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Description
High chromium cast irons are widely used for wear-parts in the mining, mineral and cement industries, due to their high abrasion resistance, relative low cost and ease of production. Carbide-forming elements addition and heat treatments were improved their properties. In this work, the effects of W addition and heat treatment on the microstructure and hardness of nominal 28 wt.%Cr high chromium irons were studied. As-cast samples were destabilised at 1050 $^o$C for 4 hours and then hardened by air cooling. Tempering after destabilisation was done at 450 $^o$C for 4 hour and then air cooling. X-ray diffractometry, light microscopy and scanning electron microscopy were used to characterize the microstructures of the irons. Vickers macro-hardness was tested. The results show that the as-cast microstructure of the iron without W addition consisted of primary austenite dendrites with eutectic M$_7$C$_3$ and eutectic austenite partially transformed to martensite. The iron with (1-4) wt.%W addition were hypereutectic contained primary M$_7$C$_3$ and eutectic M$_7$C$_3$ in an austenite matrix. In the iron with 4 wt.%W, M$_6$C carbide was found. Destabilisation treatment of the austenite matrix allowed precipitation of secondary carbides and transformation to martensite during air cooling. After tempering, more secondary and tempered carbides precipitated within the matrix and eutectic austenite. After heat treatment, phase transformation of eutectic M$_7$C$_3$ was also found in the iron with (1-4) wt.%W addition. The formation of primary M$_7$C$_3$ and M$_6$C carbides in the iron with W addition increased the as-cast macro-hardness from 500 HV$_3$$_0$ (no W) to 550 HV$_3$$_0$ (4 wt.%W). Destabilisation increased the macro-hardness up to 740 HV$_3$$_0$ (no W) and 870 HV$_3$$_0$ (4 wt.%W), respectively. This is believed that secondary carbide precipitation allowed austenite to transform to essentially martensitic matrices. Tempering after destabilisation slightly increased the macro-hardness.
Keywords: High chromium iron, Heat treatment, Microstructure, Tungsten, Hardness
