Heat Exchanger World Magazine May 2023 | Page 21

Materials

This article elaborates the need to discuss super austenitic stainless steels as an alternative to options mentioned below .

By Venkat Ramesh , Global Product Manager ( Heat Exchanger tubing ), and Shabareesh Nair , Director ( Technical Marketing Asia ), Tubacex Group

Introductory background High Nickel alloyed stainless steels are categorized as having Chromium + Nickel greater than 50 %, and most of these alloys have Nickel greater than 30 %. These materials have been used widely in petrochemical , chemical , refinery , and organic acid industries , due to their excellent overall corrosion resistance to various types of media and harsh environments involving high temperatures . In recent months , there have been challenges faced by supply chain to procure these types of materials which have seen wide fluctuations in prices , and long leadtimes . This also adds to costs and budgets of major projects involving such materials , and sometimes leads to material selection changes to inferior alloys . This has resulted in an increase in OPEX cost and unplanned shutdowns in various applications . Austenitic stainless steels , on other hand , are not subjected to such high prices and lead times , but in applications involving severe corrosion , they are not suitable for use , and if used will have very short life . Duplex steels have been used widely over past years as an alternative to austenitic steels , due to its superior corrosion resistance under certain conditions . But , the major drawback has been its limited temperature zone within which equipment can be operated . Additionally , there are also issues with stress relieving heat treatment post U-bending .

Description Super austenitic stainless steels , by definition , are steel grades with PREN number ( as calculated by formula PREN = % Cr + 3.3 x % Mo + 16 x % N ) greater than 35 . These can also be classified as Cr-Ni-Mo steels , or Cr-Ni-Mo-Cu steels , having most Chromium content between 17 and 25 %, nickel between 14 and 25 %, and Molybdenum between 3 and 7 %. Many of these grades are also alloyed with nitrogen to further increase corrosion resistance and strength . Some grades are alloyed with copper to increase resistance to certain acids . It also has fully austenitic grain structure in annealed condition , good cold and hot workability , and is easy to weld . Some of the grades this paper elaborates on in detail are super austenitic , mentioned in Table 1 .

Wet corrosion properties Pitting corrosion Pitting is localized corrosion in a pattern of rat holes penetrating through the tube wall . The initializing of pitting depends on several factors , such as presence corrosive environment ( e . g ., Halides-Chlorides , Bromides , and Fluorides ), temperature , and most importantly , the defects and weakness of the steel passive protective layer . Once pitting initiates , the propagation is much faster . Prescence of higher % of Chromium in stainless steels helps in pitting corrosion resistance as forms a strong

Steel Grade , Chemical Composition ( Key Elements )

Common Grade name

International Steel designation Typical Chemical Composition %

ASTM

UNS number

EN C max Cr Ni Mo N Cu

Duplex S31803 / 32205

1.4462 0.3 21-23 4.5-6.5 2.5-3.5 0.08-0.20

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