PSI - Issue 20

Sleptsov O.I. et al. / Procedia Structural Integrity 20 (2019) 130–135

133

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

4

the concentration of impurities within the boundary zones of the grain; leads to the emergence of a new boundary state. The according to work Arkharov V.I. (1976), newly emerging state of the boundaries with an increased concentration of impurities is explained not only by the diffusion redistribution of elements between the body and grain boundaries, but also by the fact that the formation of the chemical compound with a “rigid” crystal lattice - Me 23 C 6 in the initial boundaries, contributes to the “expulsion” of substitutional impurity atoms from those volumes of boundaries that are occupied by chemical compounds, which further saturates the newly formed border with impurities that reduce its cohesion. Thus, the phosphorus content in the grain boundaries, measured by the Auger spectroscopy method, increases dozens of times and is about 0.8–0.9%, with an average content in chromium-nickel steels not exceeding 0.02%. Table 2 shows the data on the interaction energy of phosphorus atoms with grain boundaries.

Table 2. The interaction energy of the phosphorus atom with the grain boundary depending on the intensity of temper hardening

Nickel content, %

11

13

 , h

 , h 1)

Т , ° С

° С

E

E

IGC

a , eV

Т ,

IGC

a , eV

- 1

700

3

0,31

700

2

-

0,20

0 3

4

0,24

0,37

3

0

+ 2

5

+

0,38

0,46

4

8

+

0,49

5

+

0,44

650

3

-

0,22

650

2

-

0,18

4

0

0,33

3

0

0,23

5

+

0,47

4

+

0,39

8

+

0,50

5

+

0,49

1 «–» – steel is not prone to intergranular corrosion.

2 «+» – prone to intergranular corrosion; 3 0 – time equal τ min *.

According to the data from the study Guttman M. (1975), the interaction energy for phosphorus atoms with austenite grain boundaries is within 0.12 - 1.1 eV, and the smaller the interaction energy, the thicker the layer in which an increased concentration of impurity atoms is observed. The data from Table 2 demonstrates that with an increase in the intensity of sensitization heating, the interaction energy increases accordingly, and, consequently, a gradual thinning of the segregation layer occurs and the segregation approaches the most dangerous equilibrium state, reducing the corrosion resistance of grain boundaries and increasing the tendency to form defects by the mechanisms of IGC. However, while the role of the carbide formation process in increasing the propensity for intergranular corrosion of chromium-nickel steels has been studied in sufficient detail and the role of phosphorus has also been discussed enough recently, the question of the role of silicon in the process of occurrence of the propensity for IGC has scarcely been studied. In order to analyze the role of silicon in the formation of the corrosion resistance of chromium-nickel steels, experimental melts of the Fe-C-Cr-Ni alloy were fabricated, the content of all elements being constant (wt.%): Fe is the base; C-0.12; Cr-18.0; Ni- 11.8, and the silicon content varied from 0.1 to 1.1%. It