ANISOTROPY OF THE FRACTURE TOUGHNESS IN AGED INCONEL 718

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1 ANISOTROPY OF THE FRACTURE TOUGHNESS IN AGED INCONEL 718 G. Liu, D.K. Rehbein and 0. Buk Ames Laboratory-USDOE and Material Siene and Engineering Department Iowa State University, Ames, IA INTRODUCTION INCONEL 718 is a widely used material for appliations at temperatures up to 650 C. Many researh groups have studied the properties of INCONEL 718 at different onditions[1,2]. It is known that, after several thousand hours at 650 C, INCONEL 718 shows roughly a 75% derease in the Charpy V-noth impat energy[3]. However, any anisotropi harateristis of this steel have not been investigated in adequate detail. Consequently, in order to maintain the integrity of the omponents, it is important to larify the orientation dependene of the mehanial properties of INCONEL 718. The tehniques that provide at least qualitative information on a possible orientation dependene of the mehanial properties of the materials onerned ould be the hardness measurement,the small punh ( SP) test tehnique developed at Ames Laboratory[4,5], and the aousti nondestrutive evaluation(nde) [6-8]. This paper presents our efforts to determine the anisotropi properties of aged INCONEL 718. The test results inlude hardness, small punh test dutility, yield stress, ultimate stress, frature toughness J 1, and ultrasoni attenuation. The anisotropy parameter Kii is defined to desribe the extent of orientation dependene. TEST MATERIALS AND GRIENTATION The material used for this work was supplied by Idaho National Engineering Labaratory (INEL) [3]. Two heats(heat EK11 and Heat 4766-EK21)of materials were tested. Both heats were given a onventional heat treatment onsisting of a solution anneal at 950±14 C for 1 hour, air ooling to room temperature, aging at 718±8 C for 8 hours, furnae ooling to 621 C, and aging at 621±8 C, to give a total preipitation time of 18 hours. Heat treated piees from eah of the two heats were exposed to 649 C for times of 0, 500, and 1000 hours. The diretions in whih the materials were tested are shown in Fig. 1. Review ofprogress in Quantitative Nondestrutive Evaluation, Val. 16 Edited by D.O. Thompson and D.E. Chimenti, Plenum Press, New York,

2 A Fig.1 The diagrammatial figure showing the orientation. HARDNESS ANISOTROPY OF INCONEL 718 The hardness measurements were taken along diretions A, B, and C for every heat and aging time ombination. The results are shown in Fig.2. HRC is the hardness of the tested materials on the Rokwell C-type sale. MECHANICAL PROPERTY ANISOTROPY OF INCONEL 718 The small punh(sp) test tehnique was used to measure the mehanial properties of INCONEL alloy 718. The dutility is defined by the end of plasti deformation and rak initiation[4]. The frature strain Ef was used as the dutility whih an be alulated aording to the equation (3) below. The test results are shown in Fig. 3. Semianalytial relationships among the small punh test parameters and the mehanial properties of INCONEL 718 are [9]: (.) a: "E Direlion A Direlion B Oietion C (.) a: "E Diration A Diration B. Diration C Aging 11me ( hour ) (a) Heai5118-EK 11 (b) Heal4766- EK 21 Fig. 2 Hardness anisotropy of INCONEL 718. Agingllme ( hour ) 1452

3 . >!! =5 u 0 :I ;; DirationA Diretion B Diretion C 32 >!! 30 I l 'I -.! :I 0 ;; l -DiretionA Diration B Diretion C ( a) Heat5118- EK 11 ( b) Heat4766-EK21 Fig. 3 Dutility anisotropy of INCONEL 718. O"y ( MPa) O"ult (MPa) Ef JIC (kj /ID2) 340 {Py/t 0 2 ) {Pult/t 0 2 ) {Bf /t 0 ) E2.7 where Py, Pult t 0, Bf and Ef are the SP test parameters. r, rult and J 1 are the mehanial properties. The anisotropy of ry, rult and J 1 are shown in Fig. 4, Fig. 5 and Fig. 6 respetively. THE ULTRASONIC ATTENUATION ANISOTROPY OF INCONEL 718 The ultrasoni attenuation in these samples has been measured. The frequeny dependene of attenuation in both heats in diretion B, is shown in Fig. 7. Theorientation dependene of the attenuation is shown in Fig.8, where the transduer frequeny is 20 MHz and the attenuation was measured at 16 MHz. (1) (2) (3) (4) !0 0 ::il , äi >=!KlO 800 -DirationA -DiretionA Diretion B --- Diration B Diretion C.f - Diration C I 1: ::il.s::. Ö> I!!, iii äi >= 13! oo0!KlO 800 t... _ _ :-:--<:.: ( a ) Heat5118-EK11 Aging Time ( hour l ( b ) Heat4 766-EK21 Fig. 4 Yield strength anisotropy of INCONEL

