cm247lc

EFFECT OF PROLONGED THERMAL EXPOSURE ON THE MICROSTRUCTURE OF INVESTMENT CAST NICKEL BASE SUPERALLOY - CM247LC

By

M.JayarajL.MathiyazhaganG.MuralidharanT.Sivanesan

Final Year - Metallurgy

OBJECTIVENickel base super alloys are extensively used in gas turbines for aero engine applications, which demand good mechanical properties and long life at elevated temperatures. During service,CM247LC alloy will be operated at a temperature of around 900°C in jet fuel starter (JFS) as stator and rotor. So, the work was aimed at studying micro structural stability at operating temperature

INTRODUCTION•CM-247 LC is a low Carbon, modified chemistry of Mar –M-247 alloy, specially designed for turbine blade and vane applications.

•The primary alloying modification are the reduction of ‘C’ by approximately one-half to improve carbide microstructure, stability and alloy ductility, plus the tailoring of the Zr and Ti contents to improve DS grain boundary cracking resistance without sacrificing strength.

•W and Mo levels in the alloy are slightly reduced to compensate for the lower C and Ti concentrations, thereby minimizing the formation of deleterious secondary M 6 C platelets, µ phase and /or alpha W platelets •The lower Ti content in CM-247LC compare to Mar-M247 is to significantly reduce the size of γ-γ’ eutectic nodules as well as to lower the volume fractions of eutectic form approximately 4 volume % in Mar-M247 to 3 volume % in CM-247LC directionally solidified (DS) components.

CHEMICAL COMPOSITION OF ALLOY CM-247LC ( wt% )Element Min. Max.

C 0.075 0.085

Si - 0.03

Mn - 0.01

S - 10 ppm

Ag - 5 ppm

Al 5.4 5.7

B 0.01 0.02

Bi - 0.3

Nb - 0.01

Co 9 9.5

Cr 8 8.5

Cu - 0.005

Fe - 0.15

Ga - 15 ppm

Element Min Max

Sn - 15 ppm

Sb - 50 ppm

As - 50 ppm

Zn - 50 ppm

Hg - 50 ppm

U - 50 ppm

Th - 50 ppm

Cd - 50 ppm

Ge - 0.1

V - 50 ppm

Au - 50 ppm

In - 50 ppm

Na - 50 ppm

K - 50 ppm

Element Min Max

Hf 1.4 1.6Mg - 80 ppm

Mo 0.4 0.6N - 10 ppm

Ni Bal BalO - 10 ppm

P - 0.005Pb - 2 ppmSe - 1 ppmTa 3.1 3.3Te - 0.3Ti 0.6 0.9Tl - 0.3W 9.3 9.7Zr 0.007 0.02

EXPERIMENTAL WORK•Samples of 10mm diameter and 15mm long were taken from the disc of cut-up rotor casting.•The samples were subjected to a standard solution heat treatment in a vacuum heat treatment furnace of Degussa make.•Double aging followed by thermal exposure was carried out in CRAFT’SMAN air heat treatment furnace with Silicon carbide (SiC) as heating element.•Six samples were subjected to thermal exposure at 900°C for the following hours (100,125,150,175,200 and 225) later these samples are studied under optical and scanning electron microscope (SEM).Then samples were subjected to Vickers hardness test.

Vacuum Heat Treatment Furnace

CRAFTSMAN Air Heat Treatment Furnace

Tensile sample &Integral

Standard heat-treatment cycle for solution treatment & double ageing

INVESTMENT CAST CM247LC

SOLUTION TREATMENT & DOUBLE AGEING

EXPOSURE-1100 HOURS AT 900°C

EXPOSURE-2

125 HOURS AT 900°C

EXPOSURE-3150 HOURS AT 900°C

EXPOSURE-4175 HOURS AT 900°C

EXPOSURE-5200 HOURS AT 900°C

POWER SAW CUTTING

ISOMET CUTTING

MOUNTING

SEM ANALYSIS

VICKERS’ HARDNESSEXPOSURE-5225 HOURS AT 900°C

EXPERIMENTAL WORK-FLOW CHART

SAMPLE PREPARATION

Seven samples from two equi-axed rods of CM247LC alloy casting disc were taken and cut into seven pieces using ISOMET for Metallography

OPTICAL MICROSCOPY

•All the seven samples (6 exposed & 1 unexposed) were mounted by using SIMPLIMET 3000 Machine. After mounting, the samples were metallographically grinded starting from a variety of emery paper grades like 150,320,400,600,800 and 1000, followed by diamond disc polishing with various grades like 9µ, 3µ, 1µ and 0.5µ•Etchant of following chemical composition: Hydrochloric acid - 45ml

Nitric acid - 30mlGlacial acetic acid - 30mlMethyl alcohol - For post cleaning purpose

•observed under magnifications 50x, 100x, 200x, 500x and microstructures were recorded.

