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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The σ phase is probably the most studied intermetallic compound. Treischke and Tamman (1907) investigated the Fe-Cr system and suggested the existence of an intermetallic compound in the range of 30 to 50% of Cr. Bain and Griffiths (1927) studied the Fe-Cr-Ni system and found a hard and fragile phase, identified as B phase. Jett and Foote (1936) named this fragile phase as sigma ( σ ) and Bergmann and Shoemaker (1951) determined the crystal structure of σ phase in the Fe-Cr system as body centred tetragonal with 30 atoms per unit cell. The precipitation of σ phase causes loss of tenacity and impoverishment of the matrix in Cr, decreasing the corrosion resistance. In ASSs, the precipitation of σ phase (~5% in volume) takes up to thousands hours (Weiss and Stickler, 1972). In the case of DSSs, the precipitation of σ phase can be completed in few hours, consuming the ferrite phase from the microstructure (Brandi and Padilha, 1990; Cortie and Jackson, 1997). The amount, velocity and mode of precipitation of the σ phase in FSSs depend on the chemical composition of the steel, especially the Cr and Mo contents, which increase its precipitation kinetics. In FSSs with 18% Cr, the σ phase precipitation occurs at about 550°C and might demand up to 10,000 hours for completing its transformation (Bungart et al., 1963). By contrast, for a FSS with 17% Cr and 2% Mo, the σ phase precipitation at 600°C might be completed in 200 hours (Baumel, 1964), while in SFSSs containing 28% Cr and 5% Mn, large amounts of σ phase precipitate at 900°C in just few minutes (Campbell, 1992), confirming the very high susceptibility of SFSSs to σ phase embrittlement (see Table 1). Fig. 2-a to 2-f show the precipitation the σ phase and resulting embrittlement caused by the transgranular cracking of the σ phase and the intergranular cracking along the ferrite/ σ interfaces of a DIN 1.4575 SFSS (26-30Cr, 1.8-2.5Mo, 3.0-4.5 N and 12xC  Nb  1.20) after isothermal treatments at 850°C for times between 4 and 94 hours (Pimenta, 1991).

Fig. 2. DIN 1.4575 SFSS solubilized at 1050°C for 30 minutes, water quenched and aged at 850°C. Microfractographic and microtopographic characterization of notched, pre-polished and pre-etched specimens after flexural tests. (a) Ferrite grains and intergranular σ phase after 4 hours at 850°C. (b) Ferrite grains and intergranular σ phase after 94 hours at 850°C. (c) Microfractography of test-piece aged at 850°C for 4 hours, revealing ductile fracture in the alpha phase and brittle fracture by transgranular cleavage of the σ phase and intergranular cleavage along the alpha/ σ interface. (d) Microfractography of test-piece aged at 850°C for 94 hours, revealing brittle fracture by transgranular cleavage of the σ phase and cleavage along the ferrite/ σ interface. (e) and (f) Images of the pre-polished surfaces near the fracture surfaces; (e) Test-piece aged at 850°C for 4 hours, showing typical deformation lines of BCC metals (wavy glide) and interfacial cracking; (f) Test-piece treated at 850°C for 94 hours, showing transgranular cracking of the σ phase and intergranular cracking along the ferrite/ σ interfaces, Pimenta (1991). The chi phase was first identified by Andrews (1949) in residues extracted from Cr-Ni-Mo steel. Kasper (1954) synthesized the chi phase with a composition of Fe 36 Cr 12 Mo 10 and studied its crystal structure. The chi phase can occur in Fe-Cr-Ni, Fe-Cr-Ni-Mo and Fe-Cr-Ni-Ti systems and its precipitation shows negative effects on the mechanical and corrosion properties (Kasper, 1954; Hughes and Llewelyn, 1959; Okafor and Carlson, 1978). Streicher (1974) studied the relationship between microstructure and properties of Fe-28 Cr-4 Mo and Fe-28 Cr-4 Mo-2Ni, identifying the intergranular precipitation of σ and chi phases in temperatures between 700°C and 930°C after 1 hour of aging. Thermal treatments above 1040°C caused the complete dissolution of these phases. Nana and Cortie (1996) found that precipitation of the σ and chi phases were delayed and even eliminated by Al addition. In comparison with σ phase, the chi phase is richer in Mo and poorer in Cr and its occurrence in FSSs is directly related to the Mo content (for