QUALIFICATION TESTING OF LEAN DUPLEX STAINLESS STEELS FOR THE PROCESS INDUSTRY

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Product Number: 51312-01527-SG

ISBN: 01527 2012 CP

Author: Rachel Pettersson

Publication Date: 2012

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The materials specifier needs accepted methods for qualification or assurance test to verify that supplied materials have acceptable properties. For duplex stainless steels the standard ASTM A 923 has found extensive use as a qualification test for the duplex S32205 and superduplex S32750 grades where the primary cause for concern is the precipitation of intermetallic phases. Such precipitates are usually detectable in the microstructure (Method A) so this can be used to pass material without the need for impact toughness (Method B) or corrosion testing (Method C). However lean duplex grades such as S32101 or S32304 present more of a challenge because these steels are much less sensitive to intermetallic phase precipitation than the higher alloyed duplex grades. The small microstructural changes associated with improper heat treatment are challenging to detect often require high resolution microscopy. Impact toughness acceptance limits also need definition while the corrosion test used in ASTM A 923 (immersion in 6% ferritic chloride solution) results in sub-ambient temperatures for lean duplex grades and is therefore in many instances impractical to use. An approach which is explored in the present work is the use of an inhibited 5% ferric chloride solution containing additions of 1% sodium nitrate. This leads to critical temperatures for the onset of pitting which are around or above ambient and therefore provides the possibility for a simple and practical acceptance test. Results are presented showing the influence of the level of nitrate additions on the corrosion performance and also correlated to impact toughness data. Application to various product forms is discussed together with different test criteria and test philosophies. Finally a limited laboratory intercomparison of the test method is presented. These results represent steps towards development of a new variant of ASTM A 923 for lean duplex stainless steels which is aimed to remove any remaining uncertainty when specifying such grades.