Carbide-formation processes in rotor steel 25Kh1M1FA

Abstract

1. Carbides of the type M3C, M7C3, M23C6, and M2C, which contain iron, chromium, and molybdenum atoms, as well as atoms of the carbide MC, in the metallic sublattice of which may be contained vanadium, titanium, niobium, and molybdenum atoms, form in rotor steel 25Kh1M1FA without additional alloying and with additives of vigorously carbide- and nitride-forming elements. 2. The carbides M7C3 and M2C dissolve in austenite upon heating above 900°C. The solubility of carbides of the type MC is determined by their composition. The presence of the highly disperse complex phase (Ti, V, Nb)C in steel alloyed with titanium and niobium is responsible for the lowest stability of austenite and the attainment of a ferritic-pearlitic structure with a fine (5–10 μm) ferrite grain upon cooling at a rate of 250 deg/h. 3. In the process of continuous cooling at a rate that ensures occurrence of the diffusion γ→α transformation, carbides of the type MC, M23C6, and M2C, the completeness of the segregation of which depends on the cooling rate and is maximum at a cooling rate of 100 deg/h for the MC-type carbide, and at a rate of 25 deg/h for the M23C6 and M2C carbides, are formed in addition to M2C. 4. Upon lowering the cooling rate from the austenitizing temperature, the completeness of segregation of the M7C3 and MC carbides is also reduced during tempering. Segregation of the M2C carbide during tempering is observed at high temperatures (above 700°C, τ=2 h) and for prolonged holding times (675°C, τ>5 h). Heating of steel that has been cooled at a rate that ensures the attainment of the ferritic-pearlitic structure gives rise to the additional formation of a carbide of the type M23C6.