Effects of Cr and Si on the microstructure and solidification path of austenitic stainless steel
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摘要: 以316Ti奧氏體不銹鋼為基礎，設計不同Cr和Si元素含量的合金成分，結合Thermal-Calc熱力學模擬計算與合金鑄錠凝固組織形貌、成分分析，研究了Cr和Si元素對合金凝固組織構成的影響。研究結果表明，熱力學計算能夠獲得奧氏體不銹鋼中析出δ相的臨界Cr和Si含量。合金凝固時的元素偏析和冷卻過程中的“δ→γ”相變可對δ相析出預測產生一定影響。此外，本工作還針對δ相析出評價了兩種凝固路線判據。Abstract: The lead-cooled fast reactor (LFR), which features advanced technical maturity and enhanced safety, is an important part of the fourth-generation nuclear power system of China. The superior safety of the LFR results from the choice of a relatively inert coolant, the lead or lead-bismuth eutectic (LBE), which can be rather corrosive to common metallic structural materials. Furthermore, there is basically no cladding material available for the LFR. Austenitic stainless steels feature a combination of excellent corrosion resistance, proper strength, and good workability, and materials such as 316Ti and 15-15Ti, which have been used in the sodium-cooled fast reactor (SFR), are viewed as promising candidate materials for LFR cladding applications. Elements of Cr and Si have been found capable of improving the corrosion resistance of 316Ti and 15-15Ti to LBE. However, as ferrite-forming elements, the influences of Cr and Si on the microstructural stability of 316Ti and 15-15Ti are still unclear. In this work, 316Ti-based materials with various Cr and Si contents were studied through thermodynamic simulation and microstructural characterization. Specifically, the equilibrium phase constitutions of the austenitic stainless steels were investigated by thermodynamic simulation using Thermo-Calc. The solidification microstructures and precipitates of Cr- and Si-bearing austenitic stainless steels were studied by optical microscopy (OM), scanning electronic microscopy (SEM), electronic differential system (EDS), and X-ray diffraction (XRD). The results show that Cr and Si can decrease the solidus and liquidus temperatures of alloys and induce the precipitation of δ-phase. For alloy 18Cr?2.0Si?15Ni, the maximum contents of Cr and Si are determined to be no more than 18.8% and 2.55%, respectively, which hinders δ-phase precipitation. In the ingot of 20Cr?2.0Si, δ-phase is found to be located within dendrites in a skeleton morphology, with a volume fraction of 8.6%, whereas in the ingot of 18Cr?2.5Si, δ-phase precipitates between dendrites, with a volume fraction of 3.4%. Moreover, this work also evaluates two kinds of austenitic stainless steel solidification path criteria.
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圖 1 合金熱力學計算平衡相圖（0~10%質量分數）。 （a） Cr20?Si2.0；（b） Cr18?Si2.0；（c） Cr16?Si2.0；（d） Cr18?Si2.5；（e） Cr18?Si1.5
Figure 1. Thermodynamically calculated equilibrium phase diagrams for alloys (0–10%): (a) Cr20?Si2.0; (b) Cr18?Si2.0; (c) Cr16?Si2.0; (d) Cr18?Si2.5; (e) Cr18?Si1.5%
圖 2 Cr（a）和Si（b）元素在Cr18?Si2.0基體中的偽二元相圖
Figure 2. Pseudo-binary diagrams of Cr (a) and Si (b) in the Cr18?Si2.0 matrix
圖 3 合金鑄錠凝固組織金相顯微形貌. （a） Cr20?Si2.0；（b） Cr18?Si2.5；（c） Cr18?Si2.0
Figure 3. Optical observations：(a) Cr20?Si2.0；(b) Cr18?Si2.5；(c) Cr18?Si2.0
圖 4 合金鑄錠凝固組織掃描電鏡顯微形貌。 （a） Cr20?Si2.0； （b） Cr18?Si2.5；（c） Cr18?Si2.0
Figure 4. SEM observations：(a) Cr20?Si2.0；(b) Cr18?Si2.5；(c) Cr18?Si2.0
表 1 合金設計成分（質量分數）
Table 1. Design compositions of alloys
% 試樣 C Cr Si Mo Ni Ti Cu Mn Fe Cr20?Si2.0 0.06 20 2.0 1.5 15 0.36 1.5 1.5 余量 Cr18?Si2.0 0.06 18 2.0 1.5 15 0.36 1.5 1.5 余量 Cr16?Si2.0 0.06 16 2.0 1.5 15 0.36 1.5 1.5 余量 Cr18?Si2.5 0.06 18 2.5 1.5 15 0.36 1.5 1.5 余量 Cr18?Si1.5 0.06 18 1.5 1.5 15 0.36 1.5 1.5 余量 
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表 2 合金鑄錠檢測成分（質量分數）
Table 2. Chemical-tested compositions of ingots
% 試樣 C Cr Si Mo Ni Ti Cu Mn N Al Fe Cr20?Si2.0 0.063 19.74 2.01 1.52 15.01 0.36 1.60 1.48 0.0032 0.029 余量 Cr18?Si2.0 0.063 17.71 1.99 1.51 15.30 0.37 1.53 1.48 0.0024 0.030 余量 Cr16?Si2.0 0.066 15.76 2.01 1.50 15.21 0.37 1.54 1.49 0.0022 0.032 余量 Cr18?Si2.5 0.063 17.81 2.51 1.55 15.07 0.39 1.54 1.47 0.0026 0.029 余量 Cr18?Si1.5 0.065 17.73 1.53 1.54 15.22 0.38 1.52 1.49 0.0026 0.031 余量
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表 3 設計成分合金的熱力學計算固液相線溫度與凝固溫度區間
Table 3. Thermodynamically calculated liquids and solidus temperatures of alloys
合金 液相線溫度/℃ 固相線溫度/℃ 凝固區間/℃ Cr20?Si2.0 1386 1332 54 Cr18?Si2.0 1394 1340 54 Cr16?Si2.0 1401 1344 57 Cr18?Si2.5 1385 1320 65 Cr18?Si1.5 1402 1350 52
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表 4 Cr18?Si2.5合金鑄態組織析出相成分分析（質量分數）
Table 4. EDS analysis result of the Cr18?Si2.5 alloy
% 點 C Si Ti Cr Mn Fe Ni Cu Mo 總計 1 — 1.03 0.38 15.80 1.26 58.15 19.18 2.17 2.03 100 2 — 1.63 — 21.48 2.04 57.81 11.12 1.58 4.33 100 3 21.33 — 57.13 2.60 — 6.18 2.07 — 10.69 100
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表 5 合金Ni和Cr當量以及凝固路線判據計算（質量分數）
Table 5. Calculations on the Ni and Cr equivalent contents and solidification path criteria
%
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