Austempered Ductile Iron (ADI) is a class of ductile iron subjected to a two-step heat treatment process – austenitization and austempering. The heat treatment gives to ADI a high value of tensile strength and an especially good strength-to-weight ratio. However, designers in most cases are unfamiliar with this material that can compete favorably with steel and aluminum castings, weldments and forgings. The high tensile strength of ADI is the result of its unique ausferrite microstructure. In this paper, an investigation of the influence of the austempering temperature on the tensile strength of the ADI samples is presented

A. Gigović-Gekić

H. Avdušinović

M. Janjić

M. Vukčević

S. Savićević

Z. Jurković

English

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Influece of the austempering temperature on the tensile strength of the austempered ductile iron (ADI) samples

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149METALURGIJA 56 (2017) 1-2, 149-152S. SAVIĆEVIĆ, H. AVDUŠINOVIĆ, A. GIGOVIĆ-GEKIĆ, Z. JURKOVIĆ, M. VUKČEVIĆ, M. JANJIĆINFLUECE OF THE AUSTEMPERING TEMPERATURE ON THE TENSILE STRENGTH OF THE AUSTEMPERED DUCTILE IRON (ADI) SAMPLESReceived – Prispjelo: 2016-04-11Accepted – Prihvaćeno: 2016-08-15Preliminary Note – Prethodno priopćenjeS. Savićević, M. Vukčević, M. Janjić, Faculty of Mechanical Engineer-ing, University of Montenegro, Montenegro. H. Avdušinović, A. Gigović-Gekić, Faculty of Metallurgy and Materi-als Science, University of Zenica, BiH. Z. Jurković, Faculty of Engineering, University of Rijeka, Croatia.Austempered Ductile Iron (ADI) is a class of ductile iron subjected to a two-step heat treatment process – austeniti-zation and austempering. The heat treatment gives to ADI a high value of tensile strength and an especially good strength-to-weight ratio. However, designers in most cases are unfamiliar with this material that can compete fa-vorably with steel and aluminum castings, weldments and forgings. The high tensile strength of ADI is the result of its unique ausferrite microstructure. In this paper, an investigation of the influence of the austempering tempera-ture on the tensile strength of the ADI samples is presented.Key words: ADI, austempering, temperature, tensile strength, microstructureINTRODUCTIONAustempering is a heat treatment process that is ap-plied to ferrous metals, most notably for steel and duc-tile iron. In steel it produces a bainite microstructure whereas in cast iron it produces a structure of acicular ferrite and high carbon, a stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce/eliminate distortion. It was devel-oped in the 1930s by Edgar C. Bain and Edmund S. Davenport. To begin with, it was usually applied to steel products [1, 2, 3]. Ductile iron or spheroid graphite iron was developed during the 1940s. Ductile iron with its unique graphite morphology is a material that has tensile and impact properties sufficient for many different application (ve-hicle and agriculture industry, mining, pipes, and so on). In the second half of the twentieth century the aus-tempering process began to be applied commercially to cast irons.Austempered Ductile Iron (ADI) was first commer-cialized in the early 1970s but serious research in the field of ADI application was carried out at the end of the 20th and beginning of the 21st century [4].A diagram that illustrates the Austempering process is presented in Figure 1.The most notable difference between austempering and conventional quench and tempering is that it in-volves holding the workpiece at the quenching tempera-ture for an extended period of time. The basic steps are ISSN 0543-5846METABK 56(1-2) 149-152 (2017)UDC – UDK 621.741-539.55:621.785:536.5:620.17:620.18=111the same whether applied to cast iron or steel and are as follows [5-7]: Heating to a temperature to produce austenite Quenching rapidly to avoid the formation of pearlite to a temperature above the martensite start tempera-ture (Ms temperature). The selected quench temper-ature is referred to as the austempering temperature. Holding at the selected austempering temperature