PSI - Issue 20

Sleptsov O.I. et al. / Procedia Structural Integrity 20 (2019) 130–135 Sleptsov O.I. et al./ Structural Integrity Procedia 00 (2019) 000–000

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3

in these steels varies within fairly wide limits - steel 0812 Х 18 Н 12 Т contains 11-13% nickel (GOST 5632-2014). Such a variation of alloying cannot but affect structure, mechanical and corrosion properties of the material. The formation of intergranular corrosion defects in low-temperature equipment is confirmed by a series of direct studies of low-temperature equipment, in particular, various pipelines deeply studied by Van H, Lin G. (1996) and Ermakov B.S. (2011). Notably, the defects are most likely to occur in the areas adjacent to the welded joints of the structures. This is due to both the altered state of materials in the heat-affected zone of field joints - an increase in grain sizes in the superheated areas (from the fusion zone to 1100 °C), the occurrence of grain-boundary carbide inclusions in the region of 600-700 °C. In addition to the structural features of the heat-affected zone, which reduce the resistance of the material to corrosion damage, field joints are also areas of reduced accuracy in welding work, which leads to the occurrence of welding deformations and stresses not taken into account in the strength calculations; areas of increased damages in macro- and microstructure of the metal. The deviation during assembly from the geometric design of the structure adopted in the strength calculations can also change the nature of the action of the functional stresses and lead to additional stresses, first, in the area of field joints. 3. Results and discussion It is known that the least resistance ( τ min ) against intergranular corrosion appears after provoking heating in the temperature range of 650-700 ° С , notably, with an increase of nickel content in steel, the exposure time for provoking heating, leading to the occurrence of intercrystalline corrosion – τ min , decreases. The decrease in the value of τ min with increasing nickel concentration is explained by the fact that this element increases the diffusion rate of carbon in austenite, accelerates the release of chromium carbides along the grain boundaries, which impoverishes areas of the grain-boundary solid solution over chromium and creates favorable conditions for diffusion of carbon and phosphorus from the grain to its borders. Thus, in the heat-affected zone of welded joints of chromium-nickel steels, an area with the minimal resistance to intergranular corrosion appears. The mechanical properties of steel with a minimum and maximum (intra-grade) nickel content (11 and 13%) and the effect of heating on these properties that provoke a propensity for intergranular corrosion are given in Table 1. However, it is not only the process of carbid forming which “washes out” chromium from a grain-boundary solid solution that is responsible for the process of embrittlement and occurrence of a propensity for intergranular corrosion in the zone of thermal effect of a welded joint. An important role in increasing the propensity for intergranular corrosion is played by impurity elements, in particular, phosphorus. The rate of this process depends on the temperature and heating time, at the same time it increases as the amount of elements that prevent the diffusion of solution, for example, chromium and molybdenum, decreases in solid solution marked by Ermakov B.S., Karginova V.V. (2012).

Table 1. Mechanical properties of Fe-C-Cr-Ni steel after heating provoking propensity for intergranular corrosion and imitating temperature regions of the HAZ of welded joints.

20 ° С

Minus 70 ° С

w/o defects 1

with defects 2

w/o defects 1

with defects 2

 min , h

Ni,%

R m

КС V MJ/m 2

R m

КС V MJ/m 2

R m

КС V MJ/m 2

R m

КС V MJ/m 2

MPa

MPa 575 705 590 735 630 765 730 815

MPa

MPa 570 695 590 725 615 760 710 800

11 13 11 13 11 13 11 13

3,0 3,0 3,0 2,6 2,3 1,7 1,2 1,0

- - -

- - -

3,0 3,0 2.9 2,6 2,2 1,5 1,1 1,0

- - -

- - -

0

2

720 580 760 580 745

1,3 0,5 0,5 0,2

735 590 770 590 750

1,0 0,5 0,4 0,2

4

8

0

0

1 specimens without surface defects associated with intergranular corrosion 2 specimens with defects of intergranular corrosion up to 100 microns deep on the working part of the specimen.

The occurrence of phosphorus segregations within the grain boundaries with simultaneous growth of carbide inclusions creates a complex stress state at the boundaries due to a significant difference in the crystal lattice parameters of the solid solution and the carbide phase, reduces the concentration of alloying elements and increases