The aim of the present research is to identify the manufacturing factors that exert an active influence on the graphite phase formation in mottled Nihard cast irons constituting the roll shells of duplex work rolls processed by the double pour method during centrifugal casting. The studied rolls, referred to as alloy indefinite chill, were processed at industrial scale and had a core consisting of spheroidal graphite cast iron with a matrix of ferrite and pearlite. An additional aim of this study was to evaluate the effect and extent of these factors on the dynamic toughness response of the roll shell material.The research methodology employed consisted of the application of a saturated design of experiments with seven factors, eight experiments, and resolution III. The measured responses for graphite were: the volume fraction, count number per unit area, and morphology, determined by quantitative metallography. Impact testing was characterized by Charpy tests on U-notched specimens at 350°C. The manufacturing factors studied were: the final weight percent of silicon, sulfur, and manganese; the liquidus and the casting temperatures; and, finally, inoculation with SiCaMn and A-type FeSi (with Zr).The statistical experimental method conducted allowed us to confirm the significance of factors such as the %Si, the liquidus temperature and inoculation with SiCaMn on the precipitation of graphite in a white cast iron such as the Nihard irons used in the roll shell, in agreement with the precipitation of graphite in gray cast irons widely reported in the literature. It was also shown that the development of lamellar graphite shapes were favored by an increase in the total equivalent carbon and also by the increase in the amount of A-type FeSi added. Furthermore, the impact toughness was shown to improve with the increase in both the %Si and the liquidus temperature.
In the present paper, the results of a fractional design of experiments (DOE) with 16 experiments for 7 factors and resolution IV are presented. The goal was to identify the active roll manufacturing factors that influence roll barrel hardness after a double tempering treatment, the primary austenite grain size and its volume fraction, and the volume fraction of ledeburite in the outer ring layer of duplex work rolls made of Nihard cast iron. As a primary conclusion, both the total Si percentage and the amount of SiCaMn inoculant added to the ladle were found to have a significant effect on the volume fraction of ledeburite. The statistical analysis also showed that high Si percentages and heavier ring roll layer weights were the significant factors promoting the increase in the size of the gamma grains. High Si contents present in the melt and the addition of SiCaMn as inoculant seem to have a counter effect whereby the volume fraction of stable eutectic gamma increases and the volume fraction of proeutectic gamma decreases, with a positive balance for the sum of these austenites of different origins. Finally, the factors that have an influence on roll barrel hardness are those factors that promote the chill effect, such as low Si content, small amounts of liquid being poured into the mould to build up the outer ring layer, and the use of moulds without insulating sleeves.
The present research was focused on the identification of manufacturing factors that have an active influence on the graphite phase formation in Nihard cast irons inoculated with FeB, constituting the outer layer of duplex work rolls. These rolls are used in the finishing stands of hot-strip steel mills where the following are desired: (a) between 2.5 and 4 vol % of graphite; (b) homogeneous graphite distribution across the layer section; and, (c) a reasonable high number of graphite particles across the layer. The research methodology that followed consisted of the application of a saturated design of experiments (DOE), with seven factors, eight experiments, and resolution III. The analyzed responses obtained by quantitative metallographic techniques were: the volume fraction of graphite, Vv; the number of counts per unit area of graphite, N A ; and the graphite morphology across the layer thickness. Increasing the addition of FeB from 6 to 10 kg/T reduced the graphite volume fraction and the count number, but had no influence on its morphology. However, an increase of the liquidus temperature from 1225-1230 to 1250-1255 • C, and an increase in the amount of SiCaMn added to the ladle from 0.3 to 0.6 kg/T produced the desired compact graphite morphology.
The aim of this study was the morphological optimization of graphite by generating knowledge able to determine which metallurgical manufacturing factors have a significant influence on the geometry of the graphite precipitated in spheroidal grey cast irons with a mass coefficient of 4, used in the manufacture of wind turbine hubs. The research study was conducted on an industrial scale applying a fractional Design of Experiments (DoE) with 7 factors, 16 experiments and resolution IV. The following constitute the most noteworthy novel results obtained in the study. The effect of La on the graphite “counts” is strengthened by pre-inoculation of the molten iron bath with SiC. Furthermore, graphite counts can be increased under low carbon equivalent conditions using post-inoculants containing Zr. Finally, high values of Mn lead to a reduction in the size of the precipitated graphite.
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