CHAPTER III EXPERIMENTAL PROCEDURES
3.5 SEM Observation
In this technique the surface of a small sample of Inconel 718 superalloy is first prepared through a detailed and rather lengthy procedure. The preparation process includes numerous surface grinding stages (usually eight) that remove large scratches and thin plastically deformed layers from the surface of the specimen. The grinding stage is followed with a number of polishing stages (usually eight: Sequentially from several numbers sandpaper 240, 400, 600, 800, 1000, 1200 number, 2000 number, and finally grinding to a maximum of 2500 number) that remove fine scratches formed during the grinding stage. The quality of the surface is extremely important in the outcome of the process, and generally speaking, a smooth, mirror-like surface without scratches must be produced at the end of the polishing stage to complete the polishing process. These steps are necessary to minimize topographic contrast. The polished surface is then exposed to chemical etchants. The choice of the etchant and the etching time (the time interval in which the sample will remain in contact with the etchant) are two critical factors that depend on the specific material under study, this experiment using the etching solution formula (50% HCl, 10% HNO3, 2% HF, 38% distill water) for 1-2 minutes. The atoms at the grain boundary will be attacked at a much more rapid rate by the etchant than those atoms inside the grain. This is because the atoms at the grain boundary possess a higher state of energy because of the less efficient packing. As a result, the etchant produces tiny groves along the boundaries of the grains.
A Hitachi-4700 field-emission scanning electron microscope equipped with a Horiba energy-dispersive X-ray spectrometry (EDX) system was used to examine the microstructure as shown in Figure 3-4.
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3.6 High Temperature Creep Test
In order to carry out high-temperature creep experiments, stress rupture tests were performed using an ATS (Applied test systems, Inc) series 2330 lever arm creep tester as shown in Figure 3-5. Test temperature and stress were 650ºC and 625 MPa, respectively.
According to the dimensions of ASTM E8[31] round bar specimens with gage length 1.0 inch (25.4 mm) were machined to the diameter of 0.25 inch (6.35 mm) as shown in Figure 3-6.
The role of the test is to balance the principle of leverage in order to achieve a fixed stress effect. The high-temperature furnace is a three-point thermocouple set up on the gauge length of the trial bar to precisely control the temperature, with a temperature error range of ±0.1ºC.
17
Table 3-1: Chemical compositions of recast Inconel 718 (wt.%).
C
Table 3-2: Chemical compositions of forged Inconel 718 (wt.%).
C
18
Figure 3-1: the TTT diagram of Inconel 718[27].
Figure 3-2: Heat treatment scheme.
19
Figure 3-3: Heat treatment furnace.
Figure 3-4: Photograph of a scanning electron microscope.
20
Figure 3-5: ATS series 2330 lever arm creep tester.
Figure 3-6: Creep test specimen specifications.
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CHAPTER IV RESULTS AND DISCUSSTIONS
4.1 OM Observation
The average grain size of recast Inconel 718 superalloy is much greater than that of forged one for about 20 times as shown in table 4-1. The huge grain size of recast Inconel 718 superalloy supposedly has a beneficial effect on the creep behavior. The cross sectional image of recast Inconel 718 superalloy is shown in Figure 4-1. Using the optical microscope (OM) to observe the microstructure, a large number of porosities are found in the recast Inconel 718 superalloy specimens as shown in Figure 4-2.
4.2 SEM observation and EDX
Typical SEM micrographs showing carbides of as-received material of recast and forged Inconel 718 are shown in Figures 4-3 and 4-4[28], respectively. EDX analyzer system is used to identify composition elements of the carbide precipitates as shown in Figures 4-5 and 4-6[28]. EDX results identify that NbC type carbide for both recast and forged Inconel 718 superalloys as shown in table 4-2. Using SEM to observe the microstructure of as-received materials, a large number of porosities are found in the recast Inconel 718 superalloy specimens as shown in Figure 4-7.
After heat treatment, the average grain size of both recast and forged Inconel 718 is growing and most of the carbides are dissolved back to the matrix, however, there are still some left as shown in Figures 4-8 and 4-9[28]. The energy dispersive X-ray analyzer system is used to identify composition elements as shown in Figures 4-10 and 4-11[28]. EDX results identify that NbC type carbide precipitate for both recast and forged Inconel 718 superalloys as shown in table 4-3. Some of the carbides are located inside the grain and along the grain boundary of recast Inconel 718 superalloy. Carbide morphology is very long, which along the grain boundary has harmful effect on the mechanical properties. After heat treatment, a large number of porosities are also found in the recast Inconel 718 superalloy specimens as shown in Figure 4-12.
4.3 Rockwell Hardness Measurement
In the present study, HRC specifications are chosen for Rockwell Hardness test. Nine hardness measurements located at specific positions as shown in Figure 4-13 are used to obtain the average hardness value. After heat treatment the average hardness value of recast Inconel 718 superalloy is 37.75 HRC as shown in table 4-4, which is similar to the forged Inconel 718 superalloy as shown in table 4-5. Hardness value is mainly due to both γ′ and γ″
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precipitates. The amounts are γ″ and γ′ precipitates, together with their shape and distribution, have a determining influence on the mechanical properties.
