Diffusion bonding of fine grained high carbon steels in the super-plasticity temperature range

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1 Indian Journal of Engineering & Materials Sciences Vol. 12, August 2006, pp Diffusion bonding of fine grained high carbon steels in the super-plasticity temperature range Mustafa Taşkın*, Nuri Orhan & Sermin Ozan Faculty of Technical Education, Firat University, Elazig, Turkey Received 26 October 2004; accepted 17 May 2006 In this study, diffusion bonding behaviour of two high carbon hypoeutectoid steels was investigated. The steel specimens were heat treated to induce a fine grain structure and diffusion bonded within a temperature range where these steels exhibit superplasticity. The results showed that these steels could be bonded successfully at 650 o C and 735 o C within the superplastic temperature range, but poor diffusion bonds were fabricated at a temperature of 850 o C. The poor bonding was attributed to an increase in grain size, which not only reduce grain boundary diffusion mechanisms, but also a loss in superplastic deformation behaviour could be expected. IPC Code: C21C 5/00 Superplasticity is the ability of materials to achieve extremely large uniform elongations. Grain refinement is one of the main requisites to attaining structural superplasticity in a material. Kim et al. 1 reported that even in ceramic materials tensile ductility can be improved dramatically when the grain size is reduced to 0.1 μm. The strain rate sensitivity parameter m is the main material parameter which determines the superplasticity. That parameter is a part of the consecutive equation which describes the relationship between the stress and the strain at high temperatures. The materials with the parameter m exceeding the value of 0.3 are considered to be superplastic 2. High carbon steels are commonly used in defense and automobile industries, but they are brittle and susceptible to cracking during solidification when joined using conventional welding methods. Diffusion bonding is an attractive joining technique for many advanced materials, particularly, when these are degraded by traditional fusion welding processes and when there is need for structural or thermal stability. In general, diffusion bonding is performed at relatively high temperatures of about 0.75T m (T m is the melting temperature of the material 3 ). The diffusion bonding process is analogous to that of pressure sintering. Derby and Wallach, 4 Guo 5 and Orhan 6 described the bonding process systematically and attributed the bonding process to several *For correspondence ( address: mtaskin@firat.edu.tr) mechanisms as follows: (i) surface and volume diffusion from surface sources, (ii) grain boundary diffusion to the voids in fine grained structures, (iii) diffusion along the bond interface from interfaced sources due to the stress gradient at the interface, (iv) power law creep deforming the ridge; and (v) plastic yielding deforming the ridge. Lu and Yun 7 used these mechanisms in their study on theoretical modelling of the transformation superplastic diffusion bonding of eutectoid steel. Orhan et al. 6 reported that the most effective mechanisms in void closure in diffusion bonding of fine-grained materials are grain boundary diffusion and power law creep. Lu and Yun 7 also given the similar conclusion. A very fine grain structure is beneficial in diffusion bonding because an increase in grain boundary area facilitates diffusion processes responsible for the removal of bond-line voids 8. Furthermore, the superplastic nature of some alloys has also been utilized as interlayers to join brittle materials by diffusion bonding The research literature indicates that work on the diffusion bonding of high carbon steels has focused mainly on the superplastic deformation and forming behaviour after bonding 9-12, but work has not been undertaken to investigate the factors affecting the mechanisms of bond formation in high carbon steels. Therefore in this study, considering that diffusion bonding would keep original superplastic structure identical during process, diffusion bonding behaviour of micro alloyed superplastic hypoeutectoid steels

