Study of the effect of commercial inoculants in high silicon cast iron components

High silicon alloys have experienced a significant growth in the last few years because of their homogeneous structure, very good mechanical properties and the positive behavior of these materials in aggressive environments. However, they show an important disadvantage too due to a tendency in the formation of micro-shrinkage porosities that will compromise the soundness level of the castings. 

AZTERLAN Metallurgy Research Centre has carried out a comparative study of several market inoculants to determine the influence of the inoculation process on the reduction of micro porosities with the aim of ensuring the metallurgical quality of the castings. This research work has been carried out at laboratory scale as well as on industrial trials, presenting very similar results in both environments.

In both cases, a same standard casting and a thermal cup (also known as quick-cup) inoculated with different commercial inoculants, have been taken as a reference. The main difference between the tested inoculants is mainly given by trace elements, such as lanthanum, cerium, zirconium, calcium, aluminum or manganese.

 

The study of the inoculation effect on the soundness of the parts has been carried out from complementary aspects that offer a global overview and the influence in the formation of micro shrinkage defects. These four different perspectives cover the study of internal defects by computerized axial tomography, the thermal analysis of the cooling curves with the Thermolan® system to define the metallurgical quality, as well as a complete characterization of the microstructure of the material and an exhaustive knowledge of the main nucleation points of the graphite nodules using the most advanced techniques of scanning electron microscopy.

Main results of the comparative study of innoculants

The study of the cooling curves carried out with the thermal analysis system developed by AZTERLAN (Thermolan®) allows to determine some of the main parameters that characterise the solidification process of iron castings. Obtained data show that the best results have been achieved with inoculant products that are rich in zirconium.  

Main results obtained with the Thermolan® thermal analysis system

As expected, non-inoculated samples presented a higher porosity level than the inoculated ones. In all cases commercial inoculants tested have improved the soundness of the parts. This is evident when studying their microstructure. The non-inoculated samples are characterized by a very wide graphitic distribution, with very different graphite sizes and a graphite shape far from spheroidal, being closer to the vermicular shape and showing a poor graphitic density.
 

Analysis of internal porosities of non-inoculated (left) and inoculated samples (right) by means of computerized axial tomography.

In contrast, inoculated samples show a greater nodule count, with a much more spheroidal graphite shape and a left-shifted distribution. This indicates the formation of smaller graphite nodules. It is to be assumed that this smaller graphite nodules are formed in the last moments of solidification helping to counteract the contraction of the austenite and, therefore, minimizing the formation of micro-shrinkage. All samples with this graphitic distribution have shown a better behaviour than the other ones.
 

Metallographic analysis of non-inoculated (left) and inoculated (right) samples show clearly differentiated graphite shapes and distribution.

On the other side, the study of the nucleation process carried out through mappings, line scan and spectra, has allowed to determine that the main non-metallic inclusions that behave as nucleation points of the spheroidal graphite in high silicon alloys are those (MgSiAl)N, all presenting a clear polygonal form, which can act as nucleants, alone or combined with other inclusions such as (SbCeLa)S type sulfides or titanium carbides (TiC) depending on the composition of the inoculants used.
 

The SEM nucleation study of non-inoculated (left) and inoculated samples (right) shows a better metallurgical quality in the last ones.

The main results of this study have been presented at the 125 MetalCasting Cogress (2021) organized by the American Foundry Society and published in the scientific journals International Jounal of MetalCasting and Minerals.