks and the wear that contact shoes bring. Both are HFW, and both are made to the same specification requirements.
Which one for which service
The table below puts the two side by side. Read it with one thing in mind: what a quotation calls the process, and what the mill certificate states, are not always the same thing.
| ERW, the older process | HFW, in use today | |
|---|---|---|
| Welding principle | Resistance welding of the formed coil | Resistance welding of the formed coil |
| Current frequency | Mains frequency, tens of hertz | High frequency, in the hundreds of kilohertz |
| Process designation | The historical trade name, still widely used in enquiries | The designation used in API 5L and ISO 3183 |
| Heat affected zone | Wider | Narrower |
| Heating time | Longer | Shorter |
| Wall thickness reach | Limited | Heavier wall for the same diameter |
| Line speed | Slower | Faster |
| Characteristic defect risk | Cold weld, where the edges joined below welding temperature | The same defect family, greatly reduced by concentrating the heat at the edge |
| Weld line examination | Required by specification | Required by specification, and mandatory at PSL2 |
| Filler metal | None | None |
What high frequency welding fixed, and what it did not
Resistance welded pipe earned a poor reputation in the period when the process ran at mains frequency and weld-line inspection was less rigorous. The characteristic defects were the cold weld, where the edges joined without reaching welding temperature, and the penetrator, an oxide or metal particle trapped in the joint. High frequency welding reduces the cause by putting the heat where it is needed, and full length weld line examination catches what remains.
What it did not change is the geometry. Resistance welded pipe is made from coil, so it stops at around 24 inch outside diameter and it is not made in the heaviest walls. Above that size, and for high pressure service in heavy wall, the submerged arc processes take over.
How to write it so you get it
Put the process designation in the line item, not just the grade. Write the product specification level, because PSL2 makes non-destructive examination of the weld seam mandatory. State the ends and the coating if you need them.
Then ask the mill to state the welding process and the product specification level on the mill certificate. On a resistance welded order that single line is the difference between pipe welded with enough heat and pipe welded with just enough to pass the flash test.
Frequently asked questions
Is HFW the same as ERW?
They are the same welding principle at different frequencies. In API 5L and ISO 3183 this family of pipe is designated HFW; ERW is the older trade name, and it is still the word most enquiries use. When a specification written today says ERW, the pipe supplied is normally made by the high frequency process.
Can HFW pipe be used for high pressure lines?
Yes, up to the pressure that its grade and wall thickness allow. The pipe body meets the same tensile and yield requirements as any other form of the same grade, and at PSL2 it carries mandatory weld line examination. What limits it is size, not pressure as such: resistance welding starts from coil, so the ceiling is around 24 inch, and heavy wall line pipe above that is LSAW.
What should I look for on a mill certificate for ERW or HFW pipe?
The welding process, the product specification level, the grade, the hydrostatic test pressure, and the non-destructive examination applied to the weld seam. At PSL2 the weld seam examination is mandatory and the certificate should show it. Where the order calls for third party validation, the certificate also carries an independent countersignature.
ERW and HFW: the same weld, made two ways
The two names describe one welding family. Both form a tube from strip and weld the seam by heating the edges electrically, and both are covered by the same product standards — API 5L or ASTM A53 for line and general service pipe. What separates them is the frequency at which the current is applied and, as a result, how the heat is distributed at the seam.
| ERW | HFW | |
|---|---|---|
| What the name says | Electric resistance welding: the current is brought to the strip edges by contacts and the resistance at the seam does the heating. | High frequency welding: the same principle driven at high frequency, usually above the low-frequency range. |
| Where the heat sits | The heated zone is wider, because the current spreads through the wall on its way to the seam. | The current concentrates at the edges, so the heated zone is narrower and the weld is made with less disturbance to the surrounding metal. |
| How it is ordered | Named as ERW on the order, with the standard, grade and test regime. | Named as HFW where the project wants that machine and that weld preparation. |
| Relationship between them | HFW is the high-frequency form of the same process. A project that asks for ERW is normally satisfied by a high-frequency machine; a project that names HFW explicitly is asking for that route, and a low-frequency ERW mill is not the same thing on paper. | |
What travels on the certificate and the order is the standard, the grade, the dimensions and the test regime — the weld process has to be stated with them, because the process and the tests are read together.
The practical decision is narrower than the argument suggests. If the specification names HFW, quote HFW. If it names ERW without a frequency, the mill’s route and the weld preparation are confirmed on the enquiry, and the tests that go with the grade — flattening, drift, hydrostatic, and ultrasonic or eddy current examination of the weld where the standard or the project requires it — are what decide acceptance, not the label.
Where to go next
- API 5L ERW welded steel pipe – 1/2 inch to 24 inch, PSL1 and PSL2
- API 5L seamless steel pipe – the alternative where no seam is wanted
- API 5L LSAW welded steel pipe – where the resistance process runs out of size
- API 5L SSAW spiral welded pipe – large diameter, low to medium pressure
- API 5L Grade B pipe – the base line pipe grade
- API 5L X52 pipe – the mid range line pipe grade
- ASTM A53 Grade B pipe – the general purpose specification for ERW pipe
- LSAW vs SSAW pipe – the two submerged arc alternatives
- EN 10204 3.1 vs 3.2 – what the certificate on the order actually certifies
- Request a quotation – send the size, wall, grade, quantity and port
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