PSI - Issue 17

C.R.F. Azevedo et al. / Procedia Structural Integrity 17 (2019) 331–338 C. R F. Azevedo and A. F. Padilha / Structural Integrity Procedia 00 (2019) 000 – 000

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low C content (0.007%  C  0.015%) in the USA and since then the development of FSSs has been controlled by the technological developments in steel refining to minimize the interstitials content, especially C. The first generation of FSSs was developed in the first decades of the 20th century, when steel decarburization processes were not efficient, consequently AISI 430 and AISI 446 FSSs presented high levels of C and Cr (see Table 1). The second generation of FSSs, such as AISI 409 steel, comparatively featured lower C and N contents and used the addition of C and N fixation elements, such as Ti, which additionally acted as ferrite-stabilizer element in solid solution. The third generation of FSSs, such as AISI 444 steel, was developed in the 70’ s due to the availability of more efficient steel decarburization techniques, allowing the production of steels with C and N contents around 0.02%. Addition of Ti, Nb or Mo was also used as carbon and nitrogen fixation elements due to the precipitation of carbides and nitrites. The further development of large-scale steel decarburization techniques, such as argon oxygen decarburization (AOD) and vacuum-oxygen decarburization (VOD), allowed the production of FSSs with interstitial contents around 0.01%. Additionally, the increase of Cr content to values above 25% and the addition of Mo (strong ferrite-stabilizer element) led to the development of the fourth generation of FSSs, commercially branded as super ferritic stainless steels (SFSSs). The combination of high Cr, high Mo and low C in the S FSSs allowed the addition of Ni without “destabilizing” the ferrite phase to improve their toughness and corrosion resistance (see Table 1), but increased their susceptibility to alpha- prime and sigma phase embrittlement and a decrease of their maximum service temperature to around 400°C. In this sense, SFSSs are mainly selected due to their excellent resistance to generalized corrosion, pitting and crevice corrosion and stress corrosion in media containing chloride ions and cost, but the SFSSs do not stand out from other types of stainless steels in terms of mechanical properties (Azevedo et al., 2019). The main applications of SFSSs are pressure vessels, corrosion- resisting “high - temperature” service, feedwater heater tubes, chemical processing, food processing, petroleum refining, water treatment, pollution control industries, chemical process plants, food processing, “high temperature” oxidation/sulfidation equipment, oil refineries, pulp and paper and desalination plants (Treitschke and Tammann, 1907; Bain and Griffiths, 1927; Steigerwald et al., 1978; Kiessling, 1984; Padilha, 1989; Campbell, 1992; Bavay, 1993; Davis, 1994; Wegst, 1995; Rössel, 1999; Pimenta, 2001; Cambridge, 2019). The presence of high levels of alloying elements in SFSSs, such as Cr and Mo, promotes microstructural instability above 450°C, making these steels susceptible to the precipitation of sigma ( σ ), chi ( χ ) and Laves (Fe 2 Mo) phases, and the formation of α ’ domains and precipitates. These phase transformations are promoted by the thermal exposure during processing and service conditions, deleteriously affecting the toughness and corrosion resistance of these steels. The faster atomic diffusion in the ferrite phase than in the austenite phase makes the precipitation kinetics of deleterious phases more critical to SFSSs and the dissolution of these phases during the solubilisation can cause exaggerated growth of the ferritic grains and loss of toughness. Phase diagrams are of great value in predicting the position of these deleterious stable phases’ fields , but the high number of alloying element in commercial SFSSs alloys might introduce uncertainties during the calculation of the multicomponent phase diagrams. The thermal exposure conditions during processing and service of SFSSs associated with the fast atomic diffusion in BCC metals might promote the precipitation of deleterious phases (Pimenta, 2001). The most relevant phase diagrams for the understanding of SFSSs are the Fe-Cr, Fe-Cr-Mo, Fe-Cr-Ni and Fe-Cr-Ni-Mo systems. The Fe-Cr equilibrium diagram shows the strong ferrite stabilizing character of Cr, the presence of sigma phase ( σ ) field and the immiscibility dome α phase. The latter produces a duplex microstructure composed of Cr-depleted and Cr-rich α domains/precipitates, the latter is known as alpha-prime ( α ’ ). The σ phase is a hard and fragile intermetallic compound with body centred tetragonal (BCT) crystal structure, containing 30 atoms in its unit cell (Andrews, 1949; Bergman and Shoemaker, 1951; Bungardt et al., 1963; Rivlin and Raynor, 1980; Brandi and Padilha, 1990). The addition of Mo in the Fe-Cr system stabilizes the ferrite phase, extends the σ phase field and promotes the formation of three extra intermetallic phases: the Laves (HC) phase, η (Fe 2 Mo), the μ phase (Fe 7 Mo 6 ) and the chi (BCC) phase, χ (Fe 36 Cr 12 M 10 ) (Kasper, 1954; Hughes and Llewelyn, 1959; Lyman, 1973; Kiesheyer and Brandis, 1976; Okafor and Carlson, 1978), but for the Mo contents found in commercial SFSSs, the  phase does not occur. Isothermal cross sections of the Fe-Cr-Ni-Mo diagram between 1204 and 816ºC indicate that the solubilisation of SFSSs above 1050ºC produces a ferric microstructure. Isothermal heat treatments of Fe-Cr-Ni-Mo alloys containing 4% Ni and 2% Mo below 900°C can fall either in the two-phases ( α + σ ) or in the three-phases ( α + σ + γ ) fields (Eckstein, 1990; Bechtoldt and Vacher, 1957; Folkhard, 1988). Additionally, the 2. Formation of deleterious and stable phases