Effect of Cerium on the Austenitic Nucleation and Growth of High-Mo Austenitic Stainless Steel

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 degrees 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 degrees C to 43.9 degrees C and limits the growth velocity of a single austenite grain from approximately 20 to 2 mu 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).