Influences of Soaking Time in Hot Isostatic Pressing on Strength Of

Home , Hot isostatic pressing

Materials Transactions, Vol. 47, No. 2 (2006) pp. 426 to 432 #2006 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

Inﬂuences of Soaking Time in Hot Isostatic Pressing on Strength of Inconel 718 Superalloy

Shih-Hsien Chang1;*, Shih-Chin Lee1, Tzu-Piao Tang2 and Hsin-Hung Ho3

1Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701, R. O. China 2Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan 106, R. O. China 3Materials and Electro-Optics Research Division of Chung-Shan Institute of Science and Technology, Lung-tan, Taiwan 325, R. O. China

Hot Isostatic Pressing (HIP) is widely used in the casting industry to remove the internal porosity generated during the casting process. It combines higher pressure and temperature to produce materials and parts with substantially better properties than those by other methods. This results in improved strength, ductility and fatigue life of castings. The aim of this paper is to discuss the methods and to ﬁnd a suitable soaking time of HIP for Inconel 718 superalloy castings. In this study, the HIP temperature was maintained at 1453 K, pressure was kept 175 MPa and three diﬀerent soaking time are 2, 3 and 4 h. The experiment results show that HIP treatment at 1453 K under the pressure of 175 MPa for 4 h for Inconel 718 superalloy is the optimum condition. It can decrease the porosity of Inconel 718 superalloy castings. In this study, it can reduce porosity about 86% after HIP treatment. For the tension test at a fast strain rate (0.001 s1) that it increased the tensile strength by 31% at room temperature, 27% at 813 K, and 24% at 923 K. While at a very slow strain rate (0.0001 s1), it increased the tensile strength by 24% at room temperature and 20% at 813 K. For a 3-point bending test, it showed that the optimum soaking time of HIP procedure could enhance the bending strength by 38% at room temperature and 26% at 813 K.

(Received November 1, 2005; Accepted January 6, 2006; Published February 15, 2006) Keywords: Hot Isostatic Pressing, Inconel 718 superalloy, porosity, tensile strength, bending strength

1. Introduction following heat treatment: solution treated was performed at 1253 K for 1 h followed by N2 cooling, aged at 993 K for 8 h, HIP process applies heat and pressure within an enclosed furnace cooling (330 K/h) to 893 K, soaking for 8 h, and then vessel to consolidate or densify materials such as castings. It air cooling to room temperature.10–12) The solution-treating is a common method in the industry to improve strength and and age-hardening were performed in a commercial Ger- durability of the parts and remove voids from castings. HIP manic SCHMETZ vacuum furnace. Tension and bending can be applied to a wide range of metals, alloy and ceramics. tests of mechanical properties for Inconel 718 superalloy Functions of HIP processes include defect healing of castings were conducted at room temperature as well as at a castings, removal of internal porosity, diffusion bonding of high temperature. The fracture surfaces of the tensile dissimilar materials, improvement in property scatter and specimens were examined using SEM. X-ray diffraction, mechanical properties. Its major application is for eliminat- and porosity and microstructure inspection to evaluate the ing casting defects.1–4) The development of HIP processing effects of different soaking time of HIP treatments on Inconel for superalloys started in the 1970’s when there was a need 718 superalloy. for improved materials in the hot sections of gas turbine engines. Inconel 718 superalloy is a niobium-modified 2. Experimental nickel-base superalloy that is used widely in gas turbine and related high temperature of aerospace applications.5,6) Inconel 718 nickel-based superalloys are defined as those Inconel 718 superalloy is a precipitation hardenable, nickel- alloys that have nickel as the major constituent, with a based superalloy that has high strength at room temperature significant amount of chromium. This alloy may contain and excellent creep as well as fatigue strengths at high niobium, molybdenum, aluminium and titanium as major temperatures. It used in gas turbines, rocket motors, space- alloy additions. In this study, the chemical composition craft, nuclear reactors, pumps and tooling. The alloy consist (mass%) of the HIP treated for Inconel 718 superalloy is as of several phases in the matrix including , 0, 00 and NbC. follows: 52.89% Ni, 17.3% Cr, 3.52% Mo, 5.26% Nb, 0.52% However Inconel 718 is a nickel-rich alloy strengthened by Al, 0.83% Ti, 19.36% Fe, 0.05% Mn, 0.07% C, 0.01% V, order body centered tetragonal (BCT) precipitates, 00.7–9) 0.08% W and 011% Co. The Inconel 718 superalloy casting In this experiment, the effects of different soaking time of was prepared by Vacuum Induction Melting (VIM) process. HIP treatments on Inconel 718 superalloy and their influen- After melting, the alloy was solidified into square plate tial parameters of HIP process were performed. The castings. All the specimens of tension and bending test were parameters of HIP process used including three different obtained from the castings by Wire Electronic Discharge soaking times: 2, 3 and 4 h. Beside, the HIP temperature was Machining (WEDM) treatment. The specimen sizes of the maintained at 1453 K, and pressure was kept at 175 MPa. tension and bending test are illustrated schematically in After HIP treatment, all the specimens were subjected to the Fig. 1. For the tension and bending test, a SHIMADZU universal material test machine with a maximum load of 25 *Corresponding author, E-mail: [email protected] tons was used. Influences of Soaking Time in Hot Isostatic Pressing on Strength of Inconel 718 Superalloy 427

