Control of graphite phase morphology in gray cast irons by heat treatment

Abstract

Based on the previously obtained theoretical and experimental data, the effects of isothermal holding at heating on morphology of graphite phase (GP) in industrial gray and ductile pearlite matrix cast irons were investigated. Two types of the effects were revealed: partial or full dissolution of the existing GP plates and growing up existing GP plates or spherical particles. The dissolution is emerged in forming «broken» thin plates comprising big spherical GP particles. Another dissolution variant was formation of ferrite grain boundary network of thin GP layers. The growing up process appears as increasing: whole thickness of the existing GP plates uniformly along their lengths and dimensions of the initially spherical GP particles on some local interface areas. The above GP morphology change features are attributed to different silicon contents in the cast irons: the higher silicon content stimulates the GP particles grow and vice versa. Formation of fine ferrite grains is also observed on the spherical GP particle / ferrite matrix interfaces. Considerable increase in the impact resistance was revealed after optimal isothermal holdings for all cast irons investigated. The following improvements of the impact strength were reached for gray and ductile cast irons, respectively: from KCU = 51 to 120 kJ/m²; from KCU = 89 to 440 kJ/m². The results obtained were explained by the following processes joint development: decomposition of the pearlite matrix cementite with GP formation; coagulation and spheroidizing existing GP plates; ferrite fine grain nucleation on the GP particles-matrix interfaces. In turn, the decomposition results in ultrafine GP crystals appearance and their further accumulation: in the form of spherical isolated particles; as layers on the existing GP spheroid or plate surfaces as substrates