A Comparison of Creep Rupture Strength of Ferritic/Austenitic Dissimilar Weld Joints of Different Grades of Cr-Mo Ferritic Steels K. LAHA, K.S. CHANDRAVATHI, P. PARAMESWARAN, SUNIL GOYAL, and M.D. MATHEW Evaluations of creep rupture properties of dissimilar weld joints of 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb steels with Alloy 800 at 823 K were carried out. The joints were fabricated by a fusion welding process employing an INCONEL 182 weld electrode. All the joints displayed lower creep rupture strength than their respective ferritic steel base metals, and the strength reduction was greater in the 2.25Cr-1Mo steel joint and less in the 9Cr-1Mo steel joint. Failure location in the joints was found to shift from the ferritic steel base metal to the intercritical region of the heat-affected zone (HAZ) of the ferritic steel (type IV cracking) with the decrease in stress. At still lower stresses, the failure in the joints occurred at the ferritic/austenitic weld interface. The stress-life variation of the joints showed two-slope behavior and the slope change coincided with the occurrence of ferritic/austenitic weld interface cracking. Preferential creep cavitation in the soft intercritical HAZ induced type IV failure, whereas creep cavitation at the interfacial particles induced ferritic/austenitic weld interface cracking. Micromechanisms of the type IV failure and the ferritic/austenitic interface cracking in the dissimilar weld joint of the ferritic steels and relative cracking susceptibility of the joints are discussed based on micro- structural investigation, mechanical testing, and finite element analysis (FEA) of the stress state across the joint. DOI: 10.1007/s11661-011-0957-8 Ó The Minerals, Metals & Materials Society and ASM International 2011 I. INTRODUCTION DISSIMILAR weld joints between Cr-Mo ferritic steels and austenitic stainless steels are used extensively in conventional as well as in nuclear power generating plants and petrochemical industries. In fossil fuel fired power generating plants, austenitic stainless steel tubes are used in the high-temperature sections, such as the final stages of the superheaters and reheaters, where creep rupture strength and oxidation resistance are important. In the primary boiler and heat exchanger where the temperatures are lower, Cr-Mo ferritic steels such as 2.25Cr-1Mo, 9-12 Cr ferritic steels and their variants are used. In the steam generator circuit of sodium cooled fast breeder reactors (SFRs), 316L(N) austenitic stainless steel pipes from the intermediate heat exchangers are required to join with the modified 9Cr- 1Mo ferritic steel (9Cr-1MoVNb) pipes of steam gener- ators. In such a joint, a ferritic/austenitic transition bond is formed across which the chemical composition, microstructure, stress state, physical properties, and mechanical properties are appreciably different. The mismatch in the thermal expansion coefficient across the joint is reduced with the insertion of an Alloy 800 piece and adopting an INCONEL* 182 welding electrode having thermal expansion coefficients intermediate between austenitic and ferritic steels. [1] However, pre- mature creep failure is encountered in such dissimilar weld joints. [2–7] An understanding of the microstructural changes across the weld interface between ferritic and austenitic alloys and their effects on high-temperature creep deformation and fracture behavior are of primary concern for a realistic life assessment of the dissimilar weld joints. [8–17] Furthermore, the combination of mate- rial, geometry, and size of the weld joint and loading conditions can lead to the presence of complex stress state across the joint, which greatly influences the creep rupture life of dissimilar weld joints. [18–20] This study aims to understand the creep rupture behavior of the 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb dissimilar joints with Alloy 800 welded with INCONEL 182 electrode. Detailed microstructural, microhardness, ele- mental distribution, and finite element analysis (FEA) of the stress state across the weld interface were carried out. The creep deformation and rupture properties of the joints and their constituents are compared to arrive at a comprehensive understanding of the premature K. LAHA, Head, Creep Studies Section, K.S. CHANDRAVATHI and SUNIL GOYAL, Scientific Officers ‘E’, and P. PARAMESWARAN, Scientific Officer ‘G’, Physical Metallurgy Division, and M.D. MATHEW, Head, Mechanical Metallurgy Division, are with the Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Contact e-mail: laha@igcar.gov.in Manuscript submitted January 5, 2011. Article published online December 2, 2011 *INCONEL is a trademark of Special Metals Corporation, New Hartford, NY. 1174—VOLUME 43A, APRIL 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A