Effect of Cooling Rate and Sulfur Content on Sulfide Inclusions in Invar Alloy

Invar alloy has been widely used in modern industry for its extremely low coefficients of thermal expansion (CTE). Sulfide inclusions have a significant influence on the mechanical performance and corrosion resistance of Invar alloy. To improve the mechanical properties of this alloy, which are significantly influenced by the existing sulfide inclusions, a good understanding of the characteristics, including the morphology, size, distribution, and formation mechanism of sulfide inclusions, is required. This study outlines three different cooling modes, water cooling (43.1 K/s), air cooling (16.8 K/s), and furnace cooling (3.1 K/s), to examine the characteristics of calcium sulfide (CaS) inclusions. In addition, a variety of initial sulfur contents under air cooling conditions were investigated. Both laboratory experiments and thermodynamic calculations support the research foundation. The sulfide inclusion particles were extracted through non-aqueous solution electrolysis for further morphology analysis. From the results, we conclude that the cooling rate affects the sulfide inclusion size through the local solidification time. The faster the cooling rate, the larger the average inclusion size. The cooling rate also indirectly influences the morphology of calcium sulfide inclusions that precipitated during the solidification process. In addition, the precipitation of CaS takes precedence over MnS under any circumstances. MnS can only precipitate when there is excessive S content. The different initial S content derived separate supersaturation during CaS precipitation and growth, further influencing the inclusion morphologies.