scholarly journals Sigma Phase: Nucleation and Growth

2018 ◽  
Author(s):  
Gláucio Fonseca ◽  
Priscila Mendes ◽  
Ana Silva
Keyword(s):  
Stainless Steels ◽  
Sigma Phase ◽  
Volume Fraction ◽  
Interfacial Area ◽  
Nucleation And Growth ◽  
Phase Precipitation ◽  
Phase Nucleation ◽  
First Time ◽  
Kinetics Of ◽  
Partial Path

Duplex Stainless Steels (DSS) and Superduplex Stainless Steels (SDSS) are an important class of stainless steels because they combine the benefits of austenite and ferrite phases, resulting in steels with better mechanical properties and higher corrosion resistance. Due to these characteristics are widely employed in various industries. However, the appearance of deleterious phases in their microstructure impairs the properties of DSS and SDSS. Among the deleterious phases, the main one is the sigma phase (σ), which can be nucleated when the steel is exposed to the temperature range between 650 °C and 900 °C, reducing its toughness and resistance to corrosion. In a previous work, Fonseca and collaborators used two descriptors of the microstructural path to analyze the formation of sigma phase (σ), SV, interfacial area per unit volume between sigma phase and austenite, and <λ>, mean chord length of sigma, both in function of the VV, volume fraction of sigma, known in the literature as microstructural partial path (MP). In this work, the contiguity ratio is applied for the first time to describe the microstructural path in the study of sigma phase precipitation in SDSS. The contiguity ratio showed that the distribution of the ferrite/sigma boundaries is homogeneous. Thus, it is reasonable to infer that one has a uniform distribution of sigma phase nuclei within the ferrite. About the kinetics of sigma phase formation, the DSS can be described by the classical JMAK equation, whereas for the SDSS, the kinetics tends to follow the Cahn model for grain edge nucleation. Finally, we present the 3D reconstruction of the sigma phase in SDSS. The results demonstrate that the sigma phase nucleates at the edges of the ferrite/austenite interfaces. Moreover, the sigma phase grows consuming the ferrite, but it is not fully interconnected.

scholarly journals Sigma Phase: Nucleation and Growth

Metals ◽  
2019 ◽  
Vol 9 (1) ◽  
pp. 34 ◽  
Author(s):  
Gláucio Soares da Fonseca ◽  
Priscila Sousa Nilo Mendes ◽  
Ana Carolina Martins Silva
Keyword(s):  
Stainless Steels ◽  
Sigma Phase ◽  
Volume Fraction ◽  
Three Dimensional ◽  
Interfacial Area ◽  
Intermetallic Phases ◽  
Phase Nucleation ◽  
The Mean ◽  
First Time ◽  
Kinetics Of

Duplex stainless steels (DSS) and superduplex stainless steels (SDSS) are important classes of stainless steels, because they combine the benefits of austenite and ferrite phases. This results in steels with better mechanical properties and higher corrosion resistance. Owing to these characteristics, DSS and SDSS are widely employed in industry. However, the appearance of undesirable intermetallic phases in their microstructure impairs the properties of DSS and SDSS. Among the undesirable intermetallic phases, the main one is the sigma phase (σ), which can be nucleated when the steel is exposed to the temperature range between 650 °C and 900 °C, reducing the steel’s toughness and resistance to corrosion. In a previous work, Fonseca and collaborators used two descriptors of the microstructural path to analyze the formation of sigma phase (σ), the interfacial area per unit volume between sigma phase and austenite (SV), and the mean chord length of sigma (<λ>), both as a function of VV, the volume fraction of sigma, known in the literature as the microstructural partial path (MP). In this work, the contiguity ratio is applied for the first time to describe the microstructural path in the study of sigma phase precipitation in SDSS. The contiguity ratio shows that the distribution of the ferrite/sigma boundaries is homogeneous. Thus, it is reasonable to infer that one has a uniform distribution of sigma phase nuclei within the ferrite. About the kinetics of sigma phase formation, the DSS can be described by the classical Johnson-Mehl, Avrami, and Kolmogorov (JMAK) equation, whereas for the SDSS, the kinetics tend to follow the Cahn model for grain edge nucleation. Finally, we present the three-dimensional (3D) reconstruction of the sigma phase in SDSS. The results demonstrate that the sigma phase nucleates at the edges of the ferrite/austenite interfaces. Moreover, the sigma phase grows and consumes the ferrite, but is not fully interconnected.

