Microstructure Analysis for the Detection of Intermetallic Phases Predictive of Toughness and Corrosion Resistance in Duplex and Lean Duplex Steels

摘要

Metallographic techniques for microstructure analysis are used for detection of the presence of intermetallic phases in duplex stainless steels to the extent that toughness or corrosion resistance is affected significantly. Duplex (austenitic-ferritic) stainless steels are susceptible to the formation of intermetallic compounds during exposures in the temperature range from approximately 600℉ to 1,750℉ (320°C to 955°C). ASTM A923, Standard Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels, Test Method A (Sodium Hydroxide Etch Test for Classification of Etch Structures of Duplex Stainless Steels), is used as a rapid screening method to detect the presence or absence of intermetallic phases to the extent that they are detrimental to the toughness and corrosion resistance of the material. The use of these etch structures depends on the test method for which the specimens are being screened. Recent revisions have incorporated additional alloys that may be evaluated in accordance with this specification. For lean duplex alloys (composed of 30 % to 70 % ferrite content with a typical alloy composition having chromium [Cr] greater than 17 % and molybdenum [Mo] less than 1 % and with additions of nickel, manganese, nitrogen, and controlled low carbon content as well as other alloying elements), ASTM A1084-15a, Standard Test Method for Detecting Detrimental Phases in Lean Duplex Austenitic/Ferritic Stainless Steels, provides a metallographic procedure to show the appearance and approximate amount of chromium-containing detrimental phases in lean duplex stainless steel, despite the small size of the detrimental phases and the difficulty in achieving a fully reproducible etch structure. Microstructure results are correlated with toughness test and corrosion test results on a pass/fail basis. This newer process covers lean duplex stainless steels that were not well served by ASTM A923.