High-Cycle Fatigue of Inconel 718 - National MagLab · 2018-02-08 · 0.49 0.21
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ABSTRACT Inconel 718 is a nickel-based super alloy widely used in aerospace applications due to its strength at high temperatures, weldability, and resistance to creep. Applications of the alloy extend to gas turbines, jet engine components, structural materials, and High-Cycle Fatigue of Inconel 718 Kyle Buchholz University of Florida MATERIAL Table 1. shows the chemical composition of alloy 718 as received from the manufacturer. Inconel 718 is a precipitation- strengthened material with the formation of body-centered tetragonal (BCT) ”,Ni 3 Nb, and face-centered cubic (FCC) ’, Ni 3 (Al, Ti), precipitates in the matrix. The heat treatment for the alloy per manufacturer recommendation: 990 K for 8 hours, 895 K for 8 hours, air cooled. Age hardening was performed prior to sample machining in a computer programmable 3-zone furnace with an inert sample environment, Argon. Samples were extracted from original forging via wire EDM and machined to drawing speciYications (Figure 1). TEST PROCEDURE Tensile and fatigue testing was performed under testing standards, ASTM E8 and ASTM E466 respectively. All testing was done on a servo-hydraulic MTS test machine equipped with a cryostat. For tensile testing, strain was measured using three one- inch extensometers and force was recorded using a 250 kN load cell. Tension-tension fatigue test were run on a 100 kN capacity load cell at 10 Hz in load control, constant load amplitude (R=0.1). Fatigue specimens were designed to a constant radius geometry. RESULTS AND DISCUSSION CONCLUSION This study in collaboration with references [2] and [3] reveal that Inconel 718 does not have a signiYicant brittle to ductile transition at 77 K as seen in Maraging C250. The discrepancy between the two materials is largely due to differences in composition and microstructure. Maraging C250 is a martensitic steel alloy with the formation of BCC – a geometry known to exhibit a brittle transition. Inconel 718 is an Austenitic Nickel alloy with a primary matrix of FCC – a microstructure known to have good performance at cryogenic temperatures. Additional testing on Inconel 718 age hardened samples will fully characterize the fatigue life of the alloy at 77 K. Fatigue testing at 4 K could further reveal design criteria for Inconel 718 in cryogenic application. FCGR testing could provide additional insight to Inconel 718’s toughness at cryogenic temperatures. ACKNOWLEDGEMENTS This project was supported by the National Science Foundation under DMR-1157490. Thanks to R.P. Walsh, Dr. S. Balachandran, and R.E. Goddard for their support in this project. C Mn Si P S Cr Ni Mo Nb 0.02 0.06 0.09 0.009 <0.0003 17.79 54.02 2.9 5.07 Al Co Ta B Cu Fe Mg Nb+Ta Ti 0.49 0.21 <0.01 0.004 0.04 18.29 - 5.07 0.98 Inconel 718 Table 1. Inconel 718 material certi3ication sheet chemical composition Figure 1. a) Wire EDM specimen extraction/orientation from stock. b) Tensile specimen design. c) Fatigue specimen design a) b) c) cryogenic storage tanks. Cryogenic fatigue data for Inconel 718 exists but primarily in its age hardened condition. This study aims to provide cryogenic fatigue data for Inconel 718, in its solution annealed state, and shed light on a potential ductile to brittle transition present in the alloy. High cycle fatigue characteristics of commercially available Inconel 718 in the solution annealed and age hardened condition were investigated at 77 K. REFERENCES [1] Kiefer,T.F.,Keys,R.D., and Schwartzberg,F.R.,Determination of Low-Temperature Fatigue Properties of Structural Metal Alloys [2] Low-cycle Fatigue Characteristics of precipitation- hardened Superalloys at Cryogenic Temperatures, Nobuhisa Suzuki, Toshiba Corporation, Japan [3] Data Sheet on Fracture Toughness and High-Cycle Fatigue Properties, NIMS, JAXA Inconel 718 microstructure post heat treatment. Nominal grain size of 5-25 microns. Secondary phase precipitates can be seen forming in and between grain boundaries. Small grain sizes and twining contribute to higher tensile strengths. Tensile Data reveals Inconel 718 solution annealed to have a yield strength of 670 MPa at room temperature and 830 MPa at 77 K. Ultimate Tensile Strength was 1015 MPa and 1315 MPa respectively with roughly 30% elongation and a reduction of area of 25%. Fatigue life of INC718 in the solution annealed condition was reported in this study. Fig. 4. shows the results in comparison with previous published data in the age hardened condition and solution annealed. Figure 2. Inconel 718 Age hardened grain boundaries, SEM Figure 3. Tensile data for INC718 at 295K and 77K, Solution Annealed Figure 4. Fatigue data for INC718 at 77K. Solid Fill= Age Hardened; No Fill = Solution Annealed Figure 5. Fracture surface INC718 solution annealed Figure 6. shows the relation between room temperature Yield strength versus 77 K Yield. Typical cryogenic alloys and designer alloys are grouped in yellow. Maraging C250 and Inconel 718 are commercially available alloys that have good cryogenic performance with comparable room temperature strength. Figure 6. Maraging C250 and Inconel 718 Yield Strength vs. common cryogenic alloys Fracture surface of Inconel 718, Figure 5. In the top right corner a “thumbnail” fatigue crack transitions to a ductile tear. Fatigue cracks initiate on the surface and travel across the cross section until reduction of area causes tensile failure..
