The Effect of Rejuvenation Heat Treatments on Gamma Prime Distributions in a Ni-based ... 1100... · 2015-02-24 · The Effect of Rejuvenation Heat Treatments on Gamma Prime Distributions

The Effect of Rejuvenation Heat

Treatments on Gamma Prime

Distributions in a Ni-based Superalloy

for Power Plant Applications

Zhiqi Yao1, Craig Degnan2, Mark. A.E. Jepson1

& Rachel C. Thomson1

1. Loughborough University

2. E.ON New Build & Technology

8th Charles Parsons Conference

Presentation outline

Background – Why are we doing this?

Experimental

Results & Discussion

Conclusions

Suggested further work


1st (and 2nd) stage Blades

Advanced Superalloys (cooled & coated)

Nickel

matrix

(15-20%)

0.5 μm

’ (80-85%)

Superalloy Cr Co Mo W Ta Al Ti Re Hf Ni

CMSX4 5.7 11 0.42 5.2 6.6 5.2 0.74 3 0.1 Bal.

Background


’ Precipitation Strengthening (cont…)

The microstructure is unusually stable. However, ’ precipitates do

coarsen very slowly with high temperature service exposure

’ strengthened superalloy

(virgin microstructure)

’ strengthened superalloy

(service degraded microstructure)


Why a degraded (rafted) microstructure is bad

“Stress induced directional diffusion”

or

“directional coarsening”

or

“Rafting”

Time /temp./stress

Time /temp./stress

Free path for dislocation

movement → reduced creep

resistance

More continuous weaker matrix

→ less resistance to fatigue

cracking


ACTUAL PLANT FAILURE! - Fatigue cracking along continuous Ni matrix

Rafting usually only apparent to a depth of 1-2 mm on the hottest/most stressed part of the aerofoil.

However, once initiated, fatigue cracks can propagate through the

whole blade and cause a catastrophic release event ~ £1 million/per stage


Once CMSX4 GT blading has become degraded it is usually scrapped

If we can rejuvenate the microstructure of blades (i.e.

eliminate rafted microstructure) then the potential

savings are huge

Heat treatments employed during the coating /recoating process are ineffectual at rejuvenating the blade microstructure (usually performed around 1140 ºC)

Higher temperature, complex heat treatments may be able to be used to rejuvenate blading but there are lots of pitfalls (including insipient melting, distortion, etc)


Experimental

As-received CMSX4 (cuboidal structure – solution and aged) subjected to various heat treatments to evaluate microstructural evolution and associated properties

Sample I.D Heat treatment

S1 As-received

S2 Rafted (1050 ºC for 1000 hours)

S3 Rafted and Rejuvenated

S4 Rafted, Rejuvenated and Rerafted

S5 Rafted and 1140 ºC HT

Proprietary solution and ageing treatment

Simulated “return to service”

Spontaneous degradation mechanism

Samples subjected to: Electron microscopy (FEG-SEM & TEM)

Image analysis to determine inter-γ’channel widths Samples subjected to room temperature tensile testing, elevated temperature

tensile testing, low cycle fatigue testing and creep rupture testing – only RT tensile results reported here


Results

0 5 10 15 20 25 30 35

0

200

400

600

800

1000

Elongation (%)

Str

es

s (

MP

a)

S1

S2

S3

S4

S5

S1 – As-received

S1 – Rafted & low temp. HT

S4 – Rafted, Rejuvenated & ReraftedS3 – Rafted & Rejuvenated

S2 – Rafted

As-received and rejuvenated material exhibit similar properties.

Rejuvenated material that was subsequently rerafted exhibits poorer strength than the one-time rafted material (but increased ductility)


As-received

Rejuvenated

Rafted

Rejuvenated & Rafted

Rafted + low temp. HT

Decreasing

Strength/resistance

This ranking order of properties has been shown to be repeated in exactly

the same way with hot tensile tests, low-cycle fatigue tests and creep rupture tests

It has large implications on the way we utilise refurbished blades!

We cannot expect the same life out refurbed blades as new

blades even though, from an initial inspection of their properties

and microstructures, they look identical WHY?


Microstructural Examination (FEG-SEM)

S1: As-received

S4: Rejuvenated & ReraftedS3: Rejuvenated

S2: Rafted


Microstructural Examination (TEM)

S1: As-received

S4: Rejuvenated & ReraftedS3: Rejuvenated

S2: Rafted

Lots of tertiary γ’

in channels

(>100 nm)

Tertiary γ’ in

channels too

small to resolve

(<100 nm)

Some tertiary γ’

in channels

(>100 nm)


The presence of the “large”

tertiary γ’ precipitates in the

channels will impede the

movement of dislocations

which, in turn, will increase

the UTS (but reduce

ductility)S2: Rafted

The very small tertiary γ’

precipitates offer little

obstruction to dislocation

movement and hence UTS

is reduced (but ductility

increased)

0 5 10 15 20 25 30 35

0

200

400

600

800

1000

Elongation (%)

Str

es

s (

MP

a)

S1

S2

S3

S4

S5

Rafted Rejuvenated &

Rerafted

S4: Rejuvenated & Rerafted


S 1 S 2 S 3 S 4 S 5

0

50

100

150

200

250

300

350

400

Ch

an

ne

l W

idth

(n

m)

- H

ea

ds

Sample No

Parallel

Perpendicular

Rafted +

1140 oC for 6 h

Rafted +

Rejuvenated

+Rafted

Rafted +

Rejuvenated

Rafted

As Received

γ Channel Width MeasurementMeasured, using image analysis, in the perpendicular and parallel directions with respect to the <001> orientation

As-received and rejuvenated samples both have very narrow (~30 nm) channel widths. Dislocation movement is impeded and yield/UTS is, therefore, high.

The channels in the rafted samples (S2 and S4) are wider than those in the as-received and rejuvenated materials. This allows easier movement of dislocations and hence a reduction in strength.

The rerafted sample (S4) has a significantly narrower channel width in the perpendicular direction than that of the rafted sample (S2). This should lead to an increase in strength but it doesn’t - suggests that, above a certain width,

tertiary γ’ precipitate size becomes more dominant in determining strength.


Conclusions

Tensile properties developed in CMSX4 in various states of degradation result

from changes in the secondary γ’ channel width and the precipitation of tertiary γ’ precipitates in the channels

Rejuvenated CMSX4 initially demonstrates equivalent properties to that of new material. However, mechanical properties will degrade either faster or to a greater extent when returned to service.

i.e. the expected service life of a refurbished blade will be less than that of a new blade.

Rejuvenated blading is economically very attractive but if it were to be used in turbo machinery increased inspection rates and decreased service intervals would have to be implemented


Further work

Establish if rejuvenated CMSX4 degrades more quickly than new

material or at the same rate but to a greater extent

Examine the more pragmatic elements of blade rejuvenation such as

distortion and localised heat treatments

Documents

The Effect of Rejuvenation Heat Treatments on Gamma Prime Distributions in a Ni-based ... 1100... · 2015-02-24 · The Effect of Rejuvenation Heat Treatments on Gamma Prime Distributions