Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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69 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 Pitting on the outer surface was local- ized, wide, and shallow, and progressed at a very slow rate. Pitting can act as a stress c on c entration fa ctor, st ar tin g fati gu e crack s that are som etim es dif ficult to detect, as the pits frequently start under deposits and are covered by corrosion products. No corrosion or pitting of the inner surface was observed. Deterioration of the outer surface coating could have occurred by the action of cyclic loading or during the initial construction and installa- tion of the failed pipe section. Also, the c o a t i n g m ay n o t h av e b e e n pr o p e rly applied. C ra c k e x t e n s i o n w a s f o l l o w e d b y magnetic particle testing, which showed that the end points of cracks were located 110 mm inside the nominal thickness of the pipe. Chemical Analysis and Microstructure Chemical analysis of the pipe material showed that it contains 0.134% C, 0.246% Si, 1.34% Mn, 0.019% P, and 0.006% S. The polished structure of the unfailed location showed corrosion pitting, which was scat- tered along the outer surface of the pipe. The structure of the etched unfailed loca- tions showed typical ferrite-pearlite struc- ture b and s. Th e grains were ori ent ed through the rolling direction of the pipe. The pearlite phase occupied a small area inside the microstructure spot, indicating very low carbon content on the order of 0.1%. Figure 2 shows microstructure photo- graphs of a specimen from the failure loca- tion near the minimum pipe thickness; the crack took place perpendicular to the pipe rolling direction. The structure consisted of dark pearlite in a white matrix of ferrite. No evidence of grain stretching by plastic deformation was obser ved , suggesting th e rupture took place in a short time. The pitting intensity was very high near the failed location. Material conformity was checked by mechanical testing. The tensile results of the base material were 455 and 517 for yield and ultimate strength, respectively, with an elongation of 22%, while the ulti- FIGURE 2 Microstructure photographs for a sample taken from the failed location. FIGURE 1 Macrographic views of the failed pipe segment. mate strength for the weld zone was 617. In addition , the Vickers hardness mea- surements at dif ferent locations of the pipe showed a value of 172 HV. The hard- ness values were in good conformity with both chemical composition and micro- structure of different zones of the pipeline. The mean value for the three-specimen Charpy impact test was 98 J. Based on the re su l t s of t h e c h e m i c a l a n a ly si s a n d mechanical tests, it can be concluded that the pipe material conformed to the API 5L X52 9 standard limitations, and had not contributed to the failure.

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