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High chromium cast irons are widely used as abrasion resistant wear parts in mining, mineral and cement industries. Their abrasion and corrosion resistance can be improved by destabilisation heat treatment and element addition. Mo additions to high Cr irons lead to the formation of M2C or M6C type carbides depending on the Cr/C ratio. In the present work, iron containing 28 wt.%Cr-3 wt.%C-1 wt.%Mo was investigated. The as-cast specimen was destabilised at 1000 oC for 4 h and then hardened by air cooling to room temperature. Microstructures were investigated by light microscopy, scanning electron microscopy and transmission electron microscopy. Phase identification was performed by X-ray diffraction and selected-area electron diffraction. Vickers macro-hardness testing was performed using 30 kgf load and 15 seconds indenting time. A potentiodynamic technique was used to determine aqueous corrosion resistance via analysis of the anodic polarisation characteristics in a solution of 0.5 molar sulphuric acid. The results revealed that the as-cast microstructure of the 28 wt.%Cr-3 wt.%C-1wt.%Mo iron consisted of austenite dendrites and multiple eutectic carbides including M7C3, M23C6 and fish-bone M6C were found. Carbide transitions as M7C3→M23C6→M6C were also observed. After destabilisation, secondary carbides were precipitated within the prior austenite matrix, which later transformed to martensite during air cooling. Selected area diffraction pattern confirmed that secondary carbides are M23C6. Furthermore, microstructural change was observed as eutectic M7C3 transformed to M23C6 after destabilisation heat treatment. Destabilisation heat treatment increased the Vickers macro-hardness from 520 HV30 in the as-cast iron up to 780 HV30 due to precipitation of secondary carbides which destabilised the original austenite leading to a martensite matrix on air cooling. The as-cast iron had lower values of critical current density and passive current density compared with the destabilised iron. In the as-cast iron, more severe localised attack was found only in the eutectic matrix (austenite + martensite) adjacent to the eutectic carbides. In the destabilised iron, both dendritic and eutectic matrix regions were attacked. The dendritic matrix regions were attacked due to the presence of secondary carbides.
