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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This allows estimating the relative variation of the magnitude induced by the pipe exploitation [9]. The size of ferritic grain was determined according to the ASTM E 112-96. The microhardness was measured with the help of PMT-3 device with the load onto Vickers pyramid of 0.98 N (100 g). HB hardness was measured by Psh N2 N480 hardness tester at the load of 7857 N (750 kg, the diameter of the ball was 5 mm), while the HV hardness – at the hardness tester PT-7R-1 with the load of 490.3 N (500 g; the angle between the opposite faces of the pyramid makes – 136°). Fine structure was analyzed using transmission electron microscope Philips SM-12. The fracture surfaces were investigated with the use of scanning electron microscope Philips SEM 515.

3. Investigation results

3.1. Metallographic analysis

The measurement of the pipe fragment thickness for both types of steels has shown that one for the emergency reserve makes is 7.1 mm while the one after the long-term operation does not exceed 6.7 mm. Thus, operating during 37 years has given rise thinning of the pipe walls by ~ 0.4 mm. The steel from the emergency stock has a ferrite-pearlite structure (Fig. 1, a ). The perlite content makes ~16 ± 1%; pronounced striped texture is observed. The average grain size is equal to 9.5 ± 1 μm. The steel after 37 years of operation also possesses the ferrite-pearlite structure. The perlite content makes ~ 12 ± 0.6 %; pearlitic colonies are small, located mainly along the grain boundaries (Fig. 1, d ). The average grain size of this fragment is equal to 11 ± 1 μm. Thus, during the operation, the content of the pearlite phase in the 09Mn2Si steel has decreased by ~ 5 %. It is suggested to come from hydrogenation as a result of prolonged exposure to the gaseous medium. Reduction of the carbon content was also accompanied by a slight increase in the size of ferritic grains (by ~ 10 %). In the bulk material of both steel fragments a ferrite structure with cementite plates along the grain boundaries is observed. Large uniaxial cementite particles (100-500 nm) are located both in the grain bodies as well as along their boundaries (Fig. 2, c , f ). The dislocation density for the stock steel is higher and makes 10 10 – 10 11 cm -2 while after the operation just 10 9 – 10 10 cm -2 (Fig. 2, b , e ). 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. 3.2. Transmission electron microscopy

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Fig. 1. Microstructure of 09Mn2Si steel: a ) emergency stock material; d ) after long-term operation; TEM-micrographs of steel fragments from the emergency reserve ( b, c ) and after the long-term operation ( e , f )