The origin of duplex stainless steels can be traced back to the 1930s. Aggressive process environments in pulp and papermaking mills in Sweden spurred the development of more corrosion-resistant grades by local manufacturer Avesta. Higher process temperatures and pressures, together with aggressive sulphite solutions proved to be an onerous environment.
Now that PREN values (Pitting Corrosion Resistance Equivalent Number) are an accepted indication of general pitting corrosion resistance, it is obvious that increasing chromium content will improve general performance. Duplex stainless steels were established as a product family around a chromium content of c. 22%, compared with 16-18% for standard austenitic stainless steels.
As suggested by their name, duplex grades exhibit two distinct microstructural phases, that of both austenitic and ferritic. Each microstructural phase is a particular type of crystal structure that informs its properties. In duplex stainless steel, ‘islands’ of ferrite form within a matrix of austenite. Consequently, they take the best parts of both structures properties, namely; higher strength; good toughness; relative ease of fabrication; good corrosion resistance and resistance to stress corrosion cracking. The key to maintaining this combination of favourable properties is to maintain a near 50:50 balance in the microstructure, which is achieved through the composition and process conditions.
The Schaeffler diagram is an interesting chart even for non-metallurgists. A number of elements are no known to act in a similar way to chromium, encouraging the formation of a particular microstructure (ferrite). In the same way, other elements are known to act in the same way as nickel and encourage an austenitic microstructure. By calculating the chromium and nickel ‘equivalents’, it is possible to predict the final microstructure. As shown in the chart below, the progression from duplex to super duplex, to hyper duplex makes a little more sense. Chromium (and chromium equivalent) content increases, providing increased pitting corrosion resistance. However, nickel (and nickel equivalent) content must also increase to maintain the right balance of microstructure, physical and mechanical properties.
Whilst duplex stainless steels have existed in one form or another since the 1930s, it wasn’t until the 1970’s when their uptake improved. Better steelmaking technology allowed tighter control of composition, particularly that of nitrogen. Higher levels of nitrogen improve the pitting corrosion resistance in a cost-effective manner compared with more expensive alloying elements. Also, improved knowledge of weldability, maintaining the duplex microstructure through the heat-affected zone of the weld region, meant that it could be used without risk of premature failure due to poor fabrication.
At around the same time, super duplex stainless steels were invented. Led initially by Langley Alloys unique Ferralium 255 product in 1969, it was followed by Zeron 100 (UNS S32760) in 1980 and then SAF2507 (UNS S32750) in 1988. All three super duplex alloys are based upon a 25% chromium content and are now formulated to achieve a minimum PREN of 40 as this is a requirement of the Norsok standard (M-001).
Hyper duplex alloys are now being developed as potential alternatives to more expensive nickel alloys. Sandvik has developed their Sandvik SAF 2707 HD grade, pushing the chromium content beyond 27% and the PREN to c. 48. This has been exceeded by Sandvik SAF 3207 HD grade, with a chromium level of 32% and a PREN of c.50. These compositional changes also result in an equivalent increase in yield strength of >100ksi (compared with 80ksi for S32750/32760 and 85ksi for Ferralium 255-SD50).
To date, these highly-alloyed grades have only been made available commercially as extruded seamless tubes, typically for use in umbilicals, heat exchangers, condensers and seawater coolers. Producing forged bars and rolled plates has proven more problematical – difficult processing, lower yield and manufacturing defects have precluded their launch. Matching the corrosion performance of certain nickel alloys is definitely possible, and offering a cost-effective alternative is also viable, particularly if nickel prices increased in level or volatility.
Duplex Stainless Steels
Super Duplex Stainless Steels
Hyper Duplex Stainless Steels
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