Control over the Percentage, Shape and Size of the Graphite Particles in Martensitic White Castings Alloyed with Cr, Nb and Mg

Abstract

This paper presents the results obtained regarding the control by manufacturers of the percentage, shape, and size of the precipitated graphite in the working layer of duplex work-rolls used in hot strip mill finishing stands. This working layer is manufactured in a martensitic white cast iron alloyed with Cr and Nb to promote the precipitation of M3C and MC carbides, which provide a high wear resistance. The thermal cycling behavior of this layer also has a decisive influence on its service life. In this context, the percentage of graphite and its morphology play a very important role against said thermal cycling. With the aim of studying their effect on the sphericity of graphite, the analyzed industrial manufacturing factors worth highlighting include the liquidus temperature, the %Si, the use of an FeSi inoculant with traces of Lanthanum, inoculation with different amounts of FeB and SiCaMn, and the addition of Mg. At the periphery of the working layer, it was found that the use of the FeSi inoculant with traces of La promoted an increase in the density of counts of graphite, and that inoculation with FeB and the addition of 0.02% Mg diminished the nodularity of the graphite. Furthermore, throughout the entire thickness of the working layer, an increase in the amount of SiCaMn of up to 0.6 kg/T produced an increase in the size of the graphite particles and a marked improvement in their nodularity.

Enhancement of the Quality of the Shell-Core Bond Interface in Duplex Work Rolls Manufactured by Centrifugal Casting Used in Hot Strip Mills

“…In this case, it is verified that this effect also reaches the region of the shell in contact with the core. In previous studies by the authors, it was similarly found that if the ladle is not inoculated with FeSi-La, then there is an improvement in the impact toughness inside the working layer [29,30,32]. In the present study, we have verified that this effect is maintained in the region of the shell-core bond.…”

“…The analyzed factors were: The use of FeSi alloy inoculants with traces of lanthanum; inoculation with different percentages of SiCaMn, FeB, and Mg; the percentage of Si; and the liquidus temperature. Previous studies by the authors have analyzed the influence of these factors on the mechanical and microstructural properties of the working layers [29,30]. The present paper analyzes their effect on the shell-core bond interface in work rolls.…”

Improvement in the Resistance to Wear of Work-Rolls Used in Finishing Stands of the Hot Strip Mills

“…On the other hand, it was shown that, with effective quantities of Mg (around 0.02%), a deterioration in the nodularity of the graphite was produced [23]. This same effect was observed with the inoculation of FeB [23]. In previous works, the properties of the working layer were characterized.…”

“…These carbides exhibit true continuity in the microstructure; thus, they become the true mechanical matrix [20]. Inoculation with La produces an increase in the number of beads, by unit area, of graphite, and the thinning of the carbide network [21][22][23]. B has a whitening effect, thus increasing the volume fraction of carbides and improving the resistance to wear of the material [24,25].…”

“…However, the Mg produces subcooling during eutectic solidification, which favours the formation of ledeburitic eutectics [10]. In previous studies, it was concluded that inoculation with SiCaMn, apart from increasing the volume fraction of graphite and its nodularity, produced fractioning in the continuity of the carbide network, which favours an improvement in the toughness performance of the work layer [23,28]. On the other hand, it was shown that, with effective quantities of Mg (around 0.02%), a deterioration in the nodularity of the graphite was produced [23].…”