2 TASKIN et al.: DIFFUSION BONDING OF HIGH CARBON STEELS 363 was examined by carrying out diffusion bonding with the advantage of fine grain microstructure and the resulting bonds were metallurgically and mechanically tested. With this study it was also intended to show the superiority of diffusion bonding on the conventional fusing welding methods in protecting the fine grained microstructure which is an asset for superplasticity. Materials and Methods Materials Two different steels (specimen 1 and specimen 2) were used for this study. The chemical compositions of the steels can be seen in Table 1. The steels were induction melted and cast into bars of 60 mm diameter and 200 mm in length. In order to achieve a fine-grained microstructure necessary for superplasticity, the specimens were subjected to thermo-mechanical processing by hot forging at 850 o C with a cross-sectional reduction of 94% in area and then annealed at 850 o C followed by quenching in oil. The specimens were tempered at 600 o C for 30 min and cooled to room temperature in a furnace. The tempered specimens were heat treated by holding at a temperature of 735 o C for 20 min and then quenched in oil at 200 o C. This last step was repeated 9 times and the final step involved rapid cooling in a jet of air. The fine-grained specimens were subjected to a tensile stress at elevated temperatures in order to determine the conditions for superplasticity (i.e. temperature and deformation rate). Superplastic properties were evaluated by determining the strain rate sensitivity parameter m in strain-rate change tests at different strain rates. The tensile tests were carried out using a Hounsfield tensile test machine equipped with a high temperature furnace. The specimens were subjected to tensile stresses at three different temperatures; 650, 735 and 850 o C, which corresponded to a temperature below, at and above the A1 temperature respectively. The specimens were subjected to six different cross-head speeds, 1, 2, 3, 5, 10 and 20 mm min -1. Once the maximum value for m was determined (i.e. the most suitable conditions for superplasticity) diffusion bonding was carried out using the temperatures at which max m values were obtained. Diffusion bonding experiments were performed using induction heating via water-cooled copper coils in a diffusion bonding apparatus using a vacuum. Surfaces prepared for diffusion bonding were polished using 1200 mesh emery paper and then cleaned ultrasonically in acetone. The dimensions of welded specimens are prepared diameter 12 mm and length 20 mm. The specimens were kept in a vacuum until the bonding stage. The experiments were performed under a bonding pressure of 2.5 MPa at superplastic deformation temperatures, namely 650, 735 and 850 o C (+8 o C) for 30 min in a vacuum of 10-4 torr. The heating rate was chosen as 50 o C min -1 and the cooling rate was approximately 100 o C min -1. Microstructural characterization of the thermomechanically processed steels was performed using light, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX). The diffusion bonded specimens were transversely cut through the joint region, and then polished and etched with 2% nital for metallographic examination. The average grain size, d, was estimated using the equation, d = 1.74 L, where L is the linear intercept size. The quality of the joints was assessed with respect to bond-line voids, grain size variations and microstructural changes within the bonded region. Shear tests were carried out in a screw driven Hounsfield model tension machine with an overlap of 3 mm at a cross-head speed of 5 mm/min at room temperature by adapting a shear test apparatus between the chucks of the machine to determine bond strengths. Results and Discussion It is well known that the diffusion bonding mechanisms increase when a fine grained alloy is bonded or when the material shows superplasticity 5-7. After the steels were subjected to heat treatment, SEM micrographs revealed that the smallest ferrite grains achieved in specimen 1 had average 7 μm in size see Fig. 1a, and specimen 2 had a grain size of 5 μm, as Table 1 Chemical compositions of the steels used in the experiments Alloying elements % Specimen No C Si Mn P S Cr Ni Al Cu Nb Ti V

3 364 INDIAN J ENG. MATER. SCI., AUGUST 2006 Fig. 1 SEM microstructures of the specimens after heat treatment cycle (a) shown in Fig. 1b. The steels possessed a characteristic structure of cementite network in a ferrite matrix. ASTM average grain size measurements indicated that the cementite had an average size of 1 μm in the specimen 1 and 2 after the thermo-mechanical processing. Considering the chemical compositions of specimens 1 and 2 it is possible to say that the amounts of carbide forming elements such as Nb, Ti and V in specimen 2 had a positive influence on the degree of grain refinement during the thermomechanical treatment. From EDX analyses of the thermo-mechanically processed specimens, it was seen that there were some impurity compounds and these were identified as FeMnS and (FeMnS) Al (see circled areas in Figs 1a and 1b). These compounds remain as solid inclusions within the metal and can be expected to behave as stress concentration points, which will have a detrimental affect on the superplastic behaviour of the steels. Fig. 2 Maximum m value range for the superplasticity test temperatures (a) A maximum value for the strain rate sensitivity exponent, m was recorded at 735 o C with a value of 0.46 in specimen 1 and 0.56 in specimen 2 (see Figs 2a and 2b). Surprisingly, the highest deformation rate range for superplasticity or maximum m was obtained in specimen 1 at 650 o C. This could be attributed to the existence of impurities in the specimen 2. This result suggests that superplastic behaviour is possible in a hypoeutectoid steel at a lower temperature such as 650 o C. For higher temperatures the superplasticity range is similar and satisfactory for these steels (Figs 2a and 2b) because materials which have an m value

4 TASKIN et al.: DIFFUSION BONDING OF HIGH CARBON STEELS 365 greater than 0.3 are generally accepted as showing superplastic characteristics 13. Previous workers have shown superplacticity at low strain rates of 10-3 s -1 and less in most superplastic materials Therefore, with the consideration to the m values obtained in this study, one would expect good quality diffusion bonds when joints are fabricated at 735 o C. This is because a high value for m will also indicate rapid diffusion dominated mechanisms such as power law creep and especially grain boundary diffusion 5-7. A comparison of grain sizes after the bonding process showed that for bonds made at a temperature of 650 o C, a grain size of 7 μm was obtained for specimen 1 and 5 μm for specimen 2. For bonds made at a temperature of 735 o C, the grain size was 10 μm for specimen 1 and 8 μm for specimen 2. Micrographs taken from the bond interface show that the best quality bonds were achieved when diffusion bonds were made at 735 o C, (see Figs 3-5). This particular temperature was also close to the α to γ phase transformation temperature (normally 723 o C). It is suggested that the steel was still ferritic and below the α to γ phase transformation temperature when diffusion bonding at 735 o C since there is not any microstructural changes observed in the specimens as can be seen clearly in Figs 3-5. However, when bonds were made at 850 o C the steel was austenitic and well above the α to γ phase transformation temperature. This change in phase transformation is expected to influence diffusion processes at the joint interface. The change in phase from α-iron to γ-iron corresponds to a change in crystal lattice from BCC to a more close packed FCC structure thereby decreasing diffusion rates and hence the rate at which bond-line voids are removed from the joint interface 17. Fig. 3 The interfaces of the specimens bonded at 650 C (a) Fig. 4 The interfaces of the specimens bonded at 735 C (a)

