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DISLOCATION GENERATION and CELL FORMATION AS a MECHANISM for STRESS CORROSION CRACKING MITIGATION Paper #114

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DISLOCATION GENERATION and CELL FORMATION AS a MECHANISM for STRESS CORROSION CRACKING MITIGATION Paper #114 DISLOCATION GENERATION AND CELL FORMATION AS A MECHANISM FOR STRESS CORROSION CRACKING MITIGATION Paper #114 Grant Brandal1, Y. Lawrence Yao1 1 Advanced Manufacturing Lab, Columbia University New York, NY 10027, United States Abstract Introduction The combination of a susceptible material, tensile Material failure by corrosion can often be prevented stress, and corrosive environment results in stress because corrosive products, such as rust, indicate that corrosion cracking (SCC). Under these suitable the integrity of the material has weakened. But a conditions, brittle and catastrophic failure occurs at special case of corrosion called Stress Corrosion levels much lower than the material’s ultimate tensile Cracking (SCC) behaves quite differently from strength. While several different mechanisms of conventional corrosion. SCC occurs when a failure occur for SCC, hydrogen from the corrosive susceptible material in a suitable corrosive environment penetrating into the lattice is often a environment experiences a tensile stress. The required common theme. Laser shock peening (LSP) has stress can be either externally applied or residual stress previously been shown to prevent the occurrence of from a previous manufacturing process, and levels as SCC on stainless steel. Compressive residual stresses low as 20% of the material’s yield strength have been from LSP are often attributed with the improvement, shown to cause failure [1]. Of most concern with SCC but this simple explanation does not explain the is that it causes sudden and catastrophic material electrochemical nature of SCC by capturing the effects failure. Additionally, materials generally thought of as of microstructural changes from LSP processing and being resistant to corrosion are susceptible to SCC its interaction with the hydrogen atoms on the failure in certain environments and furthermore it is microscale. quite difficult to predict when the onset of SCC is going to occur. As the hydrogen concentration of the material increases, a phase transformation from austenite to Many different industrial applications are prone to martensite occurs. This transformation is a precursor experiencing SCC. Considerable attention has been to SCC failure, and its prevention would thus help paid to the occurrence of SCC in the boiling water explain the mitigation capabilities of LSP. In this reactors found in nuclear power plants [2], where any paper, the role of LSP induced dislocations failures could result in extremely dangerous situations. counteracting the driving force of the martensitic Pressure vessels and gas pipelines have been found to transformation is explored. Stainless steel samples are be at risk [3,4], as are various types of implantable LSP processed with a range of incident laser intensities medical devices [5]. and overlapping. Cathodic charging is then applied to accelerate the rate of hydrogen absorption. Using Various physical descriptions exist for explaining the XRD, martensitic peaks are found after 24 hours in mechanisms of SCC, but they often are related to samples that have not been LSP treated. But deleterious effects of hydrogen atoms absorbed from martensite formation does not occur after 24 hours in the corrosive environment. In this case, the term LSP treated samples. Transmission electron hydrogen embrittlement is used. Hydrogen has a high microscopy is used for determining the resulting diffusivity in metals, and it is highly reactive with the structure and dislocation densities. The arrangement material’s lattice. Processes such as electroplating, of the dislocations, for example forming cell like pickling, or various types of surface cleaning can structures, is important to the hydrogen trapping further increase the levels of absorbed hydrogen within capabilities. A finite element model predicting the the lattice. Details on the physical changes to the dislocation density and cell formation is also material’s lattice and structure will be provided in the developed to aid in the interpretation. following section. To prevent material failure by SCC, several different mitigation techniques exist. Coating and plasma nitriding of the material can prevent surface reactions except that it is now hydrogen atoms causing the lattice and limit the amount of hydrogen that penetrates into distortions and internal stress. During exposure to the the lattice [6,7], but in harsh environments coatings corrosive environment, once this transformation has will eventually degrade and since they are not as tough occurred, even locally, the likelihood of brittle failure as the metal, may crack or delaminate leaving the