Characterisation and machinability of high chromium hardened white cast iron with and without the addition of niobium

摘要

High chromium white cast iron (HCWCI) is considered a hard machinable material because of its high carbide content, which when associated with a matrix, forms a rigid and very hard microstructure. This work evaluates the effect of the addition of 0.5% niobium (Nb) on the machinability of a HCWCI alloy with 25.6% chromium and 3.2% carbon. The alloys were cast, heat-treated and characterised. The characterisation was performed by X-ray diffractometry, scanning electron microscopy, hardness and carbide volumetric fraction (CVF). The machinability was analysed in materials with and without the addition of Nb during the turning process using polycrystalline cubic boron nitride cutting tools, while varying the cutting speed in dry cutting and lubri-cooling conditions. The output parameters analysed were the tool lifetime, roughness and wear mechanisms. The Nb addition promoted a decrease of CVF and hardness, also resulted in the microstructure refinement, contributing positively to the machinability of HCWCI under both cooling conditions. In general, the use of cutting fluid resulted in lower roughness values and improved life performance of cutting tools, influencing the reduction of the abrasion wear mechanism.