Differential scanning calorimetry of gray cast iron: Presence of iron nitrides after casting

Abstract In order to understand the mechanisms behind the aging phenomenon of gray cast irons, differential scanning calorimetry (DSC) was chosen to monitor the evolution and quantity of iron nitride precipitates responsible for aging. Gray cast irons were produced with varying nitrogen contents (from 50 to 150 ppm) in an induction furnace and then poured into green sand molds. DSC samples were prepared from the raw castings successively by turning, micro-sectioning, and polishing. The first challenge was to make visible, through an adapted protocol, the exothermic phase transition of iron nitrides from $$\alpha^{\prime \prime } Fe_{16} N_{2} \to \gamma^{\prime } Fe_{4} N$$ α ″ F e 16 N 2 → γ ′ F e 4 N near 200 °C, which is masked by the exothermic Curie temperature of cementite at 210 °C. The nitrogen content in the cast irons was varied to observe differences in the amplitude of the exothermic signal, in order to demonstrate that the observed transition is indeed the phase transition of iron nitrides $$\alpha^{\prime \prime } Fe_{16} N_{2} \to \gamma^{\prime } Fe_{4} N$$ α ″ F e 16 N 2 → γ ′ F e 4 N . This transition was established at 169 °C. Aging monitoring tests using DSC were conducted in parallel with free resonance measurements on cast irons produced and poured simultaneously. However, the changes in Young’s modulus occurred by the aging observed through free resonance test were not reflected in the evolution of the DSC thermograms.