Non-metallic microparticles in spheroidal graphite irons are a product of the inoculation and the Mg-treatment of the liquid melt. Besides the influence on the mechanical properties of these iron–carbon–silicon alloys, they are also responsible for the nucleation and the morphology of the graphite phase. The present investigation is undertaken to study holding time effects of a (Ba, Ca, Al)–ferrosilicon (called Ba-inoculant) and (Ca, Al)–ferrosilicon (called Ca-inoculant) inoculants on the overall distribution of microparticles. Using the 2D to 3D conversions method, which is typically used for graphite nodules, the non-metallic microparticles’ statistical parameters, such as size distributions and number densities, are quantified. The total number of particles is similar after Mg-treatment and inoculation for Ca-inoculant but not for Ba-inoculated samples, which lose approximately 25 pct of microparticles after 1 minute of holding time. Iron treated with the Ca-inoculant loses about 37 pct of its nodules after 5 minutes, while the Ba-inoculated melts maintain their performance even after 10 minutes. Based on extrapolating the trend of the undercooling, Ba-inoculated samples would reach the uninoculated undercooling values in 48 minutes, while Ca-inoculated samples in only 11 minutes. By evaluating the size distributions of the non-metallic microparticles, the Ostwald ripening hypothesis or particle aggregation can be verified. The results suggest that sulfides are more critical for graphite nucleation since they can be correlated with the graphite number densities. However, due to the small difference in the microparticle population of the uninoculated sample with Ca-inoculated samples, other aspects of the fading mechanism need to be considered, such as transient metastable states, since the central hypothesis of loss of inclusions cannot alone explain the decrease in the nucleation frequency of graphite.
The effect of preconditioning treatments on the control and improvement of spheroidal graphite iron (SGI) microstructure was evaluated. In the melt, 0.15% of Zr-(Ca, Al) FeSi preconditioner was added into different conditions. Four samples were produced for this investigation: (1) in the first melt, there was no addition of a preconditioner for comparative purposes; (2) in the second melt, the preconditioner was added at the cold charge; (3) in the third melt, the preconditioner was added before the last cold charge; and (4) in the fourth melt, the preconditioner was added at tapping from the furnace. Microstructural characterization was conducted to understand the effect of the treatment on the SGI. Optical microscopy results show that preconditioning treatment increases graphite’s nodule density, ferrite content, and nodularity. Scanning electron microscopy (SEM), energy dispersive energy (EDS), and electron backscatter diffraction (EBSD) analysis were used to identify the types of microparticles present in the graphite nodules. Some complex microparticles were identified as AlMg2.5Si2.5N6, MgS, and CaS. The microstructural characteristics of the matrix, such as grain size, crystallographic orientation, and misorientation, were also evaluated by the EBSD. The addition of the preconditioning at tapping results in a higher ferrite fraction, smaller grain size, misorientation, and hardness values. This work suggests that the different preconditioning practice has a crucial effect on the microstructural characteristics of the SGI. This knowledge is vital, allowing the microstructure tailoring to enhance the mechanical properties of SGI to obtain the best performance of these materials.
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