PSI - Issue 20

Sleptsov O.I. et al. / Procedia Structural Integrity 20 (2019) 143–147 Sleptsov O.I. et al. / Structural Integrity Procedia 00 (2019) 000–000

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appearance of defects in the metal by the mechanism of corrosion cracking. The base metal, that is the metal that was not exposed to thermal effects during welding, was in the austenitized state — heating to a temperature of 1320 K, 1 hour expo-sure, water quenching; the heat-affected zone of the field joint was simulated by the specimens processed according to the mode of austenitization and the subsequent thermal welding cycle with a maximum heating temperature of about 850 K, which, as was shown in the study Ermakov B.S., Malikov S.O. (2006), leads to the maximum embrittlement in these steels. Tests on the effect of tensile stresses on the rate of the anodic process in steel - one of the main indicators of the propensity of steel for the occurrence of corrosion-cracking defects, were performed on 15 mm wide ring specimens cut from a pipe 108x8 mm reflected in the work Evans Yu.R. (1962). In order to assess the effect of local stresses on the change in the propensity for corrosion cracking of steel, triangular marks were put on a number of specimens - cuts with the apex angle 90° and 0.2 mm rounding radius at the top. The depth of the cuts was 25% of the wall thickness of the pipe specimen. The effect of predeformation of the metal on the occurrence of propensity for corrosion cracking was studied on specimens cut from sheet steel 08 Х 18 Н 10 Т , heat treated according to the same modes and under the same conditions as the ring specimens. The required degree of deformation was set by cold fractional rolling with a deformation step of 1%. The study of corrosion cracking is most often carried out with simultaneous exposure of the metal to tensile stresses and corrosive environment. As the least complex and laborious, this method was used in the work at the first stage of testing while determining the effect of stresses on the propensity for corrosion cracking of steel 08X18H10T. In further studies, when it was necessary to model the processes of corrosion cracking in the existing low-temperature equipment, studies of corrosion cracking were carried out on pre-deformed specimens, since this method most fully reflects the processes occurring in the metal of low-temperature equipment when plastic deformation processes and corrosive effects are spaced apart from each other in times. 3. Results and discussion The effect of the stress state on the electrochemical characteristics of steel 08Kh18N10T was investigated with continuous deformation of the specimens at a rate of 1 mm/min. The kinetics of the anodic process was studied in a solution of sodium sulfate with a concentration of the last of 1 mol per liter of solution (normal solution) at an electrochemical potential of 0.6 V, which corresponds to the passive state of steel that is not prone to corrosion. The research results are summarized in Table 1.

Table 1. The rate of the anodic process in smooth (1) and notched (2) specimens of the base metal and (3) specimens that simulate the heat-affected zone of the field joint.

Stretching voltage, MPa

0

100

150

200

250

270

300

350

Rate of the anodic proces µA/ cm 2

1 2

-1,4 -1,4

-1,4 -1,4

-1,4 -1,3

-1,4 -0,4

-1,3 +0,3

-0,1 +0,4

+0,2 +0,4

+0,4

-

3

-1,3

-1,2

-0,7

+0,2

+0,4

-

-

-

It was found that a sharp jump in the rate of the anodic process on smooth (without notching) specimens of the base metal corresponds to stresses approximately equal to the temperature limit of steel - 250 - 270 MPa. The rate of the anodic process in the base metal specimens (without notching) in the elastic stress region remains almost unchanged, that is, the probability of a propensity to corrosion cracking in the base metal of the pipeline is extremely small. The occurrence of plastic flows when exceeding the yield strength in a specimen of group 1 leads to the acceleration of the anodic process and, as a consequence, to the appearance of the danger of damage by the mechanism of corrosion cracking. The zones of local stresses arising from scratches, undercuts and other mechanical defects of the base metal of the pipeline - (specimens of group 2 with pre-applied defects) - lead to a decrease in stress level at which the rate of the anodic process, indicating the risk of corrosion cracking, begins to increase abruptly. That is, the propensity to