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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precipitation of chi phase in SFSSs with less than 3% Mo and 4% Ni is unlikely (Bechtoldt and Vacher, 1957; Folkhard, 1988). There are three types of Laves phases - C14 (MgZn 2 ), C15 (Cu 2 Mg) and C36 (MgNi 2 ) - but the most frequently Laves phases in SFSSs are MgZn 2 type, such as Fe 2 Mo, Fe 2 Nb and Fe 2 Ti or any combination of them (Andrade et al., 2008; Pimenta, 2001). The addition of Ti in SFSSs causes the appearance of three phases: TiC, TiN and Ti 4 C 2 S 2 , while the addition of Nb promotes the formation of niobium carbonitride (Pimenta, 2001). The main deleterious phase transformations occurring in SFSSs are the formation of α ’ domains and σ precipitates (Bandel and Tofaute, 1942; Pimenta, 2001).
Table 1. Comparison of few properties of different generations of wrought FSSs and SFSS in the solution annealed condition, Cambridge (2019).
AISI 446 (1 st generation, FSS) Fe23-27Cr (C<0.2, Mn<1.5, Si<1, Ni<0.75, N<0.25)
AISI 409 (2 nd generation, FSS) Fe10-12Cr (C<0.08, Mn<1, Si<1, Ti<0.75, Ni<0.5)
AISI 444 (3 rd generation, FSS)
ASTM A240M 29-4 (4 th generation, SFSS) Fe28-30Cr3.5-4.2 Mo (C<0.01, Mn<0.3, N<0.02)
Properties and characteristics
Fe18-20Cr (C<0.025, Mn<1, Si<1, Ti<0.75, Ni<1, N<0.035)
Chemical composition
Yield stress (MPa) Tensile stress (MPa)
275 - 350 480 - 615
205 - 295 415 - 495
275 - 350 400 - 640
379 - 450 480 - 550
Elongation (%)
15 - 20
17 - 25
20 - 40
18 - 22
Fracture toughness (MPa.m 1/2 )
75 - 125
76 - 135
62 - 119
72 - 122
Cold forming Weldability (MIG, TIG and plasma)
Limited use
Acceptable
Limited use
Limited use
Good
Fair
Fair
Good
Maximum service temperature, air (°C) Susceptibility to 475ºC embrittlement Susceptibility to σ phase embrittlement Pitting resistance equivalent number Pitting and crevice corrosion Stress corrosion cracking (chloride)
1180
700 - 750
693 - 758
455 - 495
Very high
None
Very low
Very high
High
None
Medium
Very high
23 - 31
10.5 - 11.8
23.3 - 28.3
39 - 45
Medium
Low
Medium
Very high
Not susceptible
Not susceptible
Not susceptible
Not susceptible
Intergranular line corrosion Inorganic acids
weld
Good
Moderate
Good
Excellent
Restricted
Restricted Restricted Restricted
Restricted
Moderate
Sea water
Good
Good
Good
Sour oil and gas Cost (USD/kg)
Restricted
Restricted
Moderate 2.5 - 3.1
1.6 - 1.7
1.2 - 1.3
2.1 - 2.3
The embrittlement of FSSs, which is caused by thermal exposure of high Cr FSSs at around 475°C, has been discussed for more than half a century (Fisher et al., 1953; Chandra and Schwartz, 1971; Kuwano et al., 1991; Hedström et al., 2012). This embrittlement is attributed to the formation of α ’ domains or precipitates with sizes between 20 to 200 Å in the range of 300 to 550°C. The reduced dimensions of α ’ clusters/precipitates and the similarity between the scattering amplitudes of Cr and Fe for X-rays and electrons make their direct observation difficult, but the yield and tensile strengths are increased by the formation of α ’ , while the elongation and toughness are drastically decreased (see Fig. 1-a to 1-c) (Terada, 2003). The ferrite presents curvilinear deformation lines (wavy glide), as the
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