PSI - Issue 17

Garan Martin et al. / Procedia Structural Integrity 17 (2019) 514–519

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Garan, M.,Šulko, M.: / Structural Integrity Procedia 00 (2019) 000 – 000

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appropriate conditions in contact with other chemicals. These chemicals, which allow selective disruption of the metal lattice or grain boundaries in a sensitive state, are specific to each metal, but are not numerous. The result of corrosion cracking activity on metal is the formation of cracks which decompose perpendicularly to the tensile stress direction and have an inter-crystalline, trans-crystalline or even mixed character. The inter-crystalline character of this type of disruption is different from the conventional inter-crystalline corrosion that occurs even when the material is not tensile stressed. The difference is particularly apparent in the first case that the inter-crystalline disruption is concentrated at one or more locations (perpendicular to the tensile stresses applied), in the second case it is evenly distributed over the entire metal volume. Depending on the extent of propagation of the corrosion cracks, the actual cross-section of the solid metal is reduced, which can lead to a complete breach. The place which has been damaged by corrosion cracking, has two distinctive areas:  the region of brittle disruption due to the development of corrosion cracks, without signs of plastic deformation, which makes it possible to regard corrosion cracking as a kind of disruption of metals and alloys by brittle fracture;  an area including a residual intact cross-section in which fracture occurs only as a result of an increase in tension above the strength limit. In the area of brittle fracture in corrosion cracking, the current development of several cracks often occurs. The place where the final fracture of the material occurs is then given by the crack distribution in relation to the loading of the cross-section. From the nature of the corrosion cracking implies its binding to tensile stresses, which is consistent with the observed phenomena of this cracking in practice, which has not yet been recorded for pressure stresses in practice. This is also supported by the recognition that the artificial recall of stresses on the surface of a structural material is one of the methods used to reduce corrosion cracking. Increasing the susceptibility to brittle fracture or decreasing the ductility of the material can also occur without the influence of the external environment only due to internal processes in the material, referred to as steel aging. Ageing occurs in low carbon steels (up to 0.35% carbon). Ageing causes mainly nitrogen at normal temperatures, and it can also be carbon above 100 °C . Only nitrogen and carbon atoms that are freely atomically dissolved in ferrite are involved in ageing. Nitrogen and carbon atoms that have created nitrides and carbides have no effect on ageing. The contribution of nitrogen to ageing at 20 °C is approximately 20 times higher than carbon. This is due to factors such as  solubility - at 20 °C in ferrite is ten times higher than carbon  diffusion rate - free nitrogen content at 20 °C is approximately ten t imes higher than carbon  in the case of steel crystallization at high temperatures, when carbon is still highly diffusible, it is eliminated at slow cooling to Fe 3 C such that its residual amount at temperature 20 °C is not sufficient to induce ageing processes, conversely, at rapid cooling from maximum carbon and nitrogen solubility temperatures in ferrite (580 to 723 °C) a supersaturated solid solution is formed in which both elements can be involved in ageing at 20 °C . Ageing alone (phenomenon undesirable) is the same process as precipitation hardening (phenomenon desirable) and consists in forming of nitrides (Fe 4 N) or carbides (Fe 3 C) under the conditions described above. If these precipitates are located at grain boundaries (Fig.1), they weaken their cohesion and block the development of plastic deformations. The result is reduced notch toughness (Fig. 2), ductility (Fig. 3) and transient material temperature. 4. Ageing