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 based Superalloy containing , complex chemistry, it is difficult to achieve the desired and 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 . 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 and its distribution. But distinct treated at relatively higher temperature. 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 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 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. 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 When the annealing temperature is low (1398K), a Τ CT) Tf •

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 mobility. This can be achieved by heat Fig. 3: % Elongation of cold rolled sheets annealed at treatment cycle 1. different temperature

188 Gupta et al.

Fig. 4: Mechanical properties at 1253K of cold rolled sheets annealed at different temperature

Fig. 5: Optical photomicrograph of annealed samples corresponding to experimental cycle, a), cycle 1, b). cycle 2, c). cycle 3, d). cycle 4.

As the annealing temperature is raised without changing the soaking time, dissolution of grain boundary carbide is observed (Fig. 5 b-d). It is in line with earlier findings of Herchenvoeder and Ebihara /3/. However grain size also increases with increase in annealing temperature from 1398K to 1548K (Fig. 5 b- d). Grain size of the order of ASTM 5-6 obtained for the samples annealed through heat treatment cycle 1 (Fig. 5a) becomes ASTM 2-3 for the samples annealed at >1473K. The presence of annealing twins is also observed, which indicates prior mechanism of deformation and growth of grains by twining. There is a distinct observation in terms of room temperature elongation, which is marginally affected through change in annealing temperature. This striking revelation, that the grain size and carbide dissolution are not affecting the ductility of the alloy significantly, merits further studies. Several observations have been made by researchers in this field /9-11/. Wlodek observed that reduction of ductility is due to high temperature exposure between 923K and 1473K, which

results in precipitation of Co2W Laves phase in the grain boundaries 191. Similarly, also attributes the loss of ductility shown in their data to the

precipitation of the Co2W phase /10/. The presence of

precipitate of W3Co3C type can also affect the ductility, as observed by Jonathan et al. during high temperature exposure at 1073K and 1273K. for very long time /ll/.

Absence of Co2W as well as W3Co3C precipitates,

189 Vol. 27, No. 3, 2008 Effect of Microstructure and Mechanical Properties of Co-Cr- W-Ni Super alloy which normally originate with long time exposure as the high ductility of the alloy. Representative noted in this study, indicates less chance of reduction in fractographs of the high temperature tested specimen is ductility as observed in Fig. 3. shown in Fig. 6. Micro-dimples are also observed along Tensile properties at elevated temperature (1253K) the grain boundary apart from those noticed inside the are presented in Fig. 4. A marginal change in strength is grain. All these features support the results of observed. Elongation is found to be in the range 64- mechanical properties. 74%. It indicates effect of grain coarsening does not have significant effect on the high temperature tensile properties. Observation of low strength high elongation as compared to room temperature properties is also along the expected lines. Stress rupture properties for samples annealed at various temperatures are tabulated in Table 3. It is found that elongation at failure is almost consistent in the range of 18-25% and time to failure of specimen has a very wide range. It indicates that high temperature stress rupture properties at the nominal stress of 8 Kg/mm2 are not affected significantly by changing annealing temperatures and are more or less independent of grain size. All the above observations indicate that, though grain size increases with annealing temperature but mechanical properties are less affected in this alloy system. However, from cold workability point of view, desirable grain size along with desired mechanical properties are derived by optimized annealing cycle.

Table 3 High temperature (1223K) stress rupture properties of the various heat treatment cycles at stress of 8 Kg/mm2 Cycle Annealing cycle Propert ies No. Life in hrs. % El

1 1398K/30 min. 26-82 23-25 2 1448K/30 min. 38-78 12-21 3 1498K/30 min. 43-53 19-29 4 1598K/30 min. 37-54 18-23

The macroscopic fracture surface in both the tensile test (room temperature as well as high temperature) was at 45° to the tensile axis and fracture surface of stress rupture tested specimen showed presence of jagged edges or lips periphery to the specimen. Fractographic observation of tensile tested as well as stress rupture tested fractured specimen showed the presence of dimples throughout the fracture surface, which indicate

190 Gupta et al. High Temperature Materials and Processes

REFERENCES

1. J. R. Davis, Alloying: understanding the basics, ASM Hand book, ASM Int., Materials Park, Ohio, 2001: 540-549. 2. A. M. Beitran, Superalloys II-High temperature materials for aerospace and industrial power, edited by C. T. Sims, N. S. Stoloff and W C. Hagel, John Wiley & Sons, NY, 1987: 135-164. 3. R. B. Herchenvoeder and W. T. Ebihara, Engineering Quarterly, 313-324 (1969). 4. E. F. Bradley, Superalloy: a technical guide, ASM Fig. 6: Fractographs of the high temperature tested Int., Metals Park, Ohio (1988) 163-183. samples 5. R. K. Gupta, B. R. Ghosh, P. P. Sinha, Int. Conf. and Annual Technical Meeting, Indian Institute of Metals, Trivandrum, 17-19 Nov, 2004: 113-114. 4. CONCLUSIONS 6. G. E. Dieter, Mechanical , McGraw- Hill Book Company, London, 1988: 432-470. 1. Annealing temperature has a definite effect on the 7. M. J. Donachi Jr., Superalloy Source Book, ASM, grain coarsening and carbide dissolution of the alloy. Metals Park Ohio, 1984: 37-60. 2. Increase in grain size due to increase in annealing 8. N. Yukawa and K. Sato, Proc. Int. Conf. on temperature has a marginal effect on the change in strength of metals and alloys I (ICSMA I), Japan mechanical properties at room temperature as well Institute of Metals, Omachi, Japan, 1967: 680-686. as at high temperature. 9. S. T. Wlodek, Trans. ASM, 56, 287-303 (1963). 3. Although annealing temperature does not have a 10. Haynes International, http://www.haynesintl.com/ significant bearing on mechanical properties, except 25 alloy/H3057Cts.htm. that high temperature annealing leads to grain 11. J. Teague, E. Cerreta and M. Stout, Metall. Mater. coarsening, fine-grained structure by optimized Trans., 35A, 2767 (2004). annealing cycle is necessary from the process control point.

ACKNOWLEDGEMENTS

We are extremely thankful to M/S MIDHANI, Hyderabad, IFF, VSSC and MCD, VSSC for their cooperation during development of these heat treatment cycles and metallographic support for characterization of the alloy. We are thankful to Director, VSSC for permission to publish this work.

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