Effect of Niobium on the Microstructure of High Chromium White Cast Iron
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Effect of Niobium on the Microstructure of High Chromium White Cast Iron Cláudio. G. Oliveira1 and Ivete.P.Pinheiro1 1
Centro Federal de Educação Tecnológica de Minas Gerais, CEFET-MG. Avenida Amazonas, 5.253 - Nova Suíça, CEP.: 30.480-000, Belo Horizonte, Minas Gerais, Brasil.
ABSTRACT Equipment wear is caused by the disintegration of material due to the contact between the machines components and the ore, resulting in stress to the surface of the material. Wear causes loss of efficiency, vibration, misalignment and, in severe cases, cracks that may lead to fracture and damage to the equipment. In mining, wear is caused by operational problems in which generate high costs. Some researchers studied white cast iron alloys with high chromium and the addition of niobium for wear plates manufacturing and therefore, plates to protect structural parts of the equipment have been developed. This study presents the characterization of the microstructure of two alloys of white cast iron with high chromium containing 3.8 wt.% C and 27.1 wt.% Cr and the addition of 0.9 wt.% Nb (alloy 1) and 1.6 wt.% Nb (alloy 2), respectively. Samples of the two alloys were subjected to metallographic tests, microhardness and abrasion type rubber wheel tests, according to the ASTM: G65-91 standard. Complexes carbides have been identified in both alloys. The results of microhardness and wear resistance tests were correlated and identified the effect of niobium addition. The findings suggest that the addition of niobium in these alloys contributes to the formation of NbC and increase of Cr in the matrix; consequently increase in the hardenability of the material. The wear resistance of alloy 2 was 47.95% higher than alloy 1 in abrasion type rubber wheel tests. It demonstrates that the increase of niobium in the alloy has contributed to improve wear resistance due to the substantial change in the microstructure and distribution of NbC carbides.
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Cláudio G.Oliveira1 ()
e-mail: [email protected]
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INTRODUCTION Abrasion wear is a major cause of operational problems and generates high costs in the mining business. As a result, equipment such as conveyors, pumps, ball mills, feeders, among others are coated with white cast iron materials containing chromium according to ASTM A532 IIIA. White cast irons are alloys which basically contain iron, carbon and chromium in which present eutectic reactions during solidification [1]. For these authors, such material presents almost all carbon in combined form of iron carbide (Fe3C) showing a clear fracture surface. Its fundamental properties, precisely due to the high amount of cementite, are high hardness and wear resistance. A properly adjusted chemical composition (carbon, silicon and chromium content) and the cooling rate are the means most used to produce white cast iro
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