INFLUENCE OF AUSTEMPERING TIME ON THE MORPHOLOGY OF HEAT AFFECTED ZONE IN WELDING OF UPPER BAINITIC UNALLOYED AUSTEMPERED DUCTILE IRON

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1 INFLUENCE OF AUSTEMPERING TIME ON THE MORPHOLOGY OF HEAT AFFECTED ZONE IN WELDING OF UPPER BAINITIC UNALLOYED AUSTEMPERED DUCTILE IRON Dr. Ing. Cem AKÇA, Assoc.Prof.Dr.Ing. Ahmet KARAASLAN Yildiz Technical University Faculty: Chemical and Metallurgical Engineering, Department: Metallurgical and Materials Engineering Davutpasa Campus, Esenler, Istanbul - Turkey akca@yildiz.edu.tr Abstract Austempered ductile irons (ADI) could be produced with a wide range of mechanical properties depending on the austempering conditions. Upper bainitic ADIs exhibit advantages when compared to lower bainitic ADIs due to its bainite morphology, retained austenite content and transformability of retained austenite to martensite during service. In this study, unalloyed ferritic-pearlitic ductile iron specimens which contain 3.69% C, 2.47% Si and 0.2% Mn were austempered at 375 C to obtain upper bainitic microstructure for 20 and 100 min and then welded using Gas Tungsten Arc Welding (GTAW) without filler. The results have shown that upper bainitic ADI could be welded successfully without any crack formation in heat affected zone (HAZ). It was also observed that HAZ of specimen which is austempered for 20 min has greater hardness than those of 100 min due to high carbide content. Welding of ductile irons austempered longer times is preferable in gas tungsten arc welding due to homogeneous hardness distribution. 1. INTRODUCTION Austempered ductile iron offers the design engineer the best combination of low cost, design flexibility, good machinability, high strength-to-weight ratio and good toughness, wear resistance and fatigue strength. ADI offers this superior combination of properties because it can be cast like any other member of the Ductile Iron family, thus offering all the production advantages of a conventional Ductile Iron casting. Subsequently it is subjected to the austempering process to produce mechanical properties that are superior to conventional ductile iron, cast and forged aluminum and many cast and forged steels. Many applications of austempered ductile iron (ADI) have been reported since it offers a combination of high strength, toughness and good wear resistance with the low cost (OLIVERA, et al. 2006, SUN et al. 2005). ADI with an austenitic bainitic matrix is a new type of engineering material and has gained increasing interest in academic research and industrial applications due to its exceptional combination of tensile strength and ductility. Therefore, there is a great need for welding ADI parts during manufacturing (HAYRYNAN et al. 2006, DORAZIL 1986). The base iron chemistry and alloy additions to ductile iron play important roles in ADI technology. The addition of alloying elements during the production of ADI is often considerably higher than the levels used in the production of conventional grades of ductile irons (ROUNS et al. 1984, BAHMANI et al. 1997). In present study, ADI specimens austempered at 375 C for 20 and 100 min holding times were welded by GTAW without filler. Schematics of welding were given in Fig. 1. HAZ of upper bainitic specimens was investigated to characterize the effects of austenite content on mechanical properties of unalloyed welded ADI. Optical microscopy and image alaysis w used in microstructural observation. The hardness of specimens was determined by Vickers method. 1

2 2. EXPERIMENTAL PROCEDURE Specimens austenitized at 900 C (30 min) and austempered at 375 C (for 20 and 100 min; A and B) have been welded using GTAW to demonstrate the effect of (retained) austenite content morphology in HAZ. Chemical composition of specimens were given in Table 1. Table 1. Chemical composition of ductile iron specimens used in experiments. C Si Mn P S Mo Al Cu Mg Fe 3,69 2,47 0,20 0,014 0,008 0,001 0,014 0,09 0,030 rest Austempering was carried out in salt bath furnace. After austenitization of ductile iron specimens at 900 C temperature, austempering was carried out in neutral-atmosphere electric resistance furnace. Austempering conditions were given in Table 2. Table 2. Austempering parameters of test specimens. Austempering Temperature ( C) 375 Austempering Time (min) 20 (specimen A) 100 (specimen B) ADI specimen which upper (375 C) bainitic microstructures was welded using GTAW as illustrated in Figure 1. Dissimilar materials which have different austempering times (20 and 100 min) were welded. Figure 1 Gas tungsten arc welding method (GTAW) schematic [7]. During welding, 80 A current and 12 l/min Argon flow rate were used. After welding process, properties of heat affected zones were investigated using optical microscopy and Vickers hardness measurement. 2

