Firstly, we must distinguish between duplex, super duplex and hyper duplex stainless steels. In principle, all variants are based upon a duplex microstructure, combining both austenitic and ferritic grains. Therefore, they could all be described as duplex and offer a certain combination of corrosion, mechanical and physical properties. However, there are sufficient differences between the different grades that it is worthwhile focusing just on the more standard duplex stainless steels based upon 22% chromium content.
Explained elsewhere, there is a subtle different between the grades UNS S31803 (F51) and UNS S32205 (F60), the terminology for which is often used interchangeably, but let’s use 2205 or duplex as shorthand for simplicity.
Pitting Resistance Equivalent Number (PREN) is a quick reference to the corrosion resistance of stainless steels and other corrosion resistant alloys (CRA). For 2205 it is 34, which is a significant uplift in performance compared with 316 at 25. Increasing the chromium content from c.18% to c.22% will improve the resistance to pitting corrosion by increasing the efficacy of the passive layer that forms when the alloy is exposed to oxygen in the environment.
As a very crude ‘rule of thumb’, Alloy 316L can be used successfully in rural, urban and estuary environments, but is limited in more exposed coastal settings; 2205 duplex can be comfortably used in estuary, marine and offshore environments; but for applications with long-term immersion then a super duplex may be necessary.
Consequently, 2205 duplex is used in a wide range of applications where Alloy 316L is not sufficient, but the cost differential to super duplex stainless steels, super austenitic stainless steels and nickel alloys is too high. These include:
a) pulp and paper manufacture – duplex alloys were actually invented with this application in mind, where various equipment items will not only be exposed to water, but also the various solutions/liquors used to break down the wood pulp, process and bleach it.
b) water treatment and desalination – this has been in increasingly significant market for 2205 duplex grades. In desalination processes, the corrosive nature of seawater will decrease during the desalination process, allowing for a range of alloys to be used along the length of the process units.
c) geothermal energy – depending upon the location, and therefore the composition of the liquid being handled, duplex alloys can be used in the extraction and energy recovery from the earth. Some locations where brine is present will require super duplex or super austenitic alloys due to the more aggressive corrosion conditions.
d) chemical industry – for a great many solutions, particularly at lower concentrations and temperatures, 2205 duplex stainless steel will give acceptable performance. It is widely used to machine the body of smaller valves and occasionally as fasteners.
e) marine applications – propeller drive shafts are frequently manufactured from 2205 duplex stainless steel. A ground finish is often specified to reduce fouling, plus to improve the straightness during this additional processing step. Other applications include rudders, shaft seals, pumps, bolts, fasteners, valves and instrumentation.
f) Oil & Gas – within this very industry segment, it will be frequently used in applications such as pumps, valves, pipe, vessels, wellhead equipment, subsea equipment, separators and heat exchangers. Duplex grades offer excellent resistance to stress corrosion cracking (SCC) that can open up applications in more aggressive environments. This property is related to the presence of ferrite grains with the mixed microstructure.
g) food production – although Alloy 316L will be the default alloy choice for most food and drink-related facilities, 2205 duplex will be used in applications such as brewery piping, evaporators, hot liquor tanks and presses where the temperature is higher, or the solution more aggressive, or the strength requirements greater.
h) architectural – although most of the above applications have focused on the increased corrosion resistance, for load-bearing applications such as pedestrian footbridges, the additional strength of 2205 duplex is of benefit.
i) oil and chemical storage tankers – for this application, the additional strength (>50%) compared with Alloy 316L goes a long way to mitigate the additional cost of this grade with the higher alloy content. It can provide the same structural performance from a lighter section, reducing the amount of metal needed as well as reduce the size of supporting structures.
Solid bars are available ex-stock from ½” to > 16” diameter, whilst plate is available from our Portland USA warehouse at up to 1” thick.