Abrasive Wear Studies on Austempered Ductile Iron Castings

Transcription

1 Abrasive Wear Studies on Austempered Ductile Iron Castings Siddaraju C 1, Dr. N. D. Prasanna 2, Dr. M. K. Muralidhara 3 1 Research scholar, 2 Professor, Department of Mechanical Engineering, MSRIT Bangalore Principal, R R Institute of Technology, Bangalore Abstract-- Conventionally steels have enjoyed some kind monopoly in automotive and earthmoving applications like ripper tips and grader blades. The limitation of steels is that it is difficult to get a good combination of properties. Recent research efforts have focussed on austempered ductile iron as an alternative material which exhibits good properties. Before the usage of ADI can flourish, there is a need to thoroughly understand its mechanical and tribological behaviours.in the present Investigation successful attempts were made to subject ductile iron castings (conforming to IS 400/15 - C 3.65%, Si 2.75%, Mn 0.35max, P 0.03max, S 0.015max, Mg 0.03%min) to austempering heat treatment cycle to bring about the changes in the structure and hence the properties. Austempering variables studied were austenisation temperature 900oC for 2hours duration, austempering temperature selected were 320oC and 400oC,austempering duration selected were 40, 80, 120, 160, 200 minutes. Abrasive wear studies were carried out using sand abrasive wear tester. Flat rectangular specimen were used and test was carried out for four different loads and speeds. The results of investigation indicate that specimen austempered at 400 oc for 120 minutes duration exhibit better abrasive wear resistance property compared to the other conditions studied, indicating that higher austempering temperature exhibits better abrasion resistant behavior. Keywords-- Austempered Ductile iron (ADI),DI, Wear, UTS, BHN. I. INTRODUCTION Ductile Iron is also called nodular cast iron or nodular iron exhibit good combination of properties such as castability property with comparable strength, machinability, thermal conductivity and damping properties which make it widely employable for manufacturing different machine parts[1,2]. The recent development is to obtain a vareity of bainitic structures[3,4]. Banitic matrix can be obtained in two ways in the as cast form where baintic transformation is achieved by judicious alloying elements such as Ni, Mo and Cr, and by subjecting ductile iron to specific heat treatment known as austempering. Austempering heat treatment,a two stage treatment involves subjecting the material to austenitisation treatment ( o C) and then suddenly quenching to austempering baths (220 o C-360 o C). This iron is referred to as Austempered Ductile iron or Simply ADI. ADI has attracted has considerable interest in recent years because of its excellent properties [5,6] Further, it is reported that ADI possess advantages such as cheaper than forged steel, lighter than steel counterpart, production of ADI components needs less energy than for steel component. Hence it finds wide range of applications. Applications include digger teeth material in agricultural sector, transmission gears,crankshaft, connecting rods in automotive industry, aircraft landing gears, cannon shells in defence etc[7-10].wear is now gaining importance and is thought as an important study in characterizing a material for a particular application[11,12]. Wear has been classified by different authors based on mechanism involved, interaction between the surfaces and medium used [13,14]. In the present investigation IS400/15 grade ductile iron castings were subjected to standard austempering heat treatment cycle. Mechanical properties such as ultimate tensile strength, Brinell hardness measurements were carried out. Sand abrasive wear test was carried out carried out using ASTM G 65 Specifications for ascast condition and austempered condition. The results indicate that higher austempering temperature and longer austempering duration exhibits superior wear properties compared to other conditions. II. EXPERIMENTAL WORK The details of the experiments carried out on IS 400/15 grade ductile iron is highlighted under the following headings A. Moulding: Ordinary mangalore sand having AFS No. 60 was used in the present investigation. Green sand moulding was carried out with permeability of the sand being 115 and compression strength 1150 N/mm 2. B. Melting and pouring: Melting of the charge was carried out using 1.25T capacity, 200Hz frequency induction furnace. The charge consisting of ductile Iron, Grey Iron returns, Commercial Pig Iron ( imported Soral Pig Iron consisting low sulphur content less than 0.05%), mild steel punching were melted and superheated to a temperature range 1490 o C-1550 o C. 124

