MECHANICAL PROPERTIES AND CORROSION CHARACTERISTICS OF IS400/12 GRADE DUCTILE IRON N. D. Prasanna*, M. K. Muralidhara* Mhit Kumar Agarwal* and K. Radhakrishna** *Dept. f Mech.Engg., M. S. Ramaiah Institute f Technlgy, Bangalre-560 054. **Dept. f Mech. Engg., B.M.S. Cllege f Engg., Bangalre-560 019. ABSTRACT In the present investigatin, ductile irn castings cnfrming t IS400/12 grade were prduced using green sand mulds. The castings were prduced in a regular prductin fundry using 1.25T Inductin Furnace. The mlten metal was subjected t standard desulphurisatin and mdularisatin t get graphite in the ndular frm. The mlten metal was pured int the sand muld and allwed t slidify. Based n the earlier investigatin by the abve grup (1,4) the castings were austenitised at 900 C fr 120m and quenched in a salt bath cnsisting f ptassium nitrate and sdium nitrate in the rati f 55:45 and maintained at 310 C fr 120 minutes t bring abut austempering treatment. The result in austempered ductile irn will cnsist f graphite ndules in a matrix f bainite. The effect f austempering heat treatment n micrstructure, mechanical prperties and crrsin characteristics were studied and cmpared with the ascast cnditin. Mechanical prperties such as ultimate tensile strength, percentage elngatin, hardness and impact strength were determined. Crrsin tests were carried ut t determine the weight lss and crrsin rate f specimens; using salt spray fg type apparatus. Crrsin test was carried ut fr tw different perating temperatures viz. 35 C and 45 C. The results f the investigatin indicate that the austempered castings shw higher UTS values (34% increase), elngatin values (24.2% increase) and hardness values (12.05% increases) as cmpared t the as-cast cnditin. Frm the crrsin studies, it is seen that austempered specimens exhibit lwer weight lss (34% imprvement), lwer crrsin rate (33% imprvement) cmpared t the as-cast specimens. INTRODUCTION Ductile Irn cnsists f graphite in the frm f ndules r spherids in a matrix 1,2 f either ferrite r pearlite. The prperties f ductile irn cmbine the prperties f cast irn and steel. Irn with varying micrstructure can be btained by varying the treatment cnditins during melting, after treatment and als by heat-treating the castings. By altering any ne f the parameters, a suitable irn as per requirements and applicatin can be btained. In rder t imprve the prperties f ductile irn, the material can be treated with allying elements r can be heat-treated t bring abut change in the micrstructure f the material. Cmmnly, austempering heat treatment is carried ut n ductile irn; hence he name Austempered Ductile Irn r ADI. ADI has fund its way in a wide range f cmpnents 5,6 fr many engineering sectrs as in gears, crank shafts, transmissins, suspensins, earth-mving and cnstructin equipment, railways etc. Wear is an imprtant prperty, which is evaluated in materials t find ut its respnse t underg lss f material. It is a prgressive, unintentinal lss f material when tw surfaces cme in cntact under nrmal lad and there is relative mtin between the surfaces. Wet abrasive wear test is ne methd fr evaluating the material behaviur t wet abrasive actin. Transactins f 57 th IFC 2009 89
Different crrsin testing methds are available viz. Electrchemical test, salt spray test, ttal immersin test etc. The chice f the crrsin testing methds as reprted 7,8 depends upn many factrs such as nature f envirnment, nature f expsure, type f specimen etc. Salt Spray Fg test 6 is ne f the crrsin testing methds, which invlves the expsure f the specimens t fine spray as mist f a slutin f sdium chlride at a specified temperature. The spray particles settle upn the test surface (which is preferably inclined) and cnstantly replenish and replace the film f slutin n the surface. The extent and nature f the crrsin f the metal r cated surface after a specified perid f expsure serve as a measure f quality/crrsin resistance. T accelerate crrsin the temperature f the media r the ph f the media r the cncentratin f the media can be varied. Many Indian fundries are prducing ductile irn n a cmmercial scale since a few decades. Hwever, the prductin f ADI in India is still in infant stages. Hence, in the present investigatin ductile irn cnfrming t IS400/12 grade was prduced and austempered fr the best cnditins hence, in the present investigatin a systematic study was taken up fr structure examinatin, assessment f mechanical prperties, and crrsin prperties fr different time and temperature levels. Experimental Details i) Muld Material Used : Silica sand having a grain fineness f AFS61 was used fr making the mulds. Greensand mulds were prepared using 5% bentnite and 3.5 % water. Cylindrical test bar castings f 30mm dia and 300mm length were prduced using cpe and drag mulds. Figure 1 shws the details f the cpe and drag pattern used t prepare the test bar. ii) Melting, Casting and Treatment : Charge cnsisting f Grey Cast Irn; fundry returns; ductile Irn, cmmercial pig irn (S<0.05%), mild steel punching was used. Cnventinal techniques f melt treatment and heat treatment were carried ut as shwn in the flwchart Fig.2 t prduce