Overview of Intermetallic Sigma () Phase Precipitation in Stainless Steels

Abstract

The phase which exists in various series of stainless steels is a significant subject in steels science and engineering. The precipitation of the phase is also a widely discussed aspect of the science and technology of stainless steels. The microstructural variation, precipitation mechanism, prediction method, and effects of properties of phase are also of importance in academic discussions. In the first section, a brief introduction to the development and the precipitation characteristics (including morphologies and precipitation sites) of phase in stainless steels is presented. In the second section, the properties effect, prediction method, processing effect, elemental addition, retardation method and Thermo-Calc simulation of the phase in stainless steels are highlighted.

The Cracking Mechanism of Ferritic-Austenitic Cast Steel

“…Results obtained in the micro fractography analysis of cracks are shown in Figures 4 and 5. The microstructure analysis showed, next to the primary phase of ferrite and austenite, presence of the sigma phase, which negative interaction is generally known [13,14]. In the tested material were found two types of sigma phase precipitates in the form of single precipitates or a single colony in ferrite and in the form of a thin, more or less continuous film on the border of ferrite -austenite.…”

“…At about 936 °C begins to precipitate the intermetallic σ phase, which ends for the analyzed chemical composition of about 510 °C. This range of temperature generally correspond to literature value [13,14], a little too low is the temperature 510°C. According to the data obtained in the FactSage for the equilibrium conditions there are no other intermetallic phases.…”

Precipitation within localised chromium-enriched regions in a Type 316H austenitic stainless steel

“…For example, the presence of secondary phase precipitates, such as ferrite, has the potential to significantly alter the creep properties by changing the behaviour of creep damage accumulation [1][2][3][4][5]. The evolution of secondary phases during thermal ageing of Type 316 austenitic stainless steels has been observed experimentally [5][6][7][8][9] and predicted thermodynamically [10], driven by a favourable Gibbs energy [11]. These precipitates will nucleate at discrete sites within the overall microstructure and grow over time.…”

“…The composition corresponds to (Fe,Ni,Mo) 16 (Cr) 6 Si 2 . Previous workers have reported various compositions of the G phase in a range of austenitic stainless steels: a typical example for the compositions is (Ni, Fe, Mo, Cr) 16 (Nb, Mn, Cr, Ti) 6 Si 6 or 7 [12,14,19,23]. The crystal structure of the G phase detected in the current specimen is consistent with previous observations, although the lattice parameter is slightly smaller (a = 1.115-1.120 nm [12]).…”

“…One of the less encountered precipitates is G phase, a silicon containing FCC intermetallic phase (a = 1.115-1.120 nm [12]) with a nominal composition observed in austenitic stainless steels [12][13][14][15][16][17][18][19][20] of (Ni/Fe/Cr) 16 (Nb/ Ti) 6 Si 6 [12]. By comparison, in duplex stainless steels [12,21,22] the composition is (Fe/Ni) 16 (Mn/Cr) 6 Si 7 [19]. Other studies have found that substitution of Mo for Ni is possible [14,23].…”