5 366 INDIAN J ENG. MATER. SCI., AUGUST 2006 Fig. 6 Fracture surfaces of the specimens bonded at 650 C (a) Fig. 5 The interfaces of the specimens bonded at 850 C (a) Microstructural examination also showed that diffusion bonds made at 650 o C and at 735 o C revealed a finer grain size in specimen 2 when compared with specimen 1. This was attributed to grain boundary pinning effect due to the formation of fine carbides from the alloying elements Nb, Ti and V. At a bonding temperature of 850 o C, the grain size of the steels increased considerably, the original microstructure changed completely due to the change in phase from α-iron to γ-iron and the material would not be expected to show superplastic behaviour in this state (see Fig. 5). This grain growth can be attributed to the fine carbides loosing the pinning effect due to Ostwald ripening 18. The fractured surfaces of the specimens bonded at 650 o C, are shown in Figs 6a and 6b. The fractographs reveal that the fracture mode was predominantly cleavage-type, which indicates the brittle nature of the fracture. Carbides formed probably provided an easy path for crack propagation because of cracking and debonding of the carbides occurring along the ferrite/carbide interface 19. Fracture surface of the specimen 2 (Fig. 6b) consists of some dimples and voids as well. This could be attributed to the smaller grains and therefore more ductile nature of the structure due to the alloying elements, Nb and Ti. It was determined from the microscopic examinations that total bonded area for all specimens was greater than 80%. The bond strengths measured by lap-shear tests were between 560 Nmm -2 and 600 Nmm -2 for all specimens and temperatures. Since the superplastic nature of the microstructures disapeared completely at 850 o C, fractographic examinations were not performed for these joints.

6 TASKIN et al.: DIFFUSION BONDING OF HIGH CARBON STEELS 367 Conclusions The results of this study show that specific thermomechanical heat treatments can be used to induce superplastic behaviour in high carbon steels. The presence of alloying elements such as Nb, Ti and V in small quantities has a positive influence on grain refinement during thermo-mechanical treatment, and inhibits grain growth above the A1 temperature and between A1 and A3 temperature range. When diffusion bonding is performed within the temperature range o C good joints were produced due to a combination of especially increased grain boundary diffusion and some amount of superplastic deformation at the interface. It is also suggested that the best quality joints were produced at a higher temperature of 735 o C, but bonds must be made with the steel in the ferritic state to save the fine grained microstructure. Diffusion bonds made above the α to γ phase transformation temperature showed grain growth, and the materials could not be expected to possess superplastic behaviour any longer. Acknowledgement This project was supported by Firat University Scientific Research Unit (FÜBAP) under the project no: 399 References 1 Kim Wo-Jin, Wolfenstein J, Ruano O A, Frommeyer G & Sherby O D, Metall Trans, 23A (1992) Rusz S, Lapkowski W, Sinczak J, & Boruta J, J Mater Process Technol, 60 (1996), Somekawa H, Watanabe H, Mukai T, & Higashi K., Scr Mater, 49(9) (2003) Derby B, & Wallach E.R., Mater Sci, 16 (1982) Ridley H, Guo Z X, Mater Sci Technol, 3 (1985) Orhan N, Aksoy M, & Eroglu M, Mater Sci Eng A, 271 (1999) Lu S H, & Yun J B, J Mater Process Technol, 129 (1-3) (2002) Shewmon Paul G, Diffusion in Solids, (McGraw-Hill, USA), 1963, Meahara Y, Komizo Y, & Langdon T G, Mater Sci Technol, 6-2 (1988) Yeh M S, & Chuang T H, Scr Metall, 33 (8) (1995) Orhan N, Khan T I, & Eroglu M, Scr Metall, Scr Metall, 45 (2001) Wu H Y, Lee S, & Wang J Y, J Mater Process Technol, 75 (1-3) (1988) Orhan N, & Kurt B, J Mater Process Technol, 136 (2003) Hernandez. D, Frommeyer G, & Hofmann H, Mater Technol, 67 (1996) Sherby D & Wadswort J, Proc 9th Int Metallurgy and Materials Congress, Turkey, Istanbul, 1998, Yoder G R, & Weiss V, Metal Trans, 3 (1972) Perevezentsev V N, Rybin V V & Chuvil deev V N, Acta Metal Mater, 40 (5) (1992) Kang M K, Kim D Y, & Hwang N M, Eur Ceram Soc, 22 (2002) Putatunda S K, Mater Sci Eng A, 297 (1-2) (2001)