Reactive Boride Brazing on Low-Alloy Automotive Grade Steel
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INTRODUCTION
THE focus of the current work involves brazing boride-based cermets onto a low-alloy steel substrate using in-situ reactions. Hence, the term reactive boride brazing refers to simultaneous processes wherein a ternary boride is synthesized, the boride layer is brazed onto a substrate, and at the same time the boride layer densifies by liquid-phase sintering. Cermets possess the high hardness inherent in ceramics as well as the toughness provided by a ductile matrix. Tungsten carbide-based cermets were widely used in cutting tool applications and applications involving high wear resistance.[1] Boride-based cermets are an alternative for similar applications and are much lighter than their carbide counterparts. Though boride cermets can be a potential replacement for tungsten carbide cermets in the cutting tool industry, their good wear resistance to abrasion, erosion, and high-temperature sliding wear can also improve component life.[2] Reaction brazing is a process where precursors interact at high temperatures to develop a ceramic-metal dispersed composite structure. The synthesis involves deploying the precursors in the form of a tape cast sheet on the surface of the substrate followed by thermal cycling.[3] The process does not involve sophisticated equipment and processing steps as compared to a thermal spraying technique and, hence, is economic. The process parameters such as B. PALANISAMY, Student, formerly with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, UP, India, is now with the School of Mechanical, Industrial and Manufacturing Engineering, Oregon State University, Corvallis, OR 97331. A. UPADHYAYA, Professor, is with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology. Contact e-mail: [email protected] Manuscript submitted May 6, 2010. Article published online June 15, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
brazing temperature, holding time, and substrate composition can be modified to obtain a range of dispersoid (ceramic phase) volume fractions, which, in turn, provide a range of mechanical properties. Binary metal borides, owing to their high melting temperatures (for example, Fe2B = 1662 K (1389 C), and FeB = 1923 K (1650 C)[4]), make processing difficult. Besides, their fixed stoichiometry does not provide scope for change in the properties. It is for these reasons that the complex borides of the transition metal group are considered for cermet synthesis, because their properties can be altered based on composition and processing parameters. The wetting angle between the complex boride and the binder phase can also be altered to yield better densification, unlike that of binary borides. Yamasaki first studied the structure and properties of Mo-Ni-B-V hard alloys,[5] which led to the development of a new ternary boride-based alloy of the form M2M¢B2 (M = refractory metal, and M¢ = transition metal of iron group). Ide et al. subsequently studied the Mo2FeB2-Fe cermet and observed the app
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