EFFECT OF VARIATION IN HOMOGENIZATION TREATMENT IN AN INVESTMENT-CAST CORROSION AND HEAT RESISTANT SUPERALLOY

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1 Journal of Materials Science and Engineering with Advanced Technology Volume 9, Number 2, 2014, Pages EFFECT OF VARIATION IN HOMOGENIZATION TREATMENT IN AN INVESTMENT-CAST CORROSION AND HEAT RESISTANT SUPERALLOY SWATI BISWAS 1, M. D. GANESHACHAR 1, B. R. SRIDHAR 2, V. N. SATISH KUMAR 1 and S. RAMACHANDRA 1 1 Gas Turbine Research Establishment Bangalore India swati@gtre.drdo.in 2 SEA College of Engineering and Technology Bangalore India Abstract Corrosion and heat resistant, precipitation hardenable superalloy components are widely used for high temperature applications. They exhibit excellent mechanical properties at elevated temperatures up to which the precipitates are stable. However, difficulties are encountered when used in cast form due to larger elements segregation leading to deleterious phase formation and reduced mechanical properties. Investment cast components manufactured from a corrosion and heat resistant nickel based superalloy Su 718 were subjected to solution treatment and aging and were planned to be used in gas turbine engines. However, hardness on the samples was found to be less than the minimum requirement. Homogenization treatment was introduced for the components before subjecting it to solution treatment and aging. Higher homogenization treatment was found to be more effective in obtaining hardness almost comparable to the wrought product after the final heat treatment. In order to understand the effect of homogenization treatment on the microstructural changes in the material, scanning electron microscopic (SEM) Keywords and phrases: superalloy, casting, segregation, homogenization, solution treatment, aging, hardness. Received April 30, Scientific Advances Publishers

2 110 SWATI BISWAS et al. studies were carried out on the samples in as-cast condition and after homogenization treatments. Back scattered electron (BSE) imaging indicated presence of large compositional heterogeneity in the inter-dendritic region in ascast condition. X-ray dot mapping and X-ray line scanning revealed niobium (Nb) and molybdenum (Mo) segregation in those regions. Homogenization treatment was carried out to have uniform distribution of Nb and Mo. Heterogeneity was found to reduce with increasing homogenization temperature. 1. Introduction Precipitation hardenable nickel (Ni) based superalloys are suitable for elevated temperature application because of their capability to retain strength close to its melting temperature [1]. Their extensive usage includes components of aircraft, automotive, land-based power systems, nuclear power plants, and space-ships [1, 2]. The underlying physical metallurgy of these classes of alloys that enables them to be used for high temperature application, has been discussed extensively in literature [3-6]. Su 718 is a nickel-chromium-iron base superalloy that has been used in elevated temperature applications up to 650 C [7]. The alloy is used in both wrought and cast form in gas turbine engines [8-10]. The performance of the investment cast components to the optimum level is dependent on the microstructural constituent as the material is highly susceptible to inter-dendritic segregation of elements like niobium (Nb) and molybdenum (Mo) leading to deleterious laves phase formation [9, 11-13]. Homogenization at a suitably higher temperature [13, 14] (above 1150 C) helps in reducing the segregation as this results in uniform distribution of the above elements. Present work describes the identification of the inter-dendritic elemental segregation in the investment cast Su 718 material and effect of two different homogenization temperatures in bringing down the heterogeneity. While manufacturing investment cast Su 718 components for a gas turbine engine, the parts were subjected to solution treatment and aging, the standard practice followed for parts fabricated from wrought form. However, the final hardness achieved was less than the minimum requirement (minimum 36Rc). Therefore, homogenization treatment was introduced after casting to improve the hardness.

3 EFFECT OF VARIATION IN HOMOGENIZATION Experimental Details Samples were prepared from investment cast Su 718 material. Nominal composition of the material is presented in Table 1. Microstructural studies using scanning electron microscope (SEM) and micro-hardness survey (using 300g load and 10 seconds dwell time) was carried out on the metallographically prepared in as-cast condition sample. Elemental distribution was also recorded. Subsequently, samples were subjected to various combinations of heat treatments. This included homogenization at various temperatures followed by solution treatment (ST) and aging (A). Few samples were also solution treated and aged directly from as-cast condition. The details of the heat treatment conditions of various samples are listed in Table 2. Macro & micro hardness and microstructural studies were carried out on all these samples for comparison. Samples were also obtained from wrought form of Su 718 material in ST and solution treated and aged (STA) condition to compare the hardness variation with respect to the cast samples. Table 1. Nominal composition of the material [14] Elements Weight % Chromium Nickel Molybdenum Niobium Titanium Aluminum Cobalt Iron 1 (max) Balance

