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Effect of Microsctucture on Mechanical Properties of Co-Cr-W-Ni Superalloy

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Effect of Microsctucture on Mechanical Properties of Co-Cr-W-Ni Superalloy Effect of Microsctucture on Mechanical Properties of Co-Cr-W-Ni Superalloy R. K. Gupta3 ', Μ. K. Karthikeyan3, D. N. Bhatiab, B. R. Ghosh3, P. P. Sinha3 3Mechanical Engineering Entity, Vikram Sarabhai Space Centre, Trivandrum, India bMishra Dhatu Nigam Ltd., Kanchanbagh P.O., Hyderabad, India (Received September 9, 2008; final form September 19, 2008) ABSTRACT nature due to its chemical composition, exhibits very good cold formability and weldability. However, due to Cobalt based Superalloy containing Tungsten, complex chemistry, it is difficult to achieve the desired Nickel and Chromium as alloying elements, is combination of UTS, 0.2% YS, % Ε and grain size, each extensively used in fabrication of high temperature of which is very important for fabrication and components of launch vehicle systems. Microstructural performance of the components made out of this alloy. variation with respect to grain size has an important role After a careful correlation between the degree of to play in mechanical properties of this alloy. mechanical working and response to annealing Microstructural changes were incorporated by varying parameters, the annealing cycle could be optimized. the heat treatment temperature and effect of such The nature and morphology of phases within the changes on the mechanical properties at room alloy structure determine the mechanical properties and temperature as well as at high temperature is microstructural stability. These are governed mainly by investigated. Increase in grain size and decrease in grain size, and other metallographic features, like the strength and ductility is observed for the alloy heat presence of carbides and its distribution. But distinct treated at relatively higher temperature. carbide stringers, if present, contribute adversely during downstream processing like rolling, forming etc. as well Keywords: Co based Superalloy, M23C6 type carbide, as in service. Close control of chemical composition by grain, heat treatment selection of optimum melting process is quite essential to minimize or eliminate these detrimental micro- structural constituents. Strong carbide formers like 1. INTRODUCTION chromium and tungsten are used in the alloy, which also plays a major role in solid solution strengthening and in The Cobalt based super alloys, which are essentially improvement of corrosion and oxidation resistance of used for high temperature applications (close to 1373K), the alloy. Tungsten also provides high temperature gain strength by combination of solid solution strength to the alloy. hardening, carbide precipitation and mechanical Workability of the alloy is improved by addition of working. The microstructure of Co based alloys is a nickel, which stabilises austenitic (fee) phase by function of chemical composition, crystallographic suppressing the transformation to hep at low phases and thermo mechanical processing history. The temperatures /1, 21. Workability of the alloy is also alloy under present study, although very complex in improved by dissolving the carbides, mostly M23C6type, ' Corresponding author: Tel:+91-471-2562484; Fax: +91-471-2705427; [email protected] 185 Vol. 27. No. 3. 2008 Effect of Microstructure and Mechanical Properties of Co-Cr- W-Ni Super alloy through high temperature soaking at temperature greater environment under thermal loading for short durations, than 1423K during hot working. But grain coarsening is thereby stress rupture property of the alloy becomes rather faster and formation of embrittling 'Laves quite relevant. Considering the above points, a phases' (Co2W) takes place at this temperature range. systematic heat treatment experimental cycle was Maximum dissolution of secondary phases with planned and mechanical properties at room temperature complete recrystallization and control of the grain as well as at high temperatures were evaluated, with growth are two opposite requirements, which make it all detailed microstructural studies. The paper presents the the more difficult to select time and temperature for hot effect of grain size of cold rolled and annealed 2 mm working and annealing /3, 4/. Optimum parameters of thick sheets on both room temperature and high annealing are therefore important to obtain the desired temperature properties. microstructure and required properties for both forged and rolled products. The amount of prior mechanical working before annealing has a very important role to 2. EXPERIMENTAL play in determining the overall response of the alloy to obtain desired properties. Considering the above, The alloy under study was melted through Vacuum optimum annealing cycle was formulated 151. But, due Induction Melting (VIM) process. The primary ingots of to high temperature application of this alloy, it is VIM were refined through the 'Electro Slag