TRIBOLOGICAL PROPERTIES OF ISOTHERMALLY UPGRADED NODULAR CAST IRONS IN CONTACT WITH CARBON STEEL AND GREY PIG-IRON CASTING

Transcription

1 118 NATIONAL TRIBOLOGICAL PROPERTIES OF ISOTHERMALLY UPGRADED NODULAR CAST IRONS IN CONTACT WITH CARBON STEEL AND GREY PIG-IRON CASTING Dušan Ješić Tribotehnik, Titov trg 6/4, Rijeka, Croatia ABSTRACT This paper presents a part of the results of experimental research of tribological properties of two types of isothermally upgraded nodular castings and one type of alloyed steel in contact with carbon steel and grey pig-iron casting. The research was performed on a device with a rotating disk and a stationary pin. The obtained results of wear are presented as diagrams. KEYWORDS: wear, material, contact. 1. INTRODUCTION Tribological properties of materials are determined by measuring either the friction force in the contact zone or by measuring the value of wear of one element of the tribomechanical system. The value of friction force as well as the value of wear of one or both elements of the tribomechanical system after a certain duration of contact depend on numerous factors that define the conditions under which the contact is realised (external load, sliding velocity, lubricant type, etc.). After a certain duration of contact, tribological properties of materials are relative and dependent both on the methods that determine them as well as on the conditions under which the contact between elements of the tribomechanical system is realised. Comparison shows whether tribological properties are determined by measuring the force of friction i.e. friction coefficient or by measuring the level of wear of one of the elements of the tribological system. Here, the conditions of contact can have significant influence on the relation of the tribological properties of two materials. Measuring methods of the tribological properties of materials in laboratory conditions are based as a rule on using the tribometer "Pin on Disk" which allows all three basic contact geometries (point, line, area). Sophisticated equipment and devices fitted into the tribometers allow with accompanying instruments, precise and reliable measurement of normal force or friction force i.e. friction coefficient. In order to measure wear or pin after a certain duration of contact numerous methods are used which are based on measuring their dimensions (length of pins or disk diameters) or traces left by the wear on contact zones of the elements in contact. Reliability and accuracy of these measurements are relatively small if the contact duration is not long. In case of greater wear of the elements of the tribomechanical systems of this kind (pin or disk) which allow greater reliability of measurements, either the disk or the pin Table 1 Chemical composition of nodular cast irons NL 420 and NL 600 Chemical composition in % Nodular casting C Si Mn Mg P S Cu NL NL Table 2 Measured properties of nodular cast iron pins. Thermal processing Nodular casting T a in C/t in min T p in C/t in min Hardness in HV 5 Structure NL /60 390/90, 390/60, 390/ ferrite-perlite NL /60 390/90, 390/60, 390/ mainly perlite

2 NATIONAL 119 need to be changed when transition is made to other conditions of contact (another sliding velocity). Changing either pin or disk during a certain research programme can result in poor repeatability of measurement results and in increased number of experiments in order to insure sufficient reliability of the research results. `This paper presents one part of the study programme where two disks were used, one made of grey pig-iron casting SL 250 and the other made of steel Č 1530, whereas pins were made of nodular cast irons NL 420 and NL 600, and one pin was made of steel Č 3840 which will be mentioned in this paper. 2. PROGRAM AND CONDITIONS OF EXPERIMENTAL STUDY Experimental studies were carried out by means of tribometer "pin on disk" with line contact between the pins face and circumference of the disk area Isothermal upgrading of nodular cast irons NL 420 AND 600 Isothermal upgrading means that the specimen is suddenly cooled from the austenisation temperature to the isothermal temperature that is higher than the temperature at which martensite starts to form. It is maintained for a certain period of time at temperature T i, until the desired transformation starts, and then it is cooled in the air. The goal is to avoid transformation of austenite into martensite since this may cause strain and cracks. By maintaining at T i the so-called inter-structure bainite is obtained. By machining it provides the workpiece with extremely good mechanical properties. Isothermal upgrading was carried out at the Laboratory for Thermal Processing at the Faculty of Mechanical Engineering and Naval Architecture in Zagreb. Table 3 Comparison properties of nodular cast irons. Mechanical properties NL 420 in starting state NL 600 in starting state Tensile strength R m [N/mm 2 ] Pour point R p0.2 [N/mm 2 ] Post-fracture extension A 5 [%] Post-fracture contraction Z [%] 19 Hardness HB Fracture impact K [J] Fig. 1. Structure of NL 420 charge magnified 500 times, corroded, with ferriteperlite base Fig. 2. NL 600 charge , magnified 500 times, corroded, of mainly perlite base Fig. 4 Diagram of the isothermally upgraded specimen.