4 112 SWATI BISWAS et al. Table 2. Heat treatment details of the samples Sl. No. Sample ID Heat treatment condition As-cast As-cast + ST As-cast + STA As-cast + homogenization at 1150 C for 15 minutes followed by gas fan quench (GFQ) As-cast + homogenization at 1150 C followed by ST As-cast + homogenization at 1150 C followed by STA As-cast + homogenization at 1200 C for 15 minutes followed by GFQ As-cast + homogenization at 1200 C followed by ST As-cast + homogenization at 1200 C followed by STA Wrought (ST) Wrought (STA) Solution treatment was carried out at 980 C for 15 minutes in vacuum furnace followed by gas fan quench (GFQ). This was followed by aging treatment with a stepped cycle: i.e., heating to 720 C in a vacuum furnace, holding at the temperature for 8 hours followed by furnace cooling to 620 C and holding at 620 C for 8 hours followed by GFQ. 3. Results and Discussion 3.1. Hardness Macro-hardness results Macro-hardness measurements were carried out on all the samples using Rockwell hardness tester (C scale). The as-cast sample (sample ID 1.1) hardness was around 20Rc. No significant increase in hardness was observed after subjecting the samples to homogenization and solution treatments (sample IDs 1.2, 2.1, 2.2, 3.1, and 3.2). However, aging treatment was effective in increasing the hardness level of the samples (1.3, 2.3, and 3.3). The wrought alloy hardness was found to be 25Rc and 44Rc in ST and STA condition, respectively. The hardness values of the samples in different heat treatment conditions are presented in Table 3.

5 EFFECT OF VARIATION IN HOMOGENIZATION 113 Table 3. Rockwell hardness results Sample Identification Rockwell Hardness Value Rc Rc Rc Rc Rc Rc Rc Rc Rc Rc Rc Micro-hardness survey All the samples were prepared using metallographic technique and micro-hardness measurements were carried out at an interval of 200 micrometer distance to check the hardness variation in the samples. Results exhibited similar trends as macro-hardness measurements and can be observed in micro-hardness profile presented in Figures 1(a)-1(e). As-cast sample (ID 1.1) showed hardness variation in the range from 187 to 256 Vickers Hardness Number (VHN) (Figure 1(a)). Homogenization treatment at 1150 C temperature was found to yield hardness (sample ID 2.1) in the similar range ( VHN) (Figure 1(b)). However, 1200 C homogenization treatment (sample ID 3.1) revealed narrower band of hardness ( VHN) (Figure 1(c)). These results indicated a higher degree of casting homogeneity after 1200 C heat treatment. Solution treatment of as-cast sample (sample 1.2) and 1150 C homogenized sample (sample 2.2) was not effective in altering the hardness ( VHN (sample ID: 1.2, Figure 1(a)) and VHN (sample ID: 2.2, Figure 1(b)), respectively) significantly. On the contrary, a marginal improvement in hardness ( VHN, Figure 1(c)) was observed after

6 114 SWATI BISWAS et al. solution treatment on the sample (ID: 3.2), which was earlier homogenized at 1200 C. The wrought sample (sample 4.2) hardness was found to be in the range of VHN, (Figure 1(d)). Presence of more homogeneous structure in the wrought sample against the cast one appeared to be the reason for more or less uniform hardness profile. Aging treatment resulted in significant increase in hardness in all the samples owing to the precipitation of γ [ ( Al, Ti)] Ni 3 and γ [ Nb] phases. However, as-cast sample hardness was lowest after solution treatment and aging (sample ID: 1.3) and the corresponding variation in the hardness was also quite significant ( VHN) (Figure 1(a)). Sample 2.3 that was homogenized at 1150 C followed by solution treatment and aging exhibited higher hardness ( VHN) (Figure 1(b)) compared to the as-cast sample. But variation in hardness in this case also was high (~ 100 VHN). High hardness variation indicates large microstructural variations in the material and is unsuitable for structural applications. Sample 3.3 (homogenized at 1200 C, followed by solution treatment and aging) showed lesser hardness variation ( VHN) (Figure 1(c)) compared to sample 1.3 and 2.3. Overall hardness of this sample (3.3) was also higher compared to sample 1.3 and 2.3. The wrought sample showed highest hardness amongst all the samples ( VHN) with very less variation (Figure 1(d)). It may be noted that sample 3.3 that was homogenized at 1200 C followed by solution treatment and aging achieved hardness, which is almost comparable with the wrought sample (4.3) in similar heat treatment condition. The hardness variation in all the samples in STA condition against the as-cast sample is presented in Figure 1(e) for comparison. Ni 3

