Thermodynamic Analysis on Precipitation of Nonmetallic Inclusions in Gear Steel

摘要

In order to make nonmetallic inclusions with low melting point and improve the fatigue resistance of gear steel, thermodynamic analysis on precipitation of nonmetallic inclusions CaO(MgO)-Al_2O_3 system was investigated in this paper. The complex deoxidation equilibria of strong deoxidizing elements such as aluminum, calcium, magnesium and so on were calculated accurately with the second order and middle term of the activity coefficient expression. Thermodynamic conditions for the precipitation of spinel inclusion (MgO-Al_2O_3) is sufficient as the acid-soluble aluminum content is 0.04% and magnesium content is less than 0.0005% in the tested gear steel 20CrMoH. The relationship between precipitation of spinel and calcium aluminate inclusions in gear steel and the composition of molten steel at 1873K indicates that, the compositions of inclusions before and after calcium treatment are C_(12)A_7 and C_3A respectively, while the reactions in ladle reach the equilibrium state.