Anew lean duplex stainless steel, known as LDX 2101, offers a cost-effective alternative to conventional duplex and super martensitic steels in applications requiring high mechanical strength and corrosion resistance. First deliveries to offshore customers will be used in the construction of flexible pipeline and umbilical tubing.
The new grade, developed by Outokumpu, features a low nickel content - 1.5% - and a relatively high manganese and nitrogen content. This boosts the austenite formation, compensating for the low nickel content, according to the company’s senior research engineer, Pelle Johansson. 310 Stainless Steel Pipe
Compared with a standard austenite grade such as 304, LDX 2101 has at least double the yield strength - 450 MPa for hot rolled plate, and 530 MPa for cold rolled coil and sheet, as against 210 MPa for hot rolled 304. Given the high correlation between yield strength and fatigue strength, its fatigue strength is also significantly higher.
The corrosion resistance of LDX 2101 is also well above that of 304. Expressed in terms of the PRE value, it has a rating of 26, against 19 for 304. With regard to pitting resistance, tests have established that it has far superior resistance to 304, and qualities close to those of the 316 grade.
The new grade also has a superior stress corrosion cracking performance, on a par with that of other duplex stainless steels. Cracking occurred in 304 when subjected to the ASTM G39 U-bend stress corrosion test, but not in LDX 2101.
Because of the low nickel content, the price of the new lean duplex stainless steel is much less sensitive than 304 to rising nickel prices. Given a figure of $15,000/ton of nickel, the alloy adjustment factor for 304 is around $900 a ton, while that for LDX 2101 is less than $200. Recently, nickel has been rated above $15,000/ton at this level, the cost of LDX 2101 is about the same as 304, without even taking into account the cost saving accruing from weight reduction, Johansson says.
Outokumpu has calculated that only 36.6 tons of LDX 2101 would be needed to achieve the same mechanical properties in a tank made from 64.9 tons of 304. The weight difference is due to the new grade’s lessened wall thickness requirements; this in turn reduces manufacturing time and consumption of welding materials.
Good weldability is another advantage of the new grade. Standard welding methods can be employed, fewer consumables are required, and post-weld heat treatment is unnecessary.
Outokumpu has produced a significant tonnage of LDX 2101 since it was declared commercial in 2004. It is available in a broad range of products, including plate, coil and sheet, billet, wire rod and bar, welded tube and pipe, and rectangular hollow sections. It has been approved to various ASTM and ASME standards, and preparation of a harmonized European Union norm application is under way.
The largest order to date has been for 2,200 tons to be delivered to a desalination plant project in the Middle East. Several other orders have also been received for desalination plants, for a white liquor tank in Finland, wine tanks in Chile, palm oil tanks in The Netherlands, and marble slurry tanks in Norway.
The company has secured two significant offshore orders. One was from Technip Offshore, for construction of a flexible flowline for Total’s Dalia deepwater project off Angola. The second was for welded pipe for RathGibson in the US, to be used in the construction of zinc-coated umbilical tubes.
For flowline line-pipe, LDX 2101 is viewed as a replacement for costlier duplex grades such as 2205. It also offers an alternative to super martensitic steels. Although super martensitic steels are less costly than duplex, cracking problems, sometimes leading to catastrophic failures, have been reported in pipelines made with this material.
With respect to umbilicals, there is another compelling reason why oil companies and their umbilical suppliers are keen to find alternatives, namely that demand now exceeds the ability of the producers of seamless tube to supply it. This has boosted requests for welded tube, of which Outokumpu is an established supplier.
The German line-pipe manufacturer Butting has followed closely the development of LDX 2101 and reported on its investigations to a meeting of the Stainless Steel World Conference late last year. LDX 2101 is a promising candidate for the oil and gas business as its “mechanical properties and corrosion behavior can be positioned in between those of super martensitic steels and standard duplex stainless steels,” it said. Test results “have proven the principal suitability of longitudinally welded pipes made from lean duplex grades for line-pipes for sweet and slightly sour service.”
At the same conference, RathGibson presented the findings of tests comparing LDX 2101 with alloy 19D (UNS S32001), the present material of choice for zinc-clad lean duplex stainless steel umbilical tubing. LDX 2101 has a higher strength than alloy 19D, which “has significant operational implications for umbilicals as it will extend their existing operating envelopes,” the presentation said. “This will allow use in conjunction with the higher pressure ranges typical of modern deepwater developments for which super duplex stainless steel materials are presently the only viable alternative.”•
Stainless Steel Tubes For more information, contact Pelle Johansson, Outokumpu Stainless. Tel +46 226 810 23, fax +46 226 810 77. [email protected], www.outokumpu.com.