Dislocation generation and cell formation as a mechanism for stress corrosion cracking mitigation

Abstract

The combination of a susceptible material, tensile stress, and corrosive environment results in stress corrosion cracking (SCC). Under these suitable conditions, brittle and catastrophic failure occurs at levels much lower than the material’s ultimate tensile strength. While several different mechanisms of failure occur for SCC, hydrogen from the corrosive environment penetrating into the lattice is often a common theme. 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 with 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 hours in samples that have not been LSP treated. But martensite formation does not occur after 24 hours in LSP treated samples. Transmission electron microscopy is used for determining the resulting structure and dislocation densities. The arrangement of the dislocations, for example forming cell like structures, is important to the hydrogen trapping capabilities. A finite element model predicting the dislocation density and cell formation is also developed to aid in the interpretation.The combination of a susceptible material, tensile stress, and corrosive environment results in stress corrosion cracking (SCC). Under these suitable conditions, brittle and catastrophic failure occurs at levels much lower than the material’s ultimate tensile strength. While several different mechanisms of failure occur for SCC, hydrogen from the corrosive environment penetrating into the lattice is often a common theme. 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 with 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 th...