4.4 Porosity
Using the OM/ SEM to observe the microstructure, a large number of porosities are found in the recast Inconel 718 superalloy specimens as shown in Figures 4-2, 4-7, 4-12 and 4-15. Average porosity size and percentage are 24.7 μm and 2.683%, respectively, as shown in table 4.6. The average porosity size can be classified as meso-porosity. The percentage porosity is very high that has mainly harmful effects on creep behavior.
4.5 High Temperature Creep Behavior
After the same heat treatment, the creep curves between two kinds of Inconel 718 are shown in Figure 4-14. The creep rupture life time of forged Inconel 718 is (about 1800 hours) much longer than that of recast Inconel 718 (about 120 hours). SEM fractographs of crept specimens are shown in Figures 4-15 and 4-16 for recast and forged specimens, respectively.
Since the average porosity size and percentage are very high in the recast Inconel 718 specimens, porosity is the main reason to lead to premature creep rupture failure. Fracture of a recast Inconel 718 crept specimen by rapid crack propagation through the porosities is shown in Figure 4-17. Conversely, because the grain boundaries are the weak areas in the forged Inconel 718 superalloy, SEM fractographs indicate inter-granular fracture pattern as shown in Figure 4-16. The inter-granular facture of a forged Inconel 718 crept specimen occurs by slow crack propagation as shown in Figure 4-18.
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Table 4-1: The average grain size.
Materials Average Grain size (µm) Inconel 718 (Recast) 3392±68
Inconel 718 (Forged) 170±12.88
Table 4-2: Composition elements of carbide by EDX for as-received materials (Atomic %).
Element Recast Inconel 718 Forged Inconel 718
C 49.74 55.97 Totals 100.00 100.00
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Table 4-3: Composition elements of carbide by EDX after heat treatment (Atomic %).
Element Recast Inconel 718 Forged Inconel 718
C 49.43 60.83 Al -0.15 -0.09 Ti 6.33 5.49
V 0.08 -
Mn 0/02 -
Fe 0.42 0.37
Co -0.58 -
Ni 2.17 1.36
Cu -0.35 -
Nb 42.85 30.38 Mo -0.02 0.42
W -0.21 -
Si - 0.17 Cr - 0.63 Totals 100.00 100.00
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Table 4-4: Hardness values of recast Inconel 718 after heat treatment at nine test locations (HRC).
Locations Average Hardness Value
A 36.4
Table 4-5: Rockwell hardness value for two kinds of Inconel 718 superalloys (HRC).
Materials Average Hardness Value Recast Inconel 718 37.75
Forged Inconel 718 38.72
Table 4-6: Average porosity size and percentage of recast Inconel 718.
Process Average porosity size (µm)
Percentage porosity (%)
Recast 24.7 2.683
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Figure 4-1: Cross sectional view of a recast Inconel 718 specimen after heat treatment.
27 (a)
(b)
Figure 4-2: Optical Microscope pictures showing porosities of a recast Inconel 718 specimen, (a) 200X, (b) 500X.
28
Figure 4-3: SEM image showing carbides of as-received recast Inconel 718.
Figure 4-4: SEM image showing carbides of as-received forged Inconel 718[28].
29
Figure 4-5: SEM image with EDX position showing carbides of as-received recast Inconel 718.
Figure 4-6: SEM image with EDX position showing carbides of as-received forged Inconel 718 [28].
30 (a)
(b)
Figure 4-7: SEM images showing porosity of original material recast Inconel 718, (a) 50X, (b)100X.
31
Figure 4-8: SEM image showing carbides of recast Inconel 718 after heat treatment.
Figure 4-9: SEM image showing carbides of Forged Inconel 718 after heat treatment[28].
32
Figure 4-10: SEM image with EDX position showing carbides of recast Inconel 718 after heat treatment.
Figure 4-11: SEM image with EDX position showing carbides of forged Inconel 718 after heat treatment [28].
33 (a)
(b)
Figure 4-12: : SEM images showing porosity of recast Inconel 718 after heat treatment, (a) 50X, (b) 250X.
34
Figure 4-13: Hardness testing locations of a specimen cross section.
35
Creep Strain (%)
Time (h)
200 400 600 800 1000 1200 1400 1600 1800 2000 2
4 6 8 10 12 14 16
Recast Forged
0 0
Figure 4-14: Creep curves of recast and forged Inconel 718 specimens under constant tensile stress 625 MPa at 650°C.
36 (a)
(b)
Figure 4-15: SEM fractographs of a crept recast Inconel 718 specimen, (a) 50X, (b) 80X.
37 (a)
(b)
Figure 4-16: SEM fractographs of a crept forged Inconel 718 specimen, (a) 100X, (b) 300X.
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Figure 4-17: Fracture surface of a crept recast Inconel 718 specimen shows rapid crack propagation.
Figure 4-18: Fracture surface of a crept forged Inconel 718 specimen shows slow crack propagation.
