Materials Transactions Online

Materials Transactions, Vol.52 No.01 (2011) pp.12-19
© 2011 The Japan Institute of Metals

Microstructure and Stress Corrosion Cracking Behavior of the Weld Metal in Alloy 52-A508 Dissimilar Welds

Wei-Chih Chung1, Jiunn-Yuan Huang2, Leu-Wen Tsay3 and Chun Chen1

1Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, R. O. China
2Institute of Nuclear Energy Research, Lungtan 325, Taiwan, R. O. China
3Institute of Materials Engineering, National Taiwan Ocean University, Keelung 202, Taiwan, R. O. China

In the nuclear power industry, dissimilar metal welding is widely used for joining low alloy steel to austenite stainless steel components with nickel-base filler metals. In this study, attention was paid to the weld metal in multi-pass Alloy 52-A508 dissimilar welds. An approximately 2 mm wide transition zone was observed that consisted of a martensitic layer (10 ∼ 20 um) along the weld interface and the austenite phase region with varying degrees of dilution. After post-weld heat treatment, the microstructures near the weld interface consisted of martensite, carbides and Type II boundaries. The presence of Type II boundaries significantly reduced the resistance to stress corrosion cracking (SCC) and formed intergranular cracking under simulated reactor coolant conditions. Constant extension rate tensile (CERT) tests were performed on the notched tensile specimens in 300°C water at two extension rates, 3× 10-4 and 1× 10-6 mm/s. A fast CERT test can be regarded to have no contribution of corrosion, and its results can be used as standards for comparison. In the slow CERT tests, the ductility losses of round-bar specimens with a circumferential notch at various regions in the weld metal were ranked accordingly. The relative susceptibility to SCC in terms of the ductility loss in increasing order of severity was as follows: the undiluted weld metal, the transition zone and the weld interface. SEM fractographic observations were consistent with the SCC results, i.e., an increased ductility loss or SCC susceptibility was associated with more brittle fractures.

(Received 2010/8/31; Accepted 2010/10/12; Published 2010/11/24)

Keywords: A508 steel, Alloy 52 filler metal, dissimilar welds, microstructures, stress corrosion cracking, high-temperature water

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