2 This was desulphurized using calcium carbide (1.9% by weight basis) to reduce the sulphur in the Iron. Standard sandwich treatment methods was employed to get ductile Iron castings. After skimming off the impurities, slag, molten metal was poured into the prepared sand moulds. The carbon equivalent of the iron was aimed between to meet specification of IS 400/15. The molten metal was allowed to cool in the moulds and solidified castings taken out were used for preparation of test specimen. The moulding, melting of SG iron castings were carried out in a regular medium production foundry. C. Heat treatment : Standard austempring heat treatment cycle as shown in the fig.1. was employed in the present investigation. Austempering variables studied are listed below: Austenisation temperature: 900 o C; Duration : 120 minutes - followed by Austempering temperature: 320 o C and 400 o C ; Duration : 40, 80, 120,160, 200 minutes. Test specimen for the structure examination, assessment of mechanical properties and wear studies were machined from as cast and austempered ones. Specimen no. TABLE I DETAILS OF THE SPECIMEN Austempering temp. ( o c) Soaking time (min) The furnace used for austenitization and austempering are shown in Figure 2 and Figure 3. Figure 1: Standard austempering cycle employed Details of the conditions of the specimen are shown in the following table. Austenitization temperature C for 120 minutes. Figure 2: Furnace used for Austenitisation 125

3 Revolution counter was set for number of wheel revolutions. Sand flow rate was adjusted. Test was conducted for a preset time (15 minutes). At the end of the test, the specimen was removed from the specimen holder, cleaned thoroughly dried and weighed precisely using electronic balance. Loss in weight (difference between initial weight and final weight) has been taken as measure of wear of the specimen Fig. 4a and b shows the details of wear specimens and testing machine. Figure 3: Salt bath Furnace used for Austempering. D. Structure Examination: Structure examination was carried out employing standard metallographic procedure. Matrix and nodule count examination were made in ascast condition; whereas matrix examination were carried out in austempered condition (using 3% Nital as etchant). E. Mechanical properties : Standard procedure was employed to assess UTS, and Brinell hardness measurements on ascast and austempered specimen. An average of three readings has been considered for the analysis. F. Abrasive wear test : Steps involved in Abrasive wear test are: Cleaning: The specimen surface was cleaned thoroughly. This was done using acetone to remove all dust and grease/foreign matter present on the surface of the specimen. Initial weight of the specimen was noted down. This was carried out using an electronic balance. Specimen was secured in the specimen holder. Weights were added to leverage to develop proper force to press the specimen against the wheel. Figure 4a: Wet erosive wear specimen Figure 4.b:Schematic representation of wet abrasive wear testing machine. 126

4 III. RESULTS AND DISCUSSIONS A. Microstructure examination Figure 5a : Micrograph of Ascast ductile iron Condition : Ascast Matrix : Ferrite No. of Nodules: Nodules Magnification: X50 B. Mechanical properties: Figure 6 9 shows the variation of the ultimate tensile strength values and hardness values of the specimen for different austempered conditions. It can be seen from the figure that austempered specimen exhibit higher values of ultimate tensile strength and hardness values compared to the ascast ones. Maximum ultimate tensile strength values of 1313 N/mm 2 is observed for the specimen austempered for 120 minutes duration. With increase in austempering duration a decreasing trend in the ultimate tensile strength values is observed. Higher hardness values are obtained for the austempered specimen compared to ascast ones. Maximum hardness values of 354 BHN is observed for the austempered specimen austempered for 120 min duration. This may be due to the clear formation of bainite for that austempering duration condition Figure 6: Variation of UTS Values for specimen in ascast and austempered conditions (austempered at 320 o C) Figure 5b. SEM Image of Austempered specimen Condition: Austempered Austempering temperature: 400 o C, 120 minutes Etchant used: 3% Nital Magnification: X 3000 In the ascast condition, the nodules are uniform in shape and distributed evenly in a matix of ferrite. An average of nodules /mm 2 is observed. Upon austempering, the structure reveals the formation of bainite. Figure 7: Variation of UTS Values for specimen in ascast and austempered conditions (austempered at 400 o C). 127

5 The improvement in the wear resistance of austempered specimen may be attributed to the lubricity inherent in the graphite nodules, increase in hardness value brought about by the bainitic structure and the work hardening of the surface. Also specimens subjected to Austempering heat treatment cycle exhibit (ADI) with high hardness values due to the formation of baintic structure in the matrix, which is responsible for superior wear property. Hence from the wear studies carried out on ascast ductile iron and austempered ductile iron, ADI can be thought of as an attractive alternate material in many applications such as transmission gears, crankshafts, rolls, train car wheels, gear train for power loam industries etc., which demand superior wear resistance property. Figure 8: Variation of Hardness Values for specimen in ascast and austempered conditions (austempered at 320 o C). Figure 10: Variation of Weight Loss for different Loads, Speed 100 Figure 9: Variation of Hardness Values for specimen in ascast and austempered conditions (austempered at 400 o C). C. Abrasive wear studies Figure 10 to 11 shows the variation of weight loss of the specimen for different conditions. Referring to fig. 10 it can be seen that weight loss steadily increases with increase in the loads investigated. Ascast specimen exhibits more wear compared to the austempered conditions for all the loads investigated. This may be attributed to the lower hardness values and the soft ferritic microstructure obtained for the ascast condition. Upon Austempering, the austempered specimen exhibit improved wear resistance property. Least wear is exhibited by the specimen subjected to Austempering heat treatment ( austempered for 120 min duration). Figure 11: Variation of Weight Loss for different Loads, Speed

