CM247LC

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

By M.Jayaraj L.Mathiyazhagan G.Muralidharan T.Sivanesan Final Year - Metallurgy

OBJECTIVE Nickel 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 M6C 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.

C

Max.

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

Element

Min

Max

Hf Mg

1.4 -

1.6 80 ppm

Sn

-

15 ppm

Sb

-

50 ppm

Mo N

0.4 -

0.6 10 ppm

As

-

50 ppm

Zn

-

50 ppm

Ni O

Bal -

Bal 10 ppm

Hg

-

50 ppm

U

-

50 ppm

P Pb Se Ta Te Ti Tl W Zr

3.1 0.6 9.3 0.007

0.005 2 ppm 1 ppm 3.3 0.3 0.9 0.3 9.7 0.02

Th

-

50 ppm

Cd

-

50 ppm

Ge

-

0.1

V

-

50 ppm

Au

-

50 ppm

In

-

50 ppm

Na

-

50 ppm

K

-

50 ppm

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

EXPERIMENTAL WORK-FLOW CHART INVESTMENT CAST CM247LC

SOLUTION TREATMENT & DOUBLE AGEING

POWER SAW CUTTING

EXPOSURE-1 100 HOURS AT 900°C ISOMET CUTTING EXPOSURE-2 125 HOURS AT 900°C

EXPOSURE-3 150 HOURS AT 900°C

MOUNTING

EXPOSURE-4 175 HOURS AT 900°C SEM ANALYSIS EXPOSURE-5 200 HOURS AT 900°C

EXPOSURE-5 225 HOURS AT 900°C

VICKERS’ HARDNESS

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 30ml Glacial acetic acid 30ml Methyl 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 E 1 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

GAMMA PRIME SIZE 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)

1.2 1

   )    m0.8    µ    (    E 0.6    Z    I    S    ’ 0.4    γ

γ’ SIZE(µm)

0.2 0 STA

E1

E2

E3

E4

E5

Exposure time, hours

E6

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

VICKER’S HARDNESS RESULT EXPOSURE

VICKERS HARDNESS

HOURS

(VHN)

STA

-

360

E1

100

332

E2

125

328

E3

150

318

E4

175

316

E5

200

312

E6

225

314

CONDITION

VICKERS HARDNESS (VHN) 370 360       N 350       H 340       V   ,      s 330      s      e      n 320       d 310      r      a       H 300 290 280

VICKERS HARDNESS (VHN)

STA

E1

E2

E3

E4

E5

Therma l Exposure,Hours

E6

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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THANK YOU