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