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  • NRL Memorandum Report 5431

    Mechanical Properties and Microstructure ofCentrifugally Cast Alloy 718

    D. J. MICHr AND H. H. SMiTH

    Thernosuctwral Mauriah BranchMawrial Sciee and Technoloky Divion

    I 5

    0o I.CoN November 14, 1984

    " "T

    NAVAL RESEARCH LABORATORYAc W ashington, D.C.

    Aplxoved for public feeas; distfbutiun unlimited.'. .I8 1

    84102 0

    .".0

  • REPORT DOCUMENTATION PAGE'a QFDOA, SE-, .ASS .C.V- CN 'b 4ESTR:CTIVE MARKCINGS

    UNCLASSIFIED ...__ _ _ _ _.2~ tC~R:T ZAS:~CA~ON .- Gfl"3 S-RISUTIONAlAiLASILITY 0; REPORT

    2 C.SSF.,[ON, 3OWG5ADG SE Approved for public release; distribution unlimited.

    4 PtRFiORMiNG ORGAN;ZAT:ON qESORT NQMUItR($) 5 MONITORING ORGANIZATION REPORT %iUMSER(S)

    NRL Memorandum Report 54316 NA AME OPF MONINTORING ORGANIZATION

    Naval Research Lab orary Code 63 9 0 Naval Air Systems Command

    6c ADODESS Cry. State. dnf ZIP QOe) Ab aDDRESS ,City, State. and ZIP C0ov81

    Washington, DC 20375-50C'O. Washington, DC 20361

    Ba. NAME OF 'UNOING,SPOPJSORING Stb OFFICE SYMBOL 9 PROCJREMENT iNSTRUMENT IDENTIFICATION NUMBERORGANIZAr'ON (if j0Iicable)

    Naval Material Command MAT 064 "_" _ _SC ADORESS (City, State. and ZIP Code) 10 SOURCE OF FUNDING NUMBERS

    Washington, DC 20360 PROGRAM PROJECT TASK WORK UNITELEMENT NO NO NO. ACCESSION NO

    DN291-257 -TITLE (i'clude Scucty CIlrthficroi.n)

    Mechanical Properties and Microatructure of Centrifugally Cast Alloy 718

    1: PERSONAL. AUIOR(S)Michel, D.J. and Smith. H.H.

    '3a "'YE OF REPORT 1lb TIME COVERED '4 DATE OF REPORT (Yesr, MonPh, Olyj S *AGE COUNTFinal ;ROM 10182 ro 5/84 1984 November 14 32

    '6 SUPOLI.6MEINARY FjOTATiON

    !7 COSATI CODES IS SUBJECT TERMS 'Contifnue on reverie if necessary and identify by block number),EL.D 4ROUP iSUB-GROUP Tensile strength Fatigue Mechanical properties

    _ Creep-rupture strength Crack propagation Elevated temperatures

    "'. .- OSTRACT fConlinue On revere# if neceslury and identify by block njmberl)rTh relationship between the microstructure and mechanical properties of alloy 718 was investigated for two

    ditic" centrifugally cast at 50 and 200 rpm and %iven a duplex age heat treatment. The results of mechanicalproperty tests at temperatures from 426 to 649 C showed that the tensile yield and ultimate strength levels ofboth castings were similar. However, the creep-rupture properties were considerably enhanced for the castingproduced at 200 rpm. Comparison of the radial and transverse creep properties of each disc indicated thatcreep life was generally independent of orientation, but ductility was greatest for specimens oriented transverseto the radial direction of the casting. Fatigue crack propagation performance was not greatly influenced byorientation or mold speed parameters and was comparable to wrought Alloy 718 when compared on the basisof stres. intensity factor range. The centrifugal casting process was found to produce a homogeneous micro-structure free of porosity but with the expected segregation of solute alloying elements to Laves and carbidephases. The Pfrect of the as-cast microstructure on the mechanical behavior and the potential influence of hotisostatic pressing to improve the microstructure are discussed.

    .0 * 1 ON AVAILA8IL,1i' OP ABVSRAC' 21 ABSTRACT SECURIT' C.ASSIFICATIONSCA5S.FtE.. .Ni MrED0 S AME AS RPr [ DTIC SERS UNCLASSIFIED

    "a 'AME ')9 ESONS,BLE NOIVIDtUAL 22!3 TrELONE (include Ar@e Code) 22c. 07PI,CE SYMBOLD. J. Michel (202) 767-2621 Code 6390

    DO FORM 1473, 54 MAP 53 APR e5i.t~on may 1e 14d .. tll, svna6s1ldA!: Othef edioni AtO OBSOIlte

    iEC.IRIrY CLASSIFiCAT;ON OF 'HS PAGE

    ........................................

