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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numerous sliding systems of the bcc structure and its high stacking defect energy facilitate the occurrence of cross slop (Pimenta, 2001). However, the presence of α ’ domains/precipitates pins the dislocations, reducing the number of active sliding planes and promoting the appearance of straight-line deformation lines on the polished surface (see Fig. 1-d). Marcinkowski et al. (1964), Blackburn and Nutting (1964), Anglada et al. (1989) and Isalgué et al. (1990) consider that the ferrite containing α ’ precipitates deforms predominantly by twinning and that the straight lines of deformation are actually deformation twinning, as confirmed by Fig. 1-d to 1-f (Terada, 2003). The magnitude of the effect of α ’ on the mechanical and corrosion properties of SFSSs mainly depends on the Cr content, so the susceptibility to α ’ embrittlement is higher for higher contents of Cr. SFSSs containing from 25 to 30% Cr and aged around 475°C may double the hardness values (Bandel and Tofaute, 1942; Newel, 1946; Grobner, 1973; Jacobsson et al., 1975). There is a wide discussion as to whether the formation of α ’ is the result of a spinodal decomposition or nucleation and growth process (Bandel and Tofaute, 1942; Pimenta, 2001; Fisher et al., 1953; Anglada et al., 1989; Isalgué et al., 1990); Marcinkowski et al., 1964; Blackburn and Nutting, 1964; Grobner, 1973; Jacobsson et al., 1975). The kinetics of α ’ formation is generally presented in a TTT diagram as a C-shaped curve, whose nose is located at around 475°C for a given incubation time, which is a common feature of phase transformations by nucleation and growth. Folkhard (1998) compared the TTT curves of α’ formation in several FSSs and DSSs. In steels containing between 14% and 18% of Cr contents, there is evidence that ferrite embrittlement occurred by the nucleation and growth of α’ precipitates (Grobner, 1973; Steigerwald et al., 1978; Jacobsson et al., 1975). As the chromium content of the steel increases, the incubation time for the α’ precipitation is reduced. Evidence of α’ formation by spinodal decomposition appears in SFSSs containing more than 20% Cr (Chandra and Schwartz, 1971; Anglada et al., 1989; Isalgué et al., 1990). Hedström et al. (2012), however, showed for Fe-Cr system that the decomposed microstructure in the immiscibility dome changed with increasing Cr content continuously changed from particle-like nucleation and growth structure at 25% Cr to more spinodal-like duplex microstructure at 30% Cr at 500°C. On the other hand, Kuwano et al. (1991), using Mössbauer spectroscopy results, suggested that the addition of 5% nickel to the alloy containing 28% chromium caused modification of the α’ formation from nucleation and growth to spinodal decomposition. Finally, it should be noted that the few studies performed on the DIN W. Nr. 1.4575 SFSS present evidences that the formation of α’ in this steel occurs by spinodal decomposition, Anglada et al. (1989), Isalgué et al. (1990) although Terada (2003) showed the precipitation of α’ particles of 15 nm after aging a DIN W. Nr.1.4575 SFSS (26.0-30.0%Cr, 1.8-2.5%Mo and 3.0 4.5%Ni) at 475ºC for 811 hours (see Fig. 1-d).

Fig. 1. DIN 1.4575 SFSS solubilized at 1050°C for 30 minutes water quenched and aged at 475°C, pre-polished and pre-etched specimens after flexural tests. (a) and (b) Sample aged at 475°C for 1 hour; (a) Microfractography revealing transgranular cleavage fracture with few areas of ductile fracture, SEM, SEI; (b) Detail of the surface near the fracture, showing typical deformation lines of BCC metals (wavy glide), SEM-SEI. (c) and (d) Sample aged at 475°C for 512 hours; (c) Microfractography revealing the brittle transgranular cleavage fracture, SEM, SEI; (d) Detail of the surface near the fracture, showing marks of plastic deformation by twinning. (e) EBSD result of sample aged at 475°C for 317 hours after flexural test, showing mechanical twinning (  = 3, in grey). (f) Precipitation of α ’ particles, sample aged at 475°C for 811 hours, TEM, Terada (2003).