Control of Surface Graphite Degeneration in Ductile Iron for Windmill Applications

Abstract

The objective of this paper is to review the factors influencing the formation of degenerated graphite layers on the surfaces of ductile iron castings for chemical resin-acid moulding and coremaking systems and how to reduce this defect. In the resin mould technique the sulphur in the P-toluol sulphonic acid (PTSA), usually used as a hardener, has been identified as one factor causing graphite degeneration at the metal mould interface. Less than 0.15% S in the mould (or even less than 0.07% S) can reduce the surface layer depth. Oxygen may also have an effect, especially for sulphur containing systems with turbulent flows in the mould, water-bearing no-bake binder systems, Mg-Silica reactions, or dross formation conditions. Despite the lower level of nitrogen in the iron melt after magnesium treatment (less than 90 ppm), nitrogen bearing resins have a profound effect on the frequency and severity of surface pinholes, but a limited influence on surface graphite degeneration. The specific characteristics and optimum conditions for the manufacture of heavy sectioned ductile iron windmill castings were reviewed. The paper concludes on the optimum iron chemistry, melting procedure, Mg-alloys and specific inoculants systems, as well as the practical solutions to limit graphite degeneration to ensure castings with the highest integrity.