Super Duplex Stainless Steel

By Mr MRC Nagarajan

Some of us remember our grand mothers using wooden or earthen spatula for retrieving, mixing or any handling of pickles, which is a standard ingredient in South Indian household. When white shining stainless steel formed spatulas arrived on the scene, the eager new generation jumped into the ‘ clean’ object and replaced the wooden spatula with stainless steel. Many found to their horror that the shining object broke for no reason what-so-ever. Even to day you can walk into the kitchen and find such broken remnants, and some of the tumblers, particularly the one’s with a brim, or rolled edges having developed cracks. All these have a common service environment causing the premature failure – the common salt. Such failure is known as ‘ stress corrosion cracking. The ingredients required for failure are, locked up stress, or work hardening, and susceptible environment.This is typical of ‘ non-magnetic’ type stainless steel which is presently called ‘austenitic stainless steel. The un-exhaustible source of common salt is, of course, sea. So, a failure of normal stainless steel in off-shore environment is an expected factor. Even in this family of stainless steels there are some varieties which are resistant to such failure, and these are the duplex stainless steels. Unlike the former variety this variety has equal proportions of ferrite and austenite, both a high temperature phases, held stable at room temperature, with high enough inducements & perks, like high percentage of chromium, some nickel, molybdenum to create such ‘ crack – jack – a slat – sweet taste’ required.

These steels first developed in Sweden, and in France about seventy years back, has found large use these days after the advent of Off-shore industry. Modern research, to improve and stretch the usability – newer generation of super duplex stainless steels have been added.

Thus came into the family of 2205 grade (UNS 31803), other grades like 2505, 2705, 2707, (UNS 32750, 32760, 39274) also known as Zeron 100, and later 6 – 8 Mo. steels, (UNS 31254).

This was because, when man moved from land on to off shore, he was already in deep sea. Because this area where what you put in would not be amenable for retrieve, replace, remove, repair, but yet expected to serve for the life of the well, one had be conscious of cost, availability, and ease of handling. Presence of hydrogen sulphide had ushered in duplex stainless steels. When going still deeper he had to cope with larger amounts of hydrogen sulphide, carbon-di-oxide, besides the higher temperature and pressure, in various ranges and combinations, normal UNS 31803 was found insufficient and better material was the need of the day. Thus brought in its wake the super duplex varieties. This was because deeper environment behaved as un-predictable as a drunken monkey bitten by a scorpion.

These hybrids were like race horses. They needed handling with kid gloves, rich food and care. You could not hitch it to any cart nor could you bring it on to MG Road. It would run only on turf, with a light weight jockey astride and lot of money tagged on to its tail tip. These super duplexes need expert welders, special welding consumables and techniques for fabrication, besides control of presence of undesirables impurities at its manufacturer’s works. Many would not venture into manufacturing, and hence easy availability in all forms itself used to be a question. Further any slip in further treatment would cause it to fail prematurely and disastrously.

As it stands things are better now. More and more of such hybrids are being specified and the fabrication methods are getting stabilized. So now one can choose from 2304, 2205, to 2507, 2707, or even 6-8 Mo, alloy 254 for specific areas. Monels, and incoloy and inconels are entering in.

Normal maladies encountered are pitting and/or crevice corrosion, stress corrosion cracking, hydrogen cracking. additional alloying elements like tungsten, copper and nitrogen are featuring in the hybrids.The sea water remaining as salty as ever and corroding away what ever put into it, putting in baits as we do in fishing, by introducing the sacrificial anodes are one of the methods available for protection of important components. Once the ‘ passive layer’ is disturbed or removed for any reason, it would not heal as readily as in air where copious oxygen is available. Any local breakdown would lead to concentrated attack on that spot, as rest of the area would behave as large cathode to a localized anode, thus leading to pitting. Addition of copper would improve the corrosion resistance, the added nitrogen would improve its pitting index, and addition of tungsten would help in wider selection of welding consumables and ease of fabrication.

We had NACE MR 1035 for guidance, a compact document, for selection of materials for such arduous service. In 2004 it has gone international with an additional title of ISO 15656 and has grown into a 147 pages, with additional varieties of failures apart fro the SCC and HIC its fore runner contained. So, when in doubt use SS 316, is no more valid. Its rangability too has been curtailed to just 60 deg. cel. And this change would bring in its wake many new materials, methods and many fears and head aches for procurement and use.