The diffusion bonding of Zr-2.5 pct Nb to steel
- PDF / 1,511,204 Bytes
- 6 Pages / 594 x 774 pts Page_size
- 58 Downloads / 184 Views
I.
INTRODUCTION
THEjoining of zirconium alloys to steel is frequently required in order to reduce the cost of components in which zirconium alloys must be used. Joining of these two materials by fusion welding is undesirable because brittle intermetallic compounds such as ZrFe,_ are formed. Furthermore, the thermal expansion coefficients, allotropic phase transformations, and elastic moduli of zirconium and ferritic steel are sufficiently different that high residual stresses develop during the cooling of welds. As compared to purely mechanical (e.g., rolled) joints, fusion welds have structural continuity; this gives them the advantage of being leak tight and inspectable by such techniques as ultrasonic testing. On the other hand, this continuity provides a path for the flow of corrosion-generated hydrogen from the steel to the zirconium where it can cause embrittlement. Many of the problems of welded joints between steel and zirconium alloys can be overcome by the process of diffusion bonding. ,.2.3 Diffusion bonding involves the creation of a solid-state bond by interdiffusion across the interface between the two materials. Following appropriate surface cleaning, the surfaces to be joined are placed in contact, then heated to, typically, 0.7 of their melting points under a pressure of 0.5 to 50 MPa and held for minutes to hours. Plastic deformation occurs initially at points of contact across the interface, increasing the contact areas, decreasing the size of the interfacial voids, and breaking down surface oxide layers. As the bonding process continues, the residual interface porosity is reduced further by vacancy diffusion/ creep/sintering to a level where bond strength reaches that of the base materials. By appropriate control of temperature and pressurization cycles, narrow bonds with thin or nonexistent layers of the detrimental intermetallic compounds can be obtained. This paper reports studies carried out on the diffusion bonding of Zr-2.5 pct Nb to carbon steel using an interlayer of platinum. This system, selected after evaluation of several interlayer materials, 4 is being seriously considered for applications in the CANDU nuclear reactor system. 5 The M. L. WAYMAN, Professor of Metallurgy, and R. R. SMITH, Graduate Student, are with the Department of Mineral Engineering, University of Alberta, Edmonton, AB T6G 2G6, Canada. M.G. WRIGHT is Manager of Engineering Design and Operations, Atomic Energy of Canada Ltd., Whiteshell Nuclear Research Establishment, Pinawa, MB ROE IL0, Canada. Manuscript submitted July 30, 1984. METALLURGICALTRANSACTIONS A
choice of platinum as the interlayer material is reasonable on the basis of several criteria. While the solubility behavior is not extraordinarily good, platinum is close to the "ideal" interlayer material for this system on the basis of its thermal expansion and Young's Modulus. 6 It does not have a stable surface oxide to complicate surface preparation. Furthermore, it acts as an effective barrier to the diffusion of hydrogen from the steel to the zirconiu
Data Loading...
