Research Papers

Laser Shock Peening for Suppression of Hydrogen-Induced Martensitic Transformation in Stress Corrosion Cracking

[+] 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.

Manuscript received October 24, 2016; final manuscript received April 7, 2017; published online May 11, 2017. Assoc. Editor: Hongqiang Chen.

J. Manuf. Sci. Eng 139(8), 081015 (May 11, 2017) (10 pages) Paper No: MANU-16-1564; doi: 10.1115/1.4036530 History: Received October 24, 2016; Revised April 07, 2017

The combination of a susceptible material, tensile stress, and corrosive environment results in stress corrosion cracking (SCC). Laser shock peening (LSP) has previously been shown to prevent the occurrence of SCC on stainless steel. Compressive residual stresses from LSP are often attributed to the improvement, but this simple explanation does not explain the electrochemical nature of SCC by capturing the effects of microstructural changes from LSP processing and its interaction with the hydrogen atoms on the microscale. As the hydrogen concentration of the material increases, a phase transformation from austenite to martensite occurs. This transformation is a precursor to SCC failure, and its prevention would thus help explain the mitigation capabilities of LSP. In this paper, the role of LSP-induced dislocations counteracting the driving force of the martensitic transformation is explored. Stainless steel samples are LSP processed with a range of incident laser intensities and overlapping. Cathodic charging is then applied to accelerate the rate of hydrogen absorption. Using XRD, martensitic peaks are found after 24 h in samples that have not been LSP treated. But martensite formation does not occur after 24 h in LSP-treated samples. Transmission electron microscopy (TEM) analysis is also used for providing a description of how LSP provides mitigation against hydrogen enhanced localized plasticity (HELP), by causing tangling and prevention of dislocation movement. The formation of dislocation cells is attributed with further mitigation benefits. A finite element model predicting the dislocation density and cell formation is also developed to aid in the description.

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Fig. 1

Free energy diagram showing the suitable conditions for the formation of deformation-induced martensite [19]

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Fig. 2

XRD measurements of lattice changes from cathodic charging a specimen without LSP treatment. Prior to cathodic charging, the material is fully austenitic (a). After 24 h (b), the absorbed hydrogen has caused the formation of a martensite peak, (c) with further increases after 48 h.

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Fig. 3

Hydrogen-induced lattice expansion for 24 and 48 h of cathodic charging

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Fig. 4

XRD measurements of a selected region of the spectrum for samples after LSP processing (a) and then subjected to 24 h (b) and 48 h (c) of cathodic charging. The initially austenitic peaks experience broadening after 24 h, but no martensite formation occurs, illustrating the effectiveness of LSP processing as a mitigation tool. Some martensite does eventually form after 48 h.

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Fig. 5

(a) Untreated stainless steel sample after cathodic charging 24 h showing large amounts of martensite formation, seen as the grains with platelet like structure and (b) samples which were subject to LSP prior to cathodic charging have considerably fewer martensitic grains

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Fig. 6

Magnified images after etching the samples of Fig. 5, for an untreated sample (a) and LSP treated (b) both after cathodic charging

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Fig. 7

Increases to the dislocation density after LSP processing at 1.6 GPa, which act as an impediment to hydrogen-induced martensite formation. The largest increase is seen upon the initial incident pulse.

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Fig. 8

Decrease in the diffusion coefficient after various levels of incident pressure from LSP processing. The dotted line of dislocation density shows its inverse relationship to the diffusion coefficient.

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Fig. 9

Distribution of dislocation cell size after three LSP impacts. Symmetry is used along the boundary at the left side.

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Fig. 10

Dislocation cell size for increasing number of incident LSP pulses at four depths

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Fig. 11

Asymptotic increase of the ratio of dislocation density in cell walls to cell interior

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Fig. 12

(a) Untreated sample showing lower densities of dislocations and (b) increase of dislocation density after three LSP impacts at 2.5 GW cm−2

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Fig. 13

Pile ups of dislocations at the grain boundary. Regions of twinning, indicated by the arrow and letter “T” are also found, with the inset a diffraction image indicative of lattice twinning.

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Fig. 14

(a) Dislocation cell formation, with the inset diffraction image indicating no grain misorientation and (b) dislocation subgrain structure, where the formation of within individual grains ends at the grain boundaries

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Fig. 15

High-magnification TEM showing resolved lattice structure




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