Effect of Holding Time on Populations of Microparticles in Spheroidal Graphite Irons

L Michels,B Kroka,E Ott,E G Hoel,C A S Ribeiro,C Hartung,A J F Pires

doi:10.1007/s11663-021-02418-w

Abstract

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.

Highlights

SPHEROIDAL graphite iron (SGI) are iron–silicon–carbon alloys with several applications in society, especially in the wind energy and the automotive sectors.[1]
In the applications mentioned above, the desired state of carbon is the graphite phase from the divorced eutectic reaction
To avoid metastable solidification, it is necessary to treat the melt with additives, such as ferrosilicon-based inoculants and nodularizers.[4]

Summary

Introduction

Silicon–carbon alloys with several applications in society, especially in the wind energy and the automotive sectors.[1] This material can either solidify in a divorced stable eutectic reaction, in which the carbon phase nucleates and grows as graphite in spherical-shaped crystals called nodules, or as a metastable phase, iron-carbide (Fe3C) or as a mix of both. In the applications mentioned above, the desired state of carbon is the graphite phase from the divorced eutectic reaction. Stable to metastable transition depends on the composition, cooling rate, and melt treatments.[2] The presence of silicon changes the solidification behavior of SGI by shifting the temperatures of the eutectic and eutectoid transformations.[3] Usually, to avoid metastable solidification, it is necessary to treat the melt with additives, such as ferrosilicon-based inoculants and nodularizers.[4]

Methods

Results

Conclusion