the influence of compressive residual stress on a … 1125 grant gibson.pdfthe influence of...

Parson’s 2011Rolls-Royce University Technology Centre in Materials

The Influence of Compressive Residual Stress on a

Hot Corrosion Process Within the Gas Turbine

Grant Gibson 1, Jonathan Leggett 2 and Karen Perkins 3

1,3 Rolls-Royce University Technology Centre,

Materials Research Centre, School of Engineering,

Swansea University, SA2 8PP. United Kingdom

2 Rolls-Royce plc, P.O. Box 31, Derby, DE24 8BJ. United Kingdom

Acknowledgements – Cranfield University, Loughborough University and Metal Improvement Company


Company Background

Rolls-Royce Product Locations


Company Background

Civil Aerospace Portfolio


Introduction

Efficiency requirements

Set by the customer and Governments

Emissions targets

Set by Governments

Turbine entry temperatures have increased

Hot components spend more time at temperature

Leading to high temperature corrosion


Key Conclusions

Shot peening changes the rate of hot corrosion

Non shot peened specimens corroded less than shot peened specimens

Optimisation of shot peening for high temperature corrosion and

corrosion-fatigue desireable in the gas turbine engine


Area of Focus

Hot Section Components


Hot Corrosion

Corrosion rate also depends on deposition rate and composition of salt


Corrosion Pits

Pits forming in components may ultimately compromise integrity

CONVEX SIDE

General view of corrosion

scales

Corrosion pit with crack


Typical Pitting Mechanism


Shot Peening

Variety of methods

Introduce a compressive stress close to the surface of the component or

specimen

Shot peening is a cold work process using small shots of metal, glass or ceramic

Used industry wide to improve fatigue and corrosion-fatigue response

Little evidence in published literature with regard to hot corrosion

http://www.metalimprovement.com/shot_peening.php


Application of Shot Peening in the Gas Turbine

Hot components are shot peened for fatigue improvements

Stress relaxation

Loughborough University have shown that shot peening imparts a permanent

layer of mis-orientated grains into the surface of hardware

Shot peened layer can be redefined as the region of strain hardened material

Results from RR1000 and service show no fatigue deficit when corrosion is within

the shot peened layer


Shot Peening

Ln

(A

ltern

ati

ng

str

es

s –

Go

od

ma

n c

orr

ec

ted

)

Ln (Nf Number of Cycles to Failure)


NASA and General Electric

Pre-corroded under high salt flux at 704°C, then air fatigue tested at

704°C

60-98% reduction in fatigue life

The Effects of Hot Corrosion Pits on the Fatigue Resistance of a Disk

Superalloy, T.Gabb, J. Telesman, B. Hazel and D.P. Mourer, 2008


Surface Roughness and Depth Measurements

EBSD tool

Measurement of strain hardened layer

Directly proportional to the level of permanent grain mis-orientation

Green region indicates local grain to

grain mis-orientation

Surface of material


Shot Peening and Corrosion Trials

Shot peening improves fatigue response in pre-pitted specimens

Very little information in literature regarding shot peening and hot

corrosion

Investigations have begun to attempt to optimise shot peening for high

temperature corrosion and corrosion-fatigue in nickel based alloys


Shot Peening Matrix

Range of shot peening intensities and shot sizes used

Material – 720Li nickel based superalloy

Notation Intensity / AShot Type /

HCoverage / %

GA0 - - -

GA1-3 1-3 110 200

GA3-5 3-5 110 200

GA6-8 6-8 110 200

GA8-10 8-10 110 200

GB70 6-8 70 200

GB110 6-8 110 200

GB170 6-8 170 200

GB230 6-8 230 200

GB330 6-8 330 200

ø=5

10

All dimensions

in mm



Depth of strain hardening

increases with intensity and

shot size

Surface roughness increases

with intensity, less so with

shot size



Initial un-corroded samples highlighted 6-8A, 330H, 200% coverage

Most desireable



Type II hot corrosion trials undertaken at Cranfield University

98% Na2SO4 – 2% NaCl

300 hours, 50 hour recoats

A

B

C

Datum

24 measurementsA – Thickness of oxide

B – Pit Depth

C – Internal Corrosion Depth


Depth and Surface Roughness Measurements

0

5

10

15

20

25

30

35

0 20 40 60 80 100 120

Max

imu

m S

ect

ion

Lo

ss i

n M

icro

ns

afte

r 3

00

Ho

urs

Depth of Strain Hardened Material in Microns

Depth of Strain Hardening Against Maximum Section Loss in Microns

Unpeened

6-8A 230H

1-3A 110H

6-8A 110H

6-8A 110H

6-8A 170H

6-8A 330H

6-8A 70H

8-10A 110H

3-5A 110H6-8A, 230H, 200%

coverage identified as optimised

corrosion protection condition

6-8A, 330H, 200%

coverage one of the worst

Unpeened


Depth and Surface Roughness Measurements

0

5

10

15

20

25

30

35

0 0.5 1 1.5 2 2.5

Max

imu

m S

ect

ion

Lo

ss i

n M

icro

ns

afte

r 3

00

Ho

urs

Surface Roughness in (SA) in Microns

Surface Roughness Against Maximum Section Loss in Microns

Unpeened

6-8A 230H

1-3A 110H

6-8A 110H

6-8A 110H

6-8A 170H

6-8A 330H

6-8A 70H

8-10A 110H

3-5A 110H

6-8A, 330H, 200%

coverage one of the worst

6-8A, 230H, 200%

coverage identified as optimised

corrosion protection condition

Unpeened


Key Conclusions

Corrosion features within the shot peened layer show no debit in fatigue response

Shot peening affects the corrosion rate

Initial trials identified 6-8A 330H 200% coverage as optimised condition

Corrosion trials showed that 6-8A 230H 200% had the lowest section loss of all

the matrix of peening conditions


Future Work

Air fatigue trials at 700°C have been undertaken on a smaller matrix of conditions

Results show that the best shot peening condition for corrosion gave a similar

fatigue response to that of the standard peening condition

Investigation of a duplex peening process to assess the role of depth and surface

roughness on corrosion, fatigue and corrosion-fatigue performance

Corrosion-fatigue trials to be undertaken later this year at Swansea University on

the reduced matrix of conditions

the influence of compressive residual stress on a … 1125 grant gibson.pdfthe influence of...

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