(a) (b)

Fig. 1 The specimen sizes of tension and bending test for Inconel 718 superalloy (a) tensile specimen, (b) bending specimen.

In earlier study, the effects of the temperature and pressure parameters of HIP on Inconel 718 superalloy were discussed. 0.7 Four different HIP temperatures were used, including 1423, 0.6 1448, 1453 and 1478 K. Four different HIP pressures were used, including 100, 150, 175, 200 MPa. It showed that 0.5 optimum HIP temperature and pressure for Inconel 718 0.4 superalloy were 1453 K and 175 MPa, respectively. This paper will continue to discuss the effects of the soaking time 0.3 Porosity (%) of HIP on Inconel 718 superalloy casting. The HIP temper- 0.2 ature was maintained at 1453 K, pressure was kept at 175 MPa and three different HIP soaking time were used: 0.1 2, 3 and 4 h. HIP equipment was from Flow Autoclave 0 Pressure System, Inc. This commercial HIP equipment as-heat 2h 3h 4h provided with an Uniform Rapid Cooling (URC) system treated that offers decreased cycle time, higher productivity and combined solution heat treating. The uniform rapid cooling Fig. 2 Comparison of the porosities of as-heat treated and after different HIP soaking time treatment for Inconel 718 superalloy after solutionized rate can reach up to 290 K/min. and aged treatment. After HIP treatment, the solution and ageing treatment on Inconel 718 superalloy casting are needed. Because the Inconel 718 suoeralloy has a composite structure of austen- microstructure generated during solidification. These defects site matrix and precipitated 0 and 00 phases produced by the will degrade the mechanical properties of the superalloy.18–21) heat treatment. The strengthening phase in the alloy is the The optical micrograph of the as-heat treated Inconel 718 00 metastable body centered tetragonal Ni3Nb, . The creep superalloy after solutionized and aged treatment is shown in phenomenon can’t be ignored in the condition of HIP and Fig. 3(a). It contains a severe segregation and casting heat treatment. The mechanical properties depend on the microstructure, which was hard to remove from the casting morphology, size and contents of , 0 and 00 phases in structure. Beside, there are a few porosities existing on the Inconel 718 superalloy.13–16) In this experiment, the test microstructure of Inconel 718 superalloy casting. HIP specimens of Inconel 718 superalloy after HIP treatment process can densify the castings and improve strength, were placed in a vacuum furnace and then solutionized at ductility and fatigue life of materials with significantly less 1253 K for 1 h soaking followed by N2 rapid cooling, aged at variation from lot to lot. Figures 3(b), (c) and (d) shows the 993 K for 8 h, furnace cooling (330 K/h) to 893 K, soaking optical micrograph of Inconel 718 superalloy through the for 8 h, and finally air cooling to room temperature.12,16) different soaking time by HIP treatment. The casting microstructure has obviously improvement. Comparison the 3. Results and Discussion average grain size of as-heat treated specimens and after HIP treatment for 4 h, it showed that the average grain size is Inconel 718 superalloy is a face-centered cubic (FCC) 92 mm for as-heat treated and 75 mm for optimum HIP austensite matrix strengthened by strengthened by precip- treatment. They show the grain sizes are uniform and the itation of order intermetallic or carbide precipitates. Both of segregated structure is improved after HIP treatment. The 00 and 0 phases are found in alloy, but 00 is the predominant slender needle d precipitation and twin microstructure were strengthening agent.17–19) HIP process is always used to close disappeared in the grain boundary. pores and flaws by plastic flow. Figure 2 shows the amount of Figure 4 is the scanning electron micrographs of Inconel porosity for Inconel 718 superalloy as a function of the 718 superalloy before and after different HIP soaking time soaking time of HIP treatment. The porosity was reduced treatments. Figure 4(a) shows the grain boundary is not proportionally to the soaking time of HIP treatment. In this obvious. In Fig. 4(b) the needle d precipitation emerged on study, it can reduce porosity about 78% at 2 h, 84% at 3 h and the grain boundary and MC carbide is distributed on the 86% after HIP treatment for 4 h. Because the Inconel 718 matrix.22–24) Increasing the soaking time of HIP treated seems superalloy was used vacuum melting in production, it is hard to decrease the phase precipitation. And it will have the to avoid the segregation, porosity and non-uniformity of the clearly grain boundary and grain size are appropriate as 428 S.-H. Chang, S.-C. Lee, T.-P. Tang and H.-H. Ho

