Hot cracking susceptibility of fillers 52 and 82 in alloy 690 welding
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INTRODUCTION
ENVIRONMENTAL demands of chemical or nuclear power plants require structural materials to be both strong and corrosion resistant. For example, Ni-Cr-Fe alloys were once commonly used in the steam generators of boiling water reactors and pressurized water reactors. However, intergranular stress corrosion cracking (IGSCC) was found in 304 stainless steel during the 1960s. In response, the industrial sector implemented several methods to curb this phenomenon. These included searching for substitute materials, coating with anticorrosives, and annealing with specialized methods. A substitute material was nickel alloy 600. The heat and mechanical characteristics of alloy 600 (unified numbering system, UNS N06600) and stainless steel 304 are quite similar. However, the former resists stress corrosion cracking (SCC) better in chloride solutions.[1] Accordingly, alloy 600 was widely used beginning in 1960. However, in 1978, a serious defect was discovered in a boiling water reactor at the Duane Arnold Energy Center (Palo, IA): the safe-end forging part at the circulation pump’s input exhibited instability after extended use. Upon failure analysis, IGSCC was found to be the cause of cracks in the safe end.[1,2,3] Cracks initiated and propagated in the heat-affected zone (HAZ) of I-182 (UNS W86182, INCONEL* 182 manual weld metal) filler in alloy 600 *INCONEL is a trademark of INCO Alloys International, Inc., Huntington, WV.
base material. Similar affects also appeared in I-82 (UNS N06082, INCONEL 82 solid filler material) weld beads. Power plants then urgently required a new material to construct steam generators. This search ended in the development of nickel alloy 690 with its high resistance to SCC. Nickel-based alloy 690 (UNS N06690) has a significantly higher Cr content; specifically, the Cr content is 30 pct compared to 16 pct for alloy 600. This helps prevent
WEITE WU, Associate Professor, is with the Department of Materials Science and Engineering, I-Shou University, Taiwan 84008, Republic of China. C.H. TSAI, formerly Graduate Student, Department of Materials Science and Engineering, I-Shou University, is Specialist with Tien Tai Electrode Co., Ltd., Taiwan 707, Republic of China. Manuscript submitted January 13, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
chromium depletion caused by carbide segregation in grain boundaries.[4] Moreover, extra Cr enhances resistance to corrosion by chemical solutions or high-temperature gasses. With 60 pct Ni, alloy 690 is SCC resistant to sodium hydroxide and chloride. Further, under simulated steam generator conditions in a laboratory environment, the alloy is highly resistant to IGSCC, intergranular attack, and surface corrosion.[1–5] In addition, the research and development center at INCO Alloys International performed a series of tests on alloys 600 and 690, which concluded that alloy 690 resisted IGSCC and surface corrosion better than alloy 600.[6] With the high SCC susceptibility of I-82 and I-182 weld materials, research drove the development of fi
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