HFW(HIGH-FREQUENCY WELDED) CARBON PIPES PREFERRED, BETTER

A recently published article in the Wall Street Journal by Alison Sider referred to the oil pipeline failure in Mayflower, AR by saying the cause, according to the Pipelines and Hazardous Materials Safety Administration (PHMSA) website, can be attributed to “..some types of an early welding process called electric resistance welding.” It goes on to say the “process hasn’t been used on new pipelines since 1970…”

For the record, the electric resistance welding (ERW) process is not just an early process, it is an ongoing process still used in a significant number of pipes, mostly 24-inch and below. ERW is specified in all pipeline design (ASME B 31.4 and B 31.8, for example) and pipe-making (API 5L) specifications. There is no regulatory restriction on the use of these pipes in oil-carrying pipelines. As a result, ERW pipes remain in use.

The API 5L mandates through Table 2 and Table 3 that PSL 2 pipes be manufactured by the high-frequency welding (HFW) process. Still, more than two years after new API 5L/ISO 3183 went into effect, some pipe mills report PSL 2 pipes manufactured by the ERW process. This article highlights the difference between the two processes to show why HFW is preferred for PSL 2 manufacturing over the ERW process.

The pipe is processed by welding the edges of a plate or coil through techniques that include submerged arc welding (SAW), ERW and HFW. Pipe codes have long listed the last two processes as one, although welding engineers always treated the two as separate because they are technically far apart and seldom have anything in common, except perhaps to the untrained eye.

With the European influence on the industry, there has been increased recognition of these processes as separate. However, in the absence of any regulatory requirements, some pipe manufacturers continue to report them as ERW on their material certificate. This makes it difficult to obtain information whether the welding process for manufactured pipe was really HFW or ERW.

In the case of a Japanese manufacturing plant where I have been several times, there is only an HFW production line. However, the material test report (MTR) showed ERW; I had to obtain a letter certifying that the pipes I bought from an intermediary source were, in fact, produced by the HFW process in the mill.

I have been asked, “Why such a fuss about the difference of two process?” The answer involves two distinct reasons:

• The new versions of API 5L, which combines with the earlier EN 3183, clearly distinguishes PSL 2 pipes manufactured by the HFW process from the ERW process pipes. It may be noted that the HFW process was in place in API 5L, but no mandatory recognition was in place with the EN 3183.

• The quality of HFW pipe is far superior to ERW Pipe in many ways, notably HFW eliminates the perennial fear of lack of fusion in the ERW pipes. Also, the HFW process has much better control over weld and heat-affected zone (HAZ) hardness, often causes attributed to pipeline failure.

Both of these deficiencies in the ERW process are largely eliminated by the technological superiority of the HFW process.

How Do The Processes Differ?

To understand the advantages of HFW over ERW, it is important to know the key process parameters of the two, and then compare them.

ERW process – Electric resistance welding involves heat generated by the resistance to current flow through the abutting metals; a large current is required to electrify the entire surface of the plate or coil to the length of the weld. In the ERW process, the 50/60 Hz current flows through the entire conductor. Figure 1 shows the full section of the pipe is electrified.

HFW process – That is not the case with the high-frequency process because only a section of metal is heated by induction of the electric coil. High-frequency in these applications is generated by either AC or DC current. Constant current and constant voltage high-frequency (HF) welding generators are used.

Constant current welding machines generate power of 100-800 kHz. In the older version of the process, the conversion of 60 Hz, AC current to HF was done by using triode and resonant tank circuits. New machines use metal oxide silicon, field-effect transistors (MOSFET), often associated with parallel resonant circuit. The constant voltage version generators use insulated gate bipolar transistors (IGBT) designed to power about 2,000 kW with frequency ranges of 100-600 kHz.

HF current has two very distinct features that distinguish it from 60 Hz line current:

• When the 60Hz line frequency current flows through the entire conductor while the HF current flows only on the surface of the conductor, it is called “skin effect.”
• When two conductors carrying HF current are placed close to one another and the current concentrates on the two adjacent surfaces of the conductors, it is called “proximity effect.”

Figure 2: Typical HFW process setup

Figure 3: HFW current distribution, highlighting the skin and proximity effect.

HFW current distribution – In this process, the current flows in the skin on the metal surface or the conductor. Both conductors are placed in close proximity with most of the HF current flowing on the adjacent sides of the conductors. Note the skin and proximity effects, shown in shadowed area of Figure 3.
The HFW ‘Vee’
To successfully use the HFW process, the apex of the “Vee” should be as close to the weld point as possible (Figure 1). The degree of the Vee-opening is influenced by the type of material being welded and ranges from 30 to 70. The edges of the Vee face are kept parallel and flat as they approach one another nearing the weld point. Any variation in Vee length during the process will result in variations in heat, thus affecting weld quality.
Post Weld Heat Treatment Of Welds
The welds on the API grade of pipes are heat-treated to normalize the grain structure. Heat treatment of pipe welds is necessary to match the properties of the pipe parent metal and the weld. However, three different types of heat treatment are possible:

• Annealing and normalizing
• Double annealing with quenching
• Advance heat treatment that includes quench and temper

Annealing And Normalizing
The microstructure created in the steel due to welding by induction welded seams is annealed and normalized to ensure uniform structure is achieved in the heat-affected and weld zone. The right frequency of induction is calculated so that the weld is heated without overheating the surface. The weld is heated through the wall thickness to AC3 end of the austenitic rage, which will be about 1,445o F. Capacities exist to heat up to a 1-inch-thick plate in this controlled manner.
The hardness achieved through annealing and normalizing process is often below 300 HV10.

Double Annealing With Quenching
Annealing and quenching in water is practiced to obtain very fine grain structure; this improves the impact strength of the weld zone. The multi-annealing process, if properly carried out, increases the statistical confidence level.

The repeated transformation of the alpha (α) to gamma (γ), beginning at about 1,333oF and ending at about 1,445oF, is repeated several times. This annealing and quenching cycle greatly improves the weld properties. The quenching is carried out at the upper range of AC3 temperature. The entire process is intended to improve impact properties, and higher charpy values are obtained.

After this heat-treatment process, the resulting hardness measured in the weld zone is very encouraging; the values comparable to the parent metal can be easily obtained. In steel values of 180-200 HV10, it is easily obtained. Another advantage of induction welding and double annealing and quenching heat treatment is no adverse effect on the properties of Thermos-Mechanical Cooling Process (TMCP) plates/coils used to make the pipe.

Quench And Temper
Another heat treatment demanded of the induction-welded pipes is the quench and temper (QT). In induction heating, the thermal energy of the system is induced in the metal to achieve maximum throughput. Heat loss is minimal as compared to any of the conductive heating processes. This also reduces the process time, carburization is controlled to a negligible level and no scaling takes place. Because the induction heating is fast and uniform, a greater homogeneity of tempered structure is obtained. The hardness values are also uniform and evenly dispersed in the weld zone.

Conclusion
The introduction of the HFW welding process as a mandatory requirement for PSL 2 pipes in Tables 2 and 3 of API 5L/ISO 3183 is indeed an improvement over the inherent problems of hardness, lack of fusion and susceptibility of stress corrosion cracking (SCC) in line pipes associated with ERW process pipes. The engineers responsible for selecting material should consider these improvements during the manufacturing procedure specification (MPS) approval of the pipe procurement process.

Source from:http://www.pipelineandgasjournal.com/high-frequency-welded-pipes-preferred-better?page=3