Although duplex pipe flanges are widely used in the petrochemical industry, long-distance pipelines are still mainly built from carbon steel, which has a high level of corrosion. Currently, there is great interest in the oil and gas industry to implement duplex steel as the material of choice, owing to its strength and corrosion resistance.

One of the problems with using duplex flanges in pipelines is the increased skill required by the welder to achieve an acceptable root pass. This first layer of a multi-layer weld is the most critical, because it is the one most likely to crack or become contaminated. The preferable method for depositing root passes is GTAW (gas tungsten arc welding), which uses a specialised backing shield of inert gas, such as argon. It is tricky to use this method on duplex. Besides anything else, the gas itself causes problems at the welds.

Another disadvantage with duplex flanges is that it is difficult to achieve low-temperature toughness – essential for field welds. It is felt that if a gas-free arc welding technique could be developed, that somehow also attacked this problem, then duplex could replace carbon steel on long-distance pipelines. The benefits would be a radical improvement on corrosion of pipe flanges in pipelines, with a corresponding reduction in the tonnage of metal used each year.

We at Chemipetro supply both duplex and super duplex pipe flanges and fittings, all with excellent weldability. However, their unique and complex composition means that extra care must be taken over welding, in order not to compromise strength and corrosion resistance.