EFFECTS OF TWO STAGE AUSTEMPERING HEAT TREATMENT ON MICROSTRUCTURE AND WEAR RATES OF ADI BALLS USED IN BALL MILL FOR GRINDING OF IRON ORE

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 13, December 2018, pp , Article ID: IJMET_09_13_073 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed EFFECTS OF TWO STAGE AUSTEMPERING HEAT TREATMENT ON MICROSTRUCTURE AND WEAR RATES OF ADI BALLS USED IN BALL MILL FOR GRINDING OF IRON ORE Murthy BRN Department of Mechanical and Manufacturing, Manipal Institute of Technology, Manipal, , Karnataka, India ABSTRACT Wear conduct of Austempered Ductile Iron (ADI) balls in the grinding of iron ore in a ball mill process was examined as a function of microstructure developed by carrying out 2-stage asutempering heat treatment. Various microstructure of ADI are produced by austenitising SG (spheroidal ghaphite) iron balls at C for 60 minutes of time and given step austempering heat treatments those includes first step austenitising at C for different time intervals like 15, 30, 45, and 60 minutes followed by second step austeniting at C for 60 minutes of time for each case. The grinding wear conduct of these materials are evaluated for the wear loss under the wet milling conditions. The grinding processes were carried out for 60 minutes of time at the speed of 86 rpm and a pulp density of 60 % under sealed mill conditions.the slurry ph value is varied from 7 to 10.5 to evaluate the impact of ph on grinding wear performance of ball material. From this experimental work it is discovered that wear rate of grinding media material decrements relatively in increments with the ph value of the slurry. ADI which is austempered for 30 min in the first stage followed by 60 minutes in the second stage shows the lowest wear rate. Key words: Austmepered ductile Iron, Iron ore grinding, Wear rate, two stage Autempering Cite this Article: Murthy BRN, Effects of Two Stage Austempering Heat Treatment on Microstructure and Wear Rates of Adi Balls Used in Ball Mill for Grinding of Iron Ore, Journal of Mechanical Engineering and Technology, 9(13), 2018, pp INTRODUCTION Austempered ductile irons (ADI) are going to produce by generating a bainitic structure in the material by conducting an austempering heat treatment over spheroidal graphite (SG) irons. High strength along with better ductility, good resistance to wear and high fatigue strengths are the properties associated with these materials [1]. The exceptional properties of ADI are connected to their exclusive microstructure of ausferrite that involves ferrite along with high-carbon stable editor@iaeme.com

2 Murthy BRN austenite [2-5]. Due to the incredible combination of these properties, these materials are emerged as new class of ductile iron. ADI network consists of bainitic ferrite along with the retained austenite. The ADI mechanical properties are relies on morphology and properties of small scale constituents. Morphology of ADI could be changed by changing the heat treatment parameters such as austenising temperature, temperature of asutempering, asutempering time, temperature and time of stepped and also by alloy composition. Long back itself it was assessed that about 50% of the aggregate mineral processing cost is consuming by Ball mills processes alone [6, 7]. This is due to the wearing of grinding media materials has critical issue of prompting considerable increments in the production cost. It is not simply because of substitution of costly material but also due to the loss in production instigated by premature wear or fracture of balls. The wear of ball in grinding mills is due to the combined effect of abrasion, impact, corrosion and erosion. Therefore it is very important to select a materials which are going to offers best resistance to wear [8]. Earlier work proves that the optimum size of the grinding media offers better performance [9]. Few past works signifies that, microstructures obtained through various austempering strategies have noteworthy impact over the wear opposition of grinding media [10]. 2. EXPERIMENTAL WORK: 2.1 Collection of iron ore and SG iron balls Investigations were done to evaluate the wear rate of austempered ductile iron balls in the crushing of Iron ore which was accumulated from KISCO, Mangalore. The chemical examination is done on the collected ore to acquire the information on chemical content. The chemical analysis date is illustrated TABLE 2.1. S.G. iron balls with estimated chemical composition and which are moderately free from defects were utilized in this work. The chemical analysis of S G iron is presented in TABLE 2.2. S.G. iron balls are heat treated by austenitising at 920 C and stepped austempering was conducted by varying first stage austempering time. The second stage astempering time was held consistent. Table 2.1: Chemical analysis of iron ore Elements Fe Si Al S P Mn Composition (Wt %) Table 2.2: Chemical composition of SG iron Elements C Si P Mn Mo Ni Mg Composition (Wt %) ADI preparation Homogenizing of SG iron balls at C for the span of 10 hrs totally eliminates the carbides in the materials. To get ready ADI, balls were kept in the furnace and heated to the asutenetising temperature of C. Following one hour of austenetising process, to complete the initial step austempering, the balls were moved to a salt bath of C temperature. To complete the second step austempering, balls were taken out from the first salt bath at different time intervals like 15, 30, 45 and 60 minutes and put in to the second salt bath which was held at constant temperature editor@iaeme.com

