Effect of Preconditioning and Inoculation on Graphite Nodule Count and Their Size Distribution in Spheroidal Graphite (SG) Cast Iron: A Study to Minimise Rejection of Castings Due to Shrinkage Porosity

Abstract

This paper attempts to investigate the effect of preconditioning and inoculation on nodule count and their size distribution to eliminate shrinkage porosity in spheroidal graphite (SG) iron castings. Studies were conducted on various heats of SG iron prepared for differential case casting with and without the addition of 0.1% Al, Zr, Ca–FeSi alloy as preconditioner and Ca–Ce–FeSi alloy was used as an inoculant. It was found that the combined effect of preconditioning and inoculation significantly improves nodule count and nodule size distribution which subsequently reduces the number of castings containing shrinkage porosity. The shrinkage quantity (Q) was found to reduce from 10 to 2.4%. Graphite nodule count and their size distribution were found to be important factors for controlling shrinkage porosity, and it was observed that preconditioning substantially improves the percentage of small size (5–15 µm diameter) graphite nodules. Higher nodule count reduces chances of shrinkage but simply having higher graphite nodule count of similar size does not help in minimising or elimination of shrinkage, it is necessary to have nodules of different sizes in a significant proportion.