Microstructural effects induced by laser shock peening for mitigation of stress corrosion cracking

Stress corrosion cracking is a phenomenon that can lead to rapid, sudden failure of metallic products. In this paper we examine the mechanisms of SCC mitigation of stainless steel and brass samples using laser shock peening (LSP). The behavior of hydrogen within the crystal lattice is one of the most dominant contributors to SCC, where uptake of hydrogen strains the lattice and increases its hardness. Cathodic charging of the metallic samples in 1M sulfuric acid was performed in order to accelerate hydrogen uptake. Non-treated samples underwent hardness increases of 28%, but LSP treated samples only increased in the range of 0 to 8%, indicative that LSP keeps hydrogen from permeating into the metal. Mechanical U-bends subjected to Mattsson’s solution and MgCl₂ environments are analyzed, to determine changes in fracture morphology. Surface chemical effects are addressed via Kelvin Probe Force Microscopy, which is used for finding changes in the work function caused by LSP treatment. A finite element model of material deformation from U-bending was developed to analyze and compare the induced stresses. With LSP, there is a potential for overprocessing the samples, whereby negative effects to corrosion (grain refinement, martensite formation) can arise. Detection of any martensite phases formed is performed using x-ray diffraction. We find LSP to be beneficial for stainless steel but does not improve brass’s SCC resistance. With our analysis methods we provide a further understanding of the process whereby LSP reduces SCC susceptibility, and subsequently highlight important processing considerations for implementation of the process.