RESULTS AND DISCUSSIONS

OPTICAL MICROSCOPE•The Metallography samples revealed the various microstructural features such as grain size, grain boundary carbides and γ-γ’ eutectic at grain boundary when observed at 200x. γ’ was discernable at higher magnifications. Gamma prime was found to be coarse and irregular at eutectic regions

SCANNING ELECTRON MICROSCOPY (SEM)

•studied under SEM for calculating γ’ sizes, Morphology oberved and recorded at 10000X .• Secondary gamma prime precipitates were observed to be fairly uniform and cuboidal in shape and showed in STA sample. The tertiary gamma prime was observed in the gamma corridor between secondary gamma prime precipitates in the E1 – E6 samples. • The morphology of discrete grain boundary carbides and the composition of these carbides has been revealed mainly as TaC

SEM micrographs showing the γ׳size (0.7136µm) and cuboidel Morphology of STA exposed sample

at 10000X

SEM micrographs showing the γ׳size (0.8905µm) and Morphology of 100 h exposed sample

at 10000X

SEM micrographs showing the γ׳size (0.8805µm) and Morphology of 125 h exposed sample

at 10000X

SEM micrographs showing the γ׳size (1.0419µm) and Morphology of 150 h exposed sample

at 10000X

SEM micrographs showing the γ׳size (0.9370µm) and Morphology of 175 h exposed sample

at 10000X

SEM micrographs showing the γ׳size (0.8982µm) and Morphology of 200 h exposed sample

at 10000X

SEM micrographs showing the γ׳size (0.9561µm) and Morphology of 225 h exposed sample

at 10000X

SEM micrographs showing the size and Morphology of Grain boundary carbides at 2000X

γ’ SIZE

• Average γ’ sizes of the samples STA and E1 to E6 were measured at 10000x.

• The sides were measured in relation to the micron marker on the micrograph and the values were averaged. The values of γ’ sizes as calculated from the micrographs

CONDITION γ’ SIZE(µm)

STA 0.7136

E1 0.8905

E2 0.8827

E3 1.0419

E4 0.9370

E5 0.8982

E6 0.9561

γ’ SIZE(µm)

0

0.2

0.4

0.6

0.8

1

1.2

STA E1 E2 E3 E4 E5 E6

Exposure time, hours

γ’

SIZ

E(µ

m)

γ’ SIZE(µm)

GAMMA PRIME SIZE

VICKER’S HARDNESS TESTING

• From the graph hardness values starts to decrease from STA sample and again slightly increases at E6 (225hours).

• The data of γ’ size and hardness show opposite trends though in a very gradual manner. The exposure time appears to affect size of γ’ and at the same time decrease in hardness

VICKERS HARDNESS (VHN)

280290300310320330340350360370

STA E1 E2 E3 E4 E5 E6

Thermal Exposure,Hours

Har

dn

ess,

VH

N

VICKERSHARDNESS(VHN)

CONDITIONEXPOSURE

HOURS

VICKERS HARDNESS

(VHN)

STA - 360

E1 100 332

E2 125 328

E3 150 318

E4 175 316

E5 200 312

E6 225 314

VICKER’S HARDNESS RESULT

CONCLUSIONS

• γ’ size variations was studied with respect to exposure time. There is overall increase in γ’ size with exposure time from 0.71 at 100h to 0.95 at 225h.however Max of 1.04 at 150h.

• γ’ shape was also observed with respect to exposure time. There is overall change in γ’ shape with respect to exposure time. cuboidal morphology was observed at low exposure time. While at higher exposure time rounded, blocky and less cuboidal morphology of γ’ was observed.

• Hardness is decreasing with increasing in exposure time.

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PERFORMANCE: K.Harris, G.L.Erickson and R.E.Schwer Cannon-Muskegon Corp.7. Superalloys – Processing : Proceedings of the international conference Sept.18-

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Publishers 1977.13. Heat Treater’s Guide Practice And Procedure for Non Ferrous Alloys 199914. Worked Examples in Quantitative Metallography : R.L. Higginson and C.M Sellars 200315. Metallography Etching ,2nd Edition : Günter,Petzow16. Metallography principles and practice: George F. Vander Voort 2007.

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