3 Effects of Two Stage Austempering Heat Treatment on Microstructure and Wear Rates of Adi Balls Used in Ball Mill for Grinding of Iron Ore of C for a time of 60 minutes. After 60 minutes, the balls were taken out and placed in the open air to cool down to the room temperature. For asutempering treatment the salt bath is utilized in this work. The bath is mixture of potassium nitrate and the sodium nitrate at the ratio of 45:55. The combination of chosen temperatures and as well as time austempering treatment is introduced in TABLE 2.3 and the heat treatment setup in Fig Type Table 2.3 Austempering parameters Austenetising First stage austmepering Second stage austempering Temp. ( 0 C) Time (Min) Temp. ( 0 C) Time (Min) Temp. 0 C Time (Min) A B C D Figure 2.1 Heat treatment set up: 2.3 Hardness measurement After the various types of heat treatment the hardness of the balls are measured. The hardness was measured in Vickers hardness testing machine and further converted in to Rockwell hardness. 2.4 X-RD analysis To estimate the different components present in the ADI balls, after each cycle of austempering heat treatment process, the X-RD analysis has done. 2.5 Set-up of the experiment The setup which is used is for this investigation work is known as ball mill. The same is illustrated in Fig The shape of the ball mill resembles the barrel shaped shell. It is having a cm length and mm width. The inner side of the barrel is lined by neoprene rubber. The one end of the ball mill is shut and at the other end a separable top is fixed. There is a provision to fix this top to the barrel with the assistance of bolt and nut. A 3mm hole is provided at the center of this top to send the oxygen during the wet grinding process in order to estimate the impact of various kinds of air circulation on the wear rate of the grading media. On the opposite face another opening of 3mm diameter is given which will act as vent for either for oxygen or air at the time of wet milling process. During the operation time the ball mill kept running at a speed of 86 rpm editor@iaeme.com

4 Murthy BRN Figure: 2.2 Set-up of the ball mill: 2.6 Grinding Media A bunch of 200 balls of dia. 2.5 cm out of which 25 are stamped ones were utilized in this grinding analyses. The consequences of the ADI balls austempered at various circumstances are analysed under various trial conditions. 2.7 Wear rate estimation of grinding balls The wear test of grinding balls were conveyed in the ball mill which was lined with rubber. Iron mineral example of size mesh was utilized for the tests. Balls set of 200 numbers were chosen as the grinding balls to conduct this particular experiment. Out of these 200 numbers there are 25 balls are marked ones. During the experimentation the ball mill was spun at a speed of 86 rpm. The iron ore sample of 1.5 kg along with 1000ml of water (for wet milling process) which produces a pulp density up to 60% which is going held at various values of ph(7, 9, and 10.5) together with grinding media balls is the total charge of the grinding mill for the present experimentation. By adding the lime to the refined water, various ph values were obtained. At the end of every grinding experimentation marked 25 balls were picked by the hand and then washed thoroughly with the refined water. After this the balls were tumbled in a plate with acetone. Then the balls are dried by hot air and placed in a hot air stove a period of one hour at a temperature of C. After taken out from the stove they were kept in the airtight container for cooling. Once they cooled, the picked balls were weighed precisely using electronic balance and weight difference before and after grinding process was calculated. The wear rate was computed utilizing the equation below = / X! "# With the help of pressurised filtering tool, the obtained slurry was separated and dried by outing in a stove. The sifted water test is collected for the ph estimation. An agent test of 100 grams was taken from the powdered cake in the wake of coning and quartering and subjected to sieve examination. The experiment condition is presented in TABLE 2.4. Table 2.4: Experiment condition: Grinding Type Total mill revolutions Wet grinding (close mill condition) 5100 for 60 minutes Pulp Density 60 % editor@iaeme.com

5 Effects of Two Stage Austempering Heat Treatment on Microstructure and Wear Rates of Adi Balls Used in Ball Mill for Grinding of Iron Ore Total number of ball used 200 ( with marked 25 ball), of size 25 mmφ Size of ore sample mesh (as per ASTM) Time of Grinding 1 hour Density of ADI balls 7.2 g /cm 3 Density of En-3l Steel (forged) balls 7.6 g /cm 3 3. RESULTS AND DISCUSSIONS: 3.1 Morphology (microstructure) of ADI balls The high carbon content in the ductile iron generates the spheroidal graphite nodules inserted in the iron matrix. When the iron balls are quenched and held at austempering temperature acicular ferrite will be obtained accompanied by carbon rejection into austenite. This microstructure is also called as Ausferrite which is the blend of acicular ferrite and high carbon austenite (11). In the present work, since the first stage austempering is 300 0, the obtained microstucre is lower ausferritic microstructure. The microstructure consist of spheroidal graphite along with matrix of ausferrite needles. Microstructures obtained for various time intervals were presented in Fig It is observed that the quantity of retained austenite increased with rise in the first step austempering time and the carbon content decreased by time. (a) (b) (c) (d) Figure 3.1: Optical Photomicrograph of Ball Sample Austempered at C for 15(a), 30(b), 45(c) and 60 minutes (d), followed by C for 60 minutes, ( Etched with 3% Nital), 500 X 3.2 Estimation of Retained Austenite and its effects X-RD analysis were carried out in order to estimate the retained austenite and carbon content in the retained austenite. The X-RD patters of the balls heat-treated by the various austempering methods are illustrated in Fig The increment in the time of first stage asutempering produces more amount of retained austenite in the final microstructure. This may due to the production of more amount of stable austenite editor@iaeme.com