7 EFFECT OF VARIATION IN HOMOGENIZATION 115 Figure 1(a). Variation in micro-hardness in as-cast sample (1.1), in as-cast sample after solution treatment (1.2), and in as-cast sample after solution treatment and aging (1.3). Figure 1(b). Variation in micro-hardness in cast sample homogenized at 1150 C (2.1), in sample after homogenization at 1150 C followed by solution treatment (2.2), and in sample after homogenization at 1150 C followed by solution treatment and aging (2.3).

8 116 SWATI BISWAS et al. Figure 1(c). Variation in micro-hardness in cast sample homogenized at 1200 C (3.1), in sample after homogenization at 1200 C followed by solution treatment (3.2), and in sample after homogenization at 1200 C followed by solution treatment and aging (3.3). Figure 1(d). Variation in micro-hardness in wrought sample in ST and STA condition.

9 EFFECT OF VARIATION IN HOMOGENIZATION 117 Figure 1(e). Micro-hardness variation in cast and wrought samples in STA condition against the as-cast sample Scanning electron microscopy Back scattered electron imaging In order to understand the variation of hardness in as-cast sample vis a vis the homogenized samples, sample 1.1, 2.1, and 3.1 were studied under scanning electron microscope. Sample 1.1 (as-cast) revealed presence of interconnected bright phases (Figure 2). The structure was more or less identical for sample 2.1 (homogenized at 1150 C) (Figure 3) except that the continuity of the bright phase was slightly reduced. However, sample 3.1 (homogenized at 1200 C) exhibited isolated bright phase (Figure 4) unlike sample 1.1 and 2.1. To analyze the elemental distribution in these samples, X-ray dot mapping and X-ray line scanning were carried out. The results obtained are described in subsequent paragraphs.

10 118 SWATI BISWAS et al. Figure 2. Back-scattered electron image of sample 1.1 (as-cast). Figure 3. Back scattered electron image of sample 2.1 (homogenized at 1150 C).

11 EFFECT OF VARIATION IN HOMOGENIZATION 119 Figure 4. Back scattered electron image of sample 3.1 (homogenized at 1200 C) X-ray dot mapping and X-ray line scanning Back scattered electron (BSE) images and corresponding X-ray dot mapping revealed interconnected Nb and Mo concentration in as cast condition (Figure 5). Nb and Mo concentration was found to reduce after homogenization treatment. Homogenization at 1150 C (Figure 6) was found to be less effective compared to that carried out at 1200 C (Figure 7) temperature as the sample homogenized at 1200 C revealed isolated Nb and Mo concentration, whereas the one homogenized at 1150 C indicated presence of almost continuous network of Nb and Mo. The X-ray line scan results were also confirmatory to the X-ray dot mapping observations. Frequent Nb and Mo peaks were encountered (Figure 8) in sample 1.1 indicating their segregation in the cast sample. However, their occurrence was found to reduce after homogenization treatment at 1150 C (Figure 9). Very less number of Nb and Mo peaks were identified in sample 3.1 (Figure 10) after 1200 C homogenization treatment, which indicated that the higher homogenization treatment was effective in reducing the elemental segregation in the cast sample.

12 120 SWATI BISWAS et al. Figure 5. BSE images with corresponding X-ray dot mapping of Mo and Nb in as-cast sample.

13 EFFECT OF VARIATION IN HOMOGENIZATION 121 Figure 6. BSE images with corresponding X-ray dot mapping of Mo and Nb in sample 2.1.

14 122 SWATI BISWAS et al. Figure 7. BSE images with corresponding X-ray dot mapping of Mo and Nb in sample 3.1.

15 EFFECT OF VARIATION IN HOMOGENIZATION 123

16 124 SWATI BISWAS et al. Figure 8. X-ray line scanning result on sample 1.1: (a) Nb profile; (b) Mo profile; (c) secondary electron image; and (d) overall elemental profile.

