PSI - Issue 20

Sleptsov O.I. et al. / Procedia Structural Integrity 20 (2019) 143–147

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Sleptsov O.I. et al. / Structural Integrity Procedia 00 (2019) 000–000

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corrosion cracking in this group of specimens occurs much earlier, before reaching stresses commensurate with the yield strength of the metal, which is associated with the occurrence of local stresses in the notch zone, the value of which significantly exceeds their average values. The heat-affected zone of the welded joint (specimens of group 3) is even less protected from corrosion cracking, which should be attributed to structural changes in this zone, which lead to the appearance of carbide inclusions of type Ме 23 С 6 , within the grain boundaries of steel. The microvolume occupied by the formed carbide particle is much larger than the initial volume of the solid solution, from which excess carbide phases were released, which leads to a sharp change in the complex stress state of grain boundaries, their softening and embrittlement, the occurrence of additional tensile stresses in areas with minimal resistance against corrosive damage - grain boundaries. The change in the electrochemical characteristics of the anodic process, occurring at voltages of the order of 270, 200 and 150 MPa for specimens of groups 1,2 and 3, respectively, should apparently be attributed to the fact that when the local, in the microvolume (marks, carbide inclusions, etc.) yield point is exceeded, plastic forming of the metal begins with the formation of linear defects of the crystal lattice - dislocations. The potential energy of atoms in the nucleus of such dislocations increases by tens and hundreds of times with respect to the undeformed state, and the resulting sliding steps significantly increase the surface area of contact with the corrosive medium. When passing from the pipe ring specimen tested at a constant strain rate to the rolled sheet specimens, which were then subjected to corrosive attack, it was found that the anodic process rate increases as the level of plastic deformation increases, and the rate increase depends on the degree of structure imperfection of the material. So, for austenitized specimens, on which the process of imitation of the occurrence of local deformation zones in the base metal was performed by rolling, the anodic process rate reaches 0.2 μ A / cm 2 with plastic deformation of 15%; after 20% deformation - 0.3 μ A / cm 2 - Table 2 It should be noted that in this case the growth rate of the anodic process is equidistant to the growth of the degree of plastic deformation – it increases with the growth of the imperfection of the structure. In the case of simulating the heat-affected zone of the field weld, the situation is more complicated, and the change in the rate of the anodic process can be divided into two sections. At the first stage (approximately up to a strain of less than 10%), the rate of the anodic process grows together with an increase in the degree of deformation — when a 10% strain is reached, a jump in the growth rate of the anodic process is observed. This jump should probably be associated with the occurrence during deformation of specimens imitating a heat-affected zone, microcracks around carbide particles, and, as a result, with a sharp increase in the contact area of non-passivated metal of the crack walls with the aggressive corrosive environment (Fig. 1). Thus, it has been established that tensile stresses lead to an increase in the anodic process rate in the base metal and heat-affected zones of field joints of steel 08H18N10T and may cause metal damage not only to liquefied gas pipelines, but also to all low-temperature equipment by the mechanism of corrosion cracking. Taking into account the considerable size and number of pipelines of vessels and tanks of equipment for liquefaction of gases, the possibility of causing mechanical damage - scratches, nicks, marks during installation, scheduled inspections and repairs, the difficulty of access to a number of equipment elements, additional measures should be provided as well as particularly thorough visual inspection of the surfaces of the equipment for the detection and grinding of mechanical defects, as they may cause corrosion cracking and accelerated failure of low temperature equipment, even at voltages substantially less than the yield strength of the material. Table 2. The effect of the degree of plastic deformation on the rate of the anodic process in the base metal (1) and the heat-affected zone of the field weld (2) of steel 08 Х 18 Н 10 Т . Degree of plastic deformation,% 0 1 5 10 12 15 20 25 Rate of the anodic process, µA/ cm 2 1 -1,4 -1,3 -1,2 -0,7 -0,2 +0,2 +0,3 +0,4 2 -1,3 -1,2 -1,0 +0,3 +0,4 +0,4 +0,5 -