6 Murthy BRN Figure. 3.2: X-RD pattern of ball sample Austempered for and 60 minutes at C and followed by C for 60 minutes. The variations of retained austenite, carbon content in the retained austenite and the hardness with respect to first stage austempering time along with effect of retained austenite on the hardness are illustrated in the Fig The quantity in the retained austenite increases with increase in the first asutempering time whereas the percentage of the carbon content decreases with time and this leads to the decrement in the hardness with increase in time. Increase in the retained austenite reduces the hardness obtained Rretained austenite(vol.%) Carbon content in retained austenite(vo.%) First step austempering time(min) First step austempering time(min) Hardness (Rc) Hardness (Rc) First step austempering time (min) Retained austenite(vol.%) Figure. 3.3: Variation of retained austenite, carbon content and hardenss with respect to time and variation of hardness against retained austenite. Since the ware behaviour of a grinding media is sensitive to the ph values of the slurry used, in the present work in addition to various austempering methods wear rates at different ph values are also estimated. At lower ph values the wear behavior is sensitive to the microstructure. Due to this phenomenon we can see the higher wear rate at lower ph values on the table. The wear rate estimated for various autempering methods at the different ph values are given in TABLE 3.1. It is observed that ADIs which are asutempered at C for 30 minutes followed by C for 60 minutes (B type) possesses the lower wear rates for all ph values of water. This may be due to refined distribution of bainitic microstructure. To find out the exact reason for this phenomenon, further study regarding the effect of first and second stage austempering process parameters (temperature and time) over the obtained microstructure is needed editor@iaeme.com

7 Effects of Two Stage Austempering Heat Treatment on Microstructure and Wear Rates of Adi Balls Used in Ball Mill for Grinding of Iron Ore Table 3.1: Estimation of wear rate Austempering type Initial ph 1 ph values Final ph 2 Wear rate cm 3 /rev. ( 10 8 ) A B C D The variation of wear rates with ph values is plotted in Fig From the figure, it is clear that irrespective of austempering methods the lower value of wear rates are associated with the higher values of ph Min. 30 Min. 45 Min. En-31 Steel Wear rate ph values Figure 3. 4: Variation of wear with ph values. 4. CONCLUSIONS From the results we can draw the following conclusions. Variation in the first stage austempering temperature has significant effect on the microstructure and on the wear rate of ADI balls. Wear rate inversely proportional to the ph of the slurry used editor@iaeme.com

8 Murthy BRN The quantity of retained austenite increases with rise in first stage austempering temperature. The amount of carbon content decreases with increase in first step austempering temperature. For all ph values of the slurry, the minimum wear rate is observed with the 30 min first step austempering time. Further studies are necessary to find out the exact reason why the lower wear rates are associated with type B austempering method. REFERENCES: [1] T. Nasir, D. O. Northwood, J. Han, Q. Zou, G. Barber, X. Sun & P. Seaton Heat treatment microstructure mechanical/tribological property relationships in austempered ductile iron WIT Transactions on Engineering Sciences, Vol 71, 2011, pp [2] Bosnjak, B., Radulovic, B., Effect of austenitising temperature on austempering kinetics of Ni-Mo alloyed ductile iron. Materiali in Tehnologije, 38 (6), 2004, pp [3] Yang, J., Putatunda, S. K., Improvement in strength and toughness of austempered ductile cast iron by a novel two-step austempering process. Materials and Design, 25, 2004, pp [4] Myszka D., Austenite-martensite transformation in austempered ductile iron. Archives of Metallurgy and Materials, 52(3), 2007, pp [5] Eric, O., Jovanovic, M., Microstructure and mechanical properties of CuNi-Mo austempered ductile iron. Journal of Mining and Metallurgy, 40B (1), 2004 pp [6] Prasad Rao P.and Putatunda S. K. 2003, Investigations on the fracture toughness of austempered ductile iron alloyed with chromium, Materials Scince and Engineering, A, Vol. 349, No. 1-2, 2001, pp [7] Jain S.K. 2001, Mineral Processing 2nd edition, CBS Publisher and distributor, New Delhi, pp [8] Siddan J. B. 2014, Wear behavior of stepped austempered ductile iron balls in grinding iron ore, International Journal of Research in Engineering and Technology, Vol. 03, 2014, pp [9] Qingfei XiaoBoLi Huaibin Kang The Effect of Fine Grinding Medium Feature on Grinding Results AASRI Procedia Volume 7, 2014, Pages [10] Prabhukumar Sellamuthu, D. G. Harris Samuel, D. Dinakaran, V. P. Premkumar, Zushu Li Austempered Ductile Iron (ADI): Influence of Austempering Temperature on Microstructure, Mechanical and Wear Properties and Energy Consumption Metals, 8(1), 2018, pp editor@iaeme.com