4 Q. ::E i : g J!l. E Diration A Diration B Diration C I. Q. ::E = : g J!l. E r.::.::.:.:. -DirationA Diration B Diration C r L-o soo ooo--J ( a) Heat 5118-EK11 ( b) Heat 4766-EK21 Fig. 5 Ultimate strength anisotropy of INCONEL 718. :.s:. :> {:. e &t 96 so 96 so 7S lll 5S I. o. -e-dini!iofla --o-- DinaiiOfl B. O.ralion C "I (a) Heat5118-EK11 I 1000 so 70 ] :> {:. e lll 50 <> o40 l! LI 30 3l (b) Heat 4766-EK21 -e-dii&ciiana tmtian B.,.. Direlion 1000 Fig.6 Frature toughness anisotropy of INCONEL

5 10r , 10r , e 0.8!. 06.s " <! Heat 4766-EK21 (Oh) Heat 4766-EK21 ( 500h) Heat 4766-EK21 (1000h) I 08!. 06.s 0 04 "1; " <! Heat 5118-EK11 ( Oh) Heat 5118-EK11 (500h) Heat5118-EK11 (1000h) Frequeny (Mhz) (a) Heat 5118-EK11 Fig_ (b) Heat 4766-EK21 Frequeny dependene of attenuation. Frequeny (Mhz) 0 Or , - --Diretion A oeo -DiretionA Diration B I o--- Diretion B :s;: "":: Diration C } I r 035 I 0.s E s < _0, ,000., Aging Time ( hour ) (a ) Heat 5118-EK '--' "= '----' (b) Heat 4766-EK21 Fig.8 Attenuation anisotropy of INCONEL

6 DISCUSSION From Fig. 2-6 and 8, it is obvious that there exists anisotropy of Inonel 718 in hardness, strength, frature toughness, and ultrasoni attenuation. We define the anisotropy parameter Kii as: where i is the diretion in whih the quantity is maximum in value and j is the diretion in whih the quantity is minimum in value. Then, i and j an be A, B or C, whih means Diretion A, E or C with notations defined in Fig.1, and Q is the quantity, suh as hardness, yield stress, frature toughness, et. As the anisotropy was onerned, the maximum anisotropy is of interest. Then, the following Table 1 listed the results. It an be seen that the anisotropy in dutility and frature toughness ould be as large as 1.6. During aging proess, the diretion with minimum properties of INCONEL 718 will hange. Thus, the anisotropy should be taken into aount when the omponent life extension was assessed. The ultrasoni attenuation was best fitted by where f is the frequeny. C 1 and C 2 are onstants. D is the mean grain diameter. Thus, the attenuation in INCONEL 718 is in the regions of stohasti sattering ( A = D) and diffusion sattering( A «D ). It may be pretty aurate to measure the grain size by NDE methods. And the mehanial properties of materials depend on the mirostruture features[6]. However, the anisotropy in INCONEL 718 an be investigated by the aousti measurement, but to orrelate the mehanial properties and the attenuation oeffiient in INCONEL 718 steel needs further work. Table 1 Anisotropy K i Heat 5118-EK 11 Heat 4766-EK 21 Hardness ( HRC) K A = KA B = Dutility ( Ef ) KA = KA B = Yield Strength( O"y ) K B = KA = Frature Toughness KB = KA B = Attenuation( 16MHz) KB A = KA = ( 5) ( 6) 1456

7 CONCLUSIONS The mehanial properties( Hardness, dutility, yield stress, and frature toughness) and the ultrasoni attenuation in INCONEL 718 are anisotropi. The anisotropy parameter Kii was defined as the maximum-to-minimum value ratio. The dutility and frature toughness anisotropy may be as large as 1.6. In different heats of INCONEL alloy, the diretion with minimum frature toughness is dependent on the aging time. The small punh test parameters an be used to predit the mehanial property anisotropy in materials by using the relationship of equations (1)-(4). The ultrasoni attenuation oeffiient af ould be exploited to haraterize the anisotropy in materials. However, the relation between the ultrasoni attenuation oeffiient af and the mehanial properties is not linear and not well understood. ACKNOWLEDGMENTS Ames Labaratory is operated for the US Department of Energy by Iowa State University under ontrat No. W-7405-ENG-82. This work was supported by the Offie of Basi Energy Sienes, Division of Materials Sienes. REFERENCES 1. R.L. Kennedy, W.D. Cao and W.M. Thomas, Advaned Materials & Proesses, 33, 3/ J.W. Brooks and P.J. Bridges, Advaned Materials & Proesses,1431, 3/ W.G. Reuter, A.M. Porter, and N.M. Carlson, SPIE Conferene Proeedings, Volumes , WA: SPIE, J.M. Baik, J. Kameda and 0. Buk, Sripta Metall 17 (1983). 5. J.M. Baik, J. Kameda and 0. Buk, STP 888, eds. W.R.Corwin and G. E. Luas ( Philadelphia, PA: ASTM, 1986), Buk, Journal of Metals, p17, Ot., W.N. Reynolds and R.L. Smith, British J. of NDE,291,9, R.L. Smith, Ultrasonis, p211, Sept G. Liu, J. Kameda and 0. Buk, " Small Punh Tests for Evaluating the Mehanial Properties of INCONEL Alloy 718", (TEST REPORT). 1457