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mailto:[email protected]

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OUTLINE:

• As-recieved material characterizationChemical composition

Microstructural analysis

Mechanical properties

• Experimental Procedure

LSP parameters and annealing conditions

Surface roughness measurement

Residual stresses measurement

Fatigue test

• Experimental resultsResidual stresses

Fatigue Life Tests

• Concluding remarks

• Motivation

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1. MOTIVATION

● The enhance of fatigue resistance of metallic components due to thepresence of near-surface compressive residual stresses by laser peening hasbeen widely demonstrated.

● Many metallic parts used in industrial applications are subjected to cyclicloadings , high temperatures or both of them simultaneously.

● From a practical point of view, it is important to study the stability of theresidual stresses induced by laser peening, as well as the fatigue life of thematerial, under high temperature conditions.

● In the present communication, the level of relaxation of the residual stressesinduced by laser peening in stainless steel 316L after a thermal treatment,and its fatigue performance before and after the thermal treatment ispresented.

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2. MATERIAL CHARACTERIZATION

Hot-rolled plates of stainless steel 316L

Thickness: 6 mm

Chemical composition AISI 316L stainless steel (%)

Casting C Cr Mn Mo N Ni P S Si

T7C9 0.018 16.46 1.90 2.34 0.047 9.78 0.032 0.003 0.26

*Balance Fe (69.203%)

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• Microestructural analysis

– Optical microscopy – SEM

δ-ferritestringers


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• Microestructural analysis

– DRX

50 60 70 80 90 100 110

0

20

40

60

80

100

-Fe

(211)

-Fe

(200)

-Fe

(311)

-Fe

(220)

-Fe

(200)

Rel

. Int

ensi

ty (%

)

-Fe

(111)

3.5 wt% of -ferrite (from Rietveld refinement)


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• Mechanical properties

– Tensile test

0 5 10 15 20 25 30 35 40 45 50 55

0

100

200

300

400

500

600

700

Str

es

s (

MP

a)

Strain (%)

Specimen1

Specimen2

Specimen3

Specimen4

Tensile test base material

Young modulus (GPa) 177.205

Yield strength (MPa) 355.410

Ultimate yiled strength (MPa) 633.608

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3. EXPERIMENTAL PROCEDURE

• LSP parameters and annealing conditions

Process parameters

Wavelength (nm) 1064

Frecuency (Hz) 10

Energy (J/pulse) 2.8

Pulse width (ns) ~ 9

Spot diameter (mm) ~ 1.5

Overlapping (pulses/cm2)900

1600

Confining medium Water jet

Absorbent coating No

Thermal treatment

Temperature (°C) 500

Time (hours) 8

Atmosphere Air

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• Surface roughness measurement

– Topographic confocal microscopy

Confocal laser scanning microscope Leica DCM 3D

Measuring conditions (according to ISO 4287)

lc (cut-off) 2.5

Evaluation length (mm) 12.5

Z-step (mm) 2

Parameter Meaning Formula

RaArithmetic average of absolute values Z(x)

Rv Maximun valley depth

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• Residual stress measurement

Residual Stresses Measurement Equipment (According to ASTM E837-08)

CEA-XX -062UM -120CEA-XX -062UM -120

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• Fatigue test

“Dog-Bone” shapedSpecimens machined

according ASTM E 466

MTS 810 servo-hydraulicmachine with load cell of 100 kN

R=0.1Frecuency=10 HzRoom temperature

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4. EXPERIMENTAL RESULTS

• Surface roughness

No LSPLSP 900

LSP 1600

LSP 900 TT

LSP 1600 TT

0,00,51,01,52,02,53,03,54,04,55,05,56,06,57,07,5

Ra

(mm

)

No LSPLSP 900

LSP 1600

LSP 900 TT

LSP 1600 TT

0

2

4

6

8

10

12

14

16

18

20

Rp

(mm

)

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• Microstructure

900 pulses/cm2 1600 pulses/cm2


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• Ressidual stresses

0,0 0,2 0,4 0,6 0,8 1,0-600

-500

-400

-300

-200

-100

0

Before annealing

After annealingRe

sid

ua

l s

tre

ss

es

(M

Pa

)

Depth (mm)

900 pul/cm2

0,0 0,2 0,4 0,6 0,8 1,0-600

-500

-400

-300

-200

-100

0

Before annealing

After annealing

Re

sid

ua

l s

tre

ss

es

(M

Pa

)

Depth (mm)

1600 pul/cm2


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• Fatigue life

150

160

170

180

190

200

210

220230240250260270280290300

100000 1000000

Base material

LSP 900

LSP 1600

Cycles to failure

Sa (

MP

a)

LogN=16.33764-4.79302LogSa

R2=0.99760

LogN=22.51020-7.35620LogSa

R2=0.98967

LogN=21.09071-0.01178Sa

R2=0.87937


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• Fatigue life

150

160

170

180

190

200

210

220230240250260270280290300

100000 1000000

Mat. Base

LSP 1600

LSP 1600TT

LogN=24.55449-8.1898LogSa

R2=0.84671

Cycles to failureS

a (

MP

a)

LogN=16.33764-4.79302LogSa

R2=0.99760

LogN=20.45691-6.45103Sa

R2=0.89350

150

160

170

180

190

200

210

220230240250260270280290300

100000 1000000

Mat. Base

LSP 900

LSP 900TT

Cycles to failure

Sa (

MP

a)

LogN=16.33764-4.79302LogSa

R2=0.99760

LogN=22.51020-7.35620LogSa

R2=0.98967

LogN=19.9092-6.27327LogSa

R2=0.92514


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5. CONCLUDING REMARKS

● The effect of two representative LSP treatment intensities on the residual stresses andfatigue resistance has been analyzed for stainless steel 316L, in view of the excellentproperties of this alloy in a significant number of industrial applications.

● These two laser peening intensities induce in the material a similar compressive stressdistributions that achieve peak values between 300 and 400 MPa, and extend up to 1 mmfrom the surface.

● The significant residual stress fields induced by laser treatment improves the fatigue limitof stainless steel approximately 25 % in comparison with the unpeened material.

● The thermal treatment at 500 °C for 8 hours partially relieves the residual stresses,specially near the surface, but a significant compressive stress field remains (rangingfrom 200 to 300 MPa) between 100 and 600 microns below the surface. This can be due tothe irreversible character of a significant proportion of mechanical dislocations induced bythe LSP.

● The direct and desirable effect of this remaining RS field is a remarkable degree ofpermanence of the enhanced fatigue behaviour. Concretely, an improvement in fatiguelimit of about 12% is maintained with regard to the pristine material.

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ACKNOLEDGEMENT

The authors would like to thank the Spanish Ministry of Economy and Competitiveness(MINECO) for financial support under project MAT2012-37782.

[email protected]

THANK YOU FOR YOUR ATTENTION

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