Microstructure and Ductility-Dip Cracking Susceptibility of Circumferential Multipass Dissimilar Weld Between 20MND5 and
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INTRODUCTION
THE circumferential butt weld is an efficient type of welded joint which has been widely used in piping systems of different industries, especially in the petrochemical engineering and the construction of nuclear power plants (e.g., the dissimilar welded joints of pressure vessel nozzle to safe-end and steam generator tubes to tube-plate). Among these applications, the dissimilar welding between stainless steel and high strength low alloy (HSLA) steel,[1,2] Ni-base alloy and HSLA,[3] and Ni-base alloy and stainless steel[4,5] is frequently required for the purpose of cost reduction and/or the differentiation in material properties so as to meet the local requirements of the component. However, the dissimilar welding may result in uncertain chemical composition in the weld, which, together with the welding conditions, determines the microstructure of the weld and the possible welding defects and then influences the mechanical properties and the corrosion resistance of the weld metal. According to RCC-M (Design and Construction Rules for Mechanical Components of PWR Nuclear Islands), the Ni-base alloys (e.g., FM52, FM82, etc.) are specified as filler materials for the dissimilar welding between stainless steel and HSLA steel because they can offer the best compromise among the mechanical properties, the thermal stability, and the weldability, RENYAO QIN, Ph.D. Student, ZHAOLING DUAN, Master Student, and GUO HE, Professor, are with the State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China. Contact e-mail: [email protected] Manuscript submitted March 24, 2013. Article published online June 26, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
especially the hot cracking resistance.[1,6] It is well known that the Ni-base alloy Inconel 600 (corresponding to the filler material FM82) is susceptible to intergranular stress corrosion cracking (SCC).[7–9] Adding high chromium in the Ni-base alloy (leading to a new alloy: Inconel 690) can significantly improve the resistance to intergranular SCC.[10–12] Nowadays, Inconel 600 has almost been replaced by Inconel 690 in the new generation of PWR nuclear power plant. However, the Inconel 690 (corresponding to the filler material FM52) has been found being susceptible to ductility-dip cracking (DDC) in elevated temperature environments.[13–16] Many studies demonstrated that the DDC has some relation to the composition and the microstructure in the weld zone.[14–17] Although the possible mechanism of the DDC has been somewhat understood, the problems related to the DDC in the welding practice during the construction of nuclear engineering components are frequently occurring. In particular, the dissimilar welding causes diluted weld metal which may lead to unexpected welding defects, especially the DDC susceptibility. Thus, the investigation on the actual weldment is necessary for avoiding the welding problems. In this study, a large diameter circumferential dissimilar weld between stainless
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