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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It is known from the literature [1-3] that long-term operation of steel can give rise both to already mentioned hydrogenation but also to structural-degradation processes being expressed through deformation and crushing of cementite along the boundaries of ferrite grains, the formation of carbide precipitates there as well as evolution of dislocation substructure due to the deformation aging processes. At the initial stages the deformation aging is expressed through the formation of Cottrell's atmospheres and pitting of dislocations. The latter results from highly disperse carbides precipitation in the grain bodies. Pipes for the main gas pipeline construction are manufactured with a large safety margin for safe operation during dozens of years. It is this reason that the steel after 37 years of operation is unlikely to experience significant stresses that can give rise to severe plastic deformation or the deformation aging. The operating temperature range also fluctuated within acceptable limits which should not result in noticeable structure modification. This is confirmed by the TEM data: the carbides for the steels in both states are located in the grain bodies and along their boundaries as well. Cementite plates at the grain boundaries are not fractured. The measured tensile strength and microhardness have a scatter within a certain allowable range (see the results below). Thus, it can be assumed that deformation aging could occur only at the initial exploitation stage (formation of Cottrell's atmospheres and the dislocation pitting) which is confirmed by the approximately one order difference in the dislocation density between the reserve stock fragment and one after the operation. These changes do not exert a significant effect on mechanical properties or hardness but may affect the decrease of toughness, fatigue life, or shift of the cold embrittlement threshold towards higher temperatures.

3.3. Hardness and microhardness of 09Mn2Si steel

Interpretation of the results obtained for the steel after the long-term operation might be carried out from the point of view of the development of two competing processes: i) hardness reduction as a result of decarburization and ii) strain hardening due to dislocations pitting. Mutual compensation of these processes can maintain the hardness of steel after operation at the initial level while gives rise to a different deformation behavior as compared to the reserve stock steel fragment (Table 1).

3.4. Static tension

A yield tooth and plateau are observed at the loading diagrams of the 09Mn2Si steel specimens in both states, Fig. 2, a . Their presence is associated with the low-carbon "status" of this steel. Specimens of the reserve stock steel as well as one after the operation possess a similar value of the tensile strength. At the same time, specimens from the emergency reserve have a lower yield stress (  by 27%) but simultaneously higher ductility (  by 31 %) (Table 1). This difference, in all the likelihood, is associated with a larger pearlite phase volume as well as a slightly smaller grain size. The measured mechanical properties of 09Mn2Si steel were compared with ones from the National Standard “GOST 19282 - 73” (Fig. 2, b ). It is found that the conditional yield strength of the reserve stock steel was lower as compared from one recommended by the standard by λ σ 0.2 = 0.1, while after 37 years of operation it exceeded the recommended value by λ σ 0.2 = 0.22 (Table. 1). On the other hand, the value of the tensile strength of the reserve stock 09Mn2Si steel after the operation is almost identical and corresponds to the values recommended by the standard.

λ, %

a

b

-60 -50 -40 -30 -20 -10 0 10

σ В

δ

σ 0,2

GOST 19282-73

1 2

Fig. 2. a) Tensile diagrams for 09Mn2Si steel reserve stock (curve 1), after the long-term operation (curve2); b) data of the comparative analysis of the mechanical properties of 09Mn2Si steel (according to GOST 19282-73)