Research Papers

Material Influence on Mitigation of Stress Corrosion Cracking Via Laser Shock Peening

[+] Author and Article Information
Grant Brandal

Department of Mechanical Engineering,
Columbia University,
500 W 120th Street, Mudd Rm 220,
New York, NY 10027
e-mail: gbb2114@columbia.edu

Y. Lawrence Yao

Fellow ASME
Department of Mechanical Engineering,
Columbia University,
New York, NY 10027
e-mail: yly1@columbia.edu

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received March 4, 2016; final manuscript received July 5, 2016; published online August 8, 2016. Assoc. Editor: Hongqiang Chen.

J. Manuf. Sci. Eng 139(1), 011002 (Aug 08, 2016) (10 pages) Paper No: MANU-16-1141; doi: 10.1115/1.4034283 History: Received March 04, 2016; Revised July 05, 2016

Stress corrosion cracking is a phenomenon that can lead to sudden failure of metallic components. Here, we use laser shock peening (LSP) as a surface treatment for mitigation of stress corrosion cracking (SCC), and explore how the material differences of 304 stainless steel, 4140 high strength steel, and 260 brass affect their mitigation. Cathodic charging of the samples in 1 M sulfuric acid was performed to accelerate hydrogen uptake. Nontreated stainless steel samples underwent hardness increases of 28%, but LSP treated samples only increased in the range of 0–8%, indicative that LSP keeps hydrogen from permeating into the metal. Similarly for the high strength steel, LSP treating limited the hardness changes from hydrogen to less than 5%. Mechanical U-bends subjected to Mattsson's solution, NaCl, and MgCl2 environments are analyzed, to determine changes in fracture morphology. LSP treating increased the time to failure by 65% for the stainless steel, and by 40% for the high strength steel. LSP treating of the brass showed no improvement in U-bend tests. Surface chemical effects are addressed via Kelvin Probe Force Microscopy, and a finite element model comparing induced stresses is developed. Detection of any deformation induced martensite phases, which may be detrimental, is performed using X-ray diffraction. We find LSP to be beneficial for stainless and high strength steels but does not improve brass's SCC resistance. With our analysis methods, we provide a description accounting for differences between the materials, and subsequently highlight important processing considerations for implementation of the process.

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Fig. 1

LSP indentation profile of a stainless steel sample irradiated at 250 mJ with a spot size of 0.9 mm. The two lines are traces across perpendicular directions.

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Fig. 2

Indentation profile of brass LSP processed at 250 mJ. More surface roughening effects are visible than on the stainless steel sample.

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Fig. 3

Morphology of a patterned brass sample after LSP processing. Individual indentations are still visible because of the 0% overlap condition.

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Fig. 4

Hardness increases after cathodic charging on stainless steel samples, caused by increased hydrogen absorption into the lattice. The values on the abscissa correspond to the amount of overlapping between adjacent LSP pulses, and 2X indicates that the surface was treated with two passes. As the level of LSP processing increases, the amount of hardness changes via hydrogen decreases.

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Fig. 5

Hardness increases of AISI 4140 steel after cathodic charging. The level of LSP processing causes the hydrogen effects to be lessened, indicating mitigation to hydrogen embrittlement. The percent increases are lesser than for the stainless steel show.

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Fig. 6

SEM micrograph of the side of an untreated stainless steel U-bend specimens after 1 h of exposure to boiling magnesium chloride

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Fig. 7

Untreated (a) and LSP treated (b) images showing the edge of stainless steel U-bend samples, where the bottom part of these images is the outer surface. LSP prevents cracks from propagating onto the outer surface in (b).

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Fig. 8

Outer U-bent face for untreated stainless steel samples. Indicated by the arrow, cracking has occurred for the untreated sample, but was been prevented from occurring on the LSP treated sample.

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Fig. 9

Optical micrographs of etched stainless steel samples exposed to 1 h of boiling magnesium chloride. (a) has not been LSP treated, and shows a combination of transgranular and intergranular fracture. (b) has been LSP treated, where the fracture mechanism is now dominated by intergranular fracture. The effects of increased hydrogen penetrating the lattice in (a) may cause the transgranular failure.

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Fig. 10

Multiple cracks found in an AISI 4140 U-bend sample LSP three times at 300 mJ. Unlike the stainless steel, the high strength steel fails by many parallel cracks rather than one major failure.

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Fig. 11

Mises stress in Pascals overlaid on the final deformed shape of a U-bend specimen from FEM simulation

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Fig. 12

Tension on a path along the long direction in the center of the U-bend samples. The peak stress is higher for the brass.

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Fig. 13

Work function measurements for brass, stainless steel, and high strength steel. The center of the LSP pulse is at 0 μm, and each material changes differently in response to the incident shockwave. Brass experiences work function decreases from LSP, while the high strength steel experiences an increased work function. The scale on the high strength steel figure covers a wider range than the other two, indicating an increased response to the shockwave processing.

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Fig. 14

Rate of change of dislocation density, on the vertical axis for, varying amounts of plastic deformation. The three materials generate dislocations at varying rates. But since the yield strength of high strength steel is the largest, it will have lower dislocation generation for a given amount of deformation.

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Fig. 15

Rate of change of dislocation density for high strength steel with three different types of heat treatments. As such, the downward slope of the annealed sample above 350 MPa does not indicate decreasing dislocation density but rather that dislocations are being generated at a slower rate.




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