Evolution of Microstructure in Brazed Joints of Austenitic-Martensitic Stainless Steel with Pure Silver Obtained with Ag
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JOINING of silver to stainless steel is required in manufacturing of many components of aerospace vehicles such as in rotating parts of a turbo pump where good tribological properties are required. In such applications, stainless steel, having a good combination of corrosion resistance and strength[1] is used for structural applications, whereas pure silver, due to its good friction properties and high thermal conductivity, is used for preventing the excessive heating due to friction.[2–4] Joining of silver to steel is critical since it usually produces a poor joint. For example, Atasoy et al.[5] reported the propagation of crack at the interface between silver interlayer and low carbon steel, whereas Deng et al.[6] reported propagation of fracture through silver interlayer in their respective work of diffusion
S. GANGADHARAN, formerly Master’s Student with the Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, U.P. 208016, India, is now Manager with Tata Steel Ltd., Jamshedpur, India. D. SIVAKUMAR, Scientist SG, and T. VENKATESWARAN, Scientist SE, are with the Materials Processing and Research Group, Vikram Sarabhai Space Center, ISRO, Trivandrum, India. KAUSTUBH KULKARNI, Assistant Professor, is with the Department of Materials Science and Engineering, Indian Institute of Technology Kanpur. Contact e-mail: [email protected]. Manuscript submitted February 20, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
bonding of Ti and steels with silver as an interlayer. The poor bond between steel and silver is mainly on account of the highly positive heat of mixing between silver and iron and their poor solubility into each other as observed from the Fe-Ag binary phase diagram.[7] In addition, if high temperatures are used during the joining of these dissimilar metals, it may end up deteriorating the properties of structural steels such as austenitic-martensitic steels, which are given special heat treatments for establishing their desired properties. Hence, there is a need to develop a brazing process that would yield a strong and reliable joint without affecting the properties of the base material. Ag-Cu based alloys[8–10] are good candidates for a brazing filler material because of the presence of lower melting eutectic in this system and their compatibility with silver.[11,12] Tin is usually added in commercial Ag-Cu based filler alloys to further lower the melting temperature of the alloy and to improve the spreadability of the filler metal.[10] However, Ag-Cu-Sn ternary system contains a variety of intermetallic phases, which are brittle. Hence, the presence of intermetallics in the brazed joint may yield joints with poor strength. Thus, studying the evolution of microstructure in the brazed joints between austenitic-martensitic stainless steel and silver obtained with Ag-Cu-Sn-based brazing filler material is one of the motivations for this work. An austenitic-martensitic class stainless steel (AMSS) is a good material for cryogenic application[13] hence it
was used as one of t
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