On the possibility of replacing high manganese cast steel military vehicle track pads with ADI

Transcription

1 ARCHIVES of FOUNDRY ENGINEERING Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences ISSN ( ) Volume 10 Issue Special1/ /1 On the possibility of replacing high manganese cast steel military vehicle track pads with ADI M. Kaczorowski a), P. Skoczylas a), A. Krzyńska b) a) Institute of Mechanics and Design, b) Institute of Materials Processing, Faculty of Production Engineering, Warsaw University of Technology, ul. Narbutta 85, Warszawa, POLAND Received ; accepted in revised form Summary The theoretical considerations of possibility replacing of high manganese cast steel used for military vehicle track pads with ADI are presented. Except these considerations, comparative investigations including tensile strength tests hardness measurements and impact resistance were included. Moreover, the structure investigation was carried out using either conventional light metallography and scanning (SEM). The last one was applied for fractography investigations, the aim of which was to discover the mode of fracture. The discussion of experimental results leads to conclusion that ADI, known as high friction resistant, looks to be concurrent material with respect to high manganese cast steel used now for tang track pads. Key words: High manganese cast steel, ADI, Mechanical properties, Structure 1. Introduction Before we reach the essence of this paper let me say something about its origin. As probably many know I was one of the first in Poland who stared with ADI which was born in the end of Working many years at the division of Casting in Warsaw University of Technology I was looking for the structure mechanical properties relationships in different category of ADI. Four years ago, i.e. in 2005 I started to work at the Division of Mechanics and Armor Technology at the Institute of Mechanics and Design. It was obvious me to switch my interests to the materials used for armor application. At the end of 2009 I was invited to the seminar organized in Institute of Casting in Krakow devoted the promotion of ADI. I was really proud this invitation. Considering past and today collaboration in subject of ADI, I have decided to attend at this meeting. The result of it was the question: why not to start with ADI as a material for parts used in military equipment? Because of today s interests and good collaboration with plants producing for army, and knowing the most outstanding property of ADI which is hardness and wear resistance I took decision to begun with military vehicle track pads which now are produced from high manganese Hadfield type steel or cast steel. The first step was to receive the original material from Stalowa Wola factory where the track pads are used in many different products. 2. Characterization of the material Because of comparative character of this paper, at the beginning we had to collect information about original materials used now in track pad manufacturing. So first the specimens for mechanical testing and metallography investigations were cut from track pads used now in MTLB, 2S1 and BWP (fig.1). Fig.1. The armored transporter MTLB, 2S1 According to information given by producer the material used for track pads is high manganese cast steel denoted as a L120G13 ARCHIVES OF FOUNDRY ENGINEERING Volume 10, Special Issue 1/2010,

2 [PN-88/H-83160]. This steel is well known as one of the most popular wear resistant material. It is used either as wrought or cast material and was invented by Sir Robert Hadfield in The steel contain about 1,2%C and 12%Mn [1]. Hadfield steel is unique in that it combined high toughness and ductility with high work hardening ability, and, usually good wear resistance. These specific properties are caused by martensitic transformation proceeding under load (shearing). Hadfield s austenitic manganese steel is still used extensively with minor modification in chemical composition and heat treatment parameters mainly for manufacturing of parts where wear resistance is crucial condition. The information s concerning this material in polish text book is rather poor. The chemical composition taken from [2] is given in table 1. This steel is usually used in heat treatment state involving austenitizing from the o C Table 1. The chemical composition of L120G13 cast steel [2] Element concentration [weight %] C Mn Cr Ni Si P S ) The symbol HYS in table 2 denote High Yield Strength grade manganese steel. More information is given in book [3] but because it is very old elaboration the authors decided to look closer into more new ASM Hand Book [4]. According to [5], chemical composition of high manganese austenitic cast steel can substantially vary depends on the properties needed (table 2). a Rm & Re [MPa] Standard Cr 1%Mo 2%Mo HYS Steel grade Rm [MPa] Re [MPa] b. Table 2. Composition of some high manganese cast steels [3] Grade Element concentration [weight %] C Mn Cr Mo Other Standard Chromium %Mo %Mo HYS 1) %Ni, %V The mechanical properties of different grades cast steels given in table 2 were showed below (fig.2) Let s characterize the mechanical properties of some ADI grades taking into account that the material should posse s not high hardness but also ductility and impact resistance. In table 3 three ADI grades were shows. First of all it is easy to see that mechanical properties of ADI grades found in ASTM 897 are a little higher than those in EN Standards. Moreover, the ADI grades given by EN Standard have no information about impact resistance evaluated with notch - off specimens. Although it would not be the truth we assume that KC values for EN grade of ADI are close to those given by ASTM. It follows from table 3 that only these three grades of ADI can be considered as materials concurrent to high manganese austenitic cast steel. Tensile elongation [%] Standard Cr 1%Mo 2%Mo HYS Steel grade c Impact Izod Energy [J] Standard Cr 1%Mo 2%Mo HYS Steel grade Fig.2. Typical mechanical properties different grades high manganese cast steel: a tensile and yield strength, b tensile elongation and c Izod impact energy (According to [5]) 82 ARCHIVES OF FOUNDRY ENGINEERING Volume 10, Special Issue 1/2010, 81-86

