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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example, at 600°C this critical value is around 2%) (Kiesheyer and Brandis, 1976). In SFSSs containing 28% Cr, the chi phase is metastable in a wide range of temperatures, so after long aging times (such as 5,000 hours at 800°C) the chi phase dissolves, while the σ phase precipitates. The presence of chi phase in SFSSs depends not only on the Mo content, but also on the Cr content and the temperature and aging time (Pimenta, 2001). Fig. 3-a to 3-a show the precipitation of chi and σ phases of a 27Cr-4Mo-2Ni SFSS after isothermal aging. According to Lu et al. (2018), the precipitation of Laves, σ and chi phases after aging at 650°C are responsible for the drop in the impact toughness for more than 0.5 hour, but when the steel is aged at 650°C for 0.5 hour, the impact toughness decreases slightly from 149 to 120 J/cm 2 due to the limited precipitation of intergranular chi phase. At 850°C, the bulk intergranular precipitation of σ phase is the main responsible for the embrittlement, see Fig. 3-c to 3-e, causing a drastic drop of the impact energy to only 2.6 J/cm 2 . The fracture surface of samples aged at 800°C features a typical cleavage terrace and river pattern, typical of brittle fracture (see Fig. 3-c) (Lu et al., 2018). Fig. 4-a to 4-c show the intergranular precipitation of the Laves phase in a DIN 1.4575 SFSS after aging at 850°C from 30 min to 4 hours (Andrade et al., 2008). The presence of Ti and Nb can lead to the precipitation of Fe 2 M-type Laves (M = Ti, Nb and Mo) (Sawatani et al., 1982), as seen in the TTT diagram of Fig 3-e (Lu et al., 2018).

Fig. 3. Microstructural evolution and mechanical properties of a 27Cr-4Mo-2Ni SFSS after isothermal aging. (a) Microstructure of the steel after aging at 650°C for 4 hours, showing intergranular χ phase. (b) Sample aged at 800°C for 2 hours, bulk intergranular σ phase and submicron chi phase precipitates. (c) Fractography of the impact sample aged at 800°C for 2 hours, showing brittle fracture (mainly transgranular cleavage fracture). (d) Impact toughness results, showing the effect of Laves and χ precipitation on the toughness at 650°C and the effect of σ , Laves and χ precipitation on the toughness at 800°C. (e) Calculated TTT curves for the precipitation of stable and deleterious phases and the experimental results. All figures were adapted from Lu et al. (2018). Intergranular corrosion is a much less important phenomenon in ferritic stainless steels than in austenitic steels (see Table 1). Several reasons may contribute to this difference in behavior: i) diffusion in ferrite is about two orders of magnitude faster than in austenite, reducing the possibilities and occurrence of composition gradients; ii) the C content of FSSs is generally maintained at lower levels than that of ASSs because of their very negative effect on toughness; (iii) some ferritic stainless steels, such as SFSSs, exhibit high chromium contents above 25%. The precipitation of intergranular M 23 C 6 carbides and the consequent Cr-depletion causes the intergranular corrosion of ferritic steels (Kiesheyer and Brandis, 1976; Bond, 1969; Bäumel, 1973). Bäumel (1973) compared in a TTT diagram the intergranular corrosion behavior of two stainless steels containing similar C and Cr contents, one of them (FSS) free of Ni and another (ASS) containing 8% of Ni. The intergranular corrosion region of the FSS was shifted to shorter times and lower temperatures compared to the ASS. The precipitation of M 23 C 6 occurs so rapidly in FSS, making it inevitable to attain corrosion resistance for some chemical compositions, plate thicknesses and processing. The effect of minor alloying in a super ferritic stainless steel 26%Cr-3%Mo matrix was investigated in terms of their sensitization and intergranular corrosion susceptibility following a low-temperature anneal (620°C). Constant potential etching, electrochemical and immersion studies showed that the principal corrosion initiation site of heat-treated steels after