(a) (b)

Fig. 3 Optical micrographs of Inconel 718 for as-heat treated and after HIP treatment at diﬀerent soaking time (a) as-heat treated, (b) 2 h, (b) 3 h, (c) 4 h.

(a) (b)

Fig. 4 SEM micrographs of Inconel 718 superalloy for as-heat treated and different HIP soaking time treatment (a) as-heat treated, (b) 2 h, (c) 3 h, (d) 4 h. shown in Fig. 4(c). For HIP treatment for 4 h, more NbC and grain-boundaries sliding. 00 precipitation exist on the grain boundary, as shown in Figure 5(a) represents the XRD patterns of Inconel 718 Fig. 4(d). Increasing the soaking time of HIP will enhance before HIP treatment. The primary NbC formed during the effects of NbC and precipitation-strength 00 phases. The solidification are large and stable. The X-ray diffraction lump MC carbides appear in the grain-boundaries is helpful pattern were identified as the NbC(111), ð111Þ, ð200Þ and for stress rupture life. And they can effectually prevent the ð220Þ phases. Here only the austenite phase can be detect Influences of Soaking Time in Hot Isostatic Pressing on Strength of Inconel 718 Superalloy 429

soaking time of HIP treatment is helpful to eliminate the porosities of Inconel 718 superalloy. Increasing the soaking time of HIP will enhance the eﬀects of NbC and precipitation-strength 00 phases. The optimum tensile strength and elongation appeared for 4 h soaking time of HIP treated Inconel 718 superalloy. It can increase the tensile strength by 31% at room temperature, 27% at 813 K, and 24% at 923 K. The elongation of Inconel 718 superalloy also can increases about 100% at room temperature and 130% at 813 K and 60% at 923 K after tension tests. The hardness of as-heat treated Inconel 718 superalloy after solution and aged treatments is 36.3 HRC. After HIP treatment for 2 h, the hardness of Inconel 718 superalloy can reach 42.2 HRC. And 43.2 HRC for 3 h, 43.7 HRC for 4 h HIP treated. The hardness increasing of Inconel 718 superalloy castings is proportional to the HIP soaking time. Even after tension test at the temperatures of 813 and 923 K, the Fig. 5 The XRD patterns of Inconel 718 for as-heat treated and after hardness of HIP-ed sample is almost the same, it is slowly to diﬀerent HIP soaking time treatment (a) as-heat treated, (b) 2 h, (c) 3 h, decrease the hardness after high temperature tension test. In (d) 4 h. this experiment, after 813 K tension test (strain rate = 0.001 s1), the hardness of Inconel 718 superalloy is 42.1 HRC for 298K tensile 813K tensile 923K tensile 2 h, 43.0 HRC for 3 h, and 43.2 HRC for 4 h HIP treated. And after 923 K tension test, the hardness of Inconel 718 1300 superalloy is 41.0 HRC for 2 h, 42.9 HRC for 3 h, and 43.0 HRC for 4 h HIP treated. It shows that the HIP treatment was helpful to enhance the strength and hardness for Inconel 718 1100 superalloy casting at high temperature situation. /MPa When the strain rate of tension test change to very slowly σ (0.0001 s1), the results of the tension test for Inconel 718 900 superalloy at 298 and 813 K were shown in Fig. 7. They can increase the tensile strength by 24% at room temperature and 20% at 813 K. The elongation is increase 48% at room 700 temperature and 282% at 813 K. Both of the tensile strength

Tensile Strength, Tensile and elongation of Inconel 718 superalloy are similarly increase proportional to the soaking time of HIP treatment. 500 After a long time of 813 K tension test, the hardness of as-heat 2 h 3 h 4h Inconel 718 superalloy is still kept 42.2 HRC for 2 h, 42.6 treated HRC for 3 h, and 43.0 HRC for 4 h HIP treated. The rupture ductility is improved marginally after HIP Fig. 6 Comparison of the tensile strength of as-heat treated and after HIP treatment. Figure 8 shows the fracture surfaces of as-heat treatment at diﬀerent soaking time for Inconel 718 superalloy (strain rate = 0.001 s1). treated specimens and after HIP treatment for 4 h at room