3 Table 3. Mechanical properties some grades of ADI [6, 7] Mechanical properties Grade R m R p, 0.2 A 5 HB KC MPa MPa [%] - [J] EN-GJS ASTM 897 Grade EN-GJS ASTM 897 Grade EN-GJS ASTM 897 Grade Experiment 3.1. Material for testing To characterize the properties of the material used for track pad manufacturing the authors decided to perform experiments enabling to get the basic information on its mechanical properties. The specimens for experimental testing were taken from one of two the track pads received from Stalowa Wola Plant. The locations of the place from where the specimens were cut are shown in fig.3. Fig.3. The track pad used for the experimental studies The material was subjected to mechanical testing including tensile and compression loading. Moreover impact test and structure observation were performed. The last one included conventional metallography and SEM investigations Experimental procedure The tensile test experiment was performed on fivefold specimens using Instron 1115 machine. To evaluate the average values of R m, R p, 0.2 and A 5 three specimens were used. Except tensile strength, compression and impact test were carried out. The specimens for microstructure investigations were first grinded and then polished with automatic Tenupol equipment. The microstructure was studied with Olympus IX-70 light microscope using different magnification and observations mode. For fractography observations scanning electron microscopy (SEM) was applied. The SEM observations were done with Leo 1530 electron microscope. To compare the results of this experiment with those for austempered ductile iron, the results of earlier ADI studies were recalled. 4. Results 4.1. Mechanical properties The results of mechanical testing are given in table 4. Neglecting for the moment the analysis of the values given in table 4 it is very easy to see that the results are highly dispersed, especially if elongation and impact resistance is considered. Table 4. The mechanical properties of L120G13 R p,0.2 R m A 5 Hardness KC KCV MPa MPa [%] HB HRB [J] ]J/cm 2 ] ,4 71,42 1, ,5 174,1±3.2 89,1±0.5 67,59 1, ,1 42,28 1,06 1) ) Specimen rejected 69, The microstructure The results of metallography observations are showed in fig.4. In first photo (fig.4a) the typical as cast microstructure is clear visible. Except grain boundaries specific contrast suggesting heterogeneity of chemical composition across the grains can be discovered. The second micrograph (fig.4b) illustrates the microstructure close to the free surface of the specimen. In here characteristic needle-like microstructure appears. The needles are distributed at specific angles each to other (fig.4c) The specific microstructure at free surface of the specimen was caused by shearing stresses which as all know are responsible for austenite martensite transformation in Hadfield type austenitic steel. Careful inspection of the photo in fig.4c allows discovering very small equiaxed particles of carbides (fig.4c) very often distributed along needles of martensite [5] Fractography The aim of scanning electron microscope observations was to investigate the character of fracture surface of the specimens broken in tensile test experiment. The examples of fracture surfaces L120G13 specimens were shown in fig.5 and 6. The first (fig.5) shows the fracture surface of the specimen with the lowest (4%), while the next (fig.6) the specimen having 20% elongation. In both cases the SEM micrographs show quite ductile mode of fracture [8]. However, except areas representing high ductility intergranular fracture mode was identified. Moreover in fig.5b an example void with brittle precipitates which were identified from time to time was shown. On the other hand the feature of broken surfaces in the specimen of high ductility was characteristic intersecting lines visible on the almost flat surfaces (fig.6b). ARCHIVES OF FOUNDRY ENGINEERING Volume 10, Special Issue 1/2010,

4 a. b. c. Fig. 4. The microstructure of L120G13 cast steel: a central part of the specimen, b at the free surface and, c martensite needles observed at higher magnification a. b. Fig.5. The morphology of the fracture surface in L120G13 austenitic steel with elongation 4%: a mixed (intergranular and ductile) mode of fracture, b small particles located inside the shrinkage voids 5. Discussion and conclusions In this part of the paper the authors would like to compare the result of the experiment on L120G13 given above with the mechanical properties of ADI (table 3) and microstructure investigation obtained in earlier studies [10]. It looks to us, that from point of view replacing of L120G13 austenitic cast steel with ADI, low strength ADI than high strength would be better choice. This is why in table 3 only the low strength grade 84 ARCHIVES OF FOUNDRY ENGINEERING Volume 10, Special Issue 1/2010, 81-86