ductile irn castings and ADI castings. The cmpsitin f IS400/12 was 3.6%C, 2.8%Si, 0.35%Mn, 0.03%P(max) and 0.03%S(max) with 0.03%Mg (min). iii) Evaluatin f Prperties: Standard metallgraphic prcedures like grinding and plishing were dne fr micrstructure examinatin. Standard test prcedures were emplyed fr structure examinatin and mechanical prperty assessments. iv) Dry Sliding Wear Test: The specimens in the frm f 5mm pins were prepared in a lathe and used as a sample fr dry wear test. A hardened circular disc having hardness f 64HRC was used as cunterface. The testing was carried ut by allwing the specimen t rub against hardened disc fr a given nrmal lad and speed cnditin. Making use f pin n disc machine test was carried ut. The specimens were taken ut at regular intervals and the weight was recrded. The experiment was cnducted fr different lad, speeds and different cnditins f metal (as cast r heat treated). Figure 3a shws the details f wear testing machine used in the investigatin. 90 February 13 15, 2009, Klkata, India
Metal charge (PI,CI,Steel punching,sral PI) Inductin Furnace 1.25T, 200HZ Melt was superheated 155 and Desulphurised with 1.9% CaC 2 t reduce S level t <0.05% Desulphurised Irn was pured int a Ladle cntaining Fe-Si-Mg ally using sandwich methd Pst inculated with 0.3% FeSi & the melt was stirred. Slag remved Mlten metal pured int the mulds and cled Castings were heat treated by first austenitising at 900 C fr 120 minutrs Quenched in salt bath cntaining KNO 3 : NaNO 3 : : 55:45 maintained at a knwn temp. level and held at this temp. fr 120 minutes Castings cled The resulting Irn is ADI Flw chart fr ADI casting prductin. v) Wet Abrasive Wear Test: The specimens in the frm f blts were subjected t abrasive actin by making use f rtating disc dipt inside a slurry cnsisting f abrasive media and water maintained at given ph level. The specimens are rtated in the slurry fr knwn duratin f time and regular intervals f tw hurs the testing was stpped, specimens are taken ff and cleaned well using alchl and water and dry using a drier. The initial weight f the sample and the subsequent weight f the sample measure n cntinuus scale will give the wear f the specimen at a given time interval. It was carried ut fr 24hurs and graphs are pltted. Figure 3b shws the details f wet abrasive wear testing machine used in the investigatin. vi) Impact Studies: Impact test was carried ut using standard Izd (cantilever type) and standard Charpy (simply supprted) specimens and evaluatin f impact n specimen in as-cast cnditin and heat-treated cnditin. It is absrbed n heat treatment, the impact values will cme dwn appreciably. vii) Crrsin Studies: Crrsin studies were carried ut using standard salt spray fg type testing machine. Crrsin testing apparatus cnfrming t ASTM standard B117 specificatin was used. The crrsin rate f the specimen was calculated using the frmula Transactins f 57 th IFC 2009 91
Crrsin Rate (mpy)= (534xW) / D A T W = Weight lss in mg, D = Density f the specimen in gm/cc, A = Area in square mm, T = Expsure time in hurs mpy = millin parts per year Weight lss methd was emplyed t assess the crrsin f the specimen. At regular intervals, the specimen was taken ut and the weight is determined fr calculatin f lss in weight r crrsin. Figure 3c shws the details f the fg type crrsin apparatus used in this investigatin. Steps invlved Rectangular slab specimens f 50mm x 25mm x 6mm thick (after nting dwn the initial weight) was hung using a nyln wire inside the fg chamber.. The crrsive media cnsisting f water and sdium chlride in the rati 95:5 n weight basis was filled at the bttm f the chamber. The utlet f the cmpressr was cnnected t the nzzle and the heating cil was switched n; the crrsive slutin gets heated and the temperature inside the cabinet was held cnstant (viz. 35 C and 45 C) thrughut the duratin f the test. Cmpressed air at a cnstant pressure f 2kg/cm 2 was blwn thrugh the nzzle fr the entire duratin f the test t cause bubbling actin in the salt bath and create fg in the chamber. The specimen was expsed t crrsive atmsphere was 24 hurs, at the end f every 2 hurs, the test was stpped, the specimens were remved and cleaned with distilled water. The specimens after the test were thrughly cleaned as per ASTM G181 specificatins and the crrsin rate was determined. RESULTS AND DISCUSSION Micrstructure Examinatin Figures 4a & 4b shw the details f the micrstructure. In the as-cast cnditin, the graphite ndules, are mre r less unifrm and evenly distributed. In the heat-treated cnditin, the austempered specimens exhibit bainitic matrix. Mechanical prperties The variatin f ultimate tensile strength, percentage elngatin and hardness values f the specimen are shwn in Table-1. It can be seen that the UTS values, percentage elngatin values and hardness values f the heat-treated specimen r higher than the as cast nes. This may be due t the frmatin f bainite in the heat-treated cnditin. Table-1 details f Mechanical prperties under as cast heat-reated cnditin. 92 February 13 15, 2009, Klkata, India
