Development of Austempered Ductile Iron as a Grinding Media Material in the Ball Mill: Microstructural Aspects and Influence of Surface Coating

Abstract

In the present investigation an attempt has been made to evaluate the suitability of austempered ductile iron (ADI) as media material for grinding iron ore, chalcopyrite and coal in a laboratory sized ball mill. The spheroidal graphite (S.G) iron balls having 2.5 cm diameter of required composition were produced by sand casting method. In the first case, single step austempering was given on each set of 200 balls after austenitising at 900ºC for one hour and austempering at 280ºC and 380ºC for different time duration of 30, 60 and 90 minutes. In the second case, each set of 200 S.G.iron balls were austenitised at 9000C for 60 minutes and given low to high stepped austempering treatment (ATLH) at 2800C for 15, 30, 45 and 60 minutes followed by 3800 C for 60 minutes. Similarly in the third case, each set of 200 S.G.iron balls were austenitised at 9000C for 60 minutes and given high to low stepped austempering treatment (ATHL) at 3800C for 15, 30, 45 and 60 minutes followed by 2800C for 60 minutes. Altogether, 14 sets of 200 S.G.iron balls, each were austempered at different time and temperatures to obtain ADI. A selected set of ADI balls were given surface coating to improve their wear and corrosion resistance. The materials such as titanium nitride (TiN), titanium aluminium nitride (TiAlN) and aluminium chromium nitride (AlCrN) are coated by Plasma physical vapour deposition (PPVD) on ADI balls. Also, the nitriding was carried out by gas nitriding process to increase the surface hardness of the ADI ball material. These materials were characterised by measuring hardness, studying microstructure using optical and scanning electron microscope (SEM) and analysing the X-ray diffraction (XRD) profiles. Grinding experiments were carried out in a ball mill using these ADI balls as media material for the comminution of different ores namely, iron ore, chalcopyrite and coal. Grinding wear behaviour of these materials was assessed for wear loss in wet condition at different pH value of the mineral slurry. For the sake of comparison the grinding experiments were carried out with the conventionally used forged En31 steel balls as media material in the ball mill.The wear rate of all the category of ADI were compared with that of forged En 31 steel balls and found that the wear resistance of ADI was superior to forged En31 steel balls. Among ADI balls ones austempered in conventional single step at 2800C for 30 minutes with hardness of 497 BHN, 26% by volume of retained austenite and 1.90% by weight carbon content of retained austenite offered highest grinding wear resistance. Further, the wear rate of ADI balls decrease with increase in duration of grinding and it was found to be 39 X 10-8 cc/revolution after 40 hours of grinding which is less than that for any category of ADI balls. Corrosive wear behaviour of ADI and forged En31 steel balls were analysed by carrying out the grinding experiment using water and kerosene as slurry media separately. In both the cases it was found that the wear resistance of ADI is superior to that of forged En31 steel balls. However, this experiment revealed that the corrosion plays an important role in wear of grinding media balls. The mechanism of wear is conceived to be corrosive wear occurring by delamination phenomenon and aided by impact fatigue crack growth. The results of the investigations reveal that the surface coated ADI balls possess very high wear resistance during the initial few hours of ball milling but decreases suddenly after particular duration of grinding. A term called relative efficiency index (REI) has been developed to compare grinding efficiency of ADI balls with that of forged En 31 steel balls. The REI values of 3.7 as recorded by ADI balls compared to that for forged En 31 steel balls indicated the superior nature of ADI balls. Hence, even without surface coating the ADI balls offered very high wear resistance and can be a suitable substitute for conventionally used forged En31 steel balls as grinding media in the ball mill. Results of the investigations were discussed in detail.