Extending the application range of super-duplex stainless steels

摘要

Super-duplex stainless steels such as UNS S32750 have found extensive use in sea water applications as well as in equipment in the process industry, e.g. refinery heat exchangers. The newly developed hyper duplex grade UNS S32707 allows extension of the application range of austenitic-ferritic alloys into more aggressive conditions, in particular by increasing the maximum application temperature in chloride containing process solutions and cooling waters. The nominal PRE number of UNS S32707 is 49, which results in a significant increase in the resistance to pitting corrosion compared to e.g. UNS S32750 (typical PRE number 42). In addition, the resistance to concentrated brines or acidic chloride solutions is superior to that of super-duplex stainless steels. Successful use of the alloy in applications where condensate corrosion is a problem, such as refinery overhead condensers, is discussed. The paper also shows how the general corrosion resistance of UNS S32707 in reducing and oxidizing acids compares to that of high alloy austenitic and duplex alloys, as well as giving examples of the resistance of the alloy to chloride stress corrosion cracking and hydrogen induced stress cracking.