3 120 NATIONAL Fig. 3. Tribometer TPD 95-C "Pin on Disk" with accompanying instruments. Table 4 Testing conditions. Normal load [N] Temperature ambient ambient Sliding velocity [m/s] 2 2 Type of lubrication Boundary boundary Duration of contact [min] Table 5. Elements of tribomechanical system. Contact geometry line line Pin material NL 420, 51 HRC NL 600, 42 HRC Pin area R a = 0.23 µm R a = 0.25 µm R max = 1.7 µm R max = 2.6 µm Disk material Č 1531, 52HRC SL 250, 46 HRC Disk area R a = 0.60 µm R a = 0.50 µm R max = 6.2 µm R max = 4.2 µm Lubricant HVL-22 Modriča HVL-22 Modriča Table 6 Study results Friction force [N] Friction coefficient b pin (sr) [mm] b pin(av) δ pin(sr) [mm] δ pin(av) Disk area R a = 0.54 µm R max = 6.0 µm R a = 0.43 µm R max = 3.8 µm

4 NATIONAL 121 Fig. 4 Contact geometry and pin's surface wear that could be generalized to all studied materials. Fig. 5 Shape of friction coefficient signal at the beginning and at the end of contact between pin NL 420 and disk Č Study program The plan of the study program included the usage of two disks, one made of grey pig-iron casting SL 250 and the other made of steel Č 1530; three pins, of which two made of nodular cast irons NL 420 and NL 600 and one made of steel Č 3840, which is only partly mentioned in the paper. The study included ten experiment tests in the duration of 60 minutes of effective contact with boundary lubrication. 3. RESULTS OF RESEARCH The intensity of wear has been presented of one element of the tribomechanical system of any defined type i.e. selected parameter of wear compared to the duration of contact between its elements. As wear parameter, most often the change of dimension of one element in contact is used, and in this case the parameters of pin and disk topography are important prior to and following the formation of contact wear trace on the pin. 1. Each point presented on the diagrams of friction coefficient represents the mean value of the measured 750 points over a period of 5 seconds, 2. During the 60-minute test of continuous contact there was no destruction of the oil film. 3. The mentioned values of the width and depth of the wear trace in case of pin have been obtained by microscope measuring and by calculation, according to the given expression (Fig. 5). 4. CONCLUSION Based on the performed experiments it may be concluded that the application of isothermal upgrading of both types of nodular cast irons can improve their tribological properties measured through wear resistance. The greatest improvement of wear resistance has been realised by isothermal upgrading of nodular cast iron NL 600 by keeping it at temperature T p over a time period of 60 minutes. Friction forces, i.e. friction coefficients are lower when contact is made between nodular cast iron (pins) and steel and grey pig-iron casting (disks) than friction forces and friction coefficients that occur in contact between steel on one side and steel and grey pig-iron casting on the other side.

5 122 NATIONAL Fig. 6 Topography parameters of pin and disk area prior to and following the beginning of wear on pin NL 420. and disk Č Fig. 7 Topography parameters of pin and disk area prior to and following the beginning of wear on pin NL 600 and disk Č Fig. 8 Friction coefficient in dependence on the material of elements in contact at the sliding velocity of v=2 ms -1 and lubrication with lubricant HVL-22, measured 30 minutes after contact. Fig. 9 Width of the wear trace in dependence on the material of elements in contact at the sliding velocity of v=2 ms -1 and lubrication with lubricant HVL-22, measured 30 minutes after contact.

6 NATIONAL 123 Lower friction forces and friction coefficients were obtained when using NL 600 pin. The lowest wear trace was obtained on pins made of nodular cast irons NL 420 and NL 600. Nodular cast irons have significant advantage over steel regarding wear properties. Nodular cast irons also have a slight but not negligible advantage over steel regarding their anti-friction properties. It may be generally concluded that nodular cast irons have better tribological properties from the aspect of energy and economy than the tested steel in the given conditions. However, for their concrete implementation, every material has to be tested separately in actual operating conditions. One should also be careful and know well the tribomechanical system, its complexity, importance and responsibility of every single part and to which working conditions and strain it is subjected. Table 7 Material comparison table HRN DIN. HRN DIN Č 1531 Ck45 Č MnV8 SL 250 GG-25 NL 420 GGG-40 NL 600 GGG LIST OF SYMBOLS Tensile strength Pour point Post-fracture extension Post-fracture contraction Hardness Fracture impact Upgrading temperature Mean profile deviation Maximum depth of profile bottom Average width of the wear trace Disk radius Depth of the pin trace REFERENCES 1. Barbezat, Sulzer, 1986, GGG-100, Austempered Ductile Iron (ADI), 2nd International Conference on ADI, Mart l986, Ann Arbor, USA. 2. Ivković B., Rac A., 1995, Tribologija, Jugoslavensko Društvo za Tribologiju, Kragujevac Ješić D., 1999, Mašinski materijali, Tehnologija dobijanja i primena, Jugoslavensko Društvo za Tribologiju, Kragujevac.. 4. Ješić D., 2000, Tribological Properties of Nodular Cast Iron, Journal of the Balkan Tribological Association, Sofia.