PSI - Issue 5

S.V. Panin et al. / Procedia Structural Integrity 5 (2017) 401–408 S.V. Panin/ Structural Integrity Procedia 00 (2017) 000 – 000

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Table 2. The maximum load at impact bending and fracture toughness for 09Mn2Si steel specimens under impact loading T, ºC Impact strength KCV, J/cm 2 / P max , kN Ratio P max res / P max oper Reserve stock Running time> 37 years -70 76.5 / 4.61 34.5 / 3.65 1.26 -40 136.0 / 4.44 63 / 3.54 1.25 -20 139.5 / 4.42 92.5 / 3.32 1.33 0 143.5 / 4.35 93.5 / 3.19 1.36 20 134.5 / 4.19 91 / 3.52 1.19 The results obtained allow concluding on the following. Regardless the strength and ductility of the 09Mn2Si steel after the exploitation have changed insignificantly the structural defects accumulated during operation exert a significant influence on the bearing capacity of the gas pipeline ( P max ). This also indicates for the development of the damage accumulation process being dangerous for the gas pipeline. Thus, the decrease of the maximum load P max as well as impact toughness values of the 09Mn2Si steel after 37 years of the operation is related to the decreasing of microplasticity, lower energy of the crack start, and lower energy consumption of its growth being induced by the decarburization and structural changes at the microscopic level. The appearance of crack propagation zones at the fracture surface was examined in details (Fig. 5). The stress riser at the micrographs is located at the bottom; while the crack propagates from the bottom to upwards. At the ambient temperature, the ductile fracture pattern is characteristic for both types of steels. As the temperature drops down to T =- 40 ºC a brittle fracture character takes place for the long-term operation steel (Fig. 5, e ) while in the reserve stock specimen it remains ductile character which substantially enlarges the work of the crack propagation (Fig. 5, b ). At the lowest testing temperature T = -70 ºC both types of the steel exhibits brittle fracture pattern; in so doing large cracks (15-30 μm) being oriented normally to the crack growth direction are formed in the steel after the operation (Fig. 5, f ). Thus, the microstructural changes that occurred during the long-term operation most largely affected the impact toughness. This is related to the high rate of energy supply, the lower relaxation capacity of long-term exploitation steel due to the accumulation of diffuse structural mechanical defects and resulting in its more brittle fracture.

4. Conclusion

A comprehensive study on the strength, ductility, fatigue durability and impact toughness of 09Mn2Si steel taken from linear part of main gas pipeline “Mastakh -Berge- Yakutsk” with a diameter of 530 mm and a wall thickness of 7 mm after 37 years of operation were carried out. The results obtained have been compared with ones of the steel taken from the emergency reserve.

- 70ºС

a

b

c

20ºС

- 20ºС

- 20ºС

- 70ºС

e

20ºС

d

f

Fig. 5. SEM-micrographs of fracture surface of specimens tested at various temperatures taken in the crack growth zone; a), b), c) stock reserve steel; d), e), f) – after a long-term operation