Prediction of ^|^sigma; Phase Precipitation in Type 316FR Stainless Steel Weld Metal

Abstract

Through the isothermal ageing treatments at 873, 923, 973, and 1023K, precipitation of sigma (σ) phase in type 316FR stainless steel weld metal was examined based on the kinetics approach. Microstructural examination was performed by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The dominantly precipitated phases were sigma (σ) and chi (χ), nucleated at δ-ferrite/austenite (γ) interface or interior of the δ-ferrite grains, consuming the δ-ferrite during isothermal holding at each of ageing temperature. The total amount of intermetallic phases precipitated during isothermal ageing sigmoidally increased as a function of the ageing time. Precipitation of these intermetallic phases could be approximately followed by Johnson-Mehl equation. Based on the determined kinetic equation, precipitation behavior of intermetallic phases at service-exposure temperature could be successfully predicted.

Influence of Solution Treatment Temperature on Microstructural Properties of an Industrially Forged UNS S32750/1.4410/F53 Super Duplex Stainless Steel (SDSS) Alloy

“…It was also reported that, for a type 316FR austenitic stainless steel weld metal under isothermal ageing treatments, σ-phase usually precipitates around grain inclusions, incoherent grain boundaries and δ-Fe/ γ-Fe grain boundaries region [35]. The solution treatment temperature of 800 °C (Figure 4b) led to an increased grain-size of σ (CrFe) phase and to a higher fraction of σ (Cr-Fe) phase, with an average grain-size close to 8-9 μm.…”

“…For instance, in the case of a stabilized AISI 347 austenitic stainless steel under heat treatments, it was shown that σ (Cr-Fe) phase precipitation is a very slow process and usually small grains and very low proportions are obtained during slow cooling [34]. It was also reported that, for a type 316FR austenitic stainless steel weld metal under isothermal ageing treatments, σ-phase usually precipitates around grain inclusions, incoherent grain boundaries and δ-Fe/ γ-Fe grain boundaries region [35].…”

“…A solution treatment temperature of 1000 °C led to decreasing of both weight fraction and average grain-size of σ (Cr-Fe) phase, the average grain-size treatments, it was shown that σ (Cr-Fe) phase precipitation is a very slow process and usually small grains and very low proportions are obtained during slow cooling [34]. It was also reported that, for a type 316FR austenitic stainless steel weld metal under isothermal ageing treatments, σ-phase usually precipitates around grain inclusions, incoherent grain boundaries and δ-Fe/ γ-Fe grain boundaries region [35]. The solution treatment temperature of 800 °C (Figure 4b) led to an increased grain-size of σ (CrFe) phase and to a higher fraction of σ (Cr-Fe) phase, with an average grain-size close to 8-9 μm.…”

Microstructure and mechanical properties of the 316 stainless steel nuclear grade experimental component made by wire and arc additive manufacturing

“…Austenite is the equilibrium phase at high temperature; 6 delta-ferrite (d), intermetallic compounds sigma (s), and carbides are not equilibrium phases that can redissolve by appropriate solution treatments. 13 The high-temperature heat treatment at 1100 C with enough holding time (3 h) promotes the dissolution of hard and brittle intermetallic sigma (s) phase, 8,9 leaving mainly two phases (g and d) in the microstructure of WAAM SS316.…”

Evaluation of Optimal Forging Temperature Range for an Industrial UNS S32750 SDSS Alloy Using SEM-EBSD Analysis

“…For example, it was found that the σ and τ phases are formed due to Cr diffusion towards the grain boundaries during slow cooling after forging [19,20]. It was also shown that σ phase precipitation is a very slow process and small grain sizes and low contents of σ phase are found in the material after soft cooling [21,22], σ phase precipitating mainly in the ferrite-austenite grain boundaries area and around inclusions or incoherent grain boundaries [19,23]. Regardless of the deformation mode, the temperature can improve the hot formability of the steel.…”