Qualification Testing of Lean Duplex Stainless Steels for the Process Industry

Abstract The materials specifier needs accepted methods for qualification or assurance testing to verify that supplied materials have acceptable properties. For duplex stainless steels the ASTM A923 standard has found extensive use as a qualification test for the duplex UNS S32205 and superduplex S32750 grades, where the primary cause for concern is the precipitation of intermetallic phases. However, lean duplex grades such as UNS S32101 or UNS 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 and impact toughness acceptance limits need definition. The ferric chloride immersion test used in ASTM A923 results in sub-ambient temperatures for lean duplex grades and is therefore in many instances impractical to use. The approach 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. 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 proposed acceptance criteria. Finally, a limited laboratory intercomparison of the test method is presented. These results represent steps towards development of a new variant of ASTM A923 for lean duplex stainless steels, which is aimed to remove any remaining uncertainty when specifying such grades.