Qi Wang

The effects of Y on the solidification process of 7Mo super austenitic stainless steel (7MoSASS) under low cooling rate conditions (10 °C/min) were investigated using high-temperature confocal laser scanning microscopy (HT-CLSM). The in situ observation results indicate that Y samples promote an increase in austenite nucleation density. After 10 s of nucleation, the nucleation density increased by 149.53/mm² for the Y sample. Furthermore, variance analysis indicated that Y addition improved the uniformity of the 7MoSASS solidification microstructure under low cooling rate conditions. The Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory results showed that when the solid phase ratio was 0.5, the nucleation mode of the Y sample transitioned from saturation site nucleation to saturation site nucleation + Avrami nucleation. YAlO₃ has a low lattice disregistry value with austenite, making it a suitable heterogeneous nucleation core for promoting the early nucleation of austenite. During the late stages of solidification, Y accumulates in the residual liquid phase, providing a greater degree of compositional undercooling. SEM-EDS analysis showed that Y contributed to the refinement of the 7MoSASS solidification microstructure, with the proportion of precipitated phases decreasing by approximately 7.5%. Cr and Mo were the main elements exhibiting positive segregation in 7MoSASS, and the Cr segregation ratio increased in the Y sample, while the Mo segregation ratio decreased.

Reasonable regulation of nonmetallic inclusions in steel can significantly improve its strength, toughness, and corrosion resistance. In this paper, EH420 marine steel was treated with Mg, Ce, and Mg-Ce to modify the inclusions. The effects of different treatments on the morphology, composition, size distribution, induced intragranular ferrite (IGF) nucleation, and pitting resistance of inclusions were systematically analyzed using various methods. The results show that the Mg-Ce composite treatment can modify irregular MgAl₂O₄ inclusions into spherical Mg-Ce-O composite inclusions and MgO-dominated inclusions. The density of inclusions is increased from 74.8/mm² to 186.0/mm², and the average size of inclusions is decreased from 2.60 μm to 1.07 μm. The Mg-Ce-O composite inclusions are effective inclusions for inducing IGF. Furthermore, the pitting potential is increased from −503 mV to −487 mV, and the corrosion rate is decreased. The order of average electronic work function is ΦMgO < ΦCe₂O₃ < Φα-Fe < ΦAl₂O₃. Ce₂O₃ is hard to induce pitting corrosion due to its similar electronic work function to the steel matrix. Thus, the Mg-Ce composite treatment is better than that for Mg and Ce treatment alone, and has better application prospects.

Experiments were carried out to determine the effect of cooling rate on the nucleation, growth and microstructure evolution during the solidification process in Ce containing S31254 high Mo austenite stainless steel (HMASS). In situ observations and in situ quenching tests indicate that the Ce effects in this steel are to refine the equiaxed grain and change in the secondary phase from σ to δ, but are mainly suitable for the medium and low cooling rate. The high cooling rate is highly beneficial for austenite nucleation and significantly influence the primary austenite grain growth behavior. The growth velocity for austenite grain is increased with the increase in cooling rate. The morphology of the primary austenite would change from a crescent shape to rod shape with the cooling rate increased from 50 °C/min to 100 °C/min. Rod-shaped grains with high nucleation densities would make primary grains easy to be combined together, which would reduce the ability of the refined grain. Furthermore, a high cooling rate could slightly improve the solute redistribution, while it significantly deteriorated the micro-segregation of Mo in a solidified structure. The main reason is that high cooling rate could worsen the kinetic condition of ferrite precipitation and postpone the precipitation reaction at the end of the solidification process.