for a time sufficient to transform the austenite to the desired end microstructure (bainite for steel or aus-ferrite for cast iron) Cooling in air to room temperatureWhen speaking of performance improvements, austem-pered materials are typically compared to conven-tionally quench and tempered materials with a tem-pered martensite microstructure. In cast irons these improvements include [2]: Higher tensile strength, Higher ductility and impact resistance for a given hardness,Figure 1 I-T Diagram of the Austempering Process, [1] 150S. SAVIĆEVIĆ et al.: INFLUECE OF THE AUSTEMPERING TEMPERATURE ON THE TENSILE STRENGTH... METALURGIJA 56 (2017) 1-2, 149-152 A low distortion, repeatable dimensional response, Increased fatigue strength, Increased wear resistance for a given hardness.EXPERIMENTAL PROCEDUREThe main idea of the experiment was to produce un-alloyed ductile iron and prepare samples for heat treat-ment, heat treatment of the samples at different austem-pering temperatures, and at the end microstructure and tensile strength investigation and the comparison of the results [8]. The ductile iron base material was prepared and the chemical composition of the melt is presented in Table 1. Table 1  Chemical composition of the base material / wt. %C Si Mn S P Mg Ce3,48 2,10 0,40 0,012 0,027 0,045 4,18Malt was poured in to U blocks and after cooling the samples for heat treatments were cut. The samples were prepared according to BAS EN 10054-1/98 [9].The main task of the investigation was to determine the influence of the austempering temperature on the ten-sile strength of the treated samples. According to  pervi-ous investigations and the research literature,  the austem-pering temperature for ductile iron is in the range 250 - 420 °C. In case of this experiment nine different austem-pering temperatures were chosen (260 °C, 280 °C, 300 °C, 320 °C, 340 °C, 360 °C, 380 °C, 400 °C and 420 °C).According to the plan of the experiment, ten sets of the samples were cut from the U block (nine sets for the nine different austempering temperatures and one for the tensile test of the base material). Each sample set consists of three samples (to have the average value of the tensile strength). All the other parameters of the heat treatment (austenitization time and temperature and austempering time) were the same for all the treated samples. Prior to the heat treatment of the samples, a dilatomet-ric investigation of the base material was conduct to set the austenitization temperature. A DIL 402/C/7 (Netzsch) dilatometer was used (heating/cooling rate: 5 / K/min). The dilatometric curve is presented in Figure 2. Based on the dilatometric curve shown in Figure 2, an austenitization temperature of 850 °C was set. The austenitization time (45 / min) and austempering time (60 / min) were chosen based on the geometry and thickness of the samples.The heat treatment diagram of the samples is pre-sented in Figure 3.An induction furnace for austenitization and a KNO3 salt bath for austempering were used.The microstructure of the base and the heat treated samples were investigated using an Olympus optical microscope. For tensile strength investigation, a “Loshenhausenwerk MSU-FD-5000” universal testing machine was used.RESULTS AND DISCCUSSIONThe microstructure of the base material was a pearl-itic/ferritic metallic matrix with embedded graphite nodules. The microstructure of the base material is pre-sented in Figures 4 and 5. The tensile strength of the base material (the average value of the three samples) was 690 / MPa. According to BAS EN 1563:1997 the base material corresponds to the EN-GJS-600 class of ductile iron [10]. Figure 2 Dilatometric curve of the base materialFigure 3 The heat treatment diagramFigure 4 Microstructure of the base material, unetched, 100x151S. SAVIĆEVIĆ et al.: INFLUECE OF THE AUSTEMPERING TEMPERATURE ON THE TENSILE STRENGTH...METALURGIJA 56 (2017) 1-2, 149-152Figure 5  Microstructure of the base material, Nital etched, 100xThe results of the tensile strength investigation of the heat treated samples at the different austempering temperature