Identification and quantification of active manufacturing factors for graphite formation in centrifugally cast Nihard cast irons

Abstract

The present research arises from the need to gain knowledge to control the precipitation of interdendritic dispersed graphite in mottled Nihard cast irons used in the outer ring layer of industrial duplex work rolls. Industry desires a volume fraction of 2.5–4% graphite, distributed homogeneously across the layer thickness, with a reasonably constant, high number of particles across the layer, with a predominantly short flake-graphite shape. The objective was to ascertain the role of metallurgical manufacturing parameters and their interaction with graphite precipitation in the layer, and in the outer and inner sections of the roll collar. These rolls, used in the finishing stands of hot strip steel mills, were processed by a double pour method using vertical centrifugal casting machines. Analysis was conducted using a fractional Design of Experiments (DOE) program consisting of 16 industrial trials based on the study of 7 manufacturing factors at 2 levels. The analyzed responses, obtained by quantitative metallographic techniques, were: the volume fraction, V V, number per unit area, N A, and the graphite morphology across the layer thickness. The results confirmed that an increase from 0.5 to 0.8 kg/T of SiCaMn added as a bath inoculant in the ladle increased the fraction of precipitated graphite without affecting N A. An outstanding result was that the addition of 1 kg/T of FeTi as a bath inoculant did not modify the amount of precipitated graphite but significantly affected the morphology, favouring the development of elongated graphite shapes.