2
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
3
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.
4
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%)
5
• Microestructural analysis
– Optical microscopy – SEM
δ-ferritestringers
2. MATERIAL CHARACTERIZATION
6
• 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)
2. MATERIAL CHARACTERIZATION
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2. MATERIAL CHARACTERIZATION
• 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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3. EXPERIMENTAL PROCEDURE
• 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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3. EXPERIMENTAL PROCEDURE
• Residual stress measurement
Residual Stresses Measurement Equipment (According to ASTM E837-08)
CEA-XX -062UM -120CEA-XX -062UM -120
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3. EXPERIMENTAL PROCEDURE
• 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
4. EXPERIMENTAL RESULTS
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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
4. EXPERIMENTAL RESULTS
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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
4. EXPERIMENTAL RESULTS
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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
4. EXPERIMENTAL RESULTS
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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.
THANK YOU FOR YOUR ATTENTION
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