are presented in Table 2 and Figure 6Table 2  Tensile strength of the heat treated samples (average value of three tests)Ta / °C Rm / MPa1st test 2nd test 3rd test Average260 1 364 1 361 1 368 1 364280 1 308 1 385 1 345 1 346300 1 300 1 305 1 316 1 307320 1 230 1 258 1 214 1 234340 1 084 1 369 1 112 1 188360 970 1060 1009 1 013380 890 920 893 901400 697 794 840 777420 640 792 803 745Figure 6  Tensile strength of the heat treated samples for different austempering temperaturesThe microstructure of the sample after heat treat-ment at the corresponding austempering temperature is presented in Figures 7 to 11.CONCLUSIONSSummarizing the data of all the heat treatments, the microstructure and tensile testing investigation, the fol-lowing can be concluded:Figure 7  Microstructure of the tested sample. Austempering temperature 260 °C. Nital etched, 500XFigure 8  Microstructure of the tested sample. Austempering temperature 300 °C. Nital etched, 500XFigure 9  Microstructure of the tested sample. Austempering temperature 340 °C. Nital etched, 500X Heat treatment is a very powerful tool for improving the mechanical properties of the ductile iron The plan of the experiment was prepared correctly and the results obtained follow the research idea All the heat treated samples show an increase in ten-sile strength compared to the base material With the increasing austempering temperature, the tensile strength of the heat treated samples decreases152S. SAVIĆEVIĆ et al.: INFLUECE OF THE AUSTEMPERING TEMPERATURE ON THE TENSILE STRENGTH... METALURGIJA 56 (2017) 1-2, 149-152 The microstructure of the heat treated samples con-sists of needle like acicular ferrite and retained aus-tenite and was uniform at the cross section of the tested material. The size of the acicular ferrite needles increases as the aust empering temperature increases.AcknowledgementsThis work was supported by the Federal Ministry of Education and Science of the Federation of Bosnia and Herzegovina and the “CIMOS TMD CASTING” d.o.o. Foundry, Zenica.LITERATURE[1] A. G. F. Alabi, F. I. Aluko, Production of Austempered Ductile Iron with Optimum Sulphur Level for Effective mechanical Properties, The International journal Of Engi-neering and Science (IJES), 2 (2013) 12, pp. 67-71.[2] J. R. Keough, K. L. Hayrynen, Designing with Austempered Ductile Iron (ADI),  AFS Proceedings 2010, Schaumburg, IL USA, 2010, American Foundrymen Society, pp. 1 to 15.[3] Heat Treater’s Guide: Practices and procedures for Irons and Steels” ASM International, Harry Chandler ed., Mate-rials Park, Ohio, Second Edition,1995, p 824-835.[4] M. Ostojić, A. Beroš, Nodularni liv, Fakultet za metalurgi-ju i materijale, Zenica, 2008.[5] H. Avdušinović, A. Gigović, Heat Treatment of Nodular Cast Iron, TMT Proceedings, S. Ekinovic ed. Hammamet Tunisia, 2009, p. 669-672.[6] K. M. Kumar, P. Hariharan, P. Venkateshwaran, S. Tamila-rasan, Examination of Microstructure and Mechanical Properties of Austempered Ductile Iron (ADI) As Per Austempering Temperature and Time, Transactions of the Indian Institute of Metals, 68 (2015) 1, pp 67-71.[7]  A. Abedi, S.P.H. Marashi, K. Sohrabi, M. Marvastian, S.M.H. Mirbagheri, The Effect of Heat Treatment Parame-ters on Microstructure and Toughness of Austempered Ductile Iron (ADI), Advanced Materials Research, 264-265 (2011), pp. 409-414.[8] M. Janjić, H. Avdušinović, Z. Jurković, F. Bikić, S. Savi-ćević, Influence of austempering heat treatment on mecha-nical and corrosion properties of ductile iron samples, Metalurgija, 55 (2016) 3, pp. 325-328.[9] BAS EN 10054-1/98.[10] BAS EN 1563:1997.Note:  The person responsible for English language translation: Matt Whiffen, MA. The Faculty of Philology, University of Montene-gro., Podgorica, MontenegroFigure 10  Microstructure of the tested sample. Austempering temperature 380 °C. Nital etched, 500XFigure 11  Microstructure of the tested sample. Austempering temperature 420 °C. Nital etched, 500X

Influece of the austempering temperature on the tensile strength of the austempered ductile iron (ADI) samples

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