Super austenitic stainless steel (SASS) S31254 possesses outstanding corrosion resistance and exceptional comprehensive mechanical properties, making it a highly promising high-performance material. This study presents an in-situ investigation into the cooling rate-dependent phase transformation and sigma (σ) phase precipitation behavior in S31254 SASS, employing high-temperature confocal laser scanning microscopy (HT-CLSM) combined with thermodynamic and kinetic modeling. The solidification process and microstructural evolution were dynamically monitored across five cooling rates (30, 60, 150, 300, and 600 °C/min). The results revealed that increasing the cooling rate reduced the initial solidification temperature while widening the solidification temperature range. A key finding was the direct observation of the austenite (γ) to ferrite (δ) transformation during the late stages of solidification, with the sigma (σ) phase subsequently forming through the post-solidification eutectoid decomposition of δ (δ → σ + γ2), both coexisting in interdendritic regions. The morphology, nucleation sites, and growth kinetics of the δ phase exhibited a strong dependence on cooling rate. Slower cooling promoted the formation of larger, more equiaxed grains and a higher fraction of δ phase, while faster cooling favored dendritic growth with a pronounced tendency for longitudinal extension of the δ phase. Thermodynamic calculations using Thermo-Calc and JMatPro indicated that a reduction in nitrogen content promoted the precipitation of δ and σ phases, altering the solidification path to L → L + γ → L + γ + δ → γ + δ → γ + δ + σ + γ2 or L → L + δ → L + δ + γ → γ + δ → γ + δ + σ + γ2. This integrated experimental and modeling approach comprehensively elucidated the complex interplay between cooling rate, composition (especially N), and the resulting phase transformation pathways, offering valuable guidance for process optimization to control detrimental phase precipitation in S31254 SASS.

Cooling Rate-Dependent Phase Transformation and Sigma-Phase Precipitation in Super Austenitic Stainless Steel S31254: An In-Situ CLSM Study Combined with Thermodynamic Modeling

期刊论文
发表日期：

2026年3月11日

查看全文

Yong Wang,
Yanjing Wu,
Kun Bai,
Chengsong Liu,
Yaowu Wei,
Hua Zhang,
Hongwei Ni,
Sohei Sukenaga,
Hiroyuki Shibata

摘要

Super austenitic stainless steel (SASS) S31254 possesses outstanding corrosion resistance and exceptional comprehensive mechanical properties, making it a highly promising high-performance material. This study presents an in-situ investigation into the cooling rate-dependent phase transformation and sigma (σ) phase precipitation behavior in S31254 SASS, employing high-temperature confocal laser scanning microscopy (HT-CLSM) combined with thermodynamic and kinetic modeling. The solidification process and microstructural evolution were dynamically monitored across five cooling rates (30, 60, 150, 300, and 600 °C/min). The results revealed that increasing the cooling rate reduced the initial solidification temperature while widening the solidification temperature range. A key finding was the direct observation of the austenite (γ) to ferrite (δ) transformation during the late stages of solidification, with the sigma (σ) phase subsequently forming through the post-solidification eutectoid decomposition of δ (δ → σ + γ₂), both coexisting in interdendritic regions. The morphology, nucleation sites, and growth kinetics of the δ phase exhibited a strong dependence on cooling rate. Slower cooling promoted the formation of larger, more equiaxed grains and a higher fraction of δ phase, while faster cooling favored dendritic growth with a pronounced tendency for longitudinal extension of the δ phase. Thermodynamic calculations using Thermo-Calc and JMatPro indicated that a reduction in nitrogen content promoted the precipitation of δ and σ phases, altering the solidification path to L → L + γ → L + γ + δ → γ + δ → γ + δ + σ + γ₂ or L → L + δ → L + δ + γ → γ + δ → γ + δ + σ + γ₂. This integrated experimental and modeling approach comprehensively elucidated the complex interplay between cooling rate, composition (especially N), and the resulting phase transformation pathways, offering valuable guidance for process optimization to control detrimental phase precipitation in S31254 SASS.