Influence of the Heterogeneous Nucleation Sites on the Kinetics of Intermetallic Phase Formation in Aged Duplex Stainles
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UPLEX stainless steels (DSSs) are largely used in the oil and gas industry due to their high mechanical properties, fracture toughness, and corrosion resistance. DSSs are composed approximately by equal amounts of ferrite (a) and austenite (c) and high amounts of Cr, Ni, Mo, and N.[1,2] Despite these properties, DSSs are susceptible to deleterious phase formation, such as chromium nitride (Cr2N), chi (v), and sigma (r).[3] Those phases are usually formed between 823 K and 1273 K (550 °C and 1000 °C).[4–7] Sigma and chi are mainly formed by Fe, Cr, and Mo, but chi has higher amounts of Mo than
ELIS ALMEIDA MELO and RODRIGO MAGNABOSCO are with the Ignatian Educational Foundation (FEI), Av. Humberto Castelo Branco, 3972, Sa˜o Bernardo do Campo, SP 09850-901, Brazil. Contact e-mail: [email protected] Manuscript submitted September 18, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
sigma. The formation of these phases causes embrittlement and mainly loss of corrosion resistance due to Cr and Mo depletion of the matrix surrounding those deleterious phases.[8–10] Sigma phase formation occurs preferentially in heterogeneous nucleation sites of the matrix, such as triple junctions, or in a/c and a/a interfaces due to the high Gibbs energy associated with those sites.[11,12] Crystallographic orientation is also important in sigma precipitation. Considering the standard Kurdjumov–Sachs (K–S) relation ({110}a//{111}c and h111ia// h110ic), the higher the deviation from the K–S relation, the greater the precipitation of sigma, showing that not only the amount of interfaces but also their nature influence sigma precipitation.[13,14] Kinetics of sigma phase formation is influenced by time, temperature, nucleation rate, growth rate, nucleation site distribution, density of these sites, transformed volume diffusion fields overlapping, and adjacent transformed volumes. However, in a simplified approach, kinetics of sigma phase formation can be described by
Table I. Cr 22.07
Chemical Composition (Weight Percent) of the Studied DSS
Ni
Mo
Mn
Si
N
Cu
P
C
S
Fe
5.68
3.19
1.38
0.34
0.17
0.17
0.020
0.017
0.001
balance
the Kolmogorov–Johnson–Mehl–Avrami equation, as shown in Eq. [1][15,16]: f ¼ 1 eðkt
n
Þ
(KJMA) ½1
where n is the so-called Avrami exponent; when n values lie between 0.5 and 2.5, phase growth is usually assumed to be controlled by diffusion, and when n is greater than 2.5, phase formation occurs as discontinuous precipitation or interface controlled growth.[17] t is the transformation time; f is the formed phase fraction (0 < f < 1); and k depends on the nucleation rate and mechanism, growing rate, and temperature. k can be calculated by Eq. [2][15,16]: Qr k ¼ k0 exp ½2 RT
Fig. 1—UNS S3180 steel solution treated for 1 h at 1373 K (1100 °C), showing ferrite (a) and austenite (c) phases.
where T is the absolute temperature; k0 is a pre-exponential constant; Qr is the transformation activation energy, which includes the driving force for nucleation
Fig. 2—UNS S31803 steel solution tr
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