298K tensile 813K tensile and the 0 and 00 were absent. It was found that increased the soaking time of HIP treatment, the counts of intensity will to 1200 increase, especially for the soaking time is 4 h. The Figs. 5(b)(c)(d) shows the XRD patterns of Inconel 718 after 1100 HIP treatment for 2, 3 and 4 h. The XRD patterns were /MPa σ 1000 identified as the NbC(111), ð111Þ, ð200Þ and ð220Þ.24) All the intensities of XRD patterns are larger than those of as- 900 heat-treated. And the NbC(200), ð201Þ and ð020Þ are shown in XRD pattern after different HIP soaking time treatment. 800 Figure 6 shows the tensile strength measured at room 700 temperature, 813 and 923 K for Inconel 718 after HIP Strength, Tensile treatment at different soaking time. And the strain rate of 600 0.001 s1 is used. In this study, both of the tensile strength as-heat 2 h 3 h 4h and elongation of Inconel 718 superalloy increase propor- treated tionally to the soaking time of HIP treatment. Increase the Fig. 7 Comparison of the tensile strength of as-heat treated and after HIP soaking time of HIP treated, the tensile strength and treatment at different soaking time for Inconel 718 superalloy (strain elongation are rapidly to rise up. Because extended the rate = 0.0001 s1). 430 S.-H. Chang, S.-C. Lee, T.-P. Tang and H.-H. Ho

(a) (b)

Fig. 8 Fractographs of as-heat treated and after HIP treatment for 4 h for Inconel 718 superalloy tensile specimens at room temperature (a) as-heat treated (strain rate = 0.001 s1), (b) 4 h HIP (strain rate = 0.001 s1), (c) as-heat treated (strain rate = 0.0001 s1), (d) 4 h HIP (strain rate = 0.0001 s1).

(a) (b)

Fig. 9 Fractographs of as-heat treated and after HIP treatment for 4 h for Inconel 718 superalloy tensile specimens at 813 K (a) as-heat treated (strain rate = 0.001 s1), (b) 4 h HIP (strain rate = 0.001 s1), (c) as-heat treated (strain rate = 0.0001 s1), (d) 4 h HIP (strain rate = 0.0001 s1). temperature with different strain rate. Figures 8(a) and (c) Inconel 718 superalloy by 1453 K, 175 MPa, 4 h HIP treat- represent the brittle fracture microstructure before HIP ment. In Fig. 8(d), it shows the very fine dimpled rupture in treatment. They showed the brittle intergranular failure the interior of the grains at very slowly strain rate. It indicated mode. Figures 8(b) and (d) represent the dimpled rupture of a ductile transgranular failure mode.13,14) Figure 9 shows the Influences of Soaking Time in Hot Isostatic Pressing on Strength of Inconel 718 Superalloy 431

(a) (b)

Fig. 10 Fractographs of as-heat treated and after HIP treatment for 4 h for Inconel 718 superalloy tensile specimens at 923 K (a) as-heat treated (strain rate = 0.001 s1), (b) 4 h HIP (strain rate = 0.001 s1).