6 Figure 15: Variation of Weight Loss for different Loads, Speed 140 rpm, at 400 C Austempering Temperature. Figure 12: Variation of Weight Loss for different Loads, Speed 180 Figure 16: Variation of Weight Loss for different Loads, Speed 180 rpm, at 400 C Austempering Temperature. Figure 13: Variation of Weight Loss for different Loads, Speed 210 Figure 17: Variation of Weight Loss for different Loads, Speed 210rpm, at 400 C Austempering Temperature. Figure 14: Variation of Weight Loss for different Loads, Speed 100 rpm, at 400 C Austempering Temperature. IV. CONCLUSIONS Graphite nodules are more or less uniform in size and are distributed evenly in the matrix.upon subjecting the specimen to austempering heat treatment cycle, the microstructure shows graphite nodules with bainitic matrix. 129

7 By subjecting the specimen to austempering heat treatment cycle the Ultimate tensile strength and hardness properties are improved compared to ascast condition. From the sand abrasive wear tests it is seen that the wear of the specimen increases with increase in duration of testing. By subjecting the specimen to austempering heat treatment cycle the wear of the material is reduced compared to ascast condition. Hence, from the above investigation, it is clear that by austempering ductile iron castings better mechanical properties are realised. Austempering heat treatment results in improved wear resistance Hence, this can further increase the potential applications of ADI to be used in numerous applications. Acknowledgements The authors thank the management of MSRIT, Bangalore and M/s Bangalore metallurgical Pvt. Ltd., Hoskote for supporting the work. REFERENCES [1] N.D.Prasanna, M.K.Muralidhara, K.Radhakrishna Influence of Austempering temperature on Mechanical properties and wet abrasive wear of IS 700/2 Grade Ductile Iron Castings Congress proceedings - 52nd Indian foundry congress, P 41-44, Kolkatta. [2] S.Seshan ADI- The under- exploited wonder cast iron Transaction of Indian foundrymen, National seminar, SG 50. Golden Jubilee celebration, Vol 84-92, oct [3] K.Chetan, V.Krishnaraj, S.Seshan Unalloyed Austempered Ductile Iron (ADI) Production and Properties 36 th Annual Convention, IIF Pg [4] L Magalheas, J.Seabra Wear and Scuffing of austempered ductile iron gears Wear 215 ( 1998) Pg [5] K.H.W.Seah and S.C.Sharma Machinability of alloyed austempered dutile iron International journal of machine tool manufacture, vol 35, no. 10, 1995,pp [6] G.Sutradhar Influence Of Process Parameters On The Quality Of Ferritic And Ferrito-Pearlitic Grades Of Ductile Iron-A Case Study Trasaction of IIF, March [7] J.Zimba, D.J.Simbi, E.Navara Austempered ductile iron: an alternative material for earth moving components Cement and concrete components 25 (2003) pg [8] N.D.Prasanna,M.K.Muralidhara, K.Radhakrishna Influence of Austempering temperature on Mechanical properties and wet abrasive wear of IS 700/2 Grade Ductile Iron Castings Congress proceedings - 52nd Indian foundry congress, P 41-44, Kolkatta. [9] C.Siddaraju,N.D.Prasanna,M.K.Muralidhara Corrosion characteristics of Ductile iron castings subjected to Austempering heat treatment Indian foundry journal Vol. 59, No.1, Jan 2013, Pg [10] A.Refaey, N. Fatahalla Effect of microstructure on properties on ADI and low alloyed ductile iron Journal of Material Science, vol 38 ( 2008), [11] L.Magalhaes, J.Seabra Wear and scuffing of austempered ductile iron gears Wear 215 (1998) [12] A. Shafiee, M.Nili-Ahmadabadi Wear behavior of a Cr-Mo steel with different microstructures in comparision with austempered ductile iron( ADI) International Material Forum (2009) Vol.2 pp [13] R.D. Forresst The challenge and opportunity present to the S.G. Iron Industry by the development of austempered ductile iron.transaction of the foundryman, P , [14] Carl B. Rundman.Austempered Ductile iron; striving for continuous improvement.transactionof American foundrymens society, world conference on ADI. P1-21, March