Refining' essential to understand the effect of coarse/fine-grain (ESR) process. Ingots were then forged into slabs and microstructure on mechanical properties, which has not subsequently hot rolled to 6-7 mm thick plates. been properly documented. Ultrasonic inspection was carried out at semi stage and Creep properties of most of the alloys are found to in the 6 mm plate stage. Cold rolling was carried out on be better for coarse grained materials 161 due to less 6 mm thick plate to obtain sheets of 2 mm thickness. grain boundary sliding. Most of the aerospace The chemical composition of the alloy is presented in components are subjected to the high temperature Table 1. Table 1 Chemical composition of the alloy Elements C Mn Si S Ρ Cr Ni W Fe Co Wt. % 0.07 1.26 0.1 0.001 0.004 19.37 10.81 14.36 1.3 Balance A full annealing treatment with varying was carried out during entire experimental trials to get temperatures and fixed time followed by forced air- the benefit of high temperature fee structure of the alloy. cooling was carried out to generate test coupons with varying grain size. Soaking temperatures and holding Table 2 time for experimental trials are as given in Table 2. Heat treatment cycle selected for hot rolled Cold rolled sheets of 2 mm thickness were cut and and cold rolled sheets coupons were made for heat treatment. Sample coupons containing material of tensile test specimens, stress SI. No. Soaking Time rupture specimens and high temperature test specimens Temperature, Κ were heat-treated adopting the four different cycles as 1 1398 30 min. mentioned in Table 2. The heat treatments were carried 2 1448 30 min. out on coupons of size 40x250x2 mm. and 20x20x2 mm 3 1498 30 min. using air circulating muffle furnace. Forced air-cooling 4 1548 30 min 186 Gupta et at. High Temperature Materials and Processes Hardness measurements were carried out using using an Olympus make optical microscope. 4% H2S04 Vickers Hardness Tester with a load of 187.5 kg. was used as etching reagent and an electrolytic etching Mechanical properties were evaluated using INSTRON process was adopted. Standard tensile test specimens 4206 Universal Tensile testing machine. High and stress rupture test specimens were fabricated (Fig. temperature tensile testing was carried out using an 1) using CNC wire cut machine. Tensile test were INSTAR make tensile testing machine. High carried out both at room temperature and at 1253K. temperature stress rupture test was carried out at 1223K Fractured surfaces of the tested specimens were with a constant stress of 8 Kg/mm2 using an Ada Mel carefully cut through diamond cutter and fractographs Lhomargy machine. Optical metallographic investi- were studied using an XL-30 Phillips make Scanning gations were conducted on polished and etched samples Electron Microscope (SEM). 50.0 100.0 so.o 2.0 ö i/i rv, (a) 115 R25 J / in e -Θ- m Γ 10.0 V 40.0 20 3S.0 20. 40,0 Thick : 2 * Gauge width :10±0.1 on form -0.04 (b) 187 Vol. 27, No. 3, 2008 Effect of Micros ι ructure and Mechanical Properties of Co-Cr-W-Ni Super alloy 17, 17. R25 I J ο Ο r^i 10.0 35 . 25 65 J _J Thick : 2 Gauge width : 10±0.1 on form -0.04 (c) Fig. 1: a. Room temperature tensile test specimen, b. High Temperature tensile specimen, c. Stress rupture test specimen (All the dimensions are in mm) 3. RESULTS AND DISCUSSION g 1100 9 1000 - ω 900 • Hardness, Variation in mechanical properties with temperature g- 800 BHN 2 700 is graphically presented in Figs. 2-4. Tensile strength at α 600 —•—UTS, MPa "<5 500 4- room temperature decreases as the annealing 400 Λ —m η 300 —A— YS, MPa temperature increases, which is due to increase in grain 5 200 size and dissolution of carbide precipitates of the alloy. g 100 S 0 -J ι ι 1 ' l Τ 8 When the annealing temperature is low (1398K), a 8 CT) Tf •<r m greater amount of carbide precipitate will be present, 139 144 Temperature, Κ which retains the strength and also restricts the grain growth significantly by pinning down the boundaries. Fig. 2: Mechanical properties (room temperature) of Microstructural studies substantiate the above findings. cold rolled sheets annealed at different Grain size is found to be in the range of ASTM 5-6 for temperature samples annealed at 1398K (Fig. 5a). The presence of thin carbides along the grain boundary is observed in Fig. 5 a, which has been identified as of M23C6 phase 111. Laves phase was not observed in this study. Herchenvoeder and Ebihara also reported similarly when Si is low in the material /3/. Similar observations - % Elongation were made by Yukawa and Sato, i.e. that Co2W laves phase appears after high temperature exposure at 1323 Κ for longer duration (10 hrs), whereas M23C6 and M6C 1398 1448 1498 1548 type carbides develop very quickly (less than 2 hrs) /8/. Anealed Temperatures, M23C6 provides strong inhibition to grain boundary Κ sliding and grain growth as well as impeding the dislocation mobility. This can be achieved by heat Fig. 3: % Elongation of cold rolled sheets annealed at treatment cycle 1.
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