Transcript of High-Cycle Fatigue of Inconel 718 - National MagLab · 2018-02-08 · 0.49 0.21
ABSTRACT Inconel 718 is a nickel-based super alloy widely used in aerospace applications due to its strength at high temperatures, weldability, and resistance to creep. Applications of the alloy extend to gas turbines, jet engine components, structural materials, and
High-Cycle Fatigue of Inconel 718 Kyle Buchholz University of Florida
MATERIAL Table 1. shows the chemical composition of alloy 718 as received from the manufacturer. Inconel 718 is a precipitation-strengthened material with the formation of body-centered tetragonal (BCT) 𝛾”,Ni3Nb, and face-centered cubic (FCC) 𝛾’, Ni3(Al, Ti), precipitates in the 𝛾 matrix. The heat treatment for the alloy per manufacturer recommendation: 990 K for 8 hours, 895 K for 8 hours, air cooled. Age hardening was performed prior to sample machining in a computer programmable 3-zone furnace with an inert sample environment, Argon. Samples were extracted from original forging via wire EDM and machined to drawing speciYications (Figure 1).
TEST PROCEDURE Tensile and fatigue testing was performed under testing standards, ASTM E8 and ASTM E466 respectively. All testing was done on a servo-hydraulic MTS test machine equipped with a cryostat. For tensile testing, strain was measured using three one-inch extensometers and force was recorded using a 250 kN load cell. Tension-tension fatigue test were run on a 100 kN capacity load cell at 10 Hz in load control, constant load amplitude (R=0.1). Fatigue specimens were designed to a constant radius geometry.
RESULTS AND DISCUSSION
CONCLUSION This study in collaboration with references [2] and [3] reveal that Inconel 718 does not have a signiYicant brittle to ductile transition at 77 K as seen in Maraging C250. The discrepancy between the two materials is largely due to differences in composition and microstructure. Maraging C250 is a martensitic steel alloy with the formation of BCC – a geometry known to exhibit a brittle transition. Inconel 718 is an Austenitic Nickel alloy with a primary matrix of FCC – a microstructure known to have good performance at cryogenic temperatures. Additional testing on Inconel 718 age hardened samples will fully characterize the fatigue life of the alloy at 77 K. Fatigue testing at 4 K could further reveal design criteria for Inconel 718 in cryogenic application. FCGR testing could provide additional insight to Inconel 718’s toughness at cryogenic temperatures. ACKNOWLEDGEMENTS This project was supported by the National Science Foundation under DMR-1157490. Thanks to R.P. Walsh, Dr. S. Balachandran, and R.E. Goddard for their support in this project.
C Mn Si P S Cr Ni Mo Nb0.02 0.06 0.09 0.009 <0.0003 17.79 54.02 2.9 5.07Al Co Ta B Cu Fe Mg Nb+Ta Ti0.49 0.21 <0.01 0.004 0.04 18.29 - 5.07 0.98
Inconel718
Table1.Inconel718materialcerti3icationsheetchemicalcomposition
Figure1.a)WireEDMspecimenextraction/orientationfromstock.b)Tensilespecimendesign.c)Fatiguespecimendesign
a) b) c)
cryogenic storage tanks. Cryogenic fatigue data for Inconel 718 exists but primarily in its age hardened condition. This study aims to provide cryogenic fatigue data for Inconel 718, in its solution annealed state, and shed light on a potential ductile to brittle transition present in the alloy. High cycle fatigue characteristics of commercially available Inconel 718 in the solution annealed and age hardened condition were investigated at 77 K.
REFERENCES [1] Kiefer,T.F.,Keys,R.D., and Schwartzberg,F.R.,Determination of Low-Temperature Fatigue Properties of Structural Metal Alloys [2] Low-cycle Fatigue Characteristics of precipitation-hardened Superalloys at Cryogenic Temperatures, Nobuhisa Suzuki, Toshiba Corporation, Japan [3] Data Sheet on Fracture Toughness and High-Cycle Fatigue Properties, NIMS, JAXA
Inconel 718 microstructure post heat treatment. Nominal grain size of 5-25 microns. Secondary phase precipitates can be seen forming in and between grain boundaries. Small grain sizes and twining contribute to higher tensile strengths.
Tensile Data reveals Inconel 718 solution annealed to have a yield strength of 670 MPa at room temperature and 830 MPa at 77 K. Ultimate Tensile Strength was 1015 MPa and 1315 MPa respectively with roughly 30% elongation and a reduction of area of 25%.
Fatigue life of INC718 in the solution annealed condition was reported in this study. Fig. 4. shows the results in comparison with previous published data in the age hardened condition and solution annealed.
Figure2.Inconel718Agehardenedgrainboundaries,SEM
Figure3.TensiledataforINC718at295Kand77K,SolutionAnnealed
Figure4.FatiguedataforINC718at77K.SolidFill=AgeHardened;NoFill=SolutionAnnealed
Figure5.FracturesurfaceINC718solutionannealed
Figure 6. shows the relation between room temperature Yield strength versus 77 K Yield. Typical cryogenic alloys and designer alloys are grouped in yellow. Maraging C250 and Inconel 718 are commercially available alloys that have good cryogenic performance with comparable room temperature strength.
Figure6.MaragingC250andInconel718YieldStrengthvs.commoncryogenicalloys
Fracture surface of Inconel 718, Figure 5. In the top right corner a “thumbnail” fatigue crack transitions to a ductile tear. Fatigue cracks initiate on the surface and travel across the cross section until reduction of area causes tensile failure..
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