What's the differences between ERW and HFW steel pipe?

The first is to know the difference between ERW and HFW welding processes.

ERW welding process refers to the heat generated by the resistance of the current flowing through the connecting metal. Therefore, a large current is required to charge the entire surface of the steel plate or coil to the welding length. During ERW welding, the current flowing through the entire conductor is 50/60 Hz.

HFW welding process is different from the high frequency process, because high frequency electromagnetic welding is only part of the metal heated by the induction coil. Both alternating current and direct current produce high frequencies. In these cases, a constant current welding generator and a constant voltage high frequency welding generator are used.

Constant current welding machine can generate power of 100-800 kHz. In the earlier process, the 60 Hz, AC current was converted to HF by using a triode and a tank circuit. The new welding machine uses metal oxide silicon, field effect transistors, usually associated with parallel resonant circuits. The constant voltage generator uses insulated gate bipolar transistors. The design power of this transistor is about 2000 kW and the frequency range is 100-600 KHz.

There are two distinct features of HF current than 60 Hz line current:
While the 60 Hz line frequency current flow through the whole conductor but HF current flow only on the surface of the conductor, so it is “skin effect”.
While two conductors carrying HF current are placed close to one another an the current concentrates on the two adjacent surfaces of the conductors, it is “proximity effect”.

HFW current distribution: The current flows in the surface of the metal or the conductor. Both conductors are placed in close proximity with most of the HF current flowing on the adjacent sides of the conductors.

HFW is required in API 5L PSL2 ERW steel pipe
According the API 5L and ISO 3183 specification, for PSL 2 pipes in welded process the HFW welding is a mandatory requirements for ERW steel pipes. As it is an improvement from the inherent problems of hardness, lack of fusion and susceptibility of stress corrosion cracking (SCC).

ERW steel pipe includes HFW steel pipe

ERW is the general term for electric resistance welded steel pipe. It can be divided into two types: alternating current welding (AC) and direct current welding (DC). According to the different frequency, AC welding can be divided into low frequency welding, intermediate frequency welding, ultra high frequency welding and high frequency welding. High frequency welding (HFW) is mainly used for ordinary or thin-walled steel pipes. It includes contact welding and induction welding. DC welding is usually used for small diameter steel pipes. Generally, HFW is one of ERW production methods, which uses high-frequency welding technology. Therefore, ERW steel pipe includes HFW steel pipe.