Demand increases for the highest-strength super duplex.

The demand for highest-strength, corrosion-resistant alloys resulted in a new breed of alloys developed from duplex stainless steels. They were first dubbed 'super' in the 1970's by Langley Alloys following the development of FERRALIUM® 255 and, today, these a range of materials are termed super duplex.

Duplex stainless steels had been known about since the 1930's, but the metallurgy was poorly understood. Adding nitrogen was found to have a beneficial effect as this considerably improved the alloy's ductility and gave it the ability to be hot worked. Even so, as with most developments, the initial idea produced a rather crude first product and, over the years, increased metallurgical understanding has enabled many enhancements to be introduced.

For critical components in the offshore oil and gas industry, in marine, chemical production and defence applications, one of the key requirements demanded is consistency of product. Much emphasis is placed on strict melting practices, hot working reductions, greatly controlled processes and a high level of destructive and non-destructive testing.

The highest-strength super duplex today is FERRALIUM® 255-SD50 developed over the past 25 years by UK-based Langley Alloys. This product has resulted from work to pinpoint the specific aspects of the composition and manufacturing process which maximise the mechanical properties and corrosion resistance. 

There was much evolutionary work on the alloy. FERRALIUM® 255-SD50 derives from the 'generic' super duplex UNS S32550 but is produced on a much narrower composition range. Langley Alloys also set the objective for strict rules controlling the composition ratios between the elements. It was seen as particularly important to maintain a 45% to 55% ferrite content and PREN of over 40.  The result of the imposition of all these restraints is that the mechanical strength of FERRALIUM® 255-SD50 is 10% higher than other super duplex stainless steels - having its minimum proof stress increased from 490 N/mm2 to 600 N/mm2.

The characteristics of FERRALIUM® 255-SD50 include enhanced resistance to pitting and crevice corrosion in seawater. Adding to the alloy's corrosion resistance is the presence of copper in the material. The copper from the alloy is taken up into the aqueous solution to form copper ions which re-precipitate on the incipient pit. This process prevents the pit from developing and effectively shuts off the corrosion process at that site.  

However, partly due to cost differentials, the Super Duplex most commonly ordered by the offshore industry is Alloy UNS S32760, otherwise known as ZERON® 100, a material originally registered by Weir Materials Ltd. This has a lower proof stress at 550 N/mm2 and a lower tensile strength than FERRALIUM® 255-SD50. But its strength still significantly exceeds 22%Cr duplex stainless such as SAF® 2205.

Another member of the current super duplex group is SAF® 2507  (UNS S32750) made by Sandvik. This is the hardest super duplex at  310HBN, but its proof stress is also 550 N/mm2 compared to 600 N/mm2 for FERRALIUM® 255-SD50. Nevertheless its properties endow it with excellent ductility and impact strength at both ambient and sub-zero temperatures. 

All these alloys, but particularly FERRALIUM® 255-SD50, are used in some of the world's most aggressive environments and, over more than 20 years, they have notched up a widespread track record of success in many applications - in industrial plants where sulphuric, nitric and phosphoric acids are being used; in the particularly harsh environments found in FGD scrubber units on conventional coal burning power stations; and most predominantly in the marine and offshore oil and gas industry, where materials are expected to perform for the full service life of a structure, often designated as 25 years.

Many components are machined from solid bars which are available in a wide range of sizes. Plate and pipe are also available from some suppliers, including National Metal Distributors, based in north-western USA.

The more the in-service life of components engineered from super duplex receive favourable evaluations over long periods of use, the more the future of this group of alloys looks assured. Logic dictates that, at the extremes of performance, the combination of mechanical and metallurgical properties is only going to result in very few materials passing the test. As this story shows, sometimes the choice of material is a matter of compromise. Which are the essential performance requirements, and which can be rated as less critical? 

Super duplex alloys such as FERRALIUM, 32760, 32750 and 2507 are highly developed and complex to produce. In the situations where they are often employed, the failure of a component made from an inferior grade or material may have financial consequences far in excess of the initial marginal additional cost of selecting super duplex.

Richard Bulmer is in charge of the marketing operation for Langley Alloys in the UK. Langley Alloys is a British company with a worldwide reputation for inventing and supporting advanced stainless steels, Super Duplex, nickel and bronze alloys.