Enhanced Fracture Strength in the Working Layer of Rolls Manufactured in Ni-Hard Cast Iron Alloyed with Mo, Nb and Mg

Abstract

One of the main in-service failure mechanisms of the work-rolls used in hot strip mill finishing stands is surface spalling. The indefinite chill double-poured rolls usually comprise of a peripheral working layer made of crushed Ni-hard cast iron and a grey cast iron core, mostly pearlitic matrix with spheroidal graphite. To enhance its wear resistance, the working layer can be alloyed with Mo and Nb. The possible cracking and spalling of the surfaces of these work-rolls is strongly influenced by the presence of carbides and the continuity of their network. The flexural and impact toughness tests are reliable testing methods to assess these properties. The aim of this paper is to identify those manufacturing factors that have a significant effect on the flexural strength and toughness of this material, correlating the results with the volume fraction of precipitated carbides. It is worth highlighting, among the analysed factors are the liquidus temperature, the %Si, the use of an inoculant with traces of Lanthanum, and inoculation with different amounts of FeB, SiCa and Mg. Inoculation with SiCa is found to have a positive effect on the toughness of the material, breaking up the continuity of the carbide network, while FeB is found to act as a heterogeneous nucleant for NbC precipitation. However, high FeB contents reduce flexural strength and do not have a significant effect on the hardness of the material. To enhance the fracture toughness of the working layer, a liquidus temperature in the 1270–1275 °C range is recommended, as well as inoculating the ladle with Mg, 3 kg/T FeB and 0.6 kg/T SiCa.

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.…”

“…Setting both factors at their respective +1 (0.6 kg/T SiCaMn) and −1 (without FeSi-La) levels leads to an increase in said toughness. In a previous study by the authors, it was found that inoculation with SiCaMn promoted discontinuity of the carbide network [32], which increased the toughness of the shell [29]. In this case, it is verified that this effect also reaches the region of the shell in contact with the core.…”

“…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.…”

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

“…Table 2 shows the analyzed Factors and levels, while Table 3 displays the Array of Experiments, together with the confounding pattern. These same manufacturing factors were studied in a previous work of the authors, where the resistance of the working layer against mechanical stresses was analyzed, correlating the results with the volume fraction of precipitated carbides [29]. The set of generators associated with this array of experiments is D = AB, E = AC, and F = BC, whose resulting relation definition is I = ABD = ACD = BCF, where I is a column formed only by some (+1) [28].…”

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

“…At the same time, inoculation with FeB favours heterogeneous nucleation of the NbC. The Mg performs as a nucleating agent of the graphite, thus favouring its spheroidal morphology [28][29][30]. However, the Mg produces subcooling during eutectic solidification, which favours the formation of ledeburitic eutectics [10].…”