The influence of small amounts of cerium on the solidification phenomena of S31254 high-Mo austenitic stainless steel was investigated by in situ observations and theoretical calculations. In situ observations indicate that cerium addition in molten steel can accelerate austenitic nucleation but inhibit primary austenite grain growth. The initial nucleation temperature occur 32.3 °C in advance and the nucleation site density increases to 176/mm after 2 seconds in S31254-Ce compared to S31254. The modified Ce-containing inclusions in S31254-Ce improve the ability for austenite heterogeneous nucleation, which would significantly reduce the energy barrier of nucleation. The nucleation mechanism obtained from Johnson–Mehl–Avrami–Kologoromov (JMAK) theory is changed from site saturation nucleation to site saturation plus Avrami nucleation after cerium addition. Furthermore, cerium addition also prolongs the whole solidification temperature range from 28 °C to 43.9 °C and limits the growth velocity of a single austenite grain from approximately 20 to 2 μ m/s. The ratio of the solid phase decreases from 60 to 8.2 pct, but the grain density increases from 68.8 to 280/mm 2 , with the solidification time extended to 8 seconds after cerium addition. Thus, the main solidification behavior in the early solidification stage of S31254-Ce is nucleation instead of primary grain growth. The low growth velocity for austenite grains and significant high nucleation site density exhibit a low rate constant k during the solidification process. Moreover, cerium addition could significantly refine the solidified structure, and the grain density of the final solidified structure in S31254-Ce is 366.6/mm 2 .

中文翻译：

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铈对高钼奥氏体不锈钢奥氏体形核和生长的影响

通过原位观察和理论计算研究了少量铈对S31254高钼奥氏体不锈钢凝固现象的影响。原位观察表明，在钢水中添加铈可以加速奥氏体形核但抑制初生奥氏体晶粒长大。与 S31254 相比，S31254-Ce 的初始成核温度提前 32.3°C，成核位点密度在 2 秒后增加到 176/mm。S31254-Ce中改性的含Ce夹杂物提高了奥氏体异相形核能力，显着降低了形核能垒。从 Johnson-Mehl-Avrami-Kologoromov (JMAK) 理论获得的成核机制在添加铈后从位点饱和成核变为位点饱和加 Avrami 成核。此外，铈的添加还将整个凝固温度范围从 28 °C 延长至 43.9 °C，并将单个奥氏体晶粒的生长速度限制在大约 20 至 2 μm/s。固相比例从60%减少到8.2%，但晶粒密度从68.8%增加到280/mm 2 ，加入铈后凝固时间延长至8秒。因此，S31254-Ce 早期凝固阶段的主要凝固行为是形核而不是初级晶粒生长。奥氏体晶粒的低生长速度和显着的高成核位点密度在凝固过程中表现出低速率常数 k。此外，铈的添加可以显着细化凝固组织，S31254-Ce中最终凝固组织的晶粒密度为366.6/mm 2 。