298K bending 813K bending

2750

2500 /MPa

σ 2250

2000

Fig. 11 Schematic diagram of the 3-point bending test. 1750 Bending Strength, fracture surfaces of as-heat treated and 1453 K, 175 MPa, 4 h 1500 HIP treatment at 813 K with different strain rate. For as-heat as-heat 2 h 3 h 4h treated specimens, the micropore take place at high temper- treated ature and cause the decrease of tensile strength showed in Fig. 12 Comparison of the bending strength of as-heat treated and different Figs. 9(a) and (c). For the HIP treatment for 4 h, the fine HIP soaking time treatment for Inconel 718 superalloy casting. dimpled rupture transform to more coarse and ductile rupture was showed in Figs. 9(b) and (d). Comparison the Figs. 10(a) and (b), in Fig. 10(a) shows the micropore appear in the grain (2) The rupture ductility was improved marginally after 4 h boundaries and become more brittle at 923 K tensile tested. of HIP treatment. At a fast strain rate (0.001 s1), the For the optimum HIP treated, Fig. 10(b) shows the ductile optimum HIP treatment can increase the tensile strength rupture of Inconel 718 superalloy specimens. by 31% at room temperature, 27% at 813 K, and 24% at Figure 11 shows the schematic diagram of the 3-point 923 K. When the strain rate was reduced (0.0001 s1), it bending test. The L is 30 mm, b and h are all 5 mm in the can increase the tensile strength by 24% at room 2 equation Rbm ¼ 3FLk=2bh for the 3-point bending test. The temperature and 20% at 813 K. bending rate was kept at 0.005 s1. This test results are shown (3) The optimum HIP treatment can improve the porosity in Fig. 12. Increase the soaking time of HIP treated, the of Inconel 718 superalloy casting. In this study, it can bending strength at room temperature and 813 K are propor- reduce porosity about 86% and after 4 h of HIP tional to the soaking time. It indicates that the specimen treatment. subjected to HIP treatment for 4 h has the best bending (4) Increasing the soaking time of HIP will enhance the strength at room temperature and at a higher temperature effects of NbC and precipitation-strength 00 phases. conditions. It can increase bending strength by 38% at room Meanwhile the slender needle precipitation and twin temperature, and 26% at 813 K. microstructure were disappeared in the grain boundary. (5) For the 3-point bending test, it showed that the optimum 4. Conclusion soaking time of HIP procedure could enhance the bending strength of Inconel 718 superalloy casting (1) The experimental results show that 4 h of HIP treatment about 38% at room temperature and 26% at 813 K. for Inconel 718 superalloy was optimum. It can improve the porosity, elongation and segregation of Acknowledgments Inconel 718 superalloy than 2 or 3 h of HIP treatment. Meanwhile, it can more strengthen the tensile and This research supported by the Materials and Electro- bending properties. Optics Research Division of Chung-Shan Institute of Science 432 S.-H. Chang, S.-C. Lee, T.-P. Tang and H.-H. Ho and Technology Taiwan. The author would also like to 12) S. S. Manson: Inconel Alloy 718, Code 4103, (Aerospace Structural express his appreciation for Dr. Harvard Chen and Professor Metals Handbook, 1974) pp. 1–72. 13) G. A. Rao, M. Kumar, M. Srinivas and D. S. Sarma: Mater. Sci. Eng. A S. F. Wang. 355 (2003) 114–125. 14) G. A. Rao, M. Srinivas and D. S. Sarma: Mater. Sci. Eng. A 383 (2004) 201–212. REFERENCES 15) S. C. Medeiros, Y. V. Prasad, W. G. Frazier and R. Srinivasan: Scr. Mater. 42 (2000) 17–23. 1) T. Garvare: Proceedings of the International Conference on Hot 16) Materials Handbook Ninth Edition, Volume 4 Heat Treating, (Amer- Isostatic Pressing-Lulea/15–17 June, (The Swedish Institute of ican Society for Metals, 1981) pp. 754–759. Production Engineering Research, 1987) pp. 3–70. 17) Q. Chen, N. Kawagoishi and H. Nisitani: Mater. Sci. Eng. A 277 (2000) 2) Koizumi and Mitsue: Hot Isostatic Pressing Theory and Applications, 250–257. International Conference on Hot Isostatic Pressing, (Elsevier Applied 18) R. B. Li, M. Yao, W. C. Liu and X. C. He: Scr. Metall. 46 (2002) 635– Science, 1992) pp. 11–187. 638. 3) Flow Autoclave System, Inc.: Hot Isostatic Pressing Applications. 19) M. D. Charre: The Microstructure of Superalloys, (Gordon and Breach 4) Charles Barre: Int. J. Powder Metall. 35 (1999) 51–52. Science Publishers, 1997) pp. 29–45. 5) T. S. Byun and K. Farrell: J. Nucl. Mater. 318 (2003) 292–299. 20) S. D. Antolovich et al.: Superalloys 1992, (The Minerals & Metal & 6) C. Slama and M. Abdellaoui: J. Alloys Compd. 306 (2000) 277–284. Materials Soc., 1992) pp. 23–32. 7) Donachie and J. Matthew: Superalloys—A Technical Guide (ASM 21) S. Srinivas, K. S. Prasad, D. Gopikrishna and M. C. Pandey: Mater. Inter. 1988) pp. 47–91. Character. 35 (1995) 93–98. 8) F. J. Rizzo and J. J. Conway: Int. J. Powder Metall. 35 (1999) 53–62. 22) M. K. Miller, S. S. Babu and M. G. Burke: Mater. Sci. Eng. A 270 9) C. Dauong et al.: J. Mater. Sci. 39 (2004) 719–721. (1999) 14–18. 10) H. Lu, X. Jia, K. Zhang and C. Yao: Mater. Sci. Eng. A 326 (2002) 23) M. Gao and R. P. Wei: Scr. Metall. 37 (1997) 1843–1849. 382–385. 24) W. C. Liu, F. R. Xiao and M. Yao: Scr. Metall. 37 (1997) 53–57. 11) M. Gao and R. P. Wei: Scr. Metall. 32 (1995) 987–990.