Dry Sliding Wear The variatin f lss f weight at 3 different speeds and 3 different lads were evaluated Figs.5-8 cnslidates the details f the test carried ut fr evaluating dry wear. Wear f, as cast specimen is higher than heat-treated cnditin fr any given speed r any given nrmal lad cnditin. Dry sliding wear increases with increase in time f testing. Decrease in wear fr heat-treated specimen, varies frm 15 t 20% with respect t as cast specimen. Wet Abrasive Wear Figs.9-11 shws the variatin f wet abrasive wear as the functin f time fr different ph values and casting cnditin. Wear increases with time f testing fr all the ph levels tested. Heat-treated specimen seems t shw lwer wear as cmpared t as cast specimen. Specimen under ph8 seems t shw higher wear cmpared t ther ph levels. This behavir may be attributed t lnger expsure f specimen t abrasive, heat treated specimens develp higher hardness and strengths and neutral media seems t wear specimens faster. Table-2 shws the details f the wet abrasive wear fr as cast and heat-treated cnditin fr different ph values and at the end f 2 hrs & 24 hrs f testing. Impact Test The Izd impact results shw that the impact energy decreases n heat treatment indicating that the specimen tends t becme brittle. There is a decrease in energy level by nearly 29% as shwn in Table-3. Charpy result shws that the impact energy decreases n heat treatment. But there is a tendency t shw sudden drp. The decrease is as high as 71% this may be attributed t the type f arrangement used fr impact measurement and due t the specimen tending t becme brittle. CORROSION STUDIES Crrsin The weight lss f specimen in as-cast cnditin is mre than heat-treated cnditin fr any given temperature. The weight lss in specimens subjected t crrsin at higher temperature is mre. This may be attributed t higher reactin invlved at higher temperature f the fg mixture. The crrsin rate fr specimen in as-cast cnditin is mre than heat-treated cnditin fr any given temperature. The crrsin rate is mre at higher temperatures f the fg mixture. This may be attributed t mre resistance ffered by the heat-treated specimens and higher reactin rate invlved at higher temperature. Figure 12 shws a representative graph f crrsin v/s time at 35 C. Transactins f 57 th IFC 2009 93
Crrsin Rate The variatin f crrsin rate (in mpy) versus time f testing (in hurs) fr as-cast and heat-treated cnditins is shwn in Fig.13. It can be seen frm the graph that with increase in the duratin f testing, the crrsin rate decreases. Crrsin rate is less in the heat-treated specimens cmpared t the as-cast nes. This may be due t presence f bainitic structure, which ffers mre resistance t crrsin; whereas in the as-cast cnditin the structure cnsists f ferrite. Similar results were btained fr testing carried ut at 45 C. Table-4 shws the variatin f crrsin and crrsin rate fr 35 C & 45 C f the fg. CONCLUSIONS The results f the investigatin carried ut n as-cast ductile irn and austempered ductile irn indicate the fllwing : Graphite ndules are mre r less unifrm and are evenly distributed; upn austempering the structure shws bainitic matrix in the irn. Mechanical prperties such as ultimate tensile strength, percentage elngatin and hardness values are enhanced cmpared t the as-cast cnditin. Impact test shws that impact energy decreases n heat-treating f the specimen. Dry sliding wear shws that n heat treating the specimen shws higher resistance t wear Wet abrasive wear als shws higher resistance t wear in the heat-treated cnditin. Studies n crrsin shws that the weight lss is mre in as-cast cnditin cmpared t heat-treated cnditin. It was als bserved that crrsin is mre at higher temperature f the bath. The crrsin (rate) f the heat-treated specimen is less/reduced cmpared t the ascast cnditins. Hence, it is clear that by austempering the ductile irn castings, better mechanical prperties and crrsin resistance prperties are realised. References 1. N. D. Prasanna, Effect f prcesses variables n the structure mechanical prperties and wear characteristics f Austempred Ductile Irn, Ph.D thesis, submitted t Bangalre University, March 2000. 2. R. Grundy, The develpment and intrductin f spheridal graphite irns int an established range f malleable irns t meet current engineering requirements, Transactins f Fundrymen, Vl.72, Part-7, p141-151, July 1979. 3. M. J. Falln, Experience in the manufacture f ductile irns, Transcatins f Fundrymen, Vl.88, Part-9, p308-318, September 1995. 94 February 13 15, 2009, Klkata, India
4. A. A. Zhukr, et al., Step by step schedules f bainitic quenching f cast irns, Transactins f the Institute f Indian Fundrymen, p-26, March 1994. 5. Harding R. A, Gilbert G. N. J, Why the prperties f austempered ductile irn shuld interest engineers, The British Fundrymen, Vl.79, N.11, p489, 1986. 6. J. Panasie Wicz, C. Grupek, J. Huth, Chryslers experience with austempred ductile irn crankshafts, Transactins f American Fundrymen s Sciety, Wrld Cnference n ADI, p176-193, March 1991. 7. Philip A. Schweitzia P.E., Encyclpedia f Crrsin technlgy, 1975. 8. Standard practice fr perating start spray (fg) testing apparatus, Transactins f ASTM B117-94, p1-8, 1994. Transactins f 57 th IFC 2009 95