Investigation of the diffusion bondability of high carbon steels in superplastic conditions

VI SUMMARY PhD THESIS INVESTIGATION OF DIFFUSION BONDABDLITY OF HIGH CARBON STEELS IN SUPERPLASTIC CONDITIONS Mustafa TAŞKIN Firat university Graduate School of Science Metallurgy Education Department 2000, Page: 91 Advancing technology has presented new materials and methods in manufacturing and processing industry. The main goal of these studies is to save money and time. On the other hand some researches on materials are focused on improvement of properties of conventional materials like steels. One of the properties which can be used both in processing and bonding of materials as an advantage is superplasticity. Superplasticity is known as continuous deformation without necking under constant tension. High carbon steels can be made superplastic with thermo-mechanical processes. Bonding of high carbon steels with conventional welding processes are very difficult due to their high carbon content causing formation of a very hard and brittle phase, martensite, after welding. In addition high temperatures up to melting point in welding region change superplastic structure. Therefore it is vital to use a bonding method for joining high carbon steels with respect use of their economical use. One of the methods proper to joint high carbon steels is diffusion bonding. Diffusion bonding is a solid state bonding method. This method was used first in 1940s yet its importance was realized after 1970s. New materials developed and theVII difficulties in joining of these materials with conventional methods have increased the usage of diffusion bonding. Adavantages of diffusion bonding are relatively low bonding temperatures far below melting point, no need for heat treatment before and after bonding, controlling of undesired phase formation, convenience for the materials like composites and ceramics. Them main mechanisms of diffusion bonding are: plastic deformation under initial bonding pressure, diffusion, creep and recrystallization. All of these mechanisms can be stimulated easily in superplastic materials. Superplastic materials on the other hand can induce many high diffusion channels (grain boundaries) due to very fine grained structure. At diffusion bonding temperatures it is well expected creep to be much more effective because of superplasticity. Superplastic properties of the materials generally do not change after diffusion bonding. Therefore, in this study, high carbon-hypoeutectoidic steels alloyed Al, Ti, Nb and V in different proportions were made superplastic with thermo-mechnical processes and then the superplasticity temperatures were determined with hot tension tests. Finally The superplastic specimens were diffusion bonded in vacuum below at and above the superplastic temperatures. Thus it is aimed to determine the bonding conditions which do not harm superplastic properties. As a result it is understood that superplasticity could be used as an advantage in bonding of high carbon and low alloyed steels without changing their superplastic properties. In this study the subject is introduced in the first chapter. In the second chapter, superplasticity and its parameters are given. In addition the materials studied for superplasticity are dealt with. In the 3rd and 4th chapters information on diffusion and diffusion bonding are presented. In the 5th chapter the experimental studies, in the 6th chapter the results of the experiments are given. In the 7th and final chapter conclusions and recommendations are presented. Keywords: Superplastic, superplasticity, diffusion bonding, superplastic diffusion bonding.