In an article for Stainless Steel World James Chater explained that duplex grades represent a growing sector within the stainless steel industry. However obstacles to its greater use persist, particularly the limited availability of product forms. Oil & gas and desalination are the two industries that consume the largest amount of duplex. Other industries where use may be growing are architecture and transport.
Brine recirculation pump used in desalination, from Toroshima Pump.
Duplex stainless steel has always formed a tiny part of overall stainless steel use. However, its share of the overall stainless steel market remains surprisingly low, stuck in the range of 1-1.1%. Sales of duplex were hit especially hard in the 2008-9 recession, probably as a result of the slump in oil and gas. They have recovered, and as of 2014-5 duplex grades have a 1.1% share of stainless steel consumption, as opposed to 1% in 2012. Only time will tell if this apparent rise of 10% is part of a long-term trend or just a statistical blip.
Obstacles
The advantages of duplex grades are well-known: greater strength in relation to weight, almost no stress corrosion cracking and raw material savings. So why is it not more often used? Back in 2008, a spike in the nickel price made low-Ni alternatives such as duplex more attractive. However, with the price of nickel has dropped considerably (it is now around 4.50 USD/lb), so there is less cost incentive than before to prefer duplex over austenitic. Indeed, the share of CrNi stainless steels rose from 52.6% to 53.9% in 2014-5.
2014
2015
CrNi
52.6
53.9
CrMn
21.1
21.1
Duplex
24.5
23.3
Others
0.7
0.7
100.0
100.0
Another reason is simple conservatism and an unwillingness to depart from more familiar grades, such as easy-to-fabricate austenitics. Then there is what has been called the “unobtanium syndrome”, or lack of availability of a wide range of duplex products (pipe, rebar, plate etc.) in any given grade. A new grade is undoubtedly a good idea in principle, but limited availability across a range of products will inhibit its use. Most of the new grades are branded, but have not, so far at least, met with the success of the workhorse 2205, which is not branded and is available in many forms.
The solution to this problem is probably for end users to be vociferous and persistent in expressing their needs to suppliers, who will then be motivated to come up with solutions. Eduardo Gomes of Sandvik put it in a nutshell: “New alloys, such as the hyper duplex Sandvik SAF 2707 HD, were developed based on input from various potential users. … We need to provide a good technical solution that is also very cost efficient for our customers while providing the level of reliability and safety that their processes require.”
To receive a PDF of the full article by James Chater please contact the editor.
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