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

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.