Role of secondary austenite on corrosion and stress corrosion cracking of sensitized duplex stainless steel weldments

Abstract

The role of secondary austenite on corrosion and stress corrosion cracking in high temperature water for sensitized duplex stainless steel weldment was investigated using transmission electron microscopy and fractgraphy. Pitting potential measurements, 10 percent oxalic acid tests (American Society for Testing Materials (ASTM) A262A), Strauss tests (ASTM A262E) and Huey tests (ASTM A262C) were carried out for the corrosion assessment. For stress corrosion cracking, the slow strain rate testing (SSRT) was carried out at 562 K with 8 ppm dissolved oxygen under 8 MPa at a strain rate of 4.17 x 10(exp -6)s(exp -1). Volume fraction of gamma phase decreased with increasing peak temperature and the grain size of the gamma phase increased with increasing peak temperature. The amount of precipitation of Cr2N at the grain boundary and in the grain increased with increasing peak temperature. The region around Cr2N showed intergranular corrosion. For the sensitization at 923 K for 72 ks, sigma phase precipitated and secondary gamma phases (named gamma phase) appeared between primary gamma phase and sigma phase. The gamma phases were predominantly corroded and the pitting potentials were low. Stress corrosion cracking in high temperature water hardly occurred for the solution-treated specimen. The reduction in area of the specimen sensitized at 923 K for 72 ks was much lower than that of the solution-treated specimens and decreased with decreasing peak temperature, because the gamma phases near the M23C6 and sigma phase were predominantly corroded and dissolved.

高温水中における鋭敏化2相ステンレス鋼の溶接物の腐食および応力腐食割れに対する第2オーステナイトの役割を、透過電子顕微鏡法と破面検査により調べた。腐食試験として、孔食電位測定、10%しゅう酸試験(ASTM A262A)，応力試験(ASTM(米国材料試験協会) A262E)およびHuye試験(ASTM A262C)を行った。応力腐食割れでは、低歪速度試験(SSRT)は，562Kの、溶存酸素8ppm液中で、4.17x10{-6}/sの歪速度で、8MPaの負荷をかけて行った。ピーク温度が上がるに連れて、ガンマ相の容積比は減じ、ピーク温度の上昇と共にガンマ相の結晶粒度は大きくなった。結晶粒界および粒内でのCr2Nの析出量はピーク温度の上昇と共に増した。Cr2Nの周辺では粒界腐食が認められた。923K、72ksの鋭敏化で、シグマ相が析出し、ガンマプライム相がガンマ相とシグマ相との間に現れた。ガンマプライム相は選択的に腐食を受け、孔食電位は低下した。高温水での応力腐食割れは、溶体化処理した試料では先ず起こらない。923Kで72ks間鋭敏化した試料の面積低下は、溶体化処理した試料のそれより遥かに少なく、ピーク温度の低下につれて低下した。これはM23C6近傍のガンマプライム相およびシグマ相が優先的に腐食され、溶解するからである。