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Effect of Thermal Relaxation on LSP Induced Residual Stresses and Fatigue Life Enhancement of AISI 316L stainless steel

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OUTLINE:

• As-recieved material characterization

Chemical composition

Microstructural analysis

Mechanical properties

• Experimental Procedure

LSP parameters and annealing conditions

Surface roughness measurement

Residual stresses measurement

Fatigue test

• Experimental results

Residual stresses

Fatigue Life Tests

• Concluding remarks

• Motivation

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1. MOTIVATION

● The enhance of fatigue resistance of metallic components due to the

presence of near-surface compressive residual stresses by laser peening has

been widely demonstrated.

● Many metallic parts used in industrial applications are subjected to cyclic

loadings , high temperatures or both of them simultaneously.

● From a practical point of view, it is important to study the stability of the

residual stresses induced by laser peening, as well as the fatigue life of the

material, under high temperature conditions.

● In the present communication, the level of relaxation of the residual stresses

induced by laser peening in stainless steel 316L after a thermal treatment,

and its fatigue performance before and after the thermal treatment is

presented.

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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

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• Microestructural analysis

– Optical microscopy

– SEM

δ-ferrite stringers

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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)

Re

l.

In

te

ns

ity

(%

)

 -Fe (111)

3.5 wt% of

-ferrite (from Rietveld refinement)

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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 S tr e s s ( M P 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

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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

Ra Arithmetic average of absolute values Z(x) R

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3. EXPERIMENTAL PROCEDURE

• Residual stress measurement

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

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

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3. EXPERIMENTAL PROCEDURE

• Fatigue test

“Dog-Bone” shaped Specimens machined according ASTM E 466 MTS 810 servo-hydraulic

machine with load cell of 100 kN

R=0.1

Frecuency=10 Hz Room temperature

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4. EXPERIMENTAL RESULTS

• Surface roughness

No LSP LSP 900 LSP 1600LSP 900 TT LSP 1600 T T 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5 R a ( m m) No LSP LSP 900 LSP 1600LSP 900 TT LSP 16 00 TT 0 2 4 6 8 10 12 14 16 18 20 R p ( m m)

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• Microstructure

900 pulses/cm

2

1600 pulses/cm

2

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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 annealing R e s id u a l s tr e s s e s ( M P a ) Depth (mm)

900 pul/cm

2 0,0 0,2 0,4 0,6 0,8 1,0 -600 -500 -400 -300 -200 -100 0 Before annealing After annealing R e s id u a l s tr e s s e s ( M P a ) Depth (mm)

1600 pul/cm

2 4. EXPERIMENTAL RESULTS

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• Fatigue life

150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 100000 1000000 Base material LSP 900 LSP 1600 Cycles to failure S a ( M P 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

150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 100000 1000000 Mat. Base LSP 1600 LSP 1600TT LogN=24.55449-8.1898LogSa R2=0.84671 Cycles to failure S a ( M P a ) LogN=16.33764-4.79302LogSa R2=0.99760 LogN=20.45691-6.45103Sa R2=0.89350 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 100000 1000000 Mat. Base LSP 900 LSP 900TT Cycles to failure S a ( M P 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 and fatigue resistance has been analyzed for stainless steel 316L, in view of the excellent properties of this alloy in a significant number of industrial applications.

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

The significant residual stress fields induced by laser treatment improves the fatigue limit of 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 (ranging from 200 to 300 MPa) between 100 and 600 microns below the surface. This can be due to the irreversible character of a significant proportion of mechanical dislocations induced by the LSP.

The direct and desirable effect of this remaining RS field is a remarkable degree of permanence of the enhanced fatigue behaviour. Concretely, an improvement in fatigue limit 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.

mdiaz@upm.es

References

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