  • CONTENTS

    [NTOUTIOR........................... 1

    EXPERIMENTAL PROCEDURES ................. 1

    - - - Material-and Heat-Treatments ................ ... 1-Specimen Preparation......................................... 2IMechanical Property Evaluations................................. 2Microscopy and Analysis....................................... 4

    -- RESULTS................................................... 4

    Tensile Test Results.......................................... 4Creep-Rupture Test Results.................................... 5Fatigue Crack Propagation Teat Results............................ 5Metallographic Results........................................ 5 01Fractographic Results ........................................ 21

    DISCUSSION ................................................ 21 .--

    SUMMARY AND CONCLUSIONS................................. 26

    ACKNOWLEDGMENTS ........................................ 27

    REFERENCES............................................... 28

  • MECHANICAL PROPERTIES ANDMICROSTRUCTURE OF CENTRIFUGALLY CAST ALLOY 718

    INTRODUCTION

    S-Alloy 718- is a -complex precipitation hardened, nickel-base alloy widelyused for gas turbine engine components. Currently, alloy 718 is employed inthe form-of-highly stressed components produced by forging and- machiningoperations. As such, these components are costly in terms of labor, capitalequipment and energy input. However, the mechanical properties of alloy 718cast to shape by conventional methods is marginal to meet the strength,creep-rupture and fatigue crack propagation requirements for enginecomponents,

    The application of advanced casting techniques to improve theproperties of cast alloy 718 will permit the production of components cast pto near net shape thereby eliminating most machining operations. Previouswork has suggested that cast properties can be produced which approach thoseof forgings when the solidification rates and parameters are controlled toachieve the proper microstructure.

    The purpose of this report is to present the results from a study ofthe properties and microstructure of centrifugally cast alloy 718 producedwith two mold speeds. The results show that the fatigue properties of thecast alloys were similar to those of the wrought alloy when compared on thebasis of stress intensity factor range. Although the tensile strength ofthe cast alloy was less than for the wrought alloy, the creep properties ofthe cast alloy exceed those of the wrought alloy for creep loads determined -on the basis of fraction of the tensile yield strength of each product form.

    EXPERIMENTAL PROCEDURES

    Material and Heat Treatments

    The chemical composition of the cast alloy 718 and the heat treatmentdetails are given in Table 1. The castings were produced in the form offlat discs (approximately 2.54 cm thick by 25.4 cm outside diameter by 7.62cm inside diameter) using two different mold rotation speeds, 50 and 200rpm. The castings were given the duplex age heat treatment shown in Table Ito develop the Y" precipitate microstructure for increased strength. FigureI shows a photograph of the cast disc and a prototypic compressor impellercast by the centrifugal casting process.Manuscript approved July 9, 1984.

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    TABLE 1

    Chemical Composition and Heat Treatmentof Cast Alloy 718 :41

    Composition (weight percent)*

    C 0.056 Cr 19.60 Co 0.010

    Mn 0.010 Ni 53.40 Fe Hal

    P 0.004 Ti 1.05 Cu 0.010 . ...

    S 0.004 Al 0.48 B 0.003

    Si 0.010 Mo 3.09 Nb/Ta 5.22

    *Vendor supplied analysis.

    Heat Treatment

    Temperature Time

    718 0C (1325 0 F) 8 Hrs. Furnace cool at 560C (100 0 F) per hr.

    621 0 C (11500F) 8 Hrs. to air cool

    Specimen Preparation

    Specimen blanks were cut from the as received castings either above or

    below the mid-plane of the discs. Tensile/creep and 0.5T CT (compacttension) fatigue crack propagation specimens were machined from the blanksin accordance with ASTM Standards E8 and E647 [1,2]. The notch in the CTspecimens and gage length in the tensile/creep specimens were orientedeither in the radial direction or transverse to the radial direction of thecastings. All of the CT specimens were precracked in accordance with ASTMStandard E647 [2] such that the final Kmax during precracking did not exceedthe initial Kmax of the fatigue test.

    Mechanical Property Evaluations

    640 The tensile, creep and fatigue tests were conducted at 427, 538 and :.6490C (800, 1000 and 12000 F) in air. These temperatures were achievedthrough resistance heating except for fatigue tests at 427 and 5380C where .-.induction heating was employed. The reported data represent the mechanicalproperties of the alloy 718 cast at mold speeds of 50 and 200 rpm for radialand transverse specimen orientations.

    Tensile tests were conducted using an Instron Tensile Testing Machine.Crosshead speed was maintained at 1.27x10- 2 cm/min (5x10-3in/min) throughout "-"the test. The calculated values of 0.2% offset tensile yield strength werebased on extension derived from motion of the crosshead. This practice was Sconsidered satisfactory due to the relatively large extensions and low loads

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  • employed for these tests. Reported values of elongation are a result ofpost test measurements of change in overall specimen length.

    Creep tests were performed on lever arm creep frames using specimenswhich were identical to those used for the tensile tests. Test loads werebased on the ratio of the creep stress to the 0.2% offset tensile yieldstress as determined specifically for each condition of test temperature,casting speed and material orientation. Creep extension was recordedcontinuou