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CHAPTER V CONCLUSIONS
The important results of this study can be concluded as follows:
1. The mechanical properties are very much dependent on the microstructures and material processes. Through the use of OM and SEM, the microstructure was observed; this experiment explored a large number of porosities in recast Inconel 718 superalloy.
2. The average grain size of recast Inconel 718 superalloy is much greater than that of forged one for about 20 times
3. Hardenbility depends on the sum of strengthening phases γ'/γ" precipitated. The precipitation hardenings are γ′ and γ″ precipitates exist to improve hardness.
4. The creep rupture life of forged Inconel 718 (about 1800 hours) is much longer than that of recast Inconel 718 (about 120 hours).
5. Average porosity size and percentage are 24.7 μm and 2.683%, respectively, in the recast Inconel 718 superalloy, which have main harmful effect on the creep rupture life.
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REFERENCES
[1] http://www.fiddlersgreen.net/models/aircraft/Heinkel-178.html [2] http://en.wikipedia.org/wiki/Ernst_Heinkel
[3] R. C. Reed, The Superalloys Fundamentals and Application, Cambridge University Press, New York, NY, USA, 2006, p. 3.
[4] C. T. Sims and W. C. Hagel (Eds.), The superalloys, John Wiley & Sons, New York, USA, 1972, p. 5.
[5] C. T. Sims, N. S. Stoloff and W. C. Hagel (Eds.), Superalloys II, John Wiley & Sons, New York, USA, 1987, p. 3.
[6] M. J. Donachie and S. J. Donachie, Superalloys A Technical Guide, second edition, ASM International, USA, OH, 2002, p. 9.
[7] R. C. Reed, The Superalloys Fundamentals and Applications, Cambridge University Press, Cambridge, UK, 2006, p. 3.
[8] G. A. Rao, M. Kumar, M. Srinivas and D. S. Sarma, “Effect of standard heat treatment on the microstructure and mechanical properties of hot isostatically pressed superalloy Inconel 718,” Materials Science and Engineering, vol. 355, pp. 114–125, 2003.
[9] S. Azadian, L. Y. Wei and R. Warren, “Delta phase precipitation in Inconel 718,” Materials Characterization, vol. 53, pp. 7–16, 2004.
[10] G. D. Smith and S. J. Patel, “The role of niobium in wrought precipitation-hardened nickel-base alloys,” Proceedings of the Sixth International Symposium on Superalloys 718, 625, 706 and Derivatives, E.
A. Loria (Ed.), TMS, Pittsburgh, PA, USA, pp.149-155, 2005.
[11] L. Xiao, D. L. Chen and M. C. Chaturvedi, “Effect of Boron on the Low
Cycle Fatigue Behavior and Deformation Structure of Inconel 718 at
650ºC,” Metallurgical and Materials Transactions A, vol. 35A, pp.
3477-3487, 2004.
41
[12] W. R. Sun, S. R. Guo, D. Z. Lu and Z. O. Hu, “Effect of Sulfur on the Solidification and Segregation in Inconel 718 alloy,” Materials Letters, vol. 31, pp. 195-200, 1997.
[13] H. Song, S. Guo and Z. Hu, “Beneficial effect of phosphorus on the creep behavior of Inconel 718,” Scripta Materialia, vol. 41,
pp. 215-219,1999
.[14] C. Slama and M. Adbellaoui, “Structural characterization of the aged
Inconel 718,” Journal of Alloys and Compounds, vol. 306, pp. 277-284, 2000.
[15] C. T. Sims, N. S. Stoloff and W. C. Hagel (Eds.), Superalloys II, John Wiley & Sons, New York, USA, 1987, p. 177.
[16] L. Liu, C. Zhai, C. Lu, W. Ding, A. Hirose and K. F. Kobayashi, “Study of the effect of δ phase on hydrogen embrittlement on Inconel 718 by notch tensile tests,” Corrosion Science, vol. 47, pp. 355-367, 2005.
[17] H. Yuan and W. C. Liu, “Effect of δ phase on the hot deformation behavior of Inconel 718,” Materials Science and Engineering A, vol. 408, pp. 281-289, 2005.
[18] S. Li, J. Zhuang, J. Yang, Q. Deng and J. Du, “The effect of δ-phase on crack propagation under creep and fatigue conditions in alloy 718,” in Superalloys 718, 625, 706 and Various Derivatives, E. A. Loria (Ed.), TMS, Pittsburgh, PA, USA, pp. 545-555, 1994.
[19] M. J. Donachie and S. J. Donachie, superalloys A Technical Guide, second edition, ASM International, OH, 2002, p. 27.
[20] A. Thomas, M. El-Wahabi, J. M. Cabrera and J. M. Prado, “High temperature deformation of Inconel 718,” Journal of Materials Processing Technology, vol. 177, pp. 469-472, 2006.
[21] S. Azadian, L. Y. Wei and R. Warren, “Delta phase precipitation in Inconel 718,” Materials Characterization, vol. 53, pp. 7-16, 2004
[22] C. T. Sims and W. C. Hagel (Eds.), The Superalloys, John Wiley and Sons,
New York, USA, 1972, p. 133.
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