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 1. Mechanical properties of 09Mn2Si Material type H µ , GP а

HB

HV

Conditional yield strength ( σ 0,2 ), М P а

Tensile strength, ( σ U ), М P а

Elongation, δ, %

Contraction,  , %

Reserve stock steel After the operation GOST 19282-73

1.94±0.05 2.05±0.04

126±8 131±7

135±9 134±9

310±20 380±20

490±25 500±20

34±2 31±3

6,6±1 6,6±1

-

-

-

345

490

21

-

The parameter of the relative elongation of the reserve stock steel was λ δ = -0.62, while after the operation it makes λ δ =-0.48%. This indicates that the steel possesses a substantial reserve of ductility, Fig. 4. Even after the long-term operation, the relative elongation δ was 10 % larger as compared with one recommended by the standard. It should be emphasized that after the deformation, no signs of delamination and micropore formation are evident in the materials as is contrary to the majority of studies on this problem [1, 11, 12]. It is this reason that the interpretation of mechanisms of damage accumulation requires additional physical and mechanical justification. In our case, the analysis of standard mechanical properties of the gas pipeline material after the operation has shown that no significant changes happen and their values are close to ones for the reserves stock steel. Fatigue lifetime of 09Mn2Si steel after long-time operation has been reduced by 16 %: from N =1.25  10 5 ± 0.16  10 5 (emergency reserve) to N =1.05  10 5 ± 18.6  10 5 cycles (after operation). Thus, as compared to the impact toughness data described below (reduction by 32 % at room temperature testing) the fatigue durability after the arctic long-term operation has decreased to a less extent. The graphs illustrating the dependence of crack length versus the number of loading cycles were plotted (Fig. 3, f ). The analysis of the crack growth diagrams shows that crack initiation in the emergency stock specimen occurs later. As is seen the fatigue crack in the 09Mn2Si steel after long-term operation has originated at N I oper ~ 50  10 3 cycles, while in the reserve stock material this happens at N 3 res ~ 91  10 3 cycles. SEM-micrographs of the fatigue fracture surface for the specimens of both types have been analyzed. In doing so the notch was located to the left relative to the observation region, while the crack propagation was from the left to the right. The stage of the crack growth for the 09Mn2Si steel in both states under study is practically the same (Fig. 3, a, d ). Thus, the main differences in the fatigue lifetime of the specimens are manifested during the nucleation and growth of a short crack and are associated, apparently, with the operational degradation of steel. At the distance larger than 100 μm ahead of the notch tip microcracks are distinguishable being oriented mostly either towards the transverse (for the reserve stock steel) or the longitudinal direction (for the steel after the operation) (Fig. 3, b , e ). The presence of transverse cracks is associated with the branching of the main crack in the more ductile reserve stock steel. The presence of longitudinal cracks in the steel after the operation can be explained by its lower microplasticity [13]. The transverse contraction of the reserve steel specimen is higher by 20%. This agrees well with the greater ductility of the steel. 3.5. Fatigue durability

a

b

c

d

e

f

Fig. 3. SEM micrographs of the fracture surface of crack growth zone under cyclic loading; а ), b ) – emergency reserve; d ), e ) – after long-term operation; surface roughness ( c ), estimated with the use of optical profilometer; diagrams of fatigue crack growth versus number of cycles; reserve stock steel (curve 1); after long term operation (curve 2