5 of ADI were given. The examples of microstructure of these ADI grades are show below (fig.7). As can be seen from the pictures the microstructure consists of ferrite lath in substantial amount of austenite providing high ductility. The broken surfaces of this ADI grades show many deep dimples reflecting ductile mode of fracture (fig.8) a. b. Fig.6. The morphology of the fracture surface in L120G13 austenitic steel with elongation 20%: a mixed (intergranular and ductile) mode of fracture, b highly ductile type of fracture a. b. Fig.7. The microstructure of ADI: a EN GJS grade and b EN GJS grade [10] Fig.8. The fracture surface of high ductile ADI [10] So let us turn again to comparison of L120G13 austenitic cast steel with EN-GJS grade ADI. ADI has the ultimate yield strength and tensile strength substantially higher than cast L120G13 cast steel. The ADI hardness ( HB) is much higher than L120G13 cast steel (175HB). It is well known that mechanism assuring high wear resistance of L120G13 cast steel is caused by martensitic transformation caused by shearing. In case of ADI the wear resistance depends on its hardness which as was said is much higher than cast steel. So probably it would be more correct compare the hardness of ADI with the hardness of martensite at the surface. On the other hand, in case of hardness measurement, the material has to be partially strengthened and we can suspect that the values given in table 4 reflect the real hardness although it might be different from that during service. As follows from our experiment the impact resistance of L120G13 cast steel is in average 70J (table 4) while EN-GJS grade ADI reach the value 100J (table 3). Although both ARCHIVES OF FOUNDRY ENGINEERING Volume 10, Special Issue 1/2010,

6 materials differ in microstructure the mode of fracture is similar. Both cast steel and ADI show ductile type of fracture surface characterized with quite deep dimples [ASM]. There are some technological properties which should be taken into account. These are melting temperature, castability and shrinkage susceptibility. The casting temperature of ADI is much lower than cast steel. Castability of ductile iron from which ADI is produced is much better than cast steel [10]. Finally the susceptibility of cast steel to shrinkage is very high in case of cast steel. This force the foundryman for design special high volume risers what substantially decrease the yield. If the feeding is not assured the shrinkage may lead to discontinuities and voids, one o many found in casting being subject of these studies was shown in fig.5b. Shrinkage is not the real problem in case of ductile iron, where even riser-less sound casting can be obtained when rigid moulds were used. In case of ductile iron the expansion accompanying graphite precipitation compensate the shrinkage caused by solidification of liquid iron. One think more should be compared this is heat treatment. Both materials are heat treated. The L120G13 cast steel is used in supersaturated state. It means that after casting it subjected to austenitizing followed by rapid cooling. ADI is hat treated to. First the castings are subjected to austenitization and then isothermally quenched at given temperature. The authors did not calculate the cost of each heat treatment but it looks to us that these are not substantially different. Taking into account the considerations given above lead the authors to the final conclusion that ADI can be considered as a good substitute for L120G13 austenitic cast steel used now for track pad. The reasons pushing us to such conclusion are: 1. Comparable or complementary mechanical properties assuring good behavior in service. 2. Lower manufacturing costs following from lower melting temperature and much higher yield. 3. More silent work during service. 4. Lower casting rejected because of much better castability, and lower shrinkage susceptibility. References [1] Metals Handbook: vol. 1. Tenth Edition: Properties and Selection: Iron, Steels and High Performance Alloys, Metals Park, Ohio, 1990 [2] PN-72?H-83160, GOST [3] G. Kniaginin: Staliwo, Metalurgia i Odlewnictwo, Wyd. Śląsk, Katowice, [4] Metals Handbook vol. 19, Ninth Edition: Casting, Metals Park, Ohio, 1988 [5] Metals Handbook, vol. 9, Tenth Edition: Metallography and Microstructures, Metals Park, Ohio, 1990 [6] PN EN 1564 [7] ASTM A A897 [8] Metals Handbook vol. 12, Ninth Edition: Fractography, Metals Park, Ohio, 1987 [9] Kaczorowski M., Krzyńska A.: The study of mechanical properties and structure of austempered ductile iron (ADI) Proc. 4 -th International Conference on Advances in Production Engineering, APE 07, Warsaw, June 2007, pp [10] Krzyńska A., Kaczorowski M.: The studies of nodular graphite cast iron early stages austemperingy, Archives of Foundry Engineering, 2008, vol.8, Issue 4, p ARCHIVES OF FOUNDRY ENGINEERING Volume 10, Special Issue 1/2010, 81-86