is greatly enhanced. Therefore, prevention of the material vulnerable to SCC. A different approach to martensitic transformation would be a powerful mitigation is to actually modify the material itself, by method for mitigation of SCC failure in austenitic imparting a compressive residual stress on the stainless steels. material’s surface. One such technique is laser shock peening (LSP), which uses incident laser pulses to Olson and Cohen described the initiation sites for generate shockwaves on a material’s surface. While martensite as the intersection of shear bands and originally developed for increasing the fatigue life of identified which lattice planes the transformation will metallic components [8], recent studies have shown occur on [19]. As hydrogen from the corrosive that LSP processing helps to prevent the onset of SCC environment diffuses into austenite, it causes [9,10]. The improvement has mostly been attributed to expansion and an internal stress that acts as a driving the compressive stress counteracting the necessary force for the formation of martensite. This strain tensile stress for SCC initiation, but this cannot be energy increases the free energy of the austenite, solely attributable, as evidenced by the fact that LSP subsequently making the martensite phase more stable. processing can decrease the corrosion current of 4140 Plots of the free energy of the respective phases are steel [11], an electrochemical effect. LSP causes many shown vs. temperature in Figure 1, where ΔGch is the forms of microstructural changes to the material, difference in chemical free energy of the two phases. including the generation of lattice dislocations and sub grain dislocation cell formation [12]. Lattice dislocations act as hydrogen trapping sites and will influence the absorption and diffusion of hydrogen [13], and structural changes to the lattice symmetry will further influence SCC occurrence. In this paper we identify the underlying microstructural changes to stainless steel 304 induced by LSP that allow for it to be a beneficial mitigation process against SCC. Background Microstructural Considerations for SCC Of particular concern for SCC in stainless steels is the Figure 1 Free energy diagram showing the suitable formation of martensite, a phase that is brittle and conditions for the formation of deformation induced susceptible to fracture and corrosion [14]. The fracture martensite [20]. surfaces of initially austenitic stainless steel that has failed by SCC show that brittle failure has occurred, In stainless steels, the addition of alloying elements which is most often accompanied by the presence of promotes the formation of austenite, so that in Figure 1 martensite on the fracture surface [15,16]. Even T0 can be below room temperature. For the stress materials that initially are fully austenitic can from induced martensitic transformation to occur, the martensite through various environmental processes, internal stain energy must be equal to ΔGcrit-ΔGch. thereby weakening the material [17]. Martensite is Martensite becomes the lower energy phase with characterized as a phase that forms via a diffusionless increasing amounts of absorbed hydrogen because the transformation, which in the case of stainless steel, the BCC and HCP lattices provide more interstitial spaces initially FCC austenite transforms into martensite for the hydrogen to reside [21,22], but this is also which can be of either BCC or HCP crystal systems. accompanied by a volumetric expansion of nearly 4%. In carbon steel systems, this transformation occurs upon rapid cooling from elevated temperatures [18]. Since the martensitic transformation requires This does not allow time for carbon to diffuse, and the significant levels of hydrogen to reach the required remaining carbon atoms sit in interstitial lattice sites, strain energy, accelerated testing by cathodically causing distortions and subsequently the phase change. charging the workpiece is often performed. X-Ray In corrosive environments, the same type of lattice Diffraction, which measures lattice spacing, is a transformation from austenite to martensite occurs, preferred method for detecting phases present in a metallic sample. Narita et al. have used this method to rates. Since dislocation multiplication is often identify the formation of martensite, as well as relating considered as the result of tangling dislocations (such lattice shifts of the austenite peaks to expansion caused as Frank-Read sources), it cannot account for the by absorbed hydrogen [23]. whole phenemonon of LSP dislocation generation because it would require dislocations within the lattice Laser Shock Peening and Lattice Changes to be traveling at speeds higher than physically possible in order to keep up with the wave front. To Generating shockwaves on the surface of a metallic rectify this, Meyers and Murr proposed [26] a sample causes
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