Structural investigation of solidification and heat treatments influence on High Alloyed Cast Irons Grades with Nb-V-Ti Additions

Two High Alloyed Cast Irons (HACI) were studied, both belonging to the Fe-C-Cr-Si-X system where X was represented by Mo, Nb, Ti or V. One alloy was obtained after adding Nb, V and Ti to the chemical composition of the other alloy.
Raw materials originated from spun cast rolls for hot strip mill were submitted to different heat treatments routes such as double tempering or quenching, in order to study the influence of alloying elements on the microstructure.
Both HACI grades contained a mixture of martensite and retained austenite matrix in the as-cast conditions and after quenching.
Differential Thermal Analysis (DTA) was carried out on the heat treated samples in order to determine the phase transformations occurring during re-melting and subsequent solidification sequence. Retained austenite found in the as-quenched HACI without Nb, V and Ti additions exhibited an enhanced stability as confirmed by DTA tests.
Various type of martensite were found at the end of heat treatments routes, each having a possible influence on the material behavior while performing re-heating prior to re-austenitisation.
Bulky Nb-rich MC carbides precipitating at the beginning of the solidification process strongly influence the nature and the rate of the subsequent phase transformations, particularly for HACI grade with Nb, V and Ti additions. Such NbC carbides were also found to segregate during spin casting process.
Quantitative metallography was done to determine graphite, MC carbides, cementite and matrix volume fraction in HACI studied grades.