3 3. RESULTS AND DISCUSSION 3.1 Mechanical Properties Mechnanical properties in HAZ of austempered specimens were determined using Vickers hardness distribution. Fig. 2 shows the hardness distributions of HAZ regions from specimen A through specimen B followed by GTAW. 600 Hardness (HV0.2) Distance (mm) Figure 2 Hardness distribution of weld region. Specimens austempered for 20 min and 100 min have different amounts of retained austenite due to first stage (γ γ h + α ) of austempering heat treatment. As the austempering time increases, the amount of retained austenite formed during first stage increases as well. Thus, change in austempering time results in different hardness values. As shown in Fig. 2, hardness of base metal austempered for 20 min is ~360 HV, base metal austempered for 100 min is ~330 HV. HAZ of specimen austempered for 20 min has relatively lower amount of austenite thus hardness of HAZ of specimens austempered for 20 min and 100 min have resulted in 410 HV and 470 HV respectively. Since filler rod was not used, weld seam has a 570 HV hardness due to rapid cooling of molten metal formed during welding. The hardness of weld seam was also affected by the carbon diffusion through the weld. 3.2 Microstructures The morphology of upper bainite bears a close resemblance to Widmanstätten ferrite, as it is composed of long ferrite laths free from internal precipitation. [6]. Fig 3a-b shows the microstructures of specimens after austempering. 3

4 METAL 2008 Figure 3 Microstructures of austempered specimens at 375 C for 20 min (a) and 100 min (b). Fig. 4a-b shows the microstructures of of specimens austempered for 20 and 100 min. Figure 4 Microstructures of HAZ-weld seam transition regions austempered at 375 C for 20 min (a) and 100 min (b). 4. CONCLUSIONS In this research, the effect of austempering time on the GTAW of ADIs has been investigated to observe microstructural and mechanical properties of HAZ. Results were given below: Microstructural investigation has shown an obvious difference in terms of retained austenite content (light regions in Figure 3a and 3b), Although both have an upper bainitic structure, hardness of HAZ regions of specimens was found as 360 HV and 330 HV respectively, depending on the austenite content, Gas tungsten arc welding method could have been applied successfully for both austempering conditions in unalloyed ADI, It can be recommended using a filler electrode bearing lower carbon to reduce the hardness of weld seam. 4

5 LITERATURE REFERENCES Olivera, E., Dragan, R., Slavica, Z., Leposava, S., Milan, T. J., Microstructure and fracture of alloyed austempered ductile iron, Materials Characterization 57 (2006) Sun, D.Q., Gu, X.Y., Liu, W.H., Xuan, Z.Z., Welding consumable research for austempered ductile iron (ADI), Materials Science and Engineering A 402, 2005, Hayrynan, K.L., Brandenberg, K.R., Keough, J.R., Applications of Austempered Cast Irons, AFS Transactions , 2002, Dorazil, E.,"Mechanical Properties of Austempered Ductile Iron", Foundry Management & Technology, July, 1986, Rouns, T.N., Rundman K.B., Moore D.M., On the structure and properties of austempered ductile cast iron., AFS Trans., 1984, 90: Bahmani, M., Elliot, R., Varahram, N., "Austempered Ductile Iron: a Competitive Alternative for Forged Induction-Hardened Steel Crankshafts", International Journal of Cast Metals Research, 1997, 9,