Article by James Chater. – Header Photo: Fredric Alm.


Times have been lean for both hydrocarbon and raw materials producers. In the last few years the prices of oil, gas and many metals have languished, affecting profits and leading to production cuts, lay-offs and delays in implementing investment decisions. Now, however, with prices rising again both at the oil pump and at the mine head, it looks as though the worst is over. Producers of metals such as nickel and molybdenum will be smiling, but so also should duplex stainless steel producers, who have an opportunity to offer their lower-alloy products as an alternative to the higher-alloyed and therefore more expensive austenitic grades.
According to a recent report, commodity prices rallied in 2017 and metals are still soaring. The same report forecasts that the duplex market will grow from USD 2.70 billion in 2016 to USD 3.40 billion by 2021, in other words a CAGR of 4.7%. The increase will be led by tubes, especially those used in oil & gas, chemicals and petrochemicals. It is above all the Asia-Pacific region that will see sharp growth (1).
However, caution is required. Since the report’s appearance, the world economy has received a cold shower in the form of tariffs imposed by President Trump against many countries. The international trading climate has suddenly chilled, which could affect the report’s optimistic outlook.

Industrial sectors

With oil prices now pushing higher after a prolonged slump, oilfield activity is likely to increase, boosting demand for duplex. The offshore industry is pushing exploration and development to ever lower depths. Last year the Eurasia consortium announced a plan to drill an exploratory well up to 15 km in length, three km longer than the previous world record, the Kola Bore Hole at Murmansk, Russia. Sea depths are increasing too: in 2016 the Maersk Venturer spudded a well two miles (3,400 metres) below the ocean surface at Raya-1 offshore Uruguay.

The combination of great sea depth and high pressures makes it more or less obligatory to specify duplex for the umbilicals, the bundle of tubes that convey energy, communications, chemicals etc. between the topsides and the subsea wells. Duplex and especially super and hyper duplex combine high mechanical performance and high corrosion resistance, and as depths and pressures continue to increase, demand for these materials will increase. For these reasons they are found in several subsea applications, including process piping, separators, scrubbers, pumps, manifolds and Christmas tree components.

Numerous recent examples attest to demand for duplex across a wide range of other industries, including chemical, petrochemical, mining, storage tanks, chemical tankers and building. In this last sector there is a marked tendency towards the use of duplex grades where previously low austenitic grades such as 304 or 316 would have been preferred. Especially, the structural parts of bridges often use duplex bars, girders and plates to achieve structures that are not only beautiful but also strong: for instance, the Apaté pedestrian bridge in Stockholm; and the Ljunga Bay Bridge in Sölvesborg, Sweden, constructed of three arches that carry the higher deck section, railings and deck support structure, which are all made from Forta LDX2101® plate.
Transport is another fast-growing area in which duplex use is taking over from austenitics. Apart from buses, trains and cars, there are more unusual applications. When Kiruna Wagon was commissioned to refurbish bottom dumpers for raw material transport, duplex was specified because the steel’s strength allowed weight and thickness to be reduced. The duplex used for the inner lining had to be welded onto normal steel used on other components.

Easier fabrication

Several duplex grades have recently been introduced with the specific goal of making fabrication easier. Outokumpu’s two FDX™ grades are a case in point. Now advances in fabrication techniques are favouring increased use of duplex. For instance, BUMAX has introduced a new type of duplex fastener that is cold-formed rather than being made in one of the traditional ways such as machining or hot forging. This increases its tensile strength, ductility and fatigue resistance.

In the field of oil and gas, DNV has undertaken several standardization initiatives relating to fabrication of duplex. In particular it has developed specification for qualification of manufacturers in accordance with NORSOK M650. It is also participating in a joint industry effort to prepare requirements for parts manufactured by additive manufacturing. The King Abdulaziz Center for World Culture in Saudi Arabia is a graphic example of what it is now possible to achieve with duplex products.

The structure, which from far off resembles giant pebbles, actually has a free-form surface made up of 70,000 tubes made of 1.4462 duplex tubes. Laid out in parallel, the tubes were marked and bent to fit into their unique position, and some needed for the sunshades around the windows were crimped using a technically complex method.

Product types

The range of products made from duplex has gradually been expanding. SAF 2507, a super duplex grade resistant to extreme corrosion, pitting and crevice corrosion, is now available in seamless tube and pipe, seamless and welded tube and pipe, flanges, fittings, fasteners, plate/sheet/strip, bar steel and forged billets. In recent years Sandvik has developed duplex stainless steel mesh in SAF 2507 and 2205.

More valves and pumps are becoming available or are being ordered in duplex form. GN Solids Control has introduced a centrifugal pump with a bowl made of duplex 2304 and made by centrifugal casting for better material distribution and to eliminate air blister. In May this year BW Offshore asked PG Flow Solutions to supply water booster pumps to the POLVO FPSO offshore Brazil.

This summer Amarinth delivered API 610 OH2 super duplex stainless steel pumps to Sclumberger for a process water treatment in Kazakhstan’s Dunga oilfield. Orbinox PNEUTON™ is developing its VG series bi-directional slurry handling knife gate valve. Designed for an iron ore tailings application, the valve comes with a type 2205 duplex gate.

How are Ni and Mo doing?

Nickel and molybdenum are two crucial ingredients of most austenitics and nickel alloys. Price movements in these two metals have an influence on whether end users are likely to specify high-alloy materials or seek out lower-alloy materials such as duplex. Nickel moved from 4 to 7 USD a pound between July 2017 and mid-June 2018, but has since fallen to 6. Molybdenum oxide leapt from 7 to 12 USD in the same time frame and is holding steady. These sudden rises are bullish for duplex.

Duplex in urea manufacture

Some processes, such as desalination and urea production, use different grades of duplex for different applications. For example, a new urea plant being built in Zhijiang, Hubei province, China, will use three grades developed by Sandvik for Stamicarbon’s new, low-energy design. Safurex Infinity, a standard super duplex, will be used in the urea synthesis section; Safurex Star, also a super duplex, will be used in more extreme corrosive environments such as the HP stripper in CO2 and the thermal stripping process (including the heat exchanger tubes in the HP stripper); and Safurex Degree, a grade produced via the HIP method and designed for low temperature resilience, is used for creating HP ejectors, liquid dividers and check valves.

Duplex for tanks and tankers

Storage tanks are one other application where duplex grades are taking over from austenitic. On land, duplex is being used to store corrosive chemicals. In April this year Gpi was sanctioned to build a duplex 2205 demineralised water tank for Dow Chemicals in Terneuzen, the Netherlands. On ships duplex tanks are used to transport cryogenic fluids, chemicals and other substances.
Duplex stainless steel is now routinely specified for the cargo holds of chemical tankers. Chemical Transportation Group (CTG) has recently bought two chemical tankers and is about to acquire three more. The cargo tanks are made of duplex 2205. In April this year came news that chemical cargo ships contracted by Hudong-Zhonghua Shipbuilding are to be built with duplex stainless steel produced by TISCO.

* (1) Hecht Commodity Report.


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