Internal Friction Influenced by Hydrogen in Super Duplex Stainless Steels

2005 ◽  
Vol 502 ◽  
pp. 345-350
Author(s):  
Toshio Kuroda ◽  
Katsuyuki Nakade ◽  
Kenji Ikeuchi
Keyword(s):  
Internal Friction ◽  
Stainless Steels ◽  
Sigma Phase ◽  
Peak Height ◽  
Phase Precipitation ◽  
Internal Friction Peak ◽  
Hydrogen Charging ◽  
Super Duplex Stainless Steels ◽  
Two Peaks ◽  
Friction Analysis

The influence of microstructure concerning sigma phase on hydrogen behavior was investigated by means of internal friction analysis. After hydrogen charging, a sharp significant internal friction peak by hydrogen in austenite of as-received specimen was observed at 245K for a frequency of 1.5Hz. However, the peak height in the specimen precipitated significant sigma phase was substantially lower than in as-received specimen since hydrogen in austenite have a concentration lower by sigma phase precipitation. In addition, the broadening and scattering of the internal friction peak was clearly identified by interaction between hydrogen and sigma phase. It means that the two peaks associated with hydrogen in the both sigma phase and austenite were considered to be overlapped. Consequently, it was clearly confirmed that hydrogen entered in the sigma phase lattice and hydrogen was also trapped at sigma/austenite interfaces.

Modelling Chi-Phase Precipitation in High Molybdenum Stainless Steels

2006 ◽  
Vol 15-17 ◽  
pp. 531-536 ◽  
Author(s):  
W. Xu ◽  
D. San Martin ◽  
Pedro E.J. Rivera-Díaz-del-Castillo ◽  
Sybrand van der Zwaag
Keyword(s):  
Heat Treatment ◽  
Stainless Steels ◽  
Chromium Content ◽  
High Strength ◽  
Processing Parameters ◽  
Nucleation And Growth ◽  
Phase Precipitation ◽  
Treatment Conditions ◽  
Highly Sensitive

High molybdenum high strength stainless steels can contain the so-called Chi phase (Fe36Cr12Mo10). The presence of this phase, which normally occurs at grain boundaries, depletes the chromium content leading to intergranular corrosion. This may cause alloy embrittlement during long term use. The presence of such phase has proven to be highly sensitive to alloy processing parameters such as the cooling rate after a final heat treatment. The present work provides a model to quantify the effects of processing parameters aimed at controlling the Chi phase. The model is based on nucleation and growth classical theories involving capillarity effects for the early stages; it is applied to a range of heat treatment conditions and compared to experimental results.

scholarly journals Study of the Precipitation of Secondary Phases in Duplex and Superduplex Stainless Steel

2016 ◽  
Vol 879 ◽  
pp. 2537-2542 ◽  
Author(s):  
Núria Llorca-Isern ◽  
Isabel López-Jiménez ◽  
Héctor López-Luque ◽  
Maria Victoria Biezma ◽  
Antoni Roca
Keyword(s):  
Sigma Phase ◽  
Intermetallic Phases ◽  
Isothermal Treatment ◽  
Secondary Phases ◽  
Chromium Nitrides ◽  
Superduplex Stainless Steel ◽  
Backscattered Electron ◽  
Phase Nucleation ◽  
Suitable Technique ◽  
Kinetics Of

The aim of this work is to study the precipitation mechanism of the intermetallic phases present in duplex stainless steels (UNS S32205 and UNS S32750), as well as to find out the most suitable method for detecting and analyzing accurately these secondary phases, particularly Sigma-phase, Chi-phase, nitrides and carbides. The samples were characterized after a solution annealing at 1080oC followed by an isothermal treatment at 830oC from 1 min to 9 h, with the purpose of figuring out the mechanism of chi-phase nucleation and nitrides formation in relation with the sigma-phase. The study has two main objectives: 1) to find out the most suitable technique for the detection, identification and quantification of the secondary phases, obtaining the best results with the combination of field emission scanning electron microscopy (FESEM) and backscattered electron detector (BSE) in comparison with the optical microscopy (MO); 2) to study the influence of the chemical composition on the nucleation mechanism of the intermetallic phases. It has been concluded that molybdenum balance content in chi-phase related to sigma phase is close to two, consequently the kinetics of nucleation and growth of these phases is remarkably faster when this alloying element content in the steel is higher. Chromium nitrides and carbides were also observed to precipitate as a result of the heat treatments carried out to the specimen wherein chromium nitrides role is a favorable site for the nucleation of sigma and chi phases.

scholarly journals Overview of Intermetallic Sigma () Phase Precipitation in Stainless Steels

2012 ◽  
Vol 2012 ◽  
pp. 1-16 ◽  
Author(s):  
Chih-Chun Hsieh ◽  
Weite Wu
Keyword(s):  
Stainless Steels ◽  
Sigma Phase ◽  
Science And Technology ◽  
Prediction Method ◽  
Phase Precipitation ◽  
Science And Engineering ◽  
Precipitation Mechanism ◽  
Processing Effect ◽  
Precipitation Characteristics

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.

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