PSI - Issue 17

Flavio Pereira de Moraes et al. / Procedia Structural Integrity 17 (2019) 131–137 Flavio Pereira de Moraes et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Table 2. Chemical composition (mass %) determined using SEM-EDS of the particle identified as being sigma phase observed in the delta ferrite (DF) and at the grain boundary in comparison with the results obtained by Weiss et al. (1971). Phase Fe Cr Mo Ni Mn Si Sigma (DF) 54 33 6 5 1.3 0.4 Sigma (GB) 53 30 9 6 1.3 0.3 Sigma; Weiss et al. (1971) 55 29 11 5 --- --- Chi; Weiss et al. (1971) 52 21 22 5 --- --- Laves; Weiss et al. (1971) 38 11 45 6 --- --- M 23 C 6; Weiss et al. (1971) 18 63 14 5 --- --- Fig. 3 presents the X-ray diffraction pattern, performed on the tube cross section (polished and unetched surface). In synthesis, the interpretation of the diffraction pattern of Fig. 3 revealed the unequivocal presence of two phases (austenite and sigma) and the plausible presence of other phases in smaller quantity: M 23 C 6 carbide (designated by C); chi (  ) phase. Smaller chi, carbide phases and eventually Laves phase could not be identified in an unequivocal basis using SEM-EDS, due to their insufficient size and consequent matrix interference.

Fig. 3. X-ray diffraction pattern performed on the polished surface using CuK α 1 radiation.

The occurrence of the sigma phase in the pipe of the present work was expected. Hull (1973) developed an empirical formula solely based upon the chemical composition in order to foresee the occurrence of intermetallic phases in austenitic stainless steels. He used about 450 compositions prepared by using levitation melting, which on solidification, have been heat treated in order to obtain the precipitation of intermetallic phases. The formula of Hull 1973 has been used with success by reference works such as those of Leitnaker and Bentley (1977) and of Sasmal (1987) in order to foresee the occurrence on intermetallic phases in austenitic stainless steels. The simple comparison of the chemical composition of the analysis of the sample of the present work with the compositions and criteria of the papers mentioned in this paragraph corroborate our results.