17 EFFECT OF VARIATION IN HOMOGENIZATION 125

18 126 SWATI BISWAS et al. Figure 9. X-ray line scanning result on sample 2.1: (a) Nb profile; (b) Mo profile; (c) secondary electron image; and (d) overall elemental profile.

19 EFFECT OF VARIATION IN HOMOGENIZATION 127

20 128 SWATI BISWAS et al. Figure 10. X-ray line scanning result on sample 3.1: (a) Nb profile; (b) Mo profile; (c) secondary electron image; and (d) overall elemental profile.

21 EFFECT OF VARIATION IN HOMOGENIZATION Analysis Segregation of niobium (Nb) and molybdenum (Mo) in the interdendritic regions is likely to lead to laves phase formation and consequent mechanical property deterioration [10-12]. The resultant decrease in Nb content in the matrix leads to reduced γ precipitates content, which is the major contributing strengthening phase in Su 718 alloy [15]. Therefore, the lesser hardness attained after solution treatment and aging in case of sample 1.3 (as-cast + STA) may be attributed to this reduced γ precipitates. High variation in hardness across measurement distance also points towards inhomogeneous distribution of the hardening phase. As the homogenization treatment was carried out, the elemental segregation was reduced and hence this resulted in more uniform hardness distribution as well as an increase in the hardness value (for sample 2.3 and 3.3). The hardness value of the sample 3.3 (homogenized at 1200 C + STA) was closely comparable to the wrought alloy. Therefore, it can be stated that the homogenization treatment at 1200 C temperature was successful in reducing the elemental segregation in the casting and achieving better mechanical properties in the alloy Su Conclusion Investment cast Su 718 suffers from the problem of segregation of Nb and Mo. This results in reduced hardness of the material if the alloy is directly solution treated and aged without homogenization treatment. However, homogenization treatment can reduce the elemental segregation and helps in improving the hardness in aged condition. Homogenization at 1200 C was effective in removal of the casting heterogeneity and attaining a hardness comparable to its wrought form.

22 130 SWATI BISWAS et al. Acknowledgement The authors express their gratitude to the Defence Research and Development Organization for the support to carry out this work. The authors are thankful to Director, GTRE for permitting to publish these results. Help extended by Dr. Satyapal Singh, Scientist, DMRL by providing the material is gratefully acknowledged. The support extended by the members of Heat Treatment Facility of Prototype Fabrication Group in carrying out heat treatment of the samples is also acknowledged. References [1] Metallurgy, Processing and Properties of Superalloys, Heat Resistant Materials, ASM Specialty Handbook, ASM International, Edited by J. R. Davis, p [2] F. Decker, Strengthening mechanisms in nickel-base superalloys, Steel Strengthening Mechanisms Symposium, Zurich, Switzerland, (1969), [3] The Superalloys, Edited by C. T. Sims and W. C. Hagel, John Wiley & Sons, 1972, p. 3. [4] R. H. Kane, The Evolution of High Temperature Alloys: A Designers Perspective, Heat-Resistant Materials, Proceedings of the First International Conference, Florida, Sisconsin, USA/23-26 September, 1991, pg 1 edited by K. Natesan and D. J. Tillack. [5] Gary L. Ericson, Cannon-Muskegon Corporation, Polycrystalline Cast Superalloys, ASM Handbook, Vol. 1, p [6] Tresa M. Pollock and Sammy Tin, Nickel-based superalloys for advanced turbine engines: Chemistry, microstructure, and properties, Journal of Propulsion and Power 22(2) (2006), 361. [7] J. W. Brooks and P. J. Bridges, Metallurgical Stability of Inconel Alloy 718, Superalloys, Edited by S. Reichman, D. N. Dulhl, G. Maurer, S. Antolovich and C. Lund, (1988), 33, in English. [8] Mathew J. Donachie and Stephen J. Donachie, Superalloys, A Technical Guide, 2nd Edition, ASM International, Materials Park, OH, 2002, p 1-9. [9] X. Huang and M. C. Chaturvedi, An investigation of microstrcuture and HAZ microfissuring of cast alloy Su 718, superalloys 718, 625, 706 and various derivatives, Edited by E. A. Loria, The Minerals, Metals & Materials Society (1994),

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