Refinement mechanism of cerium addition on solidification structure and sigma phase of super austenitic stainless steel S32654

Abstract

• Ce addition could modify MgO and MnS into Ce 2 O 3 and Ce 2 O 2 S in S32654. • Ce addition leads to noticeable refinement of both dendrite structure and σ phase. • Effective Ce-bearing inclusions could serve as nucleation cores of γ and σ phase. • Solute Ce markedly enhances undercooling degree, further refining dendrite structure. • Solute Ce promotes σ phase nucleation, while dendrite refinement limits its growth. The influence of Ce addition on the solidification structure and σ phase of super austenitic stainless steel S32654 was systematically investigated via microstructural characterization and thermodynamic calculation. The results indicate that a small addition of Ce could modify MgO and MnS into Ce-bearing inclusions Ce 2 O 3 and Ce 2 O 2 S. Ce addition led to noticeable refinement of both the dendrite structure and σ phase. The refinement mechanism could be attributed to the combined actions of effective Ce-bearing inclusions and solute Ce. Effective Ce-bearing inclusions could serve as heterogeneous nucleation cores of austenite as well as σ phase, which provided a favorable prerequisite for their refinement. Solute Ce significantly enhanced the undercooling degree of the system, further promoting dendrite structure refinement. Meanwhile, solute Ce improved the eutectic precipitation conditions of σ phase and further promoted its nucleation, while the dendrite refinement limited its growth space. Finally, more fine and dispersed σ phase particles formed in S32654 with Ce addition. The refinement of dendrite structure and σ phase will reduce the temperature and time required for high-temperature homogenization, which is beneficial to the hot working of this steel.