A Study on Creep Characteristics of Ni-Base Superalloy IN738LC

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9th International Conference on Fracture & Strength of SolidsJune 9-13, 2013, Jeju, Korea

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A Study on Creep Characteristics of Ni-base Superalloy IN738LC

Dongkeun Lee 1, Siyoung Lee 1, To Kang 1,

Jea-Mean Koo 2, Chang-sung Seok 2,*, Sung-Jin Song 2 1

Graduate School of Mechanical Engineering, Sungkyunkwan University, Suwon, South Korea2 Department of Mechanical Engineering, Sungkyunkwan University, Suwon, South Korea

Abstract: Gas turbine blades experience high temperature degradation environments caused byflame and mechanical loads incurred by high speed rotation during operation. These conditions shortenthe lives of gas turbines and reduce the reliability of the equipment. For these reasons, the mechanicalcharacteristics of a superalloy blade substrate need to be studied to improve the reliability of gasturbines. Thus, creep tests on IN738LC substrate for gas turbine blades were performed. Consideringreal operation conditions, creep tests were performed at 850 with various stress conditions, andcreep curves were obtained through the tests. After performing those, the creep specimens with damageat specific stress conditions were prepared for indentation test, and the indentation tests were conducted.Finally, the relationship between the damage to and the hardness of the material was obtained from testresults indicating changes in the mechanical properties at various levels of damage, using regression

analysis. Also, the degradation mechanism was studied by micro structure analysis. The results indicatethat, it is possible not only to directly apply the method suggested above to specimens for degradationevaluation, but also to evaluate the degradation of blades in operation.Keywords : Superalloy, IN738LC, Creep, Indentation, Life prediction.

1. IntroductionSuperalloys are often used for turbine blades, which are an important component for thermal power generation plants. During operation, the blades are exposed to not only the centrifugal force caused byhigh-speed rotation (about 3600 rpm), but also the low cycle fatigue induced by repeated start-up andshut-down. Blades that operate for weeks or more at a time are particularly exposed to creep conditionscaused by high-speed rotation and flames. Also, blades are exposed to severe environments, whichinduced both thermal and mechanical fatigue (TMF) simultaneously since they consist of conditionswith high temperature flames. [1]Long-term operation engenders the possibilities of flaws or failures due to such severe operatingenvironments, and this can cause serious accidents for humans or equipment. Therefore, it is necessaryto research the reliability and durability of gas turbine blades to prevent such accidents. [2, 3]Destructive methods such as LCF, TMF, and creep tests have been used so far to predict the life of gasturbine blades. However, it is difficult to use the results directly, and impossible to apply to blades inoperation. Thus, unlike previous research, this study aims to find a method for evaluating thedegradation of blades in operation.For that purpose, creep rupture tests on IN738LC substrate for gas turbine blades were performed.Considering the actual operation conditions, creep rupture tests were performed at 850 with variousstress conditions, and creep curves were obtained. After performing those, the creep specimens withdamage at specific stress conditions were prepared for indentation tests, and the indentation tests wereconducted. Finally, the relationship between the damage to and the hardness of the material wasobtained from regression analysis of changes in the mechanical properties at various levels of damageusing the test results.

2. Creep rupture test2.1 Test equipment and condition A widely used superalloy for commercial gas turbines (IN738LC) was chosen for the creep rupture test.To obtain the test conditions, a tensile test was performed first using an electric motor-type testmachine in Shimadzu. Considering the actual operation conditions, tests were performed at 850 . [4]The tensile specimen prepared according to ASTM E8-M, [5] and a strain rate of 1mm/min was appliedto the test. Strain was measured using an extensometer.Fig. 1 shows the experimental equipments used in the creep rupture test, consisting of a control system,split type furnace, LVDT, load cell, loading jig, and DAQ system. The creep specimen had the samegeometry as the tensile specimen, with a reduced section length of 45 mm and diameter of 7.4 mm as

*Corresponding author: E-mail: [email protected] ; Tel: +82-31-290-7446; Fax: +82-31-290-7482


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shown in Fig. 2. In the creep rupture test, the specimen was maintained at 850 for 3 hours to preventthe formation of a temperature gradient according to ASTM-E139. [6] The time at which the specimencompletely separated was considered the time of fracture. Table 1 shows the chemical composition of IN738LC.

Table 1 Chemical composition of IN738LC

Components C Si Mn Cr Mo Cu Ti Al Co W Fe Cb Ta Zr B NiMin, % 0.09 - - 15.70 1.50 - 3.20 3.20 8.00 2.40 - 0.60 1.50 0.015 0.005 BAL

Max, % 0.13 0.30 0.10 16.30 2.00 0.10 3.70 3.70 9.00 2.80 0.35 1.10 2.00 0.050 0.020 BAL

Fig. 1 Creep test equipment and control system

Fig. 2 Creep test specimen

2.2 Test results The tensile strength of the tested specimen was 630 MPa at 850 , and fracture occurred after deformation of 13.5%. Based on the tensile test result, creep rupture tests were conducted at stressconditions of 450, 400, 350, 300, and 250 MPa.Table 2 and Fig. 3 show the results of the creep rupture tests. The x-axis in Fig. 3 is time to fracture of the specimen, and the y-axis is applied stress on the specimen. The creep rupture test indicates that, thetime to fracture of the specimen decreased gradually as the applied stress increased; this relationship isexpressed by equation (1), where T is the time to fracture of the specimen (min) and is applied stress(MPa).

Fig. 4 shows the creep rupture diagram for the 250 MPa stress condition. The x-axis is test time, andthe y-axis is strain on the specimen, which changes over time. Three stages of creep (transient, steadystate, and accelerating) are well classified, as shown in the diagram.


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Fig. 5 Microstructural image of damaged specimen by optical microscope

3.2 Analysis by SEM Change of was observed by SEM after microstructural analysis by optical microscope. Figs. 6~7show microstructural images captured by SEM. Unlike analysis by optical microscope, the specimenwith electrolytic etching was used for SEM analysis.SEM analysis shows that, voids within a specimen (black area in Fig. 6) increased according todegradation of material as creep time increased, these voids can cause specimen fracture. gradually

became spherical according to degradation of material as creep time increased as shown in Fig. 7. Also,average grain size increased.From these changes it can be predicted that the mechanical performance of material decreases. Thus, anindentation test on a damaged specimen was conducted to verify the change in mechanical performanceof the material.

Fig. 6 Microstructural image of damaged specimen by SEM

Fig. 7 Change of due to material degradation


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4 Indentation test on damaged IN738LC specimen4.1 Test equipment and condition After microstructural analysis, the change of hardness was measured using a Vickers hardness tester.Fig. 8 shows the indentation tester and specimen. A micro polishing was applied to all specimens for exact measurement of hardness, and hardness was measured ten times for each specimen in order toobtain average hardness.

Fig. 8 Indentation tester and specimen

4.2 Test results Fig. 9 shows the relationship between hardness and damage based on the average hardness of eachspecimen. The x-axis is damage of material, and the y-axis is average hardness. In addition, 0 %damage indicates a new specimen, and 100 % damage indicates a fractured specimen. Damageincreased as hardness decreased, this relationship is expressed in equation (2), where D is damage of specimen (%) and H is hardness of specimen (Hv).Using equation (2), the relative damage amount (vs. new specimen) can be derived by hardness test. Inaddition, the remaining life can be calculated by reducing the measured damage (%) of the specimenfrom damage of 100%.By using the method suggested above, it is possible not only to directly apply the method to specimensfor degradation evaluation, but also to evaluate the degradation of blades in operation.

74.218.7 6.14 / H e D (2)

Fig. 9 Relationship between hardness and damage


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5 Conclusions(1) The tensile strength of the IN738LC was approximately 630 MPa at 850 , and fracture occurredafter deformation of 13.5%.(2) The relationship between applied stress and rupture time of IN738LC was obtained from the creep

rupture test. As a test result, rupture time decreased as applied stress increased.(3) Microstructural analysis was conducted on damaged specimens. Voids within a specimen increasedaccording to degradation of material as creep time increased. In addition, gradually became sphericalaccording to degradation of material as creep time increased. Also, average grain size increased. Fromthese changes, it can be predicted that the mechanical performance of material decreases.(4) After microstructural analysis, indentation tests were conducted using the damage specimen.Hardness decreased as damage increased, and an equation was derived using this relationship. By usingthe method suggested above, it is possible not only to directly apply the method to specimens for degradation evaluation, but also to evaluate degradation of the blades in operation.

AcknowledgementThis work was supported by the R&D program of the Korea Institute of Energy Technology Evaluationand Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No.

20111020400020).

References[1] I.G. Wright, T.B. Gibbons. Recent developments in gas turbine materials and technology and their

implications for syngas firing. International Journal of Hydrogen Energy, 2007, 32: 3610 - 3621.[2] W.J. Evans, J.E. Screech, S.J. Williams. Thermo-mechanical fatigue and fracture of INCO718.

International Journal of Fatigue, 2008, 30: 257-267.[3] Brooks JW, Bridges PJ. Metallurgical stability of inconel alloy 718. Superalloys, 1988, 33-42.[4] Gas Turbine Blade Superalloy Material Prorerty Handbook. EPRI, 2001.[5] Standard Test Methods for Tension Testing of Metallic Materials. ASTM-E8, 2002.[6] Standard Test Methods for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic

Materials. ASTM-E139, 2006.

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A Study on Creep Characteristics of Ni-Base Superalloy IN738LC