Influence of sulfur content on precipitation of sulfide inclusions in 20CrMnTi gear steel

To investigate the effects of sulfur content variation on the morphology, size, and other characteristics of MnS inclusions in 20CrMnTi gear steel, this study conducted high-temperature experiments on 20CrMnTi gear steel using a high-temperature tube resistance furnace. The characteristics of MnS inclusions with sulfur mass fractions of 0.018 1% and 0.091 1% were analyzed by means of metallographic method and non-destructive electrolytic extraction method. Results show that when the sulfur mass fraction in 20CrMnTi gear steel is 0.018 1%, the MnS inclusions are small in size, with morphologies mainly presenting as spherical, sub-spherical, or spindle-shaped. When the sulfur mass fraction in the steel increases to 0.091 1%, the MnS inclusions grow larger, distribute more densely, and their morphologies are mainly blocky, clustered or irregular. Calculations on the nucleation kinetics of MnS inclusions reveal that the nucleation of MnS inclusions is dominated by grain boundary nucleation. When the temperature is lower than 1 728 K, some MnS inclusions precipitate through homogeneous nucleation, but the majority still precipitate predominantly via grain boundary nucleation. Thermodynamic calculations of MnS inclusion precipitation indicate that an increase in sulfur content advances the solid fraction of MnS inclusion precipitation from 0.986 to 0.831 under furnace cooling conditions, and to 0.822 under water cooling conditions. Kinetics calculations of MnS inclusion growth demonstrate that an increase in sulfur content leads to an increase in the size of MnS inclusions, while an increase in cooling rate reduces the size of MnS inclusions. The FactSage thermodynamic software was used to calculate the variation in the precipitated mass fraction of MnS inclusions during the cooling process. At 1 700 K, the mass fraction of MnS inclusions increases from 0.005 1% (at a sulfur mass fraction of 0.018 1%) to 0.172 7% (at a sulfur mass fraction of 0.091 1%). The research results can provide certain theoretical guidance for the study on the formation mechanism of MnS inclusions in 20CrMnTi gear